Saturday, June 22, 2024

Continuous Integration

Software DevelopmentContinuous Integration

I vividly remember one of my first sightings of a large software project.
I was taking a summer internship at a large English electronics company. My
manager, part of the QA group, gave me a tour of a site and we entered a
huge, depressing, windowless warehouse full of people working in cubicles.
I was told that these
programmers had been writing code for this software for a couple of years,
and while they were done programming, their separate units were now being
integrated together, and they had been integrating for several months. My
guide told me that nobody really knew how long it would take to finish
integrating. From this I learned a common story of software projects:
integrating the work of multiple developers is a long and unpredictable

I haven’t heard of a team trapped in such a long integration like this
for many years, but that doesn’t mean that integration is a painless
process. A developer may have been working for several days on a new
feature, regularly pulling changes from a common main branch into her
feature branch. Just before she’s ready to push her changes, a big change
lands on main, one that alters some code that she’s interacting with. She
has to change from finishing off her feature to figuring out how to
integrate her work with this change, which while better for her colleague,
doesn’t work so well for her. Hopefully the complexities of the change will
be in merging the source code, not an insidious fault that only shows when
she runs the application, forcing her to debug unfamiliar code.

At least in that scenario, she gets to find out before she submits her
pull request. Pull requests can be fraught enough while waiting for someone
to review a change. The review can take time, forcing her to context-switch
from her next feature. A difficult integration during that period can be very
disconcerting, dragging out the review process even longer. And that may not
even the be the end of story, since integration tests are often only run
after the pull request is merged.

In time, this team may learn that making significant changes to core code
causes this kind of problem, and thus stops doing it. But that, by
preventing regular refactoring, ends up allowing
cruft to grow throughout the codebase. Folks who encounter a crufty
code base wonder how it got into such a state, and often the answer lies in
an integration process with so much friction that it discourages people from
removing that cruft.

But this needn’t be the way. Most projects done by my colleagues
at Thoughtworks, and by many others around the world, treat
integration as a non-event. Any individual developer’s work is
only a few hours away from a shared project state and can be
integrated back into that state in minutes. Any integration errors
are found rapidly and can be fixed rapidly.

This contrast isn’t the result of an expensive and complex
tool. The essence of it lies in the simple practice of everyone on
the team integrating frequently, at least daily, against a
controlled source code repository. This practice is called “Continuous
Integration” (or in some circles it’s called “Trunk-Based Development”).

In this article, I explain what Continuous Integration is and how to do
it well. I’ve written it for two reasons. Firstly there are always new people
coming into the profession and I want to show them how they can avoid that
depressing warehouse. But secondly this topic needs clarity because
Continuous Integration is a much misunderstood concept. There are many
people who say that they are doing Continuous Integration, but once they describe
their workflow, it becomes clear that they are missing important pieces. A
clear understanding of Continuous Integration helps us communicate, so we know
what to expect when we describe our way of working. It also helps folks
realize that there are further things they can do to improve their experience.

I originally wrote this article in 2001, with an update in 2006. Since
then much has changed in usual expectations of software development teams.
The many-month integration that I saw in the 1980s is a distant memory,
technologies such as version control and build scripts have become
commonplace. I rewrote this article again in 2023 to better address the
development teams of that time, with twenty years of experience to
confirm the value of Continuous Integration.

Building a Feature with Continuous Integration

The easiest way for me to explain what Continuous Integration is and how it works is to
show a quick example of how it works with the development of a small
feature. I’m currently working with a major manufacturer of magic potions, we
are extending their product quality system to calculate how long the
potion’s effect will last. We already have a dozen potions supported in
the system, and we need to extend the logic for flying potions. (We’ve
learned that having them wear off too early severely impacts customer
retention.) Flying potions introduce a few new factors to take care of,
one of which is the moon phase during secondary mixing.

I begin by taking a copy of the latest product sources
onto my local development environment. I do this by checking out the
current mainline from the central repository with
git pull.

Once the source is in my environment, I execute a command to build
the product. This command checks that my environment is set up correctly, does
any compilation of the sources into an executable product, starts the
product, and runs a comprehensive suite of tests against it. This should
take only a few minutes, while I start poking around the code to
decide how to begin adding the new feature. This build hardly ever fails,
but I do it just in case, because if it does fail, I want to know before I
start making changes. If I make changes on top of a failing build, I’ll
get confused thinking it was my changes that caused the failure.

Now I take my working copy and do whatever I need to do to deal with
the moon phases. This will consist of both altering the product code, and
also adding or changing some of the automated tests. During that time I
run the automated build and tests frequently. After an hour or so I have
the moon logic incorporated and tests updated.

I’m now ready to integrate my changes back into the central repository. My
first step for this is to pull again, because it’s possible, indeed
likely, that my colleagues will have pushed changes into the mainline
while I’ve been working. Indeed there are a couple of such changes, which
I pull into my working copy. I combine my changes on top of them and run
the build again. Usually this feels superfluous, but this time a test
fails. The test gives me some clue about what’s gone wrong, but I find it
more useful to look at the commits that I pulled to see what changed. It
seems that someone has made an adjustment to a function, moving some of its
logic out into its callers. They fixed all the callers in the mainline
code, but I added a new call in my changes that, of course, they couldn’t
see yet. I make the same adjustment and rerun the build, which passes this

Since I was a few minutes sorting that out, I pull again, and again
there’s a new commit. However the build works fine with this one, so I’m
able to git push my change up to the central repository.

However my push doesn’t mean I’m done. Once I’ve pushed to the mainline
a Continuous Integration Service notices my commit, checks out the changed
code onto a CI agent, and builds it there. Since the build was
fine in my environment I don’t expect it to fail on the CI Service,
but there is a reason that “works on my machine” is a well-known
phrase in programmer circles. It’s rare that something gets missed that
causes the CI Services build to fail, but rare is not the same
as never.

The integration machine’s build doesn’t take long, but it’s long enough
that an eager developer would be starting to think about the next step in
calculating flight time. But I’m an old guy, so enjoy a few minutes to
stretch my legs and read an email. I soon get a notification from the CI
service that all is well, so I start the process again for the next part of
the change.

Practices of Continuous Integration

The story above is an illustration of Continuous Integration that
hopefully gives you a feel of what it’s like for an ordinary programmer to
work with. But, as with anything, there’s quite a few things to sort out
when doing this in daily work. So now we’ll go through the key practices
that we need to do.

Put everything in a version controlled mainline

These days almost every software team keeps their source code in a
version control system, so that every developer can easily find not just
the current state of the product, but all the changes that have been
made to the product. Version control tools allow a system to be rolled
back to any point in its development, which can be very helpful to
understand the history of the system, using Diff Debugging to find bugs. As I write this, the dominant
version control system is git.

But while version control is commonplace, some teams fail to
take full advantage of version control.
My test for full version control is that I should be able to walk
up with a very minimally configured environment – say a laptop with no
more than the vanilla operating system installed – and be able to easily
build, and run the product after cloning the repository. This means the
repository should reliably return product source code, tests, database
schema, test data, configuration files, IDE configurations, install
scripts, third-party libraries, and any tools required to build the

I should be able to walk up with a laptop loaded with only an
operating system, and by using the repository, obtain everything I need to
build and run the product.

You might notice I said that the repository should return all
of these elements, which isn’t the same as storing them. We don’t have
to store the compiler in the repository, but we need to be able to
get at the right compiler. If I check out last year’s product sources, I
may need to be able to build them with the compiler I was using last year,
not the version I’m using now. The repository can do this by storing a
link to immutable asset storage – immutable in the sense that once an
asset is stored with an id, I’ll always get exactly that asset back
again. I can also do this with library code, providing I both trust the
asset storage and always reference a particular version, never “the latest

Similar asset storage schemes can be used for anything too large,
such as videos. Cloning a repository often means grabbing everything,
even if it’s not needed. By using references to an asset store, the
build scripts can choose to download only what’s needed for a particular

In general we should store in source control everything we need to
build anything, but nothing that we actually build. Some people do keep
the build products in source control, but I consider that to be a smell
– an indication of a deeper problem, usually an inability to reliably
recreate builds. It can be useful to cache build products, but they
should always be treated as disposable, and it’s usually good to then
ensure they are removed promptly so that people don’t rely on them when
they shouldn’t.

A second element of this principle is that it should be easy to find
the code for a given piece of work. Part of this is clear names and URL
schemes, both within the repository and within the broader enterprise.
It also means not having to spend time figuring out which branch within
the version control system to use. Continuous Integration relies on
having a clear mainline – a single,
shared, branch that acts as the current state of the product. This is
the next version that will be deployed to production.

Teams that use git mostly use the name “main” for the mainline
branch, but we also sometimes see
“trunk” or the
old default of “master”. The mainline is that branch on the central repository,
so to add a commit to a mainline called main I need to first commit to my
local copy of main and then push that commit to the central server. The
tracking branch (called something like origin/main) is a copy of the
mainline on my local machine. However it may be out of date, since in a
Continuous Integration environment there are many commits pushed into
mainline every day.

As much as possible, we should use text files to define the product
and its environment. I say this because, although version-control
systems can store and track non-text files, they don’t usually provide any
facility to easily see the difference between versions.
This makes it much harder to understand what change was made.
It’s possible that in the future we’ll see more storage formats
having the facility to create meaningful diffs, but at the moment clear
diffs are almost entirely reserved for text formats. Even there we need
to use text formats that will produce comprehensible diffs.

Automate the Build

Turning the source code into a running system can often be a
complicated process involving compilation, moving files around, loading
schemas into databases, and so on. However like most tasks in this
part of software development it can be automated – and as a result
should be automated. Asking people to type in strange commands or
clicking through dialog boxes is a waste of time and a breeding ground
for mistakes.

Computers are designed to perform simple, repetitive tasks. As soon
as you have humans doing repetitive tasks on behalf of computers, all
the computers get together late at night and laugh at you.

— Neal Ford

Most modern programming environments include tooling for automating
builds, and such tools have been around for a long time. I first encountered
them with make, one of the earliest Unix

Any instructions for the build need to be stored in the repository,
in practice this means that we must use text representations. That way
we can easily inspect them to see how they work, and crucially, see
diffs when they change. Thus teams using Continuous Integration avoid
tools that require clicking around in UIs to perform a build or to
configure an environment.

It’s possible to use a regular programming language to automate
builds, indeed simple builds are often captured as shell scripts. But as
builds get more complicated it’s better to use a tool that’s designed
with build automation in mind. Partly this is because such tools will
have built-in functions for common build tasks. But the main reason is
that build tools work best with a particular way to organize their logic
– an alternative computational model that I refer to as a Dependency Network. A dependency network organizes
its logic into tasks which are structured as a graph of dependencies.

A trivially simple dependency network might say that the “test” task is
dependent upon the “compile” task. If I invoke the test task, it will
look to see if the compile task needs to be run and if so invoke it
first. Should the compile task itself have dependencies, the network will look to see if
it needs to invoke them first, and so on backwards along the dependency
chain. A dependency network like this is useful for build scripts
because often tasks take a long time, which is wasted if they aren’t
needed. If nobody has changed any source files since I last ran the
tests, then I can save doing a potentially long compilation.

To tell if a task needs to be run, the most common and
straightforward way is to look at the modification times of files. If any
of the input files to the compilation have been modified later than the
output, then we know the compilation needs to be executed if that task
is invoked.

A common mistake is not to include everything in the automated build.
The build should include getting the database schema out of the
repository and firing it up in the execution environment. I’ll elaborate
my earlier rule of thumb: anyone should be able to bring in a clean
machine, check the sources out of the repository, issue a single
command, and have a running system on their own environment.

While a simple program may only need a line or two of script file to
build, complex systems often have a large graph of dependencies, finely
tuned to minimize the amount of time required to build things. This
website, for example, has over a thousand web pages. My build system
knows that should I alter the source for this page, I only have to build
this one page. But should I alter a core file in the publication
tool chain, then it needs to rebuild them all. Either way, I invoke the
same command in my editor, and the build system figures out how much to do.

Depending on what we need, we may need different kinds of things to
be built. We can build a system with or without test code, or with
different sets of tests. Some components can be built stand-alone. A
build script should allow us to build alternative targets for different

Make the Build Self-Testing

Traditionally a build meant compiling, linking, and all the
additional stuff required to get a program to execute. A program may
run, but that doesn’t mean it does the right thing. Modern statically
typed languages can catch many bugs, but far more slip through that net.
This is a critical issue if we want to integrate as frequently as
Continuous Integration demands. If bugs make their way into the product,
then we are faced with the daunting task of performing bug fixes on a
rapidly-changing code base. Manual testing is too slow to cope with the
frequency of change.

Faced with this, we need to ensure that bugs don’t get into the
product in the first place. The main technique to do this is a
comprehensive test suite, one that is run before each integration to
flush out as many bugs as possible. Testing isn’t perfect, of course,
but it can catch a lot of bugs – enough to be useful. Early computers I
used did a visible memory self-test when they were booting up, which led
me referring to this as Self Testing Code.

Writing self-testing code affects a programmer’s workflow. Any
programming task combines both modifying the functionality of the
program, and also augmenting the test suite to verify this changed
behavior. A programmer’s job isn’t done merely when the new
feature is working, but also when they have automated tests to prove it.

Over the two decades since the first version of this article, I’ve
seen programming environments increasingly embrace the need to provide
the tools for programmers to build such test suites. The biggest push
for this was JUnit, originally written by Kent Beck and Erich Gamma,
which had a marked impact on the Java community in the late 1990s. This
inspired similar testing frameworks for other languages, often referred
to as Xunit frameworks. These stressed a
light-weight, programmer-friendly mechanics that allowed a programmer to
easily build tests in concert with the product code. Often these tools
have some kind of graphical progress bar that is green if the tests pass,
but turns red should any fail – leading to phrases like “green build”,
or “red-bar”.

A sound test suite would never allow a mischievous imp to do
any damage without a test turning red.

The test of such a test suite is that we should be confident that if the
tests are green, then no significant bugs are in the product. I like to
imagine a mischievous imp that is able to make simple modifications to
the product code, such as commenting out lines, or reversing
conditionals, but is not able to change the tests. A sound test suite
would never allow the imp to do any damage without a test turning
red. And any test failing is enough to fail the build, 99.9% green is
still red.

Self-testing code is so important to Continuous Integration that it is a
necessary prerequisite. Often the biggest barrier to implementing
Continuous Integration is insufficient skill at testing.

That self-testing code and Continuous Integration are so tied
together is no surprise. Continuous Integration was originally developed
as part of Extreme Programming and testing has always
been a core practice of Extreme Programming. This testing is often done
in the form of Test Driven Development (TDD), a practice that
instructs us to never write new code unless it fixes a test that we’ve
written just before. TDD isn’t essential for Continuous Integration, as
tests can be written after production code as long as they are done
before integration. But I do find that, most of the time, TDD is the best
way to write self-testing code.

The tests act as an automated check of the health of the code
base, and while tests are the key element of such an automated
verification of the code, many programming environments provide additional
verification tools. Linters can detect poor programming practices,
and ensure code follows a team’s preferred formatting
style, vulnerability scanners can find security weaknesses. Teams should
evaluate these tools to include them in the verification process.

Of course we can’t count on tests to find everything. As it’s often
been said: tests don’t prove the absence of bugs. However perfection
isn’t the only point at which we get payback for a self-testing build.
Imperfect tests, run frequently, are much better than perfect tests that
are never written at all.

Everyone Pushes Commits To the Mainline Every Day

No code sits unintegrated for more than a couple of hours.

— Kent Beck

Integration is primarily about communication. Integration
allows developers to tell other developers about the changes
they have made. Frequent communication allows people to know
quickly as changes develop.

The one prerequisite for a developer committing to the
mainline is that they can correctly build their code. This, of
course, includes passing the build tests. As with any commit
cycle the developer first updates their working copy to match
the mainline, resolves any conflicts with the mainline, then
builds on their local machine. If the build passes, then they
are free to push to the mainline.

If everyone pushes to the mainline frequently, developers quickly find out if
there’s a conflict between two developers. The key to fixing problems
quickly is finding them quickly. With developers committing every few
hours a conflict can be detected within a few hours of it occurring, at
that point not much has happened and it’s easy to resolve. Conflicts
that stay undetected for weeks can be very hard to resolve.

Conflicts in the codebase come in different forms. The easiest to
find and resolve are textual conflicts, often called “merge conflicts”,
when two developers edit the
same fragment of code in different ways. Version-control tools detect
these easily once the second developer pulls the updated mainline into
their working copy. The harder problem are Semantic Conflicts. If my colleague changes the
name of a function and I call that function in my newly added code,
the version-control system can’t help us. In a statically typed language
we get a compilation failure, which is pretty easy to detect, but in a
dynamic language we get no such help. And even statically-typed
compilation doesn’t help us when a colleague makes a change to the body
of a function that I call, making a subtle change to what it does. This
is why it’s so important to have self-testing code.

A test failure alerts that there’s a conflict between changes, but we
still have to figure out what the conflict is and how to resolve it.
Since there’s only a few hours of changes between commits, there’s only
so many places where the problem could be hiding. Furthermore since not
much has changed we can use Diff Debugging to help us find the

My general rule of thumb is that every developer should commit to the
mainline every day. In practice, those experienced with Continuous
Integration integrate more frequently than that. The more frequently we
integrate, the less places we have to look for conflict errors, and the
more rapidly we fix conflicts.

Frequent commits encourage developers to break down their
work into small chunks of a few hours each. This helps
track progress and provides a sense of progress. Often people
initially feel they can’t do something meaningful in just a few
hours, but we’ve found that mentoring and practice helps us learn.

Every Push to Mainline Should Trigger a Build

If everyone on the team integrates at least daily, this ought to mean
that the mainline stays in a healthy state. In practice, however, things
still do go wrong. This may be due to lapses in discipline, neglecting
to update and build before a push, there may also be environmental
differences between developer workspaces.

We thus need to ensure that every commit is verified in a reference
environment. The usual way to do this is with a Continuous Integration
Service (CI Service)
that monitors the mainline. (Examples of CI
Services are tools like Jenkins, GitHub Actions, Circle CI etc.) Every time
the mainline receives a commit, the CI service checks out the head of the
mainline into an integration environment and performs a full build. Only
once this integration build is green can the developer consider the
integration to be complete. By ensuring we have a build with every push,
should we get a failure, we know that the fault lies in that latest
push, narrowing down where have to look to fix it.

I want to stress here that when we use a CI Service, we only use it on
the mainline, which is the main branch on the reference instance of the
version control system. It’s common to use a CI service to monitor and build
from multiple branches, but the whole point of integration is to have
all commits coexisting on a single branch. While it may be useful to use
CI service to do an automated build for different branches, that’s not
the same as Continuous Integration, and teams using Continuous
Integration will only need the CI service to monitor a single branch of
the product.

While almost all teams use CI Services these days, it is
possible to do Continuous Integration without one. Team members can
manually check out the head on the mainline onto an integration machine
and perform a build to verify the integration. But there’s little point
in a manual process when automation is so freely available.

(This is an appropriate point to mention that my colleagues at
Thoughtworks, have contributed a lot of open-source tooling for
Continuous Integration, in particular Cruise Control – the first CI

Fix Broken Builds Immediately

Continuous Integration can only work if the mainline is kept in a
healthy state. Should the integration build fail, then it needs to be
fixed right away. As Kent Beck puts it: “nobody has a
higher priority task than fixing the build”. This doesn’t mean
that everyone on the team has to stop what they are doing in
order to fix the build, usually it only needs a couple of
people to get things working again. It does mean a conscious
prioritization of a build fix as an urgent, high priority

Usually the best way to fix the build is to revert the
faulty commit from the mainline, allowing the rest of the team to
continue working.

Usually the best way to fix the build is to revert the latest commit
from the mainline, taking the system back to the last-known good build.
If the cause of the problem is immediately obvious then it can be fixed
directly with a new commit, but otherwise reverting the mainline allows
some folks to figure out the problem in a separate development
environment, allowing the rest of the team to continue to work with the

Some teams prefer to remove all risk of breaking the mainline by
using a Pending Head (also called Pre-tested, Delayed,
or Gated Commit.) To do this the CI service needs to set things up so that
commits pushed to the mainline for integration do not immediately go
onto the mainline. Instead they are placed on another branch until the
build completes and only migrated to the mainline after a green build.
While this technique avoids any danger to mainline breaking, an
effective team should rarely see a red mainline, and on the few times it
happens its very visibility encourages folks to learn how to avoid

Keep the Build Fast

The whole point of Continuous Integration is to provide rapid
feedback. Nothing sucks the blood of Continuous Integration
more than a build that takes a long time. Here I must admit a certain
crotchety old guy amusement at what’s considered to be a long build.
Most of my colleagues consider a build that takes an hour to be totally
unreasonable. I remember teams dreaming that they could get it so fast –
and occasionally we still run into cases where it’s very hard to get
builds to that speed.

For most projects, however, the XP guideline of a ten
minute build is perfectly within reason. Most of our modern
projects achieve this. It’s worth putting in concentrated
effort to make it happen, because every minute chiseled off
the build time is a minute saved for each developer every time
they commit. Since Continuous Integration demands frequent commits, this adds up
to a lot of the time.

If we’re staring at a one hour build time, then getting to
a faster build may seem like a daunting prospect. It can even
be daunting to work on a new project and think about how to
keep things fast. For enterprise applications, at least, we’ve
found the usual bottleneck is testing – particularly tests
that involve external services such as a database.

Probably the most crucial step is to start working
on setting up a Deployment Pipeline. The idea behind a
deployment pipeline (also known as build
or staged build) is that there are in fact
multiple builds done in sequence. The commit to the mainline triggers
the first build – what I call the commit build. The commit
is the build that’s needed when someone pushes commits to the
mainline. The commit build is the one that has to be done quickly, as a
result it will take a number of shortcuts that will reduce the ability
to detect bugs. The trick is to balance the needs of bug finding and
speed so that a good commit build is stable enough for other people to
work on.

Once the commit build is good then other people can work on
the code with confidence. However there are further, slower,
tests that we can start to do. Additional machines can run
further testing routines on the build that take longer to

A simple example of this is a two stage deployment pipeline. The
first stage would do the compilation and run tests that are more
localized unit tests with slow services replaced by Test Doubles, such as a fake in-memory database or
a stub for an external service. Such
tests can run very fast, keeping within the ten minute guideline.
However any bugs that involve larger scale interactions, particularly
those involving the real database, won’t be found. The second stage
build runs a different suite of tests that do hit a real database and
involve more end-to-end behavior. This suite might take a couple of
hours to run.

In this scenario people use the first stage as the commit build and
use this as their main CI cycle.
If the secondary build fails, then this may not have
the same ‘stop everything’ quality, but the team does aim to fix such
bugs as rapidly as possible, while keeping the commit build running.
Since the secondary build may be much slower, it may not run after every
commit. In that case it runs as often as it can, picking the last good
build from the commit stage.

If the secondary build detects a bug, that’s a sign that the commit
build could do with another test. As much as possible we want to ensure
that any later-stage failure leads to new tests in the commit build that
would have caught the bug, so the bug stays fixed in the commit build.
This way the commit tests are strengthened whenever something gets past
them. There are cases where there’s no way to build a fast-running test
that exposes the bug, so we may decide to only test for that condition
in the secondary build. Most of the time, fortunately, we can add suitable
tests to the commit build.

Another way to speed things up is to use parallelism and multiple
machines. Cloud environments, in particular, allow teams to easily spin
up a small fleet of servers for builds. Providing the tests can run
reasonably independently, which well-written tests can, then using such
a fleet can get very rapid build times. Such parallel cloud builds may
also be worthwhile to a developer’s pre-integration build too.

While we’re considering the broader build process, it’s worth
mentioning another category of automation, interaction with
dependencies. Most software uses a large range of dependent software
produced by different organizations. Changes in these dependencies can
cause breakages in the product. A team should thus automatically check
for new versions of dependencies and integrate them into the build,
essentially as if they were another team member. This should be done
frequently, usually at least daily, depending on the rate of change of
the dependencies. A similar approach should be used with running
Contract Tests. If these dependency
interactions go red, they don’t have the same “stop the line” effect as
regular build failures, but do require prompt action by the team to
investigate and fix.

Hide Work-in-Progress

Continuous Integration means integrating as soon as there is a little
forward progress and the build is healthy. Frequently this suggests
integrating before a user-visible feature is fully formed and ready for
release. We thus need to consider how to deal with latent code: code
that’s part of an unfinished feature that’s present in a live

Some people worry about latent code, because it’s putting
non-production quality code into the released executable. Teams doing
Continuous Integration ensure that all code sent to the mainline is
production quality, together with the tests that
verify the code. Latent code may never be executed in
production, but that doesn’t stop it from being exercised in tests.

We can prevent the code being executed in production by using a
Keystone Interface – ensuring the interface that
provides a path to the new feature is the last thing we add to the code
base. Tests can still check the code at all levels other than that final
interface. In a well-designed system, such interface elements should be
minimal and thus simple to add with a short programming episode.

Using Dark Launching we can test some changes in
production before we make them visible to the user. This technique is
useful for assessing the impact on performance,

Keystones cover most cases of latent code, but for occasions where
that’s not possible we use Feature Flags.
Feature flags are checked whenever we are about to execute latent code,
they are set as part of the environment, perhaps in an
environment-specific configuration file. That way the latent code can be
active for testing, but disabled in production. As well as enabling
Continuous Integration, feature flags also make it easier for runtime
switching for A/B testing and Canary Releases. We then make sure we remove this logic promptly once a
feature is fully released, so that the flags don’t clutter the code

Branch By Abstraction is another technique for
managing latent code, which is particularly useful for large
infrastructural changes within a code base. Essentially this creates an
internal interface to the modules that are being changed. The interface
can then route between old and new logic, gradually replacing execution
paths over time. We’ve seen this done to switch such pervasive elements
as changing the persistence platform.

When introducing a new feature, we should always ensure that we can
rollback in case of problems. Parallel Change (aka
expand-contract) breaks a change into reversible steps. For example, if
we rename a database field, we first create a new field with the new
name, then write to both old and new fields, then copy data from the
exisitng old fields, then read from the new field, and only then remove
the old field. We can reverse any of these steps, which would not be
possible if we made such a change all at once. Teams using Continuous
Integration often look to break up changes in this way, keeping changes
small and easy to undo.

Test in a Clone of the Production Environment

The point of testing is to flush out, under controlled
conditions, any problem that the system will have in
production. A significant part of this is the environment
within which the production system will run. If we test in a
different environment, every difference results in a risk that
what happens under test won’t happen in production.

Consequently, we want to set up our test environment to be
as exact a mimic of our production environment as
possible. Use the same database software, with the same
versions, use the same version of the operating system. Put all
the appropriate libraries that are in the production
environment into the test environment, even if the system
doesn’t actually use them. Use the same IP addresses and
ports, run it on the same hardware.

Virtual environments make it much easier than it was in the past to
do this. We run production software in containers, and reliably build
exactly the same containers for testing, even in a developer’s
workspace. It’s worth the effort and cost to do this, the price is
usually small compared to hunting down a single bug that crawled out of
the hole created by environment mismatches.

Some software is designed to run in multiple environments, such as
different operating systems and platform versions. The deployment
pipeline should arrange for testing in all of these environments in

A point to take care of is when the production environment isn’t as
good as the development environment. Will the production software be
running on machines connected with dodgy wifi, like smartphones? Then ensure a test
environment mimics poor network connections.

Everyone can see what’s happening

Continuous Integration is all about communication, so we
want to ensure that everyone can easily see the state of the
system and the changes that have been made to it.

One of the most important things to communicate is the
state of the mainline build. CI Services have dashboards that allow
everyone to see the state of any builds they are running. Often they
link with other tools to broadcast build information to internal social
media tools such as Slack. IDEs often have hooks into these mechanisms,
so developers can be alerted while still inside the tool they are using
for much of their work. Many teams only send out notifications for build
failures, but I think it’s worth sending out messages on success too.
That way people get used to the regular signals and get a sense for the
length of the build. Not to mention the fact that it’s nice to get a
“well done” every day, even if it’s only from a CI server.

Teams that share a physical space often have some kind of always-on
physical display for the build. Usually this takes the form of a large
screen showing a simplified dashboard. This is particularly valuable to
alert everyone to a broken build, often using the red/green colors on
the mainline commit build.

One of the older physical displays I rather liked were the use of red
and green lava lamps. One of the features of a lava lamp is that after
they are turned on for a while they start to bubble. The idea was that
if the red lamp came on, the team should fix the build before it starts
to bubble. Physical displays for build status often got playful, adding
some quirky personality to a team’s workspace. I have fond memories of a
dancing rabbit.

As well as the current state of the build, these displays can show
useful information about recent history, which can be an indicator of
project health. Back at the turn of the century I worked with a team who
had a history of being unable to create stable builds. We put a calendar
on the wall that showed a full year with a small square for each day.
Every day the QA group would put a green sticker on the day if they had
received one stable build that passed the commit tests, otherwise a red
square. Over time the calendar revealed the state of the build process
showing a steady improvement until green squares were so common that the
calendar disappeared – its purpose fulfilled.

Automate Deployment

To do Continuous Integration we need multiple environments, one to
run commit tests, probably more to run further parts of the deployment
pipeline. Since we are moving executables between these environments
multiple times a day, we’ll want to do this automatically. So it’s
important to have scripts that will allow us to deploy the application
into any environment easily.

With modern tools for virtualization, containerization, and serverless we can go
further. Not just have scripts to deploy the product, but also scripts
to build the required environment from scratch. This way we can start
with a bare-bones environment that’s available off-the-shelf, create the
environment we need for the product to run, install the product, and run
it – all entirely automatically. If we’re using feature flags to hide
work-in-progress, then these environments can be set up with all the
feature-flags on, so these features can be tested with all immanent interactions.

A natural consequence of this is that these same scripts allow us to
deploy into production with similar ease. Many teams deploy new code
into production multiple times a day using these automations, but even
if we choose a less frequent cadence, automatic deployment helps speed
up the process and reduces errors. It’s also a cheap option since it
just uses the same capabilities that we use to deploy into test

If we deploy into production automatically, one extra capability we find
handy is automated rollback. Bad things do happen from time to time, and
if smelly brown substances hit rotating metal, it’s good to be able to
quickly go back to the last known good state. Being able to
automatically revert also reduces a lot of the tension of deployment,
encouraging people to deploy more frequently and thus get new features
out to users quickly. Blue Green Deployment allows us
to both make new versions live quickly, and to roll back equally quickly
if needed, by shifting traffic between deployed versions.

Automated Deployment make it easier to set up Canary Releases, deploying a new version of a
product to a subset of our users in order to flush out problems before
releasing to the full population.

Mobile applications are good examples of where it’s essential to
automate deployment into test environments, in this case onto devices so
that a new version can be explored before invoking the guardians of the
App Store. Indeed any device-bound software needs ways to easily get new
versions on to test devices.

When deploying software like this, remember to ensure that version
information is visible. An about screen should contain a build id that
ties back to version control, logs should make it easy to see which version
of the software is running, there should be some API endpoint that will
give version information.

Styles of Integration

Thus far, I’ve described one way to approach integration, but if it’s
not universal, then there must be other ways. As with anything, any
classification I give has fuzzy boundaries, but I find it useful to think
of three styles of handling integration: Pre-Release Integration, Feature
Branches, and Continuous Integration.

The oldest is the one I saw in that warehouse in the 80’s –
Pre-Release Integration. This sees integration as a phase of
a software project, a notion that is a natural part of a Waterfall Process. In such a project work is divided into
units, which may be done by individuals or small teams. Each unit is
a portion of the software, with minimal interaction with other
units. These units are built and tested on their own (the original use of
the term “unit test”). Then once the units are ready, we integrate them
into the final product. This integration occurs once, and is followed by
integration testing, and on to a release. Thus if we think of the work, we
see two phases, one where everyone works in parallel on features,
followed by a single stream of effort at integration.

work on features

work on integration

The frequency of integration in
this style is tied to the frequency of release, usually major versions of
the software, usually measured in months or years. These teams will use a
different process for urgent bug fixes, so they can be released
separately to the regular integration schedule.

One of the most popular approaches to integration these days is to use
Feature Branches. In this style
features are assigned to individuals or small teams, much as units in the
older approach. However, instead of waiting until all the units are done
before integrating, developers integrate their feature into the mainline
as soon as it’s done. Some teams will release to production after each
feature integration, others prefer to batch up a few features for

Teams using feature branches will usually expect everyone to pull from
mainline regularly, but this is semi-integration. If Rebecca and I
are working on separate features, we might pull from mainline every day,
but we don’t see each other’s changes until one of us completes our
feature and integrates, pushing it to the mainline. Then the other will
see that code on their next pull, integrating it into their working copy.
Thus after each feature is pushed to mainline, every other developer will
then do integration work to combine this latest mainline push with
their own feature branch.

when a developer completes a feature…

…all others need to integrate

This is only semi-integration because each developer combines the
changes on mainline to their own local branch. Full integration can’t
happen until a developer pushes their changes, causing another round of
semi-integrations. Even if Rebecca and I both pull the same changes from
mainline, we’ve only integrated with those changes, not with each other’s

With Continuous Integration, every day we are all pushing our changes
to the mainline and pulling everyone else’s changes into our own work.
This leads to many more bouts of integration work, but each bout is much
smaller. It’s much easier to combine a few hours work on a code base than
to combine several days.

Benefits of Continuous Integration

When discussing the relative merits of the three styles of integration,
most of the discussion is truly about the frequency of integration. Both Pre-Release
Integration and Feature Branching can operate at different frequencies and
it’s possible to change integration frequency without changing the style
of integration. If we’re using Pre-Release Integration, there’s a big
difference between monthly releases and annual releases. Feature Branching
usually works at a higher frequency, because integration occurs when each
feature is individually pushed to mainline, as opposed to waiting to batch
a bunch of units together. If a team is doing Feature Branching and all
its features are less than a day’s work to build, then they are
effectively the same as Continuous Integration. But Continuous Integration
is different in that it’s defined as a high-frequency style.
Continuous Integration makes a point of setting integration frequency as a
target in itself, and not binding it to feature completion or release

It thus follows that most teams can see a useful improvement in the
factors I’ll discuss below by increasing their frequency without changing
their style. There are significant benefits to reducing the size of
features from two months to two weeks. Continuous Integration has the
advantage of setting high-frequency integration as the baseline, setting
habits and practices that make it sustainable.

Reduced risk of delivery delays

It’s very hard to estimate how long it takes to do a complex
integration. Sometimes it can be a struggle to merge in git, but then
all works well. Other times it can be a quick merge, but a subtle
integration bug takes days to find and fix. The longer the time between
integrations, the more code to integrate, the longer it takes – but
what’s worse is the increase in unpredictability.

This all makes pre-release integration a special form of nightmare.
Because the integration is one of the last steps before release, time is
already tight and the pressure is on. Having a hard-to-predict phase
late in the day means we have a significant risk that’s very difficult
to mitigate. That was why my 80’s memory is so strong, and it’s hardly the
only time I’ve seen projects stuck in an integration hell, where every
time they fix an integration bug, two more pop up.

Any steps to increase integration frequency lowers this risk. The
less integration there is to do, the less unknown time there is before a
new release is ready. Feature Branching helps by pushing this
integration work to individual feature streams, so that, if left alone,
a stream can push to mainline as soon as the feature is ready.

But that left alone point is important. If anyone else pushes
to mainline, then we introduce some integration work before the feature
is done. Because the branches are isolated, a developer working on one
branch doesn’t have much visibility about what other features may push,
and how much work would be involved to integrate them. While there is a
danger that high priority features can face integration delays, we can
manage this by preventing pushes of lower-priority features.

Continuous Integration effectively eliminates delivery risk. The
integrations are so small that they usually proceed without comment. An
awkward integration would be one that takes more than a few minutes to
resolve. The very worst case would be conflict that causes someone to
restart their work from scratch, but that would still be less than a
day’s work to lose, and is thus not going to be something that’s likely
to trouble a board of stakeholders. Furthermore we’re doing integration
regularly as we develop the software, so we can face problems while we
have more time to deal with them and can practice how to resolve

Even if a team isn’t releasing to production regularly, Continuous
Integration is important because it allows everyone to see exactly what
the state of the product is. There’s no hidden integration efforts that
need to be done before release, any effort in integration is already
baked in.

Less time wasted in integration

I’ve not seen any serious studies that measure how time spent on
integration matches the size of integrations, but my anecdotal
evidence strongly suggests that the relationship isn’t linear. If
there’s twice as much code to integrate, it’s more likely to be four
times as long to carry out the integration. It’s rather like how we need
three lines to fully connect three nodes, but six lines to connect four
of them. Integration is all about connections, hence the non-linear
increase, one that’s reflected in the experience of my colleagues.

In organizations that are using feature branches, much of this lost
time is felt by the individual. Several hours spent trying to rebase on
a big change to mainline is frustrating. A few days spent waiting for a
code review on a finished pull request, which another big mainline
change during the waiting period is even more frustrating. Having to put
work on a new feature aside to debug a problem found in an integration
test of feature finished two weeks ago saps productivity.

When we’re doing Continuous Integration, integration is generally a
non-event. I pull down the mainline, run the build, and push. If
there is a conflict, the small amount of code I’ve written is fresh in
my mind, so it’s usually easy to see. The workflow is regular, so we’re
practiced at it, and we’re incentives to automate it as much as

Like many of these non-linear effects, integration can easily become
a trap where people learn the wrong lesson. A difficult integration may
be so traumatic that a team decides it should do integrations less
often, which only exacerbates the problem in the future.

What’s happening here is that we are seeing much closer collaboration
between the members of the team. Should two developers make decisions
that conflict, we find out when we integrate. So the less time
between integrations, the less time before we detect the conflict, and
we can deal with the conflict before it grows too big. With high-frequency
integration, our source control system becomes a communication channel,
one that can communicate things that can otherwise be unsaid.

Less Bugs

Bugs – these are the nasty things that destroy confidence and mess up
schedules and reputations. Bugs in deployed software make users angry
with us. Bugs cropping up during regular development get in our way,
making it harder to get the rest of the software working correctly.

Continuous Integration doesn’t get rid of bugs, but it does make them
dramatically easier to find and remove. This is less because of the
high-frequency integration and more due to the essential introduction of
self-testing code. Continuous Integration doesn’t work without
self-testing code because without decent tests, we can’t keep a healthy
mainline. Continuous Integration thus institutes a regular regimen of
testing. If the tests are inadequate, the team will quickly notice, and
can take corrective action. If a bug appears due to a semantic conflict,
it’s easy to detect because there’s only a small amount of code to be
integrated. Frequent integrations also work well with Diff Debugging, so even a bug noticed weeks later can be
narrowed down to a small change.

Bugs are also cumulative. The
more bugs we have, the harder it is to remove each one. This is partly
because we get bug interactions, where failures show as the result of
multiple faults – making each fault harder to find. It’s also
psychological – people have less energy to find and get rid of bugs when
there are many of them. Thus self-testing code reinforced by Continuous
Integration has another exponential effect in reducing the problems
caused by defects.

This runs into another phenomenon that many
people find counter-intuitive. Seeing how often introducing a change
means introducing bugs, people conclude that to have high reliability
software they need to slow down the release rate. This was firmly
contradicted by the DORA research
program led by Nicole Forsgren. They found that elite teams
deployed to production more rapidly, more frequently, and had a
dramatically lower incidence of failure when they made these changes.
The research also finds that teams have higher levels of performance
when they have three or fewer active branches in the application’s code
repository, merge branches to mainline at least once a day, and don’t have
code freezes or integration phases.

Enables Refactoring for sustained productivity

Most teams observe that over time, codebases deteriorate. Early
decisions were good at the time, but are no longer optimal after six
month’s work. But changing the code to incorporate what the team has
learned means introducing changes deep in the existing code,
which results in difficult merges which are both time-consuming and full
of risk. Everyone recalls that time someone made what would be a good
change for the future, but caused days of effort breaking other people’s
work. Given that experience, nobody wants to rework the structure of
existing code, even though it’s now awkward for everyone to build on,
thus slowing down delivery of new features.

Refactoring is an essential technique to attenuate and indeed reverse
this process of decay. A team that refactors regularly has a
disciplined technique to improve the structure of a code base by using
small, behavior-preserving transformations of the code. These
characteristics of the transformations
greatly reduce their chances of introducing bugs, and
they can be done quickly, especially when supported by a foundation of
self-testing code. Applying refactoring at every opportunity, a team can
improve the structure of an existing codebase, making it easier and
faster to add new capabilities.

But this happy story can be torpedoed by integration woes. A two week
refactoring session may greatly improve the code, but result in long
merges because everyone else has been spending the last two weeks
working with the old structure. This raises the costs of refactoring to
prohibitive levels. Frequent integration solves this dilemma by ensuring
that both those doing the refactoring and everyone else are regularly
synchronizing their work. When using Continuous Integration, if someone
makes intrusive changes to a core library I’m using, I only have to
adjust a few hours of programming to these changes. If they do something
that clashes with the direction of my changes, I know right away, so
have the opportunity to talk to them so we can figure out a better way

So far in this article I’ve raised several counter-intuitive notions about
the merits of high-frequency integration: that the more often we
integrate, the less time we spend integrating, and that frequent
integration leads to less bugs. Here is perhaps the most important
counter-intuitive notion in software development: that teams that spend a
lot of effort keeping their code base healthy deliver features faster and cheaper. Time
invested in writing tests and refactoring delivers impressive returns in
delivery speed, and Continuous Integration is a core part of making that
work in a team setting.

Release to Production is a business decision

Imagine we are demonstrating some newly built feature to a
stakeholder, and she reacts by saying – “this is really cool, and would
make a big business impact. How long before we can make this live?” If
that feature is being shown on an unintegrated branch, then the answer
may be weeks or months, particularly if there is poor automation on the
path to production. Continuous Integration allows us to maintain a
Release-Ready Mainline, which means the
decision to release the latest version of the product into production is
purely a business decision. If the stakeholders want the latest to go
live, it’s a matter of minutes running an automated pipeline to make it
so. This allows the customers of the software greater control of when
features are released, and encourages them to collaborate more closely
with the development team

Continuous Integration and a Release-Ready Mainline removes one of the biggest
barriers to frequent deployment. Frequent deployment is valuable because
it allows our users to get new features more rapidly, to give more
rapid feedback on those features, and generally become more
collaborative in the development cycle. This helps break down the
barriers between customers and development – barriers which I believe
are the biggest barriers to successful software development.

When we should not use Continuous Integration

All those benefits sound rather juicy. But folks as experienced (or
cynical) as I am are always suspicious of a bare list of benefits. Few
things come without a cost, and decisions about architecture and process
are usually a matter of trade-offs.

But I confess that Continuous Integration is one of those rare cases
where there’s little downside for a committed and skillful team to utilize it. The cost
imposed by sporadic integration is so great, that almost any team can
benefit by increasing their integration frequency. There is some limit to
when the benefits stop piling up, but that limit sits at hours rather
than days, which is exactly the territory of Continuous Integration. The
interplay between self-testing code, Continuous Integration, and
Refactoring is particularly strong. We’ve been using this approach for two
decades at Thoughtworks, and our only question is how to do it more
effectively – the core approach is proven.

But that doesn’t mean that Continuous Integration is for everyone. You
might notice that I said that “there’s little downside for a
committed and skillful team to utilize it”. Those two adjectives
indicate the contexts where Continuous Integration isn’t a good fit.

By “committed”, I mean a team that’s working full-time on a product. A
good counter-example to this is a classical open-source project, where
there is one or two maintainers and many contributors. In such a situation
even the maintainers are only doing a few hours a week on the project,
they don’t know the contributors very well, and don’t have good visibility
for when contributors contribute or the standards they should follow when
they do. This is the environment that led to a feature branch workflow and
pull-requests. In such a context Continuous Integration isn’t plausible,
although efforts to increase the integration frequency can still be

Continuous Integration is more suited for team working full-time on a
product, as is usually the case with commercial software. But there is
much middle ground between the classical open-source and the full-time
model. We need to use our judgment about what integration policy to use
that fits the commitment of the team.

The second adjective looks at the skill of the team in following the
necessary practices. If a team attempts Continuous
Integration without a strong test suite, they will run into all sorts of
trouble because they don’t have a mechanism for screening out bugs. If they don’t
automate, integration will take too long, interfering with the flow of
development. If folks aren’t disciplined about ensuring their pushes to
mainline are done with green builds, then the mainline will end up
broken all the time, getting in the way of everyone’s work.

Anyone who is considering introducing Continuous Integration has to
bear these skills in mind. Instituting Continuous Integration without
self-testing code won’t work, and it will also give a inaccurate
impression of what Continuous Integration is like when it’s done well.

That said, I don’t think the skill demands are particularly hard. We don’t
need rock-star developers to get this process working in a team. (Indeed
rock-star developers are often a barrier, as people who think of themselves
that way usually aren’t very disciplined.) The skills for these technical practices
aren’t that hard to learn, usually the problem is finding a good teacher,
and forming the habits that crystallize the discipline. Once the team gets
the hang of the flow, it usually feels comfortable, smooth – and fast.

Common Questions

Where did Continuous Integration come from?

Continuous Integration was developed as a practice by Kent Beck as
part of Extreme Programming in the 1990s. At that time pre-release
integration was the norm, with release frequencies often measured in
years. There had been a general push to iterative development, with
faster release cycles. But few teams were thinking in weeks between
releases. Kent defined the practice, developed it with projects he
worked on, and established how it interacted with the
other key practices upon which it relies.

Microsoft had been known for doing daily builds (usually
overnight), but without the testing regimen or the focus on fixing
defects that are such crucial elements of Continuous

Some people credit Grady Booch for coining the term, but he only
used the phrase as an offhand description in a single sentence in his
object-oriented design book. He did not treat it as a defined practice,
indeed it didn’t appear in the index.

What is the difference between Continuous Integration and Trunk-Based Development?

As CI Services became popular, many people used
them to run regular builds on feature branches. This, as explained
above, isn’t Continuous Integration at all, but it led to many people
saying (and thinking) they were doing Continuous Integration when they
were doing something significantly different, which causes a lot of confusion.

Some folks decided to tackle this Semantic Diffusion by coining a new term: Trunk-Based
Development. In general I see this as a synonym to Continuous Integration
and acknowledge that it doesn’t tend to suffer from confusion with
“running Jenkins on our feature branches”. I’ve read some people
trying to formulate some distinction between the two, but I find these
distinctions are neither consistent nor compelling.

I don’t use the term Trunk-Based Development, partly because I don’t
think coining a new name is a good way to counter semantic diffusion,
but mostly because renaming this technique rudely erases the work of
those, especially Kent Beck, who championed and developed Continuous
Integration in the beginning.

Despite me avoiding the term, there is a lot of good information
about Continuous Integration that’s written under the flag of
Trunk-Based Development. In particular, Paul Hammant has written a lot
of excellent material on his website.

Can we run a CI Service on our feature branches?

The simple answer is “yes – but you’re not doing Continuous
Integration”. The key principle here is that “Everyone Commits To the
Mainline Every Day”. Doing an automated build on feature branches is
useful, but it is only semi-integration.

However it is a common confusion that using a daemon build in this
way is what Continuous Integration is about. The confusion comes from
calling these tools Continuous Integration Services, a better term
would be something like “Continuous Build Services”. While using a CI
Service is a useful aid to doing Continuous Integration, we shouldn’t
confuse a tool for the practice.

What is the difference between Continuous Integration and Continuous

The early descriptions of Continuous Integration focused on the
cycle of developer integration with the mainline in the team’s
development environment. Such descriptions didn’t talk much about the
journey from an integrated mainline to a production release. That
doesn’t mean they weren’t in people’s minds. Practices like “Automate
Deployment” and “Test in a Clone of the Production Environment” clearly
indicate a recognition of the path to production.

In some situations, there wasn’t much else after mainline
integration. I recall Kent showing me a system he was working on in
Switzerland in the late 90’s where they deployed to production, every
day, automatically. But this was a Smalltalk system, that didn’t have
complicated steps for a production deploy. In the early 2000s at
Thoughtworks, we often had situations where that path to production was
much more complicated. This led to the notion that there was an
activity beyond Continuous Integration that addressed that path. That
activity came to knows as Continuous Delivery.

The aim of Continuous Delivery is that the product should always be
in a state where we can release the latest build. This is essentially
ensuring that the release to production is a business decision.

For many people these days, Continuous Integration is about
integrating code to the mainline in the development team’s environment,
and Continuous Delivery is the rest of the deployment pipeline heading
to a production release. Some people treat Continuous Delivery as
encompassing Continuous Integration, others see them as closely linked
partners, often with the moniker CI/CD. Others argue that
Continuous Delivery is merely a synonym for Continuous Integration.

How does Continuous Deployment fit in with all this?

Continuous Integration ensures everyone integrates their code at
least daily to the mainline in version control. Continuous Delivery
then carries out any steps required to ensure that the product is
releasable to product whenever anyone wishes. Continuous Deployment
means the product is automatically released to production whenever it
passes all the automated tests in the deployment pipeline.

With Continuous Deployment every commit pushed to mainline as part
of Continuous Integration will be automatically deployed to production
providing all of the verifications in the deployment pipeline are
green. Continuous Delivery just assures that this is possible (and is
thus a pre-requisite for Continuous Deployment).

How do we do pull requests and code reviews?

Pull Requests, an artifact of GitHub,
are now widely used on software projects. Essentially they provide a
way to add some process to the push to mainline, usually involving a
pre-integration code review, requiring
another developer to approve before the push can be accepted into the
mainline. They developed mostly in the context of feature branching in
open-source projects, ensuring that the maintainers of a project can
review that a contribution fits properly into the style and future
intentions of the project.

The pre-integration code review can be problematic for Continuous
Integration because it usually adds significant friction to the
integration process. Instead of an automated process that can be done
within minutes, we have to find someone to do the code review,
schedule their time, and wait for feedback before the review is
accepted. Although some organizations may be able to get to flow
within minutes, this can easily end up being hours or days – breaking
the timing that makes Continuous Integration work.

Those who do Continuous Integration deal with this by reframing how
code review fits into their workflow. Pair Programming is popular because it creates a continuous
real-time code review as the code is being written, producing a much
faster feedback loop for the review. The Ship / Show / Ask process encourages teams
to use a blocking code review only when necessary, recognizing that
post-integration review is often a better bet as it doesn’t interfere
with integration frequency. Many teams find that Refinement Code Review is an important force to maintaining a
healthy code base, but works at its best when Continuous Integration
produces an environment friendly to refactoring.

We should remember that pre-integration review grew out of an
open-source context where contributions appear impromptu from weakly
connected developers. Practices that are effective in that environment
need to be reassessed for a full-time team of closely-knit staff.

How do we handle databases?

Databases offer a specific challenge as we increase integration
frequency. It’s easy to include database schema definitions and load
scripts for test data in the version-controlled sources. But that
doesn’t help us with data outside of version-control, such as
production databases. If we change the database schema, we need to
know how to handle existing data.

With traditional pre-release integration, data migration
is a considerable challenge, often spinning up special teams just to
carry out the migration. At first blush, attempting high-frequency
integration would introduce an untenable amount of data migration work.

In practice, however, a change in perspective removes this problem.
We faced this issue in Thoughtworks on our early projects using
Continuous Integration, and solved it by shifting to an Evolutionary Database Design approach, developed
by my colleague Pramod Sadalage. The key to this methodology is to
define database schema and data through a series of migration scripts,
that alter both the database schema and data. Each migration is small,
so is easy to reason about and test. The migrations compose naturally,
so we can run hundreds of migrations in sequence to perform
significant schema changes and migrate the data as we go. We can store
these migrations in version-control in sync with the data access code
in the application, allowing us to build any version of the software,
with the correct schema and correctly structured data. These
migrations can be run on test data, and on production databases.

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