Continuous Delivery

When you do Continuous Delivery you can deploy whenever you want: you made it as easy as possible and you have become very good at it. Everyone in the Engineering and Operations teams knows how to deploy your application to any environment. Product can always see the latest bells and whistles as they are built because you have Stable servers running the latest versions of the application. You implement big changes gradually and show them to the Product Team while keeping the customer’s experience stable. When you decide to release, you have made sure all things will work and you know how to react if nevertheless they break, without fires or panic.

Continuous Deployment also forces you to do many right things: repeatable builds; the exact same deployment process in all environments, including the developer’s machines and a development environment that is as close as possible to Production; backwards-compatible database changes; easy rollbacks; code that is split into components; good tests…

Taming the Software Lion


It’s the 1800s. The lion is beaten into submission through fear, brute force and confusion. That famous tamer wielding a chair—the lion is not scared of the chair, it’s confused of it: why is this chair floating here? And why is this guy holding it?

Modern times

Nowadays tamers understand the lion’s psychology. They condition the lion to behave as they want, they tie behaviors to signals and reward the right behaviors. They build up trust.

Taming the Software Lion—recap


It’s the 1800s. Code is beaten into submission through iterations of half-working attempts at hand-made deployments; with fear, because it works and we barely understand it, so don’t touch it because it works; with weapons, because when it breaks it’s hacked some more until it works (install a missing dependency, copy the forgotten templates…); and with confusion: it’s difficult to see what’s actually installed, how, what part of it is needed and what extra changes are needed to make it work.

Modern times

Nowadays software engineers understand that deploying software is hard, and so they must get very good at it and automate it with clean procedures so that it’s repeatable and debugable. They use continuous integration, which rewards them with a green light when the build passes. They take care of the health of the build. They deploy it frequently to a staging server that the stakeholders can see. When software breaks, they know how to act.

Engineers build up trust: they can trust that the software works and that it does what it’s supposed to do; and the stakeholders trust them in that the product is built to their expectations.

What You Need to implement Continuous Delivery

To implement Continuous Delivery, you need the following:

  • A team
  • Working software
  • A repeatable build
  • An automated deployment
  • A way to rollback
  • An automated release


Every single member of the Team must be committed to quality—process can’t compensate for lack of commitment. This commitment includes the constant learning of best practices and ways to improve.

Everyone in the Team must know how to deploy and release software (also in Live) and how to maintain the deployment and release scripts. Everyone is responsible for these scripts: there cannot be a deployment guru. For this, the deployment scripts must be clear, concise and simple.

All environments must be as similar as possible; this includes the development machines. The Team must deploy the software in their development machines using the exact same deployment and release scripts that are used in Production and Staging.

In addition to this, everyone in the Team is responsible for:

  • Having working Stable and Production environments.
  • Having a green CI.
  • Never committing broken code.
  • Adding sufficient tests.
  • Having good quality code.

Working Software

Software, even software that works, is not working software unless it has automated tests:

  • Unit Tests
  • Functional Tests
  • Acceptance Tests (testing from the user’s viewpoint, not from a lower layer)
  • Infrastructure and Configuration Tests (for example, testing that the server must be able to send e-mail).

Tests are not second-class citizens: the standards of their cleanness, readability and maintainability must be as high as those of the rest of the software. This quality must be maintained: tests must not be let rot when changes accumulate.

All these tests must be run locally before committing changes and also automatically using Continuous Integration. Because tests are executed often they should be kept fast.

Simple mistakes, such as the ones that pylint catches, should be checked even before running any tests.

The build must be kept green at all times. Engineers should check in their changes often, and be ready to rollback if the change (which passed the local tests) breaks the build. Many small changes are preferable to a single big change: they are easier to debug and to rollback.

Both Engineering and QA are responsible for the quality of the software (this includes the tests).

When a test breaks, it must be fixed. There are two possible moments for fixing it:

  • Right now. If the failure is legit, you must drop what you are doing and fix the it.
  • As soon as possible. If the failure is due to a false positive and it’s not possible to fix it right now, the test must be fixed as soon as possible. This should not be later than the end of the day.

If a test breaks because of changes that are being made, either the test must be fixed right now or the changes must be reverted.

Tests cannot be disabled to be fixed later. Later won’t come any time soon.

Any code must be peer-reviewed before being merged into the Stable branch.

Repeatable Build

The build must be automated, and used by all members of the Team in all environments. The build process must contain no manual steps or changes.

Deployment Script

As for the build, the deployment script must be automated and used by all members of the Team in all environments. Deploying the software should be accomplished by a single command:

./ <environment> <version>

The only way to deploy is to follow the pipeline: tests, peer-review, merge, test, automated build, automated deploy. This includes emergencies: many problems come from skipping the pipeline and hacking a solution out of urgency.

If the pipeline is skipped and software is deployed by hand, the system is left on an unknown state. If the hack fails it will be very difficult to duplicate it and investigate what went wrong. Most of the time of fixing a problem is usually spent in searching its cause.


When a deployment fails it must be easy to rollback. There are many strategies to accomplish this, for example Blue-Green Deployments and Canary Deployments.

Blue-Green Deployments

Have two separate environments: green is where the customers go when they go to Production; blue is not.

  1. Deploy the new version on blue.
  2. Test blue and do manual acceptance.
  3. Switch blue to green and green to blue: now Blue is serving Production

If the deployment goes wrong, rolling back is a matter of switching green and blue. It’s easy to investigate what went wrong because blue is still running the new code.

Canary Deployment

Deploy the new version on a fraction of the servers and have it run alongside the old version. Once it’s confirmed that it works as expected, extend the deployment to the rest of the servers.

This strategy can also be used to do A/B testing or assessing the performance impact of new features.

Rollback the Database

For rollbacks to be possible, the database changes must be kept backwards compatible. There is no way around this. When this is not possible, make a plan on how to rollback.

Automated Release

When all the previous steps are in place, an automated release is just the last step on the chain. While an automated release to Production may not be desirable in all cases, an automated release to a Staging or Integration environment will allow the stakeholders to use the latest version of the software while it’s being developed and before it goes to Production.

Frequent, smaller changes are preferred to a big release: small releases have shorter Time to Recover: if it goes wrong it will be easier to find what went wrong if the amount of changes is small.

Releasing is hard. If it hurts, do it more often.

Hidden Features

In some cases it is useful to release features but keep them inaccessible or only accessible to a few users. There are several tools to make this easy; gargoyle is a popular one for Django.


Split In Components

Split your software in components that can be deployed independently.

A component:

  • Is reusable
  • Is replaceable with something else that implements the same API.
  • Is independently deployable.
  • Encapsulates a coherent set of behaviors and responsibilities of the system.

Splitting your software in components encourages a clear delineation of responsibilities and makes understanding and changing the code easier.

Rehearse Releases

Releasing is hard. Rehearse it and get very good at it.

Manage your Infrastructure

Write tests that verify that your infrastructure behaves as you expect and provides the necessary functionality.

Automate all infrastructure changes that can be automated, and document the rest.

Equal Environments

All environments must be as similar as possible. Use vagrant to develop.

Automate Everything

A process that is automated is repeatable and easier to debug. Automate everything that can be automated.

I gave a talk about this at DjangoCon Europe 2013. Here are the slides; the video will be available soon.

If this interests you, you may want to check these books: