can i deploy

Before you deploy a new version of an application to a production environment, you need to know whether or not the version you're about to deploy is compatible with the versions of the other apps that already exist in that environment. The old-fashioned way of managing these dependencies involved deploying sets of pre-tested applications together, creating a bottleneck, and meaning that speedy development and testing on one application may be negated by slow development and testing on another.

The Pact way of managing these dependencies is to use the Pact Matrix - this is the matrix created when you create a table of all the consumer and provider versions that have been tested against each other using Pact. (When a pact is published, the version of the consumer that generated the pact is recorded. When a pact is verified against a provider, the verification results are published to the Pact Broker, along with the version of the provider that verified the pact. When you put all of the consumer versions and provider versions that have been tested against each other into a table, you end up with the "Pact Matrix".)

You can view the Pact Matrix for any pair of applications by clicking on the little grid icon for your pact in the Pact Broker index page.

Imagine the pact verification matrix for consumer Foo and provider Bar. It shows the consumer version, the provider version, and whether or not the verification passed.

Foo version (consumer)	Bar version (provider)	Verification success?
22	56	true
23	56	true
23	57	false
23	58	true
24	58	true
25	58	false

So how does this help us? Well, if we know that version 56 of Bar is already in our prod environment, looking at the matrix tells us that we are safe to deploy version 22 or 23 to prod, but not any of the versions after. Conversely, if we know that version of 24 of Foo is in prod, then we know we're safe to deploy version 58 of Bar, but not any of the versions before.

Let's see what this looks like in practice.

In the deployment script for each application that uses Pact, we need to add a step that checks the Pact Matrix to make sure we're safe to deploy. It's recommended to do this for each environment, not just production. The tool that we use to check the matrix is called can-i-deploy. It is part of the Pact Broker client command line interface, and is available via Docker, or an executable that you can install via a script.

Here is how we would check to see if we were safe to deploy Foo version 23 to production, given that we know version 56 of Bar is in production. (The Pact Broker url and credentials have been skipped for clarity.)

$ can-i-deploy --pacticipant Foo --version 24 --pacticipant Bar --version 56