Talk from Oct 11th 2016 at LondonCD (https://www.meetup.com/London-Continuous-Delivery)
Autonomy and isolation are some of the core values of microservices, allowing for independent changes and independent deployments. As loosely coupled services interact on interfaces managed under different life-cycles and even different teams, making sure that a simple change did not break the application can turn into an integration nightmare.
Consumer-Driven Contracts testing brings an alternative integration testing approach for distributed systems, relying less on live-like integration environments and more on making interactions explicit and quickly verifiable.
This talk cover how Newsweaver (https://www.newsweaver.com/email-overview) has made CDCs part of its pipeline with Pact and how it improved collaboration and confidence between teams when designing APIs.
Been working for >8 years in Newsweaver SaaS company, Internal Comms platform for large enterprises improve and unify communication across channels by giving them tools to manage their content and measuring engagement across channels Worked 6 years on large J2EE app Could be qualified as Monolithic app Still going great, making for 90% of the company’s income 2 years ago, moved to a new team New product, greenfield, analytics based Focus on Continuous Delivery Microservices emerged as our architecture of choice Now all of our new work is on microservices / splitting the monolith
Rarely start from scratch - even if we do, we’ve usually been working on this system (left) - large, with components (if lucky), on one single DB etc - typically called monolith
Then moving to microservices, look like this (right) - boundaries are clearer, concepts more clearly separate
What’s missing is the interactions - in monolith, implicit, compile time, hard to break - in micro, more central, less validation (almost none!), easy to break if not careful
Different dev cycles Different teams Different languages Different dependencies
Breaking example of API backward compatibility: Provider tests are all passing in build Provider deploys new version Some services then start throwing errors, even though they have not changed Why? Interactions got broken How do we test the interactions
What are we testing? Can we login? Are we breaking an interaction? Who should test that? At what stage? Testing is there to stop us from doing something stupid We want to know as early as possible if we’re doing something stupid
The problem with end to end testing Why was this wrong? - team responsible for users need to concern themselves more than what they should - environment will take time to come up
This is turning into a full e2e acceptance test. - there might still be value for e2e AT in the pipeline - but this is not what we’re trying to test here - temptation to think e2e AT cover everything
Can we test that small thing here earlier? - anything we get faster feedback on is a win for our pipeline time
In general e2e integration is really not practical - illusion of coverage (explosion of potential branches as nb of services go up)- slow, painful to maintain (e.g. UI based) - if it’s slow, this is stopping us from deploy (or even fixing forward) How do we get rid of this? This is where contracts testing comes in...
We wanted to test the interaction - the idea behind CDC is to put the interaction at the center of the problem. By creating a contract between the 2 services, it makes things more explicit
This is not an API specification that we write once and drift away from as the weeks go by!
Consumers will have to ensure that they are using the provider in the way the contract specified Providers will have to ensure that they work the way the contract specified These contracts need to be tested and validated
I will talk more specifically about PACT, framework we are using Some people roll their own thing, and it’s perfectly fine - PACT has worked great for our workflow, so why reinvent the wheel Pact terminology = a Pact (or Pact file) is a CDC between 2 services
Pact spec = what the pact file contains, will go in more details, but quite simple - specify how interactions are defined Verif as Tolerant Reader / Postel’s Law: allows for flexibility in API evolution - ex. If provider starts sending back more fields, consumers shouldn’t be considered as broken (should ignore the new fields) - steers away for API versioning nightmares
Implementation guidelines for the verification. Support in a lot of languages - on both sides of the puzzle (consumer / provider)
Both parties are responsible for the interactions Consumer can define their expectations Provider can check that they are fulfilling them Contracts: Consumer define a contract, which is a set of interactions Interaction = request, response (in short) Provider verifies interactions by replaying requests and checking responses Benefits: Share the work, with each side keeping their concerns. Each step can be done separately, repeatedly and on their own time
What’s inside the PACT file?
Request: also query params, POST body...
How do we share the PACTs? For small number of services/teams, start by copy/pasting - we did this for more than a year and it was fine (with 2 teams) When it starts becoming painful, Pact Broker will help to scale up Pact Broker Consumers push the contracts they have generated Providers can query the broker for all contracts consumers have with them
So it’s just some database of contracts? Kind of, but with a few added bonuses…
Provider build workflow > Fast feedback during development > Find out unindented breaking change before commit
Deployment workflow > Important if gap of time between build and deploy > Important if deploying to different environment (in that case there can be different PROD tags)
Prevented breaking before deploying anything! (didn’t even break a test server!)
HEAD pacts are “tentative” contracts that need to be verified by each related providers before the service can go forward. Provider pact tests are still unit tests, very quick to run - still no need to deploy anything.
Original advantage: cover more interactions, faster, increase confidence in having backward compatible API. Extra finding: PACTs become very central to our way to work Team discuss them first thing when planning a new interaction
Pacts against services that rely on state (e.g. with saved data) will require some setup logic (catered for with the assumptions piece) Large number of interactions and assumed starting state can put some pressure on the provider
Confidence from the Pacts comes from interaction coverage Consumer tests must interact with mock and assert results, not check responses directly Beware of only testing happy paths. Errors are also interactions. But! Beware of being overzealous - this is probably not a place to test the Provider’s business logic (e.g. validation criteria)
Integration in Build/Deploy workflow is key to scale Automating the verification prevents mistakes (CD book is all about this) Can be trickier when dealing with multiple live environments but tags are very powerful with the Broker Take the time to think about your flow and where the tests should sit best
Increased in confidence when coding & deploying > Break fast, without having to deploy anything anywhere > Saved us many times (forgot consumers, forgot use case…) Relying less and less on full e2e env and tests > Reserved for acceptance and synthetic API design at the forefront > Interactions are discussed early “let’s see what the pact would look like” Consumer-driven benefits > API are designed with consumer in mind > Consumers are responsible for documenting their usage in PACT (“please don’t break this”) API documents itself as it’s being used > New consumers can check out existing pacts for examples of usage
Consumer-driven contracts: avoid microservices integration hell! (LondonCD - Oct 2016)
Authentication Team Users Team
Given that user 'pierre' exists
"name": "Pierre Vincent",
✓ REST Api
Implement changes Get Pacts from Broker
Replay & Verify
Deploy to EU
Get Pacts from Broker
Replay & Verify
Stop deployment of
Stop introduction of
Push Pacts to
Each Provider verifies Pacts Deploy Service Tag Pacts
Deploy to EU
Stop deployment of
Caution: side-effects may include teams
collaborating on API design
Provider state setup for each interaction
Confidence comes from coverage:
don’t limit to happy paths
deployment pipeline isn’t trivial
What’s the catch…?
After 2 years of CDCs...
Increased confidence when coding & deploying
Collaborative API design
Living API documentation
Pact docs.pact.io, github.com/pact-foundation
Pact Broker github.com/bethesque/pact_broker
Try it out!
“Pact Matrix” (Beth Skurrie) rea.tech/enter-the-pact-matrix-or-how-to-decouple-the-release-
Why should you use CDC for microservices integration tests
Sharing CDCs with Pact Broker