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Use contract tests at service boundaries to check that independently developed microservices still agree on the messages they exchange. In a consumer-driven Pact workflow, the consumer’s tests define concrete interactions, and provider verification checks those interactions against the provider’s implementation. Automate both checks and make their results available before deployment; a consumer-side mock test alone does not show that the real provider satisfies the contract.
What contract testing checks
A contract test checks an integration boundary: whether an application sends or handles messages in the way another application expects. For HTTP services, those messages are requests and responses. For asynchronous systems, they can be messages read from or written to a queue.
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The test focuses on a particular interaction between a consumer and a provider rather than requiring the whole system to be deployed. Pact describes a contract as a collection of interactions. These are concrete examples of behavior, not a complete inventory of every state an API could support.
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Use the direction of the interaction, not a service’s organizational label, to assign the roles:
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- HTTP: the consumer initiates the request; the provider returns the response.
- Messaging: the consumer reads the message; the provider writes or produces it.
This distinction matters in event-driven systems, where “client” and “server” can obscure who produces and who consumes a message. Map each integration you want to protect, including the relevant HTTP calls and message flows, then prioritize boundaries where a change could disrupt another service or team.
Build a consumer-driven contract workflow
- Choose a real consumer interaction. Identify a request and the response fields the consumer actually uses, or the message content it needs to handle. Keep the contract focused on those needs rather than incidental details.
- Write consumer tests against a mock provider. Exercise the consumer code using an expected interaction. The test checks that the consumer makes the expected request and handles the expected response or message.
- Generate the contract by running the tests. In Pact’s consumer-driven approach, the consumer tests generate the contract. Creating it separately or hand-authoring it would defeat the purpose of tying the agreement to tested consumer behavior.
- Share the resulting contract. Make it available to the team responsible for the provider, or use a Pact Broker to coordinate contract publication and retrieval across CI pipelines.
- Verify the provider implementation. Run provider verification against the provider’s real code and arrange the provider state needed to produce each expected response or message. This checks whether the provider fulfills the recorded interactions.
- Use verification evidence before deployment. Make results available to the teams deciding which versions can be deployed together. A passing consumer test and a passing provider verification are complementary evidence for that boundary.
What belongs in the contract
Include behavior the consumer relies on
For an HTTP interaction, include the relevant request details and the minimum response data the consumer requires. For messaging, include the expected message content and the behavior needed to consume it. Each interaction should represent a real use of the boundary.
Leave incidental details out
Avoid asserting response fields, formatting, or other details that the consumer does not use. Extra assertions make a contract sensitive to changes that may not affect the consumer. Conversely, do not mistake a consumer-driven contract for a schema of every valid response or message; it records selected examples of actual consumer needs.
Automate contract checks in CI/CD
Make consumer contract generation and provider verification repeatable in the teams’ development and release workflows. Share the contract and the corresponding verification evidence so deployment decisions can take account of the versions being considered. A Pact Broker can help coordinate contract publication and retrieval between pipelines; its availability and terms depend on the offering, so check current details before choosing one.
There is no single pipeline shape that suits every organization. Fit the checks into existing build and release practices, and decide deliberately when provider verification runs. Pact’s guidance notes that contract-change-triggered verification may be run separately from the provider’s other CI build so another team’s change does not unexpectedly disrupt that build.
Set up provider state deliberately
Provider verification needs the provider to be in a state that can return the expected response or produce the expected message. Define a repeatable way to establish that state for each interaction. Avoid relying on a live public API to set up provider state unless the trade-off is acceptable: doing so can make verification slower and more brittle than ordinary provider verification.
Choose the assurance method that matches the question
| Approach | What it checks | What it does not establish by itself |
|---|---|---|
| Consumer-driven interaction contracts, such as Pact | Concrete interactions current consumers use, with provider verification against those interactions. | Every valid API state, whole-system behavior, operational reliability, or business semantics across an entire workflow. |
| Provider conformance to a static API specification, such as OpenAPI | Whether provider implementation aligns with the published specification. | Whether consumers call the provider correctly or whether the provider meets all consumer expectations. |
These approaches can complement each other: interaction contracts address observed consumer needs, while specification conformance addresses alignment with a broader published schema. Choose according to the assurance goal rather than treating either as a universal replacement for the other.
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- They provide evidence about compatibility at a particular boundary, not proof that an entire distributed workflow behaves correctly.
- They do not establish operational reliability or business correctness across multiple services.
- A passing consumer test against a mock provider does not prove that provider code fulfills the interaction; provider verification is needed for that check.
- They do not enumerate every possible API state, and they do not remove the need for other tests that cover end-to-end or broader system concerns.
Troubleshoot common contract-test failures
The consumer test passes, but provider verification fails
The consumer test exercised a mock, not the real provider. Compare the recorded request and response or message with the provider’s implementation, then check whether the provider state used for verification can produce the expected interaction.
Verification fails because the provider is in the wrong state
Make provider-state setup explicit and repeatable for the interaction under test. If setup depends on calling a public API, consider a controlled local or test environment instead; live setup can add latency and brittleness.
Small provider changes break many contracts
Review whether contracts assert details consumers do not rely on. Keep interactions focused on real consumer needs and remove incidental expectations, while preserving fields or message content that the consumer actually requires.
A team’s change unexpectedly breaks another team’s build
Review when and where contract-change-triggered verification runs. Coordinate it with the provider’s CI process; running verification separately from the provider’s other build may avoid making another team’s change an unexpected failure in that build.
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FAQ
Can contract testing cover asynchronous messaging?
Yes. The consumer is the application that reads a message, and the provider is the application that produces it. Define the message interaction and verify the provider can produce it in the required state.
Should every service boundary have a contract test?
Prioritize the HTTP and messaging boundaries where changes can affect another service or team. The contract should reflect an interaction a consumer actually uses, rather than an exhaustive description of every possible behavior.
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