Consumer-Driven Contract Testing

Also Known As

TL;DR

Explanation

In microservices, integration tests require both services running simultaneously — slow, fragile, and hard to maintain. Consumer-Driven Contract Testing (CDC) with Pact solves this: the consumer writes tests defining what request/response it expects, generating a contract (pact file). The provider then runs its own verification against the contract without needing the consumer running. Both can deploy independently once the contract is satisfied.

Diagram

sequenceDiagram
    participant CONS as Consumer
    participant BROKER as Pact Broker
    participant PROV as Provider
    CONS->>CONS: Write consumer test<br/>define expected request/response
    CONS->>BROKER: Publish contract pact file
    Note over CONS,BROKER: Consumer CI passes
    PROV->>BROKER: Fetch contract
    PROV->>PROV: Verify provider meets contract
    PROV-->>BROKER: Verification result
    BROKER-->>CONS: Can I deploy? yes/no

Common Misconception

Why It Matters

Common Mistakes

• Provider teams writing contracts rather than consumers — the whole point is the consumer defines what it needs.
• Not publishing contracts to a broker (Pact Broker) — contracts stored locally cannot be verified by the provider CI.
• Contracts that are too strict — specifying exact response body structure breaks on innocuous additions.
• Not running contract verification in the provider's CI — contracts only help if the provider actually checks them.

Code Examples

// No contract testing — provider changes break consumer silently:
// Provider team renames 'user_id' to 'id' in response
// Consumer code: $response['user_id'] — now null
// Discovered in production, not in CI
// Both teams deployed independently with no coordination

// Consumer defines contract (Pact PHP):
$builder->uponReceiving('a request for user 42')
    ->with(['method' => 'GET', 'path' => '/users/42'])
    ->willRespondWith([
        'status' => 200,
        'body' => ['user_id' => 42, 'name' => Matchers::like('Alice')]
    ]);
// Pact file published to broker
// Provider CI verifies: GET /users/42 returns user_id (not id)

References

• ↗ https://docs.pact.io/
• ↗ https://github.com/pact-foundation/pact-php

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