Microservice architectures create a testing challenge that monolithic applications do not face: ensuring that independently deployed services can actually communicate correctly. Both Pact and Keploy address this challenge, but they approach it from opposite directions. Pact prevents integration failures through explicit contracts. Keploy prevents integration failures through replaying real-world interactions. Understanding this philosophical difference is key to choosing the right tool.
Pact's consumer-driven approach starts from the API caller's perspective. The consumer service defines what it expects from the provider — specific endpoints, request formats, and response structures. These expectations become a contract that is shared with the provider team. The provider then verifies that its implementation satisfies the contract. If a provider change would break a consumer's expectations, the verification fails before deployment. This catches breaking changes at build time rather than in production.
Keploy's traffic-based approach starts from real-world behavior. It captures actual API calls from production or staging traffic, including request/response pairs and external dependency interactions. These captured interactions become test cases with automatically generated assertions and mock responses for external dependencies. The tests replay real behavior rather than testing against human-defined expectations, catching issues that contract definitions might miss.
The cold start problem differs. Pact requires someone to write contracts — consumer tests that define expectations, which are then shared and verified. This upfront effort scales with the number of consumer-provider relationships. Keploy generates tests from traffic with no manual test writing — start the capture agent, let traffic flow, and tests appear automatically. For teams with many services and limited testing resources, Keploy's automatic generation reduces the barrier to coverage.
Accuracy and coverage have different characteristics. Pact contracts test exactly what consumers depend on — nothing more, nothing less. This precision means contracts are stable and meaningful but only cover explicitly defined interactions. Keploy tests cover whatever traffic was captured, which may include edge cases and error conditions that contract authors would not think to specify, but may also include irrelevant interactions that add noise to the test suite.
The contract broker (Pact Broker or PactFlow) enables coordination between teams. Contracts are published by consumers, verified by providers, and the broker tracks compatibility across all service versions. The can-i-deploy check queries the broker to determine whether a specific version of a service is compatible with its deployed consumers and providers. This coordination layer is essential for large-scale microservice deployments where dozens of services interact.