When all you have is a hammer everything looks like a nail...
Like every tool, there are situations that Pact is great for, and situations where other tools would be better. In the right situation, Pact should make testing your integrations less painful than traditional integration tests. In the wrong situation, it will probably be just as painful!
Pact is most valuable for designing and testing integrations where:
You (or your team/organisation/partner organisation) control the development of both the consumer and the provider.
The consumer and provider are both under active development.
The requirements of the consumer(s) are going to be used to drive the features of the provider.
There is a small enough number of consumers for a given provider that the provider team can manage an individual relationship with each consumer team.
The primary advantages are:
You can continuously evolve your codebases knowing that Pact will guarantee contracts are met.
You can find out before you deploy whether or not your applications will work together - there is no need to wait for slow e2e tests.
Pact is fantastic tool for developing and testing intra-organisation microservices.
Testing APIs where the team maintaining the other side of the integration will not also being using Pact
Testing APIs where the consumers cannot be individually identified (eg. public APIs).
Situations where you cannot load data into the provider without using the API that you're actually testing (eg. public APIs). Why?
Testing new or existing providers where the functionality is not being driven or altered by the needs of particular consumers (eg. a public API or an OAuth provider where the API is completely stable)
Testing providers where the consumer and provider teams do not have good communication channels.
Performance and load testing.
Functional testing of the provider - that is what the provider's own tests should do. Pact is about checking the contents and format of requests and responses.
Testing "pass through" APIs, where the provider merely passes on the request contents to a downstream service without validating them. Why?
Use as a general purpose mocking or stubbing tool for browser driven tests. Why?
If you're unsure whether Pact will be valuable for your team, see this this page in the FAQ.