Fault proxy

A fault proxy stands between the service under test and one of its dependencies, so a test can make that dependency slow, unreachable or broken on purpose. It works at the network level, so it serves a service in any stack.

Finding it

• The test setup: a proxy container in the Compose file or the test fixtures, and its client library among the test dependencies.
• The service's test config: the dependency's address points at the proxy's port, not at the dependency.

When there is none, it starts as one more container beside the dependency it sits in front of.

Using it

• Add a fault in the test that needs it, through the client or the proxy's HTTP API: latency, a timeout, a reset connection, limited bandwidth, or the proxy disabled to cut the dependency off.
• Reset after each test (POST /reset in Toxiproxy), so one test's fault cannot break the next.
• Assert on what the service does: it times out within its limit, falls back, or returns a clear error.

Examples: Toxiproxy; WireMock's delays and faults, for a dependency reached over HTTP.