Yulin AWS Simulator

Yulin is a local AWS simulator for testing Node.js applications. It simulates AWS system behaviour for fast, isolated testing, local development, and CI.

Yulin runs in the same single process as your tests and application under test. No network, containers, or external I/O are involved unless you explicitly choose to serve a simulation on localhost.

This isolated system approach makes it practical to test meaningful behaviour across AWS services without slow or fragile infrastructure setup.

Test setup is simple, with no containers or extra services to manage.

Each SimAws instance is cheap and encapsulated, so tests can create isolated AWS environments freely.

You can combine Yulin with other mocks and simulators, such as nock .

You can test behaviour across multiple simulated AWS services in one process and step through the whole system in a debugger.

AWS state is simulated internally, so you can test realistic interactions with multiple AWS services.

The word yǔlín (雨林) means “rainforest” — a roundabout reference to “Amazon” as in Amazon Web Services.