Basic Concepts

API Simulations

API Simulator™ is a versatile tool for easily modeling and running API simulations.


API Simulations are stand-in substitutes that imitate the behavior and data of real APIs or APIs yet to be developed.

API simulations by API Simulator go beyond remote stubs and mocks. The API simulations support dynamically changing responses even for the same request, test data management capabilities, scripting for extending the functionality if needed, and more.


An API simulation consists of a set of simlets. For synchronous request/response type of APIs exposed over HTTP:


A simlet simulates the HTTP response to an HTTP request.

API Simulator uses an intuitive Domain Specific Language (DSL) developed specifically for modeling simlets via simple configuration constructs: name/value pairs, lists, and maps.

Request Matching

API Simulator inspects requests by applying matching rules. It maps a matched request to the simlet that will produce the simulated response for that request.

There are two options for when no simlet from the simulation matches an input:

  • Use a default fallback simlet, or

  • Forward unrecognized requests to their actual destination and pass back the live responses.

Response Rendering

There are two types of responses with respect to their content: static stubs/mocks and dynamic responses.

Static stubs/mocks are canned responses - responses that do not change between requests for the same simlet.

Dynamic responses produced by a simlet can change even for the same request to include dynamic values like date and time, computations, parameters from the request, lookups from external data stores, random values, tokens from a pattern, and more. API Simulator’s Simula template engine makes it easy to model dynamic responses.

Moreover, by supporting several probability distributions of random values, API Simulator enables the injecting of various random delays in sending a response to simulate latency and API slowness for even more realistic API simulations.

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Happy API Simulating!