Sim

Why

Make the dev loop crazy fast.

What

Sim is straight-forward API simulation tool that's tiny, fast, secure and scalable.

Sim turns OpenAPI specs into executable API specs.

Sim doesn't just mock APIs, it allows you to specify scripts for each API operation which have access to a key-value database that allows APIs to save state between requests.

Most of today's API mocking tools run in virtual machines such as the JVM or NPM. Sim is a single binary with zero dependencies.

• It's orders of magnitude smaller binary and memory usage. Which much lower CPU usage. Each process can simulation multiple APIs.
• Running on Kubernetes? Three pods could simulate every API in your organization with high-availability.

Sim was written with extensive help from AI.

Install

Like jq, sim is a tiny (16Mb) standalone binary. You can download it from the releases page.

If you're on MacOS, you can use brew:

brew tap kitproj/sim --custom-remote https://github.com/kitproj/sim
brew install sim

Otherwise, you can use curl:

curl -q https://raw.githubusercontent.com/kitproj/sim/main/install.sh | sh

We do not support go install.

As Docker image:

docker run --rm -ti -v examples:/apis ghcr.io/kitproj/sim

Usage

Create a directory containing files named *.yaml.

Then run:

sim apis

Mocking

Simulations are described by their API specification. For simple mocking, specify your examples in the OpenAPI spec:

openapi: 3.0.0
info:
  title: Hello API
  version: 1.0.0
servers:
  - url: http://localhost:8080
paths:
  /hello:
    get:
      responses:
        '200':
          description: OK
          content:
            application/json:
              example: { "message": "Hello, world!" }

Scripting

openapi: 3.0.0
info:
  title: Teapot API
  version: 1.0.0
servers:
  - url: http://localhost:4040
# This script is executed when the spec is loaded.
# You can use it to specify global variables and functions that are available to all scripts.
x-sim-script: |
  var status = 418
paths:
  /teapot:
    get:
      # This script is executed whenever the request is serviced.
      # The last variable is the response object.
      x-sim-script: |
        response = {
           "status": status,
           "headers": {
             "Teapot": "true"
           },
           "body": { "message": "I'm a teapot" }
         }
      responses:
        '200':
          description: OK

Scripting With A Database

openapi: 3.0.0
info:
  title: Document API
  version: 1.0.0
servers:
  - url: http://localhost:4040
paths:
  /documents:
    post:
      x-sim-script: |
        var uuid = randomUUID();
        db.put("/documents/" + uuid, request.body)
        response = {
          "status": 201,
            "headers": {
                "Location": "/documents/" + uuid
            }
        }
      responses:
        '200':
          description: OK
    get:
      x-sim-script: |
        response = {
          "body": db.list("/documents")
        }
      responses:
        '200':
          description: OK
  /documents/{id}:
    get:
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: string
      x-sim-script: |
        var document = db.get("/documents/" + request.pathParams.id);
        if (document) {
          response = {
            "body": document
          }
        } else {
          response = {
              "status": 404,
          }
        }
      responses:
        '200':
          description: OK

    delete:
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: string
      x-sim-script: |
        db.delete("/documents/" + request.pathParams.id)
        response = {}
      responses:
        '204':
          description: OK

Scripting With HTTP Requests

A script can make a HTTP request:

#!/usr/bin/env sim
openapi: 3.0.0
info:
  title: Proxy API
  version: 1.0.0
servers:
  - url: http://localhost:5050
paths:
  /proxy:
    get:
      x-sim-script: |
        hello = http({"url": "http://localhost:8080/hello"})
        response = {
           "status": hello["status"],
           "headers": {
             "Proxy": "true"
           },
           "body": hello.body
         }
      responses:
        '200':
          description: OK

Executable Specs

You can make you spec an executable program, add the following line:

#!/usr/bin/env sim

Make your YAML executable with chmod +x.

Reference

In you script you have access to the following:

request

An object containing the HTTP request, e.g.

{
  "method": "PUT",
  "path": "/documents/bar",
  "pathParams": {
    "id": "bar"
  },
  "queryParams": {
    "foo": "bar"
  },
  "headers": {
    "Content-Type": "application/json"
  },
  "body": {
    "baz": "qux"
  }
}

For example:

var body = request.body;

randomUuid()

A function that generates a random UUID. For example:

var uuid = randomUUID();

db

A service that allows you to persist and access data:

// get an object, maybe null
var value = db.get(key);
// put an object (idempotent)
var existed = db.put(key, value);
// delete an object (idempotent)
var deleted = db.delete(key)
// return an array of all objects
var list = db.list(keyPrefix);

For example:

var obj = db.get("/my-api/" + id)
if (obj) {
    response = {status: 404}
} else {
    response = {body: obj}
}