API Mocker

A tiny JSON rule based API mocker.

The API Mocker intends to simulate an API behavior, supported by a group of rules written as a JSON file structure.

Dependencies

go 1.17+

Running

• Clone this project
• Prepare your rule file
• from the project folder run go run . -rules=my_custom_rules.json

The mock runs on localhost:8000 by default. These attributes can be changed by using -host and -port flags.

Rule file

The rule file follows a JSON structure with the following minimum fields:

{
  "rules": [
    {
      "endpoint": "/api/auth",
      "method": "GET",
      "items": [
        {
          "response": {
            "status": 200,
            "body": ""
          }
        }
      ]
    }
  ]
}

where:

• endpoint: (mandatory) an expression path to be included in the url routes. Follows go HttpRouter Lib schema
• method: (mandatory) any http method (GET, POST, PUT, PATCH, DELETE, OPTIONS).
• items: (mandatory) an array of rule items where each item is composed by:
  • queryString (optional) a regex expression to match against the URL query strings, If not provided, any query string will be accepted.
  • body: (optional) a regex expression to match against the request body. If not provided, any body will be accepted.
  • counter: (optional) counter matches the number of calls a request should match (starting from zero). If not provided the matching request will return every time.
  • response: (mandatory) a structure containing the response to be sent back if the request rules above match, where:
    • status: (mandatory) a HTTP code
    • delay: (optional) delay (in ms) to take before sending the response
    • headers: (optional) an array containing a "key:value" list of headers to be sent on the response
    • body: (optional) a string with the response, with optional dynamic parsable rules based on go gjson lib

Rules by example

• A GET method where it matches a specific query string

  {
    "endpoint": "/api/auth",
    "method": "GET",
    "items": [
      {
        "queryString": "foo=1.*bar=2.*|bar=2.*foo=1.*",
        "response": {
          "status": 200
        }
      }
    ]
  }

• A GET method simulating an error, after waiting for 5 seconds

    {
      "endpoint": "/api/auth",
      "method": "GET",
      "items": [
        {
          "response": {
            "status": 413,
            "delay": 5000
          }
        }
      ]
    }

• A POST method where the body match specific content

    {
      "endpoint": "/api/auth",
      "method": "POST",
      "items": [
        {
          "body": "\"name\":",
          "response": {
            "status": 200
          }
        }
      ]
    }

• A POST method where the body match multiple content, in this case must have user and password words

    {
      "endpoint": "/api/auth",
      "method": "POST",
      "items": [
        {
          "body": ".*user.*password",
          "response": {
            "status": 200
          }
        }
      ]
    }

• A POST method simulating error returning with json

    {
      "endpoint": "/api/auth",
      "method": "POST",
      "items": [
        {
          "response": {
            "status": 417,
            "headers": [
              "Content-Type: application/json"
            ],
            "body": "{\"message\": \"Failed to respond\"}"
          }
        }
      ]
    }

• A POST method with dynamic answer, parsing 1st element from request body. Dynamic parsing based on go gjson lib

    {
      "endpoint": "/api/auth",
      "method": "POST",
      "items": [
        {
          "response": {
            "status": 417,
            "headers": [
              "Content-Type: application/json"
            ],
            "body": "{\"requested-hash\": \"{{sha256|@keys|0}}\"}"
          }
        }
      ]
    }

• A POST method where the body match specific content with counter. In the first call it returns 200, second matching call 400 and all subsequent call 202 with a json body.

    {
      "endpoint": "/api/auth",
      "method": "POST",
      "items": [
        {
          "counter": 0,
          "body": ".*user",
          "response": {
            "status": 200
          }
        },
        {
          "counter": 1,
          "body": ".*user",
          "response": {
            "status": 400
          }
        },
        {
          "body": ".*user",
          "response": {
            "status": 202,
            "headers": [
              "Content-Type: application/json"
            ],
            "body": "{\"message\": \"ok\"}"
          }
        }
      ]
    }

Docker

Starting the api mocker on docker is very simple:

docker build -t api-mocker:latest .
docker run -v "/$(pwd)/rules:/rules" -p 8000:8000 api-mocker:latest  -rules=rules/test-rule.json

note that you always need to define the volume and path for the rules.

It is also possible to use it directly from Docker Hub: docker pull nordsec/api-mocker

Via docker-compose

Example docker-compose.yml:

version: "3.7"

services:
  api-mocker:
    build:
      context: .
      target: api-mocker
    command: ["-rules", "rules/test-rule.json"]
    ports:
      - "8000:8000"
    volumes:
      - ./rules:/rules

run docker-compose up wait for it to initialize completely, and visit http://localhost:8000/any-path-defined-on-rules-file