Note: this is a work in progress!

Ersatz is a tool for mocking RESTful JSON API requests written in Go. It uses simple directory and file structures to emulate the requests and responses that might be associated with a live API server. Response endpoints can be 'varied' (that is, issue different response codes, headers and bodies) using a very simple RESTful command API endpoint.

Assuming you have the Go toolchain installed, run go get github.com/homemade/ersatz. The command line utility ersatz will be added to your $GOPATH/bin directory. If you've set up Go in the standard way, that will be included in your $PATH, you can now run ersatz by typing ersatz.

The server can be started with the command ersatz server <port> </path/to/definitions/directory>.

Ersatz requires a drectory of endpoint definitions. The expected file layout is simple and consistent: {definitions/dir}/<{some/api/endpoint}>/{HTTP_VERB}/{variation}.json. Often you won't want to use an explicit variation, so the tool defauls to default. An example directory tree might look like this:

    |-- /path/to/definitions/directory
        |-- products
            |-- GET
                |-- default.json
                |-- no-products.json
            |-- POST
                |-- default.json
                |-- validation-failed.json
            |-- 123
                |-- GET
                    |-- default.json
                    |-- not-found.json
                    |-- permission-denied.json
                |-- PUT
                    |-- default.json
                    |-- not-found.json
                    |-- permission-denied.json

Using that definitions directory, ersatz would serve the following, unvaried endpoints:

GET /products
POST /products
GET /products/123
PUT /products/123 

The JSON files follow a fairly simple format:

{  
    "response_code": 200, # Any valid HTTP response code
    "headers":{  
        "header-name":"header-value"
    },
    "body":{  
        "any":"legal",
        "json":"values",
        "go":"here"
    }
}

If the JSON above were in the file /path/to/definitions/directory/products/GET/default.json, then ersatz would serve GET /products with one additional header, and the value of body from the JSON.

Ersatz has a special command endpoint, /__ersatz. This endpoint currently only accepts POST requests, and can be used to vary the next request made to an endpoint.

For example, the request POST /__ersatz, with the raw request body

{
    "command": "vary",
    "endpoint": {
        "url": "/products/123",
        "method": "GET",
        "variant": "permission-denied"
    }
}

instructs the ersatz to respond to the next request to GET /products/123 with the JSON file /path/to/definitions/directory/products/123/GET/permission-denied.json instead of the default /path/to/definitions/directory/products/123/GET/default.json. This is the core of the API mocking operation: allowing clients to control the response they expect.

This is a very early release of ersatz. Everything is very likely to change and, if you use it, breaks will no doubt occur as updates are rolled out!