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API Gateways

Architecture With An API Gateway

A contract represents the interface provided by the API provider to the API consumer.

Where the consumer calls the provider directly, there are just two parties.

However an API gateway changes this picture. API gateways subtly modify incoming requests, by adding headers or rewriting the URL. As a result, the contract seen by the consumer is slightly different from that provided by the API.

There are also direct consumers of the API lying behind the API gateway.

In this scenario, there are four parties: the external consumer, the API gateway, the interal consumer, and the API provider.

four party model

Specmatic’s approach is to write the contract the way the API provider and internal consumer sees it.

This way, the API provider can run contract tests using the contract as is, and the internal consumer can stub it out.

That leaves the external consumer, which needs to use the contract for service virtualisation.

Specmatic provides a hook named stub_load_contract in which the necessary modifications can be made to the provider contract, so that it looks exactly the way a consumer sees it.

Register it in specmatic.json in the section on hooks.

The hooks snippet in specmatic.json would look like this:

  "hooks": {
    "stub_load_contract": "python load.py"
  }

It can be any command, above is just an example.

Instead of reading the contract directly, Specmatic runs “python load.py” with the environment variable CONTRACT_FILE set to the path of the contract to be read.

load.py must read the contract, modify it as needed, and print it to standard output.

Specmatic reads the contract from standard output and proceeds from there onward as normal.

Here’s a sample of what the load.py file could look like.

Supposting that

  • the API provider has an API /List, which the API gateway exposes as /customer/list
  • the API provider expects a header x-security, which is added by API gateway, and is not sent by the external consumer

The contract might look like this:

---
openapi: "3.0.1"
info:
  title: "Customers"
  version: "1"
paths:
  /List:
    get:
      summary: "List customers"
      parameters:
      - name: "x-security"
        in: "header"
        required: true
        schema:
          type: "string"
      responses:
        "200":
          description: "List customers"
          content:
            text/plain:
              schema:
                type: "array"
                items:
                  type: "number"

The script below removes provider-only headers, and changes the provider paths to the ones accepted by the API gateway.

#filename: load.py

import yaml
import os

def cleanup_path(contract_file_name, path_map):
    with open(contract_file_name) as f:
        data = yaml.safe_load(f)

    paths = data["paths"]

    for pathName in path_map:
        if pathName in paths:
            newPathName = path_map[pathName]
            data["paths"][newPathName] = data["paths"][pathName]
            del data["paths"][pathName]


    return data

def remove_provider_only_headers(data):
    for pathName in data["paths"]:
        path = data["paths"][pathName]
        for methodName in path:
            method = path[methodName]
            if methodName in ["post", "get"] and "parameters" in method:
                params = method["parameters"]
                deletable = [i for i in range(len(params)) if params[i]["$ref"].endswith("x-security")]
                
                for i in reversed(deletable):
                    del params[i]
    
    return data

contract_file_name = os.environ["CONTRACT_FILE"]

path_map = {
    "/List": "/customer/list"
}

contract = cleanup_path(contract_file_name, path_map)

contract = remove_provider_only_headers(contract)

print(yaml.dump(contract))

The contract actually seen by Specmatic after passing it through this script would be:

---
openapi: "3.0.1"
info:
  title: "Customers"
  version: "1"
paths:
  /customer/list:
    get:
      summary: "List customers"
      parameters: []
      responses:
        "200":
          description: "List customers"
          content:
            text/plain:
              schema:
                type: "array"
                items:
                  type: "number"

Note: the hook should be used when there are very slight differences between the contract seen by the consumer and provided by the API.