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Consume External Services

Kong Gateway provides capabilities around security, traffic management, monitoring, and analytics. Additionally, Kong Gateway can be used as an ancillary development layer for your business logic. A common use case may be the desire to decorate API responses from your upstream services with data from a 3rd party service.

Prerequisites

This page is the fourth chapter in the Getting Started guide for developing custom plugins. These instructions refer to the previous chapters in the guide and require the same developer tool prerequisites.

Step by Step

The following are step by step instructions to show you how to consume data from external services using an http client and parsing JSON values. With the parsed data we will show you how to add values to the response data prior to returning to your API gateway clients.

Including HTTP and JSON support

Start by importing two new libraries to the handler.lua file giving us access to HTTP and JSON parsing support.

You are going to use the lua-resty-http library for HTTP client connectivity to the 3rd party service. The library provides a simple interface to make single HTTP requests that we will use here, but see the documentation for all options when using the library.

For JSON support, use the lua-cjson library which provides fast and standards compliant JSON support. lua-cjson supports a safe variant of the library that allows for exception free encoding and decoding support.

Add the new includes at the top handler.lua:

local http  = require("resty.http")
local cjson = require("cjson.safe")

Invoke 3rd party http request

The lua-resty-http library provides a simple HTTP request function (request_uri) we can use to reach out to our 3rd party service. Here we show invoking a GET request to the httpbin.org/anything API which will echo back various information in the response.

Add the following to the top of the MyPluginHandler:response function inside the handler.lua module:

local httpc = http.new()

local res, err = httpc:request_uri("https://httpbin.konghq.com/anything", {
  method = "GET",
})

If the request to the 3rd party service is successful, the res variable will contain the response. Before showing how to process the successful response, what about error events? Errors will be provided in the err return value, let’s see what options there are for handling them.

Handle response errors

The Kong Gateway Plugin Development Kit provides you with various functions to help you handle error conditions.

In this example you are processing responses from the upstream service and decorating your client response with values from the 3rd party service. If the request to the 3rd party service fails, you have a choice to make. You can terminate processing of the response and return to the client with an error, or you could continue processing the response and not complete the custom header logic. In this example, we show how to terminate the response processing and return a 500 internal server error to the client.

Add the following to the MyPluginHandler:response function immediately after the httpc:request_uri call:

if err then
  return kong.response.error(500,
    "Error when trying to access 3rd party service: " .. err,
    { ["Content-Type"] = "text/html" })
end

If you choose to continue processing instead, you could log an error message and return from the MyPluginHandler:response function, similar to this:

if err then
    kong.log("Error when trying to access 3rd party service:", err)
    return
end

Process JSON data from 3rd party response

This 3rd party service returns a JSON object in the response body. Here we are going to show how to parse and extract a single value from the JSON body.

Use the decode function in the lua-cjson library passing in the res.body value received from the request_uri function:

local body_table, err = cjson.decode(res.body)

The decode function returns a tuple of values. The first value contains the result of a successful decoding and represents the JSON as a table containing the parsed data. If an error has occurred, the second value will contain error information (or nil on success).

Same as during the HTTP processing above, in the event of an error return an error response to the client and stop processing. Add the following to the MyPluginHandler:response function after the previous line:

if err then
  return kong.response.error(500,
    "Error while decoding 3rd party service response: " .. err,
    { ["Content-Type"] = "text/html" })
end

At this point, given no error conditions, you have a valid response from the 3rd party you can use to decorate your client response. The httpbin service returns a variety of fields including a url field which is an echo of the URL requested. For this example pass the url field to the client response by adding the following after the previous error handling:

kong.response.set_header(conf.response_header_name, body_table.url)

We’ve broken down each section of the code. The following is a full code listing for the handler.lua file.

Full code listing

local http  = require("resty.http")
local cjson = require("cjson.safe")

local MyPluginHandler = {
  PRIORITY = 1000,
  VERSION = "0.0.1",
}

function MyPluginHandler:response(conf)

  kong.log("response handler")

  local httpc = http.new()

  local res, err = httpc:request_uri("https://httpbin.konghq.com/anything", {
    method = "GET",
  })

  if err then
    return kong.response.error(500,
      "Error when trying to access 3rd party service: " .. err,
      { ["Content-Type"] = "text/html" })
  end

  local body_table, err = cjson.decode(res.body)

  if err then
    return kong.response.error(500,
      "Error when decoding 3rd party service response: " .. err,
      { ["Content-Type"] = "text/html" })
  end

  kong.response.set_header(
    conf.response_header_name,
    body_table.url)

end

return MyPluginHandler

Update testing code

At this stage, if you re-run the pongo run command to execute the integration tests previously built, you will receive errors. The expected value in the header has changed from response to https://httpbin.konghq.com/anything. Update the spec/my-plugin/01-integration_spec.lua file to assert the new value in the header.

-- validate the value of that header
assert.equal("https://httpbin.konghq.com/anything", header_value)

Re-run the test with pongo run and verify success:

...
[----------] Global test environment teardown.
[==========] 2 tests from 1 test file ran. (23171.28 ms total)
[  PASSED  ] 2 tests.

This guide provides examples to get you started. In a real plugin development scenario, you would want to build integration tests for 3rd party services by providing a test dependency using a mock service instead of making network calls to the actual 3rd party service used. Pongo supports test dependencies for this purpose. See the Pongo documentation for details on setting test dependencies.

What’s Next

Deploying Kong Gateway plugins is the final step in building an end to end development pipeline. The next chapter will provide the options for deployment and give you a step by step guide.