Developing a simple API backend

Genie makes it very easy to quickly set up a REST API backend. All it takes is a few lines of code. Add these to a rest.jl file:

# rest.jl
using Genie
import Genie.Renderer.Json: json

Genie.config.run_as_server = true

route("/") do
  (:message => "Hi there!") |> json


The key bit here is Genie.config.run_as_server = true. This will start the server synchronously (blocking) so the up() function won't return (won't exit) keeping the Julia process running. As an alternative, we can skip the run_as_server = true configuration and use up(async = false) instead.

The script can be run directly from the command line:

$ julia rest.jl

If you run the above code in the REPL, there is no need to set up run_as_server = true or up(async = false) because the REPL will keep the Julia process running.

Accepting JSON payloads

One common requirement when exposing APIs is to accept POST payloads. That is, requests over POST, with a request body as a JSON encoded object. We can build an echo service like this:

using Genie, Genie.Renderer.Json, Genie.Requests
using HTTP

route("/echo", method = POST) do
  message = jsonpayload()
  (:echo => (message["message"] * " ") ^ message["repeat"]) |> json

route("/send") do
  response = HTTP.request("POST", "http://localhost:8000/echo", [("Content-Type", "application/json")], """{"message":"hello", "repeat":3}""")

  response.body |> String |> json

up(async = false)

Here we define two routes, /send and /echo. The send route makes a HTTP request over POST to /echo, sending a JSON payload with two values, message and repeat. In the /echo route, we grab the JSON payload using the Requests.jsonpayload() function, extract the values from the JSON object, and output the message value repeated for a number of times equal to the repeat value.

If you run the code, the output should be

  echo: "hello hello hello "

If the payload contains invalid JSON, the jsonpayload will be set to nothing. You can still access the raw payload by using the Requests.rawpayload() function. You can also use rawpayload if for example the type of request/payload is not JSON.