How to get POST data with Plug

I feel really dumb asking this, but I’ve bopped all over the internet, tried various things, and checked out some posts on this and I’m perplexed at why I can’t figure out how to simply retrieve data passed to my program via POST. Here is my Router module

defmodule Kahwiz.Router do
  use Plug.Router

  plug(Plug.Parsers, parsers: [:urlencoded])


  get "/" do

  post "/" do
    # IO.inspect(conn.params, label: "The params")
    {:ok, data, _conn} = read_body(conn)
    IO.inspect(data, label: "The data")
    send_resp(conn, 200, "Got it #{data}")

  match _ do
    send_resp(conn, 404, "oops")

And my CURL command to send data to the server:
curl -X POST http://localhost:4001?param1=1234

In its current condition, data is empty, but on the command line I do get “Got it”. If you use the IO.inspect line to get the parameters passed to it, I see the data in the inspect function but also a large amount of errors:

The params: %{"param1" => "1234"}

20:59:47.224 [error] #PID<0.892.0> running Kahwiz.Router (connection #PID<0.891.0>, stream id 1) terminated
Server: localhost:4001 (http)
Request: POST /?param1=1234
** (exit) an exception was raised:
    ** (Protocol.UndefinedError) protocol String.Chars not implemented for %{"param1" => "1234"} of type Map. This protocol is implemented for the following type(s): Decimal, NaiveDateTime, Atom, Date, BitString, Float, Integer, List, Time, Version.Requirement, Version, URI, DateTime
        (elixir) lib/string/chars.ex:3: String.Chars.impl_for!/1
        (elixir) lib/string/chars.ex:22: String.Chars.to_string/1
        (kahwiz) lib/router.ex:20: anonymous fn/2 in Kahwiz.Router.do_match/4
        (kahwiz) lib/plug/router.ex:259: Kahwiz.Router.dispatch/2
        (kahwiz) lib/router.ex:1: Kahwiz.Router.plug_builder_call/2
        (plug_cowboy) lib/plug/cowboy/handler.ex:12: Plug.Cowboy.Handler.init/2
        (cowboy) /usr/home/courtney/Documents/code/Elixir/kahwiz/deps/cowboy/src/cowboy_handler.erl:41: :cowboy_handler.execute/2
        (cowboy) /usr/home/courtney/Documents/code/Elixir/kahwiz/deps/cowboy/src/cowboy_stream_h.erl:320: :cowboy_stream_h.execute/3

My goal in the end is to be able to submit the URL above and get “Got it 1234”

I’ve looked through the Plug docs on hexdocs but I’m having a hard time solving this. I’ve accomplished what I’m trying to do in the past with Phoenix but I’m trying to learn Elixir without a framework.

In your code you are trying to retrieve the body but you have not sent any body. The params are normal GET parameters which are retrieved another way. To post a body:

curl -X POST -d "param1=1234" http://localhost:4001

You can probably use Plug.Conn.fetch_query_params/2 to get the parameters from the query string rather than the body if that is what you are after.

You might also want to look into to read the parameters from the body in a more structured way.


You’re right. I’m dumb. I need to learn to IO.inspect more before I speak. I thought that conn.params contained the info initially and got nothing so I tried a variety things. It turned out to be the query_params as you suggest. In the end I changed to POST but I’m glad I learned a lot here and am remembering things. I’m picking up web development again after 3 years of stagnation and doing it with Elixir so I’m also drudging through remembering the intricacies of functional programming, since my only exposure to functional programming was using Racket 4 years ago :slight_smile:


Am asking what if I want to get some kind of Map so that I can get param details such as “username” and password in any http requests. So that I can use in backed.

The way am seeing at top, I get data as “bitsring” which I cannot use easily.

“name=John& password=opensesami” some string like that.

Looks like form encoded data, you should be able to use Plug.Conn.Query.decode/4 to decode it.