Guided Tour through aiocoap¶
This page gets you started on the concepts used in aiocoap; it will assume rough familiarity with what CoAP is, and a working knowledge of Python development, but introduce you to asynchronous programming and explain some CoAP concepts along with the aiocoap API.
If you are already familiar with asynchronous programming and/or some other concepts involved, or if you prefer reading code to reading tutorials, you might want to go after the Usage Examples instead.
If you want to give aiocoap a try without any installation, you can also run it from your web browser – albeit with some limitations. pyodide and Jupyter describes how to use it and its differences from regular operation. You can still follow this guided tour; any changes will be noted to pyodide users.
First, some tools¶
Before we get into programming, let’s establish tools with which we can probe a server, and a server itself. If you have not done it already, install aiocoap for development.
pyodide users just install aiocoap their regular way. You can not run the commands we are using in this section; just read through it, expected outputs are displayed anyway.
Start off with the sample server by running the following in a terminal inside the aiocoap directory:
$ sign indicates the prompt; you enter everything after it in
a terminal shell. Lines not starting with a dollar sign are the program
output, if any. Later on, we’ll see lines starting with
>>>; those are
run inside a Python interpreter.
I recommend that you use the IPython interpreter. One useful feature for
following through this tutorial is that you can copy full lines (including
>>> parts) to the clipboard and use the
%paste IPython command
to run it, taking care of indentation etc.
This has started a CoAP server with some demo content, and keeps running until you terminate it with Ctrl-C.
In a separate terminal, use the aiocoap-client tool to send a GET request to the server:
$ ./aiocoap-client coap://localhost/.well-known/core application/link-format content was re-formatted </.well-known/core>; ct="40", </time>; obs, </other/block>, </other/separate>; title="A large resource", </whoami>, <https://christian.amsuess.com/tools/aiocoap/#version-0.4.3.post0>; rel="impl-info"
The address we’re using here is a resource on the local machine (
at the well-known location
.well-known/core, which in CoAP is the go-to
location if you don’t know anything about the paths on the server beforehand.
It tells that there is a resource at the path
/time that has the
observable attribute, a resource at the path
/.well-known/core, and more at
Getting all lines in a single
row or no color? Then there are third party modules missing. Run
-m aiocoap.cli.defaults to see which they are, or just go back to the
installation step and make sure to include
There can be a “(No newline at end of message)” line below your output. This just makes sure your prompt does not start in the middle of the screen. I’ll just ignore that.
Let’s see what
/time gives us:
$ ./aiocoap-client coap://localhost/time 2021-12-07 10:08
The response should have arrived immediately: The client sent a message to the
server in which it requested the resource at
/time, and the server could
right away send a message back. In contrast,
/other/separate is slower:
$ ./aiocoap-client coap://localhost/other/separate Three rings for the elven kings [abbreviated]
The response to this message comes back with a delay. Here, it is simulated by the server; in real-life situations, this delay can stem from network latency, servers waiting for some sensor to read out a value, slow hard drives etc.
In order to run a similar request programmatically, we’ll need a request message.
>>> from aiocoap import * >>> msg = Message(code=GET, uri="coap://localhost/other/separate") >>> print(msg) <aiocoap.Message at 0x0123deadbeef: no mtype, GET (no MID, empty token) remote None, 2 option(s)>
The message consists of several parts. The non-optional ones are largely handled by aiocoap (message type, ID, token and remote are all None or empty here and will be populated when the message is sent). The options are roughly equivalent to what you might know as HTTP headers:
>>> msg.opt <aiocoap.options.Options at 0x0123deadbef0: URI_HOST: localhost, URI_PATH: other / separate>
You might have noticed that the Uri-Path option is shown with some space around the slash. This is because paths in CoAP are not a structured byte string with slashes in it (as they are in HTTP), but actually repeated options of a (UTF-8) string, which are represented as a tuple in Python:
>>> msg.opt.uri_path ('other', 'separate')
Now to send that network as a request over the network, we’ll need a network protocol object. That has a request method, and can give a response (bear with me, these examples don’t actually work):
>>> protocol.request(msg).response <Future pending cb=[Request._response_cancellation_handler()]>
That is obviously not a proper response – yet. If the protocol returned a finished response, the program couldn’t do any work in the meantime. Instead, it returns a Future – an object that will (at some time in the future) contain the response. Because the Future is returned immediately, the user can start other requests in parallel, or do other processing in the meantime. For now, all we want is to wait until the response is ready:
>>> await protocol.request(msg).response <aiocoap.Message at 0x0123deadbef1: Type.CON 2.05 Content (MID 51187, token 00008199) remote <UDP6EndpointAddress [::ffff:127.0.0.1]:5683 with local address>, 186 byte(s) payload>
Here, we have a successful message (“2.05 Content” is the rough equivalent of HTTP’s “200 OK”, and the 186 bytes of payload look promising). Until we can dissect that, we’ll have to get those asynchronous things to work properly, though.
To work interactively with asynchronous Python, start your Python interpreter like this:
$ python3 -m asyncio >>>
Users of the highly recommended IPython can continue in their existing session, as support for the asynchronous shell is always available there.
pyodide users should rather use
and change later URIs accordingly.
That is an online service that runs the same server,
and can be used without any local installation.
>>> from aiocoap import * >>> protocol = await Context.create_client_context() >>> msg = Message(code=GET, uri="coap://localhost/other/separate") >>> response = await protocol.request(msg).response >>> print(response) <aiocoap.Message at 0x0123deadbef1: Type.CON 2.05 Content (MID 51187, token 00008199) remote <UDP6EndpointAddress [::ffff:127.0.0.1]:5683 with local address>, 186 byte(s) payload>
protocol object could also be created – we need to start that
once to prepare a socket for all the requests we’re sending later. That doesn’t
actually take a long time, but could, depending on the operating system.
If you want to pack any of the code into functions, these functions need to
be asynchronous functions. When working in a
.py file, the
keyword is not available outside, and you’ll need to kick off your program
The same code as above packed up in a file would look like this:
import asyncio from aiocoap import * async def main(): protocol = await Context.create_client_context() msg = Message(code=GET, uri="coap://localhost/other/separate") response = await protocol.request(msg).response print(response) asyncio.run(main())
The response obtained in the main function is a message like the request message, just that it has a different code (2.05 is of the successful 2.00 group), incidentally no options (because it’s a very simple server), and actual data.
The response code is represented in Python by an enum with some utility functions; the remote address (actually remote-local address pair) is an object too:
>>> response.code <Successful Response Code 69 "2.05 Content"> >>> response.code.is_successful() True >>> response.remote.hostinfo '[::ffff:127.0.0.1]' >>> response.remote.is_multicast False
The actual response message, the body, or the payload of the response, is accessible in the payload property, and is always a bytestring:
>>> response.payload b'Three rings for the elven kings [ abbreviated ]'
aiocoap does not yet provide utilities to parse the message according to its
content format (which would be accessed as
This is currently the end of the guided tour; see the
documentation for the server side until the tour covers that is complete.