CoAP is lightweight and its implementation both on a client and server basis should take few resources. Here, we use the Python-based aiocoap library. More can be read on aiocoap in:  Amsüss, Christian, and Wasilak, Maciej. aiocoap: Python CoAP Library. Energy Harvesting Solutions, 2013–. http://github.com/chrysn/aiocoap/. Many other free CoAP clients and servers exist with several written in low-level C code for extremely constrained sensor environments. Here, we use a Python environment for brevity.

The client implementation is:

#!/usr/bin/env python3
import asyncio            #necessary for asynchronous processing in Python
from aiocoap import *     #using the aiocoap library

The following is main loop for the client. The client uses PUT to broadcast the temperature to known URI:

async def main():
 context = await Context.create_client_context()

 await asyncio.sleep(2)                         #wait 2 seconds after initialization

 payload = b"20.2 C"
 request = Message(code=PUT, payload=payload)
 
 request.opt.uri_host = '127.0.0.1'             #URI for localhost address
 request.opt.uri_path = ("temp", "celcius")     #URI for path to /temp/celcius

 response = await context.request(request).response  
 print('Result: %s
%r'%(response.code, response.payload))

if __name__ == "__main__":
 asyncio.get_event_loop().run_until_complete(main())

The server implementation is:

#!/usr/bin/env python3
import asyncio                         #necessary for asynchronous processing in Python
import aiocoap.resource as resource    #using aiocoap library
import aiocoap

The following code illustrates the services for PUT and GET methods:

class GetPutResource(resource.Resource):

 def __init__(self):
 super().__init__()
 self.set_content(b"Default Data (padded) "


 def set_content(self, content):                #Apply padding
 self.content = content
 while len(self.content) <= 1024:
 self.content = self.content + b"0123456789
"

 async def render_get(self, request):            #GET handler
 return aiocoap.Message(payload=self.content)    

 async def render_put(self, request):            #PUT handler
 print('PUT payload: %s' % request.payload)
 self.set_content(request.payload)               #replaces set_content with received payload
 return aiocoap.Message(code=aiocoap.CHANGED, payload=self.content) #set response code to 2.04

The main loop is:

def main():
 root = resource.Site()        #root element that contains all resources found on server

 root.add_resource(('.well-known', 'core'),              #this is the typical .well-known/core
 resource.WKCResource(root.get_resources_as_linkheader)) #resource list for .well-known/core
 root.add_resource(('temp', 'celcius'), GetPutResource())) #adds the resource /tmp/celcius

 asyncio.Task(aiocoap.Context.create_server_context(root))

 asyncio.get_event_loop().run_forever()

if __name__ == "__main__":
 main()