CoapObserve - CoAP resource observation (server push)

Layer: L7 Application · Build flags: PC_ENABLE_COAP, PC_ENABLE_COAP_OBSERVE

What this example teaches

CoAP Observe (RFC 7641) turns request/response into publish/subscribe: a client sends a GET with the Observe option and the server keeps pushing updates as the resource changes - the CoAP equivalent of server-sent events, but over UDP. This serves an observable /count resource; every second it increments the counter and notifies all observers. It builds on the plain CoAP server in CoAP.

Register an ordinary resource, then notify on change. The handler is the same shape as any CoAP handler; what makes it observable is that observers are tracked by the library and you call pc_coap_notify() when the representation changes:

pc_coap_server_add_resource("/count", CoapMethodMask::COAP_ALLOW_GET, h_count);
pc_coap_server_begin(5683);

void loop() {
    if (/* once a second */) {
        g_count++;
        pc_coap_notify("/count"); // push the new value to every observer
    }
}

Each notification carries an increasing Observe sequence number so a client can detect reordering. Observe with coap-client -m get -s 30 coap://<ip>/count or aiocoap-client --observe coap://<ip>/count.

Build and run

pio ci --board=esp32dev --project-option="framework=arduino" \
  --project-option="build_flags=-DPC_ENABLE_COAP=1 -DPC_ENABLE_COAP_OBSERVE=1" \
  --lib="." examples/L7-Application/CoapObserve/CoapObserve.ino

coap-client -m get -s 30 coap://<ip>/count   # libcoap, -s = observe for 30s
aiocoap-client --observe coap://<ip>/count

Annotated source

The complete sketch (CoapObserve.ino), reproduced verbatim with added explanatory comments:

// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
#define PC_ENABLE_COAP 1
#define PC_ENABLE_COAP_OBSERVE 1
 
#include "protocore.h"
#include "network_drivers/physical/physical.h"
#include "services/iot/coap/coap.h"
 
static const char *SSID = "YOUR_SSID";
static const char *PASSWORD = "YOUR_PASSWORD";
 
static uint32_t g_count = 0;
 
// GET /count -> the current counter value (also pushed to observers on change).
void h_count(const CoapRequest *req, CoapResponse *resp)
{
    (void)req;
    int n = snprintf((char *)resp->payload, resp->payload_cap, "%lu", (unsigned long)g_count);
    resp->payload_len = (n > 0) ? (size_t)n : 0;
    resp->content_format = CoapContentFormat::COAP_CF_TEXT;
    resp->code = (uint8_t)CoapResponseCode::COAP_RSP_CONTENT;
}
 
void setup()
{
    Serial.begin(115200);
 
    init_wifi_physical(SSID, PASSWORD);
    Serial.print("Connecting to WiFi");
    while (!wifi_ready())
    {
        delay(250);
        Serial.print('.');
    }
    uint32_t ip = pc_net_egress_ip(); // library egress IP (network byte order), no Arduino WiFi
    Serial.printf("IP: %u.%u.%u.%u\n", (unsigned)(ip & 0xFF), (unsigned)((ip >> 8) & 0xFF),
                  (unsigned)((ip >> 16) & 0xFF), (unsigned)((ip >> 24) & 0xFF));
 
    pc_coap_server_reset();
    pc_coap_server_add_resource("/count", CoapMethodMask::COAP_ALLOW_GET, h_count);
    pc_coap_server_begin(5683);
    Serial.println("CoAP server on :5683, observe coap://<ip>/count");
}
 
void loop()
{
    static uint32_t last = 0;
    if (millis() - last >= 1000)
    {
        last = millis();
        g_count++;
        pc_coap_notify("/count"); // push the new value to every observer
    }
}