PicoMQTT
This is a lightweight and easy to use MQTT library for ESP8266 and ESP32 devices.
Features:
- Client and broker mode supported
- Intuitive API
- MQTT 3.1.1 implementation
- Publishing and consuming of arbitrary sized messages
- High performance -- the broker can deliver thousands of messages per second -- see benchmarks
- Easy integration with the ArduinoJson library to publish and consume JSON messages -- see examples
- Low memory usage
Limitations:
- Client only supports MQTT QoS levels 0 and 1
- Broker only supports MQTT QoS level 0, ignores will and retained messages.
- Currently only ESP8266 and ESP32 boards are supported
Installation instructions
Additionally, PicoMQTT requires a recent version of the board core: ** For ESP8266 core version 3.1 or later ** For ESP32 core version 2.0.7 or later
Quickstart
To get started, try compiling and running the code below or explore examples.
Client
#include <Arduino.h>
#include <PicoMQTT.h>
PicoMQTT::Client mqtt("broker.hivemq.com");
void setup() {
// Usual setup
Serial.begin(115200);
WiFi.mode(WIFI_STA);
WiFi.begin("MyWiFi", "password");
// Subscribe to a topic pattern and attach a callback
mqtt.subscribe("#", [](const char * topic, const char * payload) {
Serial.printf("Received message in topic '%s': %s\n", topic, payload);
});
// Start the client
mqtt.begin();
}
void loop() {
// This will automatically reconnect the client if needed. Re-subscribing to topics is never required.
mqtt.loop();
if (random(1000) == 0)
mqtt.publish("picomqtt/welcome", "Hello from PicoMQTT!");
}
Broker
#include <Arduino.h>
#include <PicoMQTT.h>
PicoMQTT::Server mqtt;
void setup() {
// Usual setup
Serial.begin(115200);
WiFi.mode(WIFI_STA);
WiFi.begin("MyWiFi", "password");
// Subscribe to a topic pattern and attach a callback
mqtt.subscribe("#", [](const char * topic, const char * payload) {
Serial.printf("Received message in topic '%s': %s\n", topic, payload);
});
// Start the broker
mqtt.begin();
}
void loop() {
// This will automatically handle client connections. By default, all clients are accepted.
mqtt.loop();
if (random(1000) == 0)
mqtt.publish("picomqtt/welcome", "Hello from PicoMQTT!");
}
Publishing messages
To publish messages, the publish and publish_P methods can be used. The client and the broker have both the same
API for publishing.
#include <PicoMQTT.h>
PicoMQTT::Client mqtt("broker.hivemq.com"); // or PicoMQTT::Server mqtt;
void setup() { /* ... */ }
void loop() {
mqtt.loop();
mqtt.publish("picomqtt/simple_publish", "Message");
mqtt.publish("picomqtt/another_simple_publish", F("Message"));
const char binary_payload[] = "This string could contain binary data including a zero byte";
size_t binary_payload_size = strlen(binary_payload);
mqtt.publish("picomqtt/binary_payload", (const void *) binary_payload, binary_payload_size);
}
Notes:
- It's not required to check if the client is connected before publishing. Calls to
publish()will have no effect and will return immediately in such cases. - More examples available here
Subscribing and consuming messages
The subscribe methods can be used with client and broker to set up callbacks for specific topic patterns.
#include <PicoMQTT.h>
PicoMQTT::Client mqtt("broker.hivemq.com"); // or PicoMQTT::Server mqtt;
void setup() {
/* ... */
mqtt.subscribe("picomqtt/foo", [](const char * payload) { /* handle message here */ });
mqtt.subscribe("picomqtt/bar", [](const char * topic, const char * payload) { /* handle message here */ });
mqtt.subscribe("picomqtt/baz", [](const char * topic, const void * payload, size_t payload_size) { /* handle message here */ });
// Pattern subscriptions
mqtt.subscribe("picomqtt/+/foo/#", [](const char * topic, const char * payload) {
// To extract individual elements from the topic use:
String wildcard_value = mqtt.get_topic_element(topic, 1); // second parameter is the index (zero based)
});
mqtt.begin();
}
void loop() {
mqtt.loop();
}
Notes:
- New subscriptions can be added at any point, not just in the
setup()function. - All strings (
const char *parameters) are guaranteed to have a null terminator. It's safe to treat them as strings. - Message payloads can be binary, which means they can contain a zero byte in the middle. To handle binary data, use a callback with a
size_tparameter to know the exact size of the message. - The topic and the payload are both buffers allocated on the stack. They will become invalid after the callback returns. If you need to store the payload for later, make sure to copy it to a separate buffer.
- By default, the maximum topic and payload sizes are is 128 and 1024 bytes respectively. This can be tuned by using
#definedirectives to override values from config.h. Consider using the advanced API described in the later sections to handle bigger messages. - If a received message's topic matches more than one pattern, then only one of the callbacks will be fired.
PicoMQTT::Serverwill not deliver published messages locally. This means that if you set up aPicoMQTT::Serverand usesubscribe, your callback will only be called when messages from clients are received. Messages published on the same device will not trigger the callback.- Try to return from message handlers quickly. Don't call functions which may block (like reading from serial or network connections), don't use the
delay()function. - More examples available here
Arbitrary sized messages
It is possible to send and handle messages of arbitrary size, even if they are significantly bigger than the available memory.
Publishing
auto publish = mqtt.begin_publish(
"picomqtt/advanced", // topic
1000000 // payload size
);
// The returned publish is a Print subclass, so all Print's functions will work:
publish.println("Hello MQTT");
publish.println(2023, HEX);
publish.write('c');
publish.write((const uint8_t *) "1234567890", 10);
// We can always check how much space is left
size_t remaining_size = publish.get_remaining_size();
// ...
// Once all data is written, we have to send the message
publish.send();
Consuming
mqtt.subscribe("picomqtt/advanced", [](const char * topic, PicoMQTT::IncomingPacket & packet) {
// at any point we can check the remaining payload size
size_t payload_size = packet.get_remaining_size();
// packet is a Stram object, so we can use its methods
int val1 = packet.read();
char buf[100];
packet.read(buf, 100);
// it's OK to not read the whole content of the message
});
Notes
- When consuming or producing a message using the advanced API, don't call other MQTT methods. Don't try to publish multiple messages at a time or publish a message while consuming another.
- Even with this API, the topic size is still limited. The limit can be increased by overriding values from config.h.
Json
It's easy to publish and subscribe to JSON messages by integrating with ArduinoJson. Of course, you can always simply use serializeJson and deserializeJson with strings, but it's much more efficient to use the advanced API for this. Check the examples below or try the arduinojson.ino example.
Subscribing
mqtt.subscribe("picomqtt/json/#", [](const char * topic, Stream & stream) {
// declare a StaticJsonDocument or DynamicJsonDocument as usual
StaticJsonDocument<1024> json;
// Deserialize straight from the Stream object
if (deserializeJson(json, stream)) {
// don't forget to check for errors
Serial.println("Json parsing failed.");
return;
}
// work with the object as usual
int value = json["foo"].as<int>();
});
Publishing
// declare a StaticJsonDocument or DynamicJsonDocument and add data as usual
StaticJsonDocument<1024> json;
json["foo"] = "bar";
json["millis"] = millis();
// publish using begin_publish()/send() API
auto publish = mqtt.begin_publish(topic, measureJson(json));
serializeJson(json, publish);
publish.send();
Benchmarks
Charts in this section show PicoMQTT how many messages a broker running on the ESP8266 and ESP32 was able to deliver per second per client depending on the payload size and the number of subscribed clients.
- Test were executed using the library version from commit 406e879e8b25b84c1488c1e2789e4b3719dd1496
- The library was using default configuration values (as defined in config.h)
- Measurements were done on a PC using scripts in benchmark/
- The broker was configured to do nothing but forward the messages to subscribed clients, see benchmark.ino
- The ESPs were connecting to a router just next to them to avoid interference. The test PC was connected to the same router using an Ethernet cable.
ESP8266
ESP32
License
This library is open-source software licensed under GNU LGPLv3.