* Multiple instances (multiple clients in one application)
* Support subscribing, publishing, authentication, last will messages, keep alive pings, and all 3 Quality of Service (QoS) levels (it should be a fully functional client)
Messages with QoS 0 will be sent only once, QoS 1 and 2 have a different behavior since the protocol requires extra steps to complete the process.
The ESP-MQTT library opts to always retransmit unacknowledged QoS 1 and 2 PUBLISH messages to avoid losses in faulty connections, even though the MQTT specification
requires the re-transmission only on reconnect with Clean Session flag been set to 0 (set :cpp:member:`disable_clean_session <esp_mqtt_client_config_t::session_t::disable_clean_session>` to true for this behavior).
Messages that could need retransmission, QoS 1 and 2, are always enqueued, but first transmission try occurs immediately if :cpp:func:`esp_mqtt_client_publish <esp_mqtt_client_publish>` is used. A transmission retry for unacknowledged messages will occur after :cpp:member:`message_retransmit_timeout <esp_mqtt_client_config_t::session_t::message_retransmit_timeout>`. After :ref:`CONFIG_MQTT_OUTBOX_EXPIRED_TIMEOUT_MS` messages will expire and deleted. If :ref:`CONFIG_MQTT_REPORT_DELETED_MESSAGES` is set an event is sent to notify the user.
The configuration is made by setting fields in :cpp:class:`esp_mqtt_client_config_t` struct. The configuration struct has the following sub structs to configure different aspects of the client operation.
Broker address can be set by usage of :cpp:class:`address <esp_mqtt_client_config_t::broker_t::address_t>` struct. The configuration can be made by usage of :cpp:member:`uri <esp_mqtt_client_config_t::broker_t::address_t::uri>` field or the combination of :cpp:member:`hostname <esp_mqtt_client_config_t::broker_t::address_t::hostname>`, :cpp:member:`transport <esp_mqtt_client_config_t::broker_t::address_t::transport>` and :cpp:member:`port <esp_mqtt_client_config_t::broker_t::address_t::port>`. Optionally, :cpp:member:`path <esp_mqtt_client_config_t::broker_t::address_t::path>` could be set, this field is useful in WebSocket connections.
For secure connections with TLS used, and to guarantee Broker's identity, the :cpp:class:`verification <esp_mqtt_client_config_t::broker_t::verification_t>` struct must be set.
The broker certificate may be set in PEM or DER format. To select DER, the equivalent :cpp:member:`certificate_len <esp_mqtt_client_config_t::broker_t::verification_t::certificate_len>` field must be set. Otherwise, a null-terminated string in PEM format should be provided to :cpp:member:`certificate <esp_mqtt_client_config_t::broker_t::verification_t::certificate>` field.
*:cpp:member:`username <esp_mqtt_client_config_t::credentials_t::username>` pointer to the username used for connecting to the broker, can also be set by URI
*:cpp:member:`client_id <esp_mqtt_client_config_t::credentials_t::client_id>`: pointer to the client ID, defaults to ``ESP32_%CHIPID%`` where ``%CHIPID%`` are the last 3 bytes of MAC address in hex format
It's possible to set authentication parameters through the :cpp:class:`authentication <esp_mqtt_client_config_t::credentials_t::authentication_t>` field. The client supports the following authentication methods:
*:cpp:member:`password <esp_mqtt_client_config_t::credentials_t::authentication_t::password>`: use a password by setting
*:cpp:member:`certificate <esp_mqtt_client_config_t::credentials_t::authentication_t::certificate>` and :cpp:member:`key <esp_mqtt_client_config_t::credentials_t::authentication_t::key>`: mutual authentication with TLS, and both can be provided in PEM or DER format
*:cpp:member:`use_secure_element <esp_mqtt_client_config_t::credentials_t::authentication_t::use_secure_element>`: use secure element available in ESP32-WROOM-32SE
*:cpp:member:`ds_data <esp_mqtt_client_config_t::credentials_t::authentication_t::ds_data>`: use Digital Signature Peripheral available in some Espressif devices
MQTT allows for a last will and testament (LWT) message to notify other clients when a client ungracefully disconnects. This is configured by the following fields in the :cpp:class:`last_will <esp_mqtt_client_config_t::session_t::last_will_t>` struct.
*:cpp:member:`topic <esp_mqtt_client_config_t::session_t::last_will_t::topic>`: pointer to the LWT message topic
*:cpp:member:`msg <esp_mqtt_client_config_t::session_t::last_will_t::msg>`: pointer to the LWT message
*:cpp:member:`msg_len <esp_mqtt_client_config_t::session_t::last_will_t::msg_len>`: length of the LWT message, required if :cpp:member:`msg <esp_mqtt_client_config_t::session_t::last_will_t::msg>` is not null-terminated
*:cpp:member:`qos <esp_mqtt_client_config_t::session_t::last_will_t::qos>`: quality of service for the LWT message
*:cpp:member:`retain <esp_mqtt_client_config_t::session_t::last_will_t::retain>`: specifies the retain flag of the LWT message
-:ref:`CONFIG_MQTT_TRANSPORT_SSL` and :ref:`CONFIG_MQTT_TRANSPORT_WEBSOCKET`: enable specific MQTT transport layer, such as SSL, WEBSOCKET, and WEBSOCKET_SECURE
The following events may be posted by the MQTT client:
*``MQTT_EVENT_BEFORE_CONNECT``: The client is initialized and about to start connecting to the broker.
*``MQTT_EVENT_CONNECTED``: The client has successfully established a connection to the broker. The client is now ready to send and receive data.
*``MQTT_EVENT_DISCONNECTED``: The client has aborted the connection due to being unable to read or write data, e.g. because the server is unavailable.
*``MQTT_EVENT_SUBSCRIBED``: The broker has acknowledged the client's subscribe request. The event data will contain the message ID of the subscribe message.
*``MQTT_EVENT_UNSUBSCRIBED``: The broker has acknowledged the client's unsubscribe request. The event data will contain the message ID of the unsubscribe message.
*``MQTT_EVENT_PUBLISHED``: The broker has acknowledged the client's publish message. This will only be posted for QoS level 1 and 2, as level 0 does not use acknowledgements. The event data will contain the message ID of the publish message.
*``MQTT_EVENT_DATA``: The client has received a publish message. The event data contains: message ID, name of the topic it was published to, received data and its length. For data that exceeds the internal buffer, multiple ``MQTT_EVENT_DATA`` will be posted and :cpp:member:`current_data_offset <esp_mqtt_event_t::current_data_offset>` and :cpp:member:`total_data_len<esp_mqtt_event_t::total_data_len>` from event data updated to keep track of the fragmented message.
*``MQTT_EVENT_ERROR``: The client has encountered an error. The field :cpp:type:`error_handle <esp_mqtt_error_codes_t>` in the event data contains :cpp:type:`error_type <esp_mqtt_error_type_t>` that can be used to identify the error. The type of error will determine which parts of the :cpp:type:`error_handle <esp_mqtt_error_codes_t>` struct is filled.
