MqttMessageQueue

Extends: MessageQueue

Implements: IReferenceable, IUnreferenceable, IConfigurable, IOpenable, ICleanable

Description

The MqttMessageQueue class allows you to create message queues that send and receive messages via an MQTT message broker.

Configuration parameters

• topic: name of MQTT topic to subscribe
• connection(s):
  • discovery_key: (optional) key to retrieve the connection from IDiscovery
  • host: host name or IP address
  • port: port number
  • uri: resource URI or connection string with all parameters in it
• credential(s):
  • store_key: (optional) key to retrieve the credentials from ICredentialStore
  • username: username
  • password: user’s password
• options:
  • serialize_envelope: (optional) true to serialize entire message as JSON, false to send only message payload (default: true)
  • autosubscribe: (optional) true to automatically subscribe on option (default: false)
  • qos: (optional) quality of service level aka QOS (default: 0)
  • retain: (optional) retention flag for published messages (default: false)
  • retry_connect: (optional) turns on/off automated reconnect when connection is lost (default: true)
  • connect_timeout: (optional) number of milliseconds to wait for connection (default: 30000)
  • reconnect_timeout: (optional) number of milliseconds to wait on each reconnection attempt (default: 1000)
  • keepalive_timeout: (optional) number of milliseconds to ping the broker while inactive (default: 3000)

References

• *:logger:*:*:1.0 - (optional) ILogger components to pass log messages
• *:counters:*:*:1.0 - (optional) ICounters components to pass collected measurements
• *:discovery:*:*:1.0 - (optional) IDiscovery services
• *:credential-store:*:*:1.0 - (optional) ICredentialStore to resolve credentials
• *:connection:mqtt:*:1.0 - (optional) shared connection to MQTT service

Constructors

Creates a new instance of the message queue.

MqttMessageQueue([String? name])

• name: String? - (optional) queue’s name.

Fields

Instance methods

abandon

Returnes a message into the queue and makes it available for all subscribers to receive it again. This method is usually used to return a message which could not be processed at the moment to repeat the attempt. Messages that cause unrecoverable errors shall be removed permanently or/and send to dead letter queue.

• Important: This method is not supported by MQTT.

@override

Future abandon(MessageEnvelope message)

• message: MessageEnvelope - message to return.

clear

Clears a component’s state.

@override

Future clear(IContext? context)

• context: IContext - (optional) a context to trace execution through a call chain.

close

Closes a component and frees used resources.

@override

Future close(IContext? context)

• context: IContext - (optional) a context to trace execution through a call chain.

complete

Permanently removes a message from the queue. This method is usually used to remove the message after successful processing.

• Important: This method is not supported by MQTT.

@override

Future complete(MessageEnvelope message)

• message: MessageEnvelope - message to remove.

configure

Configures a component by passing its configuration parameters.

@override

void configure(ConfigParams config)

• config:: ConfigParams - configuration parameters to be set.

endListen

Ends listening for incoming messages. When this method is call, listen unblocks the thread and execution continues.

@override

void endListen(IContext? context)

• context: IContext - (optional) a context to trace execution through a call chain.

fromMessage

Returns the topic and the data of a message.

Map<String, dynamic>? fromMessage(MessageEnvelope? message)

• message: MessageEnvelope? - message
• returns: Map<String, dynamic>? - topic and data

getTopic

Obtains the topic.

String getTopic()

• returns: String - topic

isOpen

Checks if the component is open.

@override

bool isOpen()

• returns: bool - true if the component is open and false otherwise.

listen

Listens for incoming messages and blocks the current thread until the queue is closed.

See IMessageReceiver

@override

void listen(IContext? context, IMessageReceiver receiver)

• context: IContext - (optional) a context to trace execution through a call chain.
• receiver: IMessageReceiver - receiver used to receive incoming messages.

moveToDeadLetter

Permanently removes a message from the queue and sends it to dead letter queue.

• Important: This method is not supported by MQTT.

@override

Future moveToDeadLetter(MessageEnvelope message)

• message: MessageEnvelope - message to be removed.

onMessage

Checks if the message comes from the right topic. If this is the case, deserializes and sends it to the receiver if it’s set. Otherwise, puts it into the queue.

@override

void onMessage(String topic, String data, packet)

• topic: String - topic
• data: String - data
• packet: dynamic - packet

open

Opens the component.

@override

Future open(IContext? context)

• context: IContext - (optional) a context to trace execution through a call chain.

peek

Peeks a single incoming message from the queue without removing it. If there are no messages available in the queue, it returns null.

@override

Future<MessageEnvelope?> peek(IConext? context)

• context: IContext - (optional) a context to trace execution through a call chain.
• returns: Future<MessageEnvelope?> - peeked message.

peekBatch

Peeks multiple incoming messages from the queue without removing them. If there are no messages available in the queue, it returns an empty list.

• Important: This method is not supported by MQTT.

@override

Future<List<MessageEnvelope>> peekBatch(IContext? context, int messageCount)

• context: IContext - (optional) a context to trace execution through a call chain.
• messageCount: int - maximum number of messages to peek.
• returns: Future<List<MessageEnvelope>> - list with peeked messages.

readMessageCount

Reads the current number of messages in the queue to be delivered.

@override

Future<int> readMessageCount()

• *returns: Future<int> - number of messages in the queue.

receive

Receives an incoming message and removes it from the queue.

@override

Future<MessageEnvelope?> receive(IContext? context, int waitTimeout)

• context: IContext - (optional) a context to trace execution through a call chain.
• waitTimeout: int - timeout in milliseconds to wait for a message to come.
• returns: Future<MessageEnvelope?> - received message or null if nothing was received.

renewLock

Renews a lock on a message that makes it invisible from other receivers in the queue. This method is usually used to extend the message processing time.

• Important: This method is not supported by MQTT.

@override

Future renewLock(MessageEnvelope message, int lockTimeout)

• message: MessageEnvelope - message to extend its lock.
• lockTimeout: int - locking timeout in milliseconds.

send

Sends a message into the queue.

@override

Future send(IContext? context, MessageEnvelope message)

• context: IContext - (optional) a context to trace execution through a call chain.
• message: MessageEnvelope - message envelop to be sent.

setReferences

Sets references to dependent components.

@override

void setReferences(IReferences references)

• references: IReferences - references to locate the component’s dependencies.

subscribe

Subscribes to a topic.

Future subscribe(IContext? context)

• context: IContext - (optional) a context to trace execution through a call chain.

_toMessage

If the message is null, it returns null. Otherwise, it returns the message.

MessageEnvelope? _toMessage(String topic, String? data, packet)

• topic: String - topic
• data: String? - data
• packet: dynamic - packet
• returns: MessageEnvelope? - Null if the message has no data. Otherwise, it returns the message.

unsetReferences

Unsets (clears) previously set references to dependent components.

@override

void unsetReferences()

Examples

var queue = MqttMessageQueue();
queue.configure(ConfigParams.fromTuples([
  'topic', 'mytopic',
  'connection.protocol', 'mqtt'
  'connection.host', 'localhost'
  'connection.port', 1883
]));

await queue.open('123');
    ...

await queue.send('123', MessageEnvelope(null, 'mymessage', 'ABC'));

var message await = queue.receive('123')
if (message != null) {
   ...
   await queue.complete('123', message);
}

See also

• MessageQueue

• MessagingCapabilities