MqttMessageQueue

Inherits: CachedMessageQueue, IMqttMessageListener

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.

public MqttMessageQueue(string name = null)

• name: string - (optional) queue’s name.

Fields

Instance methods

AbandonAsync

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.

public override Task AbandonAsync(MessageEnvelope message)

• message: MessageEnvelope - message to return.

Clear!

TODO: this method is not implemented

Clears a component’s state.

CloseAsync

Closes a component and frees used resources.

public override Task CloseAsync(IContext context)

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

CompleteAsync

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.

public override Task CompleteAsync(MessageEnvelope message)

• message: MessageEnvelope - message to remove.

Configure

Configures a component by passing its configuration parameters.

public override void Configure(ConfigParams config)

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

EndListen!

Note: this method is not implemented

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

FromMessage

Returns the topic and the data of a message.

private MqttApplicationMessage FromMessage(MessageEnvelope message)

• message: MessageEnvelope - message
• returns: MqttApplicationMessage - topic and data

GetTopic

Obtains the topic.

private string GetTopic()

• returns: string - topic

Listen!

Note: this method is not implemented

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

See IMessageReceiver

IsOpen

Checks if the component is open.

public override bool IsOpen()

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

MoveToDeadLetterAsync

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

• Important: This method is not supported by MQTT.

public override async Task MoveToDeadLetterAsync(MessageEnvelope message)

• message: MessageEnvelope - message to be removed.

OnMessage

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

public void OnMessage(MqttApplicationMessage msg)

• msg: MqttApplicationMessage - message

OpenAsync

Opens the component.

public override Task OpenAsync(IContext context)

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

Peek!

Note: this method is not implemented

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

PeekBatch!

Note: this method is not implemented

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.

ReadMessageCount!

Note: this method is not implemented

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

Receive!

Note: this method is not implemented

Receives an incoming message and removes it from the queue.

RenewLockAsync

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.

public override Task RenewLockAsync([MessageEnvelope](../../../messaging/queues/message_envelope message, long lockTimeout)

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

SendAsync

Sends a message into the queue.

public override Task SendAsync(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.

public override void SetReferences(IReferences references)

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

SubscribeAsync

Subscribes to a topic.

protected override Task SubscribeAsync(IContext context)

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

ToMessage

If the message has no data, it returns null. Otherwise, it returns the message.

private MessageEnvelope ToMessage(MqttApplicationMessage msg)

• msg: MqttApplicationMessage - message
• returns: MessageEnvelope - Null if the message has no data. Otherwise, it returns the message.

UnsetReferences!

Note: this method is not implemented

Unsets (clears) previously set references to dependent components.

Examples

var queue = new MqttMessageQueue();
queue.Configure(ConfigParams.FromTuples(
  "subject", "mytopic",
  "connection.protocol", "mqtt"
  "connection.host", "localhost"
  "connection.port", 1883
));

await queue.OpenAsync("123");
await queue.SendAsync("123", new MessageEnvelope("", "mymessage", "ABC"));
var message = await queue.ReceiveAsync("123");

if (message != null) {
    ...
    await queue.CompleteAsync("123", message);
}

See also

• MessageQueue

• MessagingCapabilities