JavaScript Client - New Feature Plan : Automatic Reconnect and Disconnected Buffering

[miketran78727]

Automatic Reconnect and Disconnected Publishing Plan

Currently, the Paho JavaScript client is lacking two major areas of functionality: Automatic Reconnect and Disconnected (or Offline) Publishing.

The goal is to implement these features in time for the next release Neon.

This issue aims to outline the plan for adding this new functionality into the JavaScript client. The design and implementation will be closely matched to the Java client's plan

Recap: Possible Client States

There are 5 main potential states that the client can be in. The User will usually desire the client to either be in the connected or disconnected states.

• never connected: This is the initial state of the client where:
  • The client has not yet sent a connect request.
  • The client has never received a CONNACK after it's initial connect request.
• connecting: A connect request is in progress.
• connected: The client is connected and ready to send and receive messages.
• disconnecting: A disconnect request is in progress.
• disconnected: The client goes from connected to disconnected state when:
  • A disconnect request has been completed
  • As a result of a networking error, or the server is no longer reachable.

    What does it do?

    Automatic Reconnect

Will automatically attempt to reconnect to the broker (or one of the servers in the host list) while the client is in disconnected state.

• The client will not perform automatic reconnection if it is not in disconnected state.
• When the connection is lost, the connectionLost callback is called before the client starts the reconnect process. Since the state of the client is disconnected, the application is allowed to call the connect function with new connection options if they wish.
• When disconnect is called while connected, the client goes to the disconnected state and automatic reconnect is disabled.
• If the client application calls connect after it had reconnected, an invalid state error will be thrown.
• The client library does not subscribe for the application after it successfully reconnects. A callback will be provided to notify the application when it has been reconnected allowing it to make any subscriptions to topics itself.

  Disconnected Publishing

Will allow the client to save messages in a buffer whilst the client is in the disconnected state.

• Once the client is reconnected (To the same or different broker), the buffered messages are scheduled to be sent.
• To maintain order of messages, the client library must send buffered messages before sending new messages.
• The client library does not save any messages while the client is in the never connected state. So it cannot send any messages before it connects for the first time.
• When disconnect is called while connected, the client goes to disconnected state and Disconnected Publishing remains active if enabled.

  API Changes

  Automatic Reconnect

The following optional attributes will be added to the Connection Options object:

• reconnect : boolean. If present and True, the client will attempt to reconnect
• reconnectInterval : number. Time interval in seconds between connect attempts. Default: 10 (seconds).

The following new callback will be added:

• onConnect - called when the connection is established. A response object is passed to the onConnect callback containing the following fields:
  • reconnect - boolean. True if the connection is the result of a reconnect operation
  • uri - string. Fully qualified WebSocket URI of the server

The existing onConnectionLost callback will be changed as follows:

• onConnectionLost - called when a connection has been lost. A single response object parameter is passed to the onConnectionLost callback containing the following fields:
  • errorCode - number.
  • errorMessage - string.
  • reconnect : boolean. If present and True, the client will perform automatic reconnection while it is disconnected.

    Disconnected Publishing

The following optional attributes will be added to the Messaging Client object:

• disconnectedPublishing : boolean. If present and True, the client will save messages while it is disconnected. This attribute is independent from the reconnect attribute mentioned above. Thus the application can set this flag to buffer messages and try to reconnect at a later time.
• disconnectedBufferSize : number. The maximum number of messages that will be stored in memory while the client is disconnected. Default : 5000
• deleteOldestBufferedMessages : boolean. If present and True, the client will delete oldest messages in the buffer to make room for newer messages.
• persistDisconnectedBuffer : boolean. If present an True, the client will persist the buffered messages.
  • A persistent buffered message is stored by the client until it is deleted by the application or it is sent after the client reconnected.
  • If persistDisconnectedBuffer is not present or False, the client will save messages in memory.
• bufferedMessages : number. This read-only attribute is the number of buffered messages.

The following methods will be added:

• getBufferedMessage(index) : Returns the buffered Message object at the index location (0 based)
• deleteBufferedMessage(index) : Deletes the buffered message at the index location (0 based)

  New Error

When the buffer is full and deleteOldestBufferedMessages is not present or False, the following error will be thrown:

• errorCode : 18
• errorMessage : AMQJS0018E Message buffer is full, maximum buffer size: ${0}.

  Sample application

This application wants to use both Automatic Reconnect and Disconnected Publising. The Application does not want to persist buffered messages.

    var client;
    var host = "iot.eclipse.org";
    var port = 1883;
    var clientId = "sample-id";
    var sentMsgs = 0;

    // This function creates a new Messaging client and initiates a connect request.
    function doConnect() {
        client = new Messaging.Client(server, port, clientId);
        client.onConnected = onConnected;            // Callback when connected
        client.onConnectionLost = onConnectionLost;  // Callback when lost connection
        client.disconnectedPublishing = true;        // Enable disconnected publishing
        client.disconnectedBufferSize = 100;         //  Buffer size : 100

        client.connect(
            {
                cleanSession : false, 
                onSuccess : onConnectSuccess, 
                onFailure : onFailedConnect, 
                keepAliveInterval: 30, 
                reconnect : true,         // Enable automatic reconnect
                reconnectInterval: 10     // Reconnect attempt interval : 10 seconds
            }
        );
    }

    // This function is called when the initial connect request is successful.
    function onConnectSuccess(resObj) {
        console.log("Initial connect request succeeded.");
    }

    // This function is called when the intial connect request failed.
    function onFailedConnect(err) {
        console.log("Initial connect request failed. Error message : " + err.errorMessage); 
    }

    // This function is called for all successful connection
    function onConnected(resObj) {
        console.log("Connected to " + resObj.uri);
        if (resObj.reconnect) {
            console.log("It was a result of automatic reconnect.");
        }
    }

    // This function is called when the connection is lost.
    function onConnectionLost(resObj) {
        console.log("Lost connection to " + resObj.uri + "\nError code: " + resObj.errorCode + "\nError text: " + resObj.errorMessage);
        if (resObj.reconnect) {
            console.log("Automatic reconnect is currently active.");
        } else {
            alert("Lost connection to host.");.
        }
    }

    function doSend() {
        // Starts timer to send a message every second
        publishTimer = setInterval(function () {sendTimer()}, 1000);
    }

    function sendTimer() {
        var message = new Messaging.Message("Test Message " + sentMsgs);
        message.destinationName = "/test/topic1";
        message.qos = 1;
        try {
            client.send(message);
            sentMsgs++;
            if (client.bufferedMessages > 0) {
                console.log("Buffered messages : " + client.bufferedMessages);
            }
        } catch (err) {
            console.log("Error occured after " + sentMsgs + " messages sent. Error : " + err.message);
        }
    }

[Fabryprog]

+1

[Dovitch]

exactly what I need! As we say here:"ça gère grave la fougère!"

[seesiva]

Is this still open or part of neon ?

[miketran78727]

@seesiva You can check out the pull request. https://github.com/eclipse/paho.mqtt.javascript/pull/72 This is still under review.

[jpwsutton]

The PR was merged in manually here: https://github.com/eclipse/paho.mqtt.javascript/commit/258524896c90139e55c9d26d9376e25536b6149d

@miketran78727 could you check that this new functionality works correctly in the develop branch before we close this off please? Just to make sure that my merge went properly :)

[miketran78727]

@jpwsutton Will do!

[miketran78727]

@jpwsutton Sorry James.. I have been busy with work in the last few weeks. I will find some gap this week to run my manual tests for the develop branch.

[miketran78727]

@jpwsutton Offline publishing does not work as expected, My test expects the number of delivered messages = number of published messages after successful reconnect

Messages sent: 57
Messages delivered: 41

I will look into the problem.

[miketran78727]

I forgot to set the offline publishing properties in the previous test:

        client.disconnectedPublish = true;
        client.disconnectedBufferSize = disconnectedBufferSize;

So, the code is working as expected. If reconnect = true but client.disconnectedPublishing = false, only automatic reconnect is enabled.

[celalo]

When can we expect this one to be released?

[ghost]

hy mike,

the application using node or native javascript?

[cjy37]

+1

[benydc]

@jpwsutton how can I use this feature for auto-reconnect?

[gautam-bmdi]

@miketran78727 @celalo

Is this feature available now?

[Talbot3]

@gautam-bmdi when I use it , test error. I not found it on master . come back to use mqtt.js