fix: Better resilience towards MQTT connection loss and errors (#168)

example/main.ts

@@ -67,7 +67,7 @@ async function main() {
         const device = devices.DevicesObj[status.deviceId];
         const watts = status.current !== undefined ? status.current * device.Voltage : undefined;
         rootLogger.info(
-          `'${device.Name}' status changed: ${status.temperature}°C, ${status.humidity}%, ${watts ?? 'na'}W`
+          `[${status.deviceId}] '${device.Name}' status changed: ${status.temperature}°C, ${status.humidity}%, ${watts ?? 'na'}W`
         );
       } catch (error) {
         rootLogger.error(error, `Error processing status update for device '${status.deviceId}'`);

src/api/Errors.ts

@@ -32,3 +32,22 @@ export class MysaApiError extends Error {
     this.statusText = apiResponse.statusText;
   }
 }
+
+/** Error thrown when an MQTT publish ultimately fails after retry attempts. */
+export class MqttPublishError extends Error {
+  /**
+   * Creates a new MqttPublishError instance.
+   *
+   * @param message - A human-readable description of the publish failure.
+   * @param attempts - The number of attempts that were made before giving up.
+   * @param original - The original error object thrown by the underlying MQTT library (optional).
+   */
+  constructor(
+    message: string,
+    public attempts: number,
+    public original?: unknown
+  ) {
+    super(message);
+    this.name = 'MqttPublishError';
+  }
+}

src/api/MysaApiClient.ts

@@ -21,7 +21,7 @@ import {
 import { iot, mqtt } from 'aws-iot-device-sdk-v2';
 import dayjs from 'dayjs';
 import duration from 'dayjs/plugin/duration.js';
-import { MysaApiError, UnauthenticatedError } from './Errors';
+import { MqttPublishError, MysaApiError, UnauthenticatedError } from './Errors';
 import { Logger, VoidLogger } from './Logger';
 import { MysaApiClientEventTypes } from './MysaApiClientEventTypes';
 import { MysaApiClientOptions } from './MysaApiClientOptions';
@@ -29,6 +29,14 @@ import { MysaDeviceMode } from './MysaDeviceMode';
 
 dayjs.extend(duration);
 
+/** Options for MQTT publish operations. */
+export interface MqttPublishOptions {
+  /** Maximum number of publish attempts before failing (default: 5). */
+  maxAttempts?: number;
+  /** Base delay in milliseconds used for exponential backoff calculation (default: 500). */
+  baseDelayMs?: number;
+}
+
 const AwsRegion = 'us-east-1';
 const CognitoUserPoolId = 'us-east-1_GUFWfhI7g';
 const CognitoClientId = '19efs8tgqe942atbqmot5m36t3';
@@ -210,7 +218,7 @@ export class MysaApiClient {
   async getDevices(): Promise<Devices> {
     this._logger.debug(`Fetching devices...`);
 
-    const session = await this.getFreshSession();
+    const session = await this._getFreshSession();
 
     const response = await this._fetcher(`${MysaApiBaseUrl}/devices`, {
       headers: {
@@ -249,7 +257,7 @@ export class MysaApiClient {
   async getDeviceSerialNumber(deviceId: string): Promise<string | undefined> {
     this._logger.debug(`Fetching serial number for device ${deviceId}...`);
 
-    const session = await this.getFreshSession();
+    const session = await this._getFreshSession();
 
     // Get AWS credentials for IoT client
     const credentialsProvider = fromCognitoIdentityPool({
@@ -292,7 +300,7 @@ export class MysaApiClient {
   async getDeviceFirmwares(): Promise<Firmwares> {
     this._logger.debug(`Fetching device firmwares...`);
 
-    const session = await this.getFreshSession();
+    const session = await this._getFreshSession();
 
     const response = await this._fetcher(`${MysaApiBaseUrl}/devices/firmware`, {
       headers: {
@@ -317,7 +325,7 @@ export class MysaApiClient {
   async getDeviceStates(): Promise<DeviceStates> {
     this._logger.debug(`Fetching device states...`);
 
-    const session = await this.getFreshSession();
+    const session = await this._getFreshSession();
 
     const response = await this._fetcher(`${MysaApiBaseUrl}/devices/state`, {
       headers: {
@@ -367,7 +375,7 @@ export class MysaApiClient {
     const device = this._cachedDevices.DevicesObj[deviceId];
 
     this._logger.debug(`Initializing MQTT connection...`);
-    const mqttConnection = await this.getMqttConnection();
+    const mqttConnection = await this._getMqttConnection();
 
     const now = dayjs();
 
@@ -378,7 +386,7 @@ export class MysaApiClient {
       time: now.unix(),
       ver: '1.0',
       src: {
-        ref: this.session!.username,
+        ref: this.session?.username ?? '',
         type: 100
       },
       dest: {
@@ -405,7 +413,13 @@ export class MysaApiClient {
       }
     });
 
-    await mqttConnection.publish(`/v1/dev/${deviceId}/in`, payload, mqtt.QoS.AtLeastOnce);
+    try {
+      await this._publishWithRetry(mqttConnection, `/v1/dev/${deviceId}/in`, payload, mqtt.QoS.AtLeastOnce);
+      this._logger.debug(`Device state publish succeeded for '${deviceId}'`);
+    } catch (error) {
+      this._logger.error(`Failed to set device state for '${deviceId}'`, error);
+      throw error;
+    }
   }
 
   /**
@@ -438,11 +452,11 @@ export class MysaApiClient {
     }
 
     this._logger.debug(`Initializing MQTT connection...`);
-    const mqttConnection = await this.getMqttConnection();
+    const mqttConnection = await this._getMqttConnection();
 
     this._logger.debug(`Subscribing to MQTT topic '/v1/dev/${deviceId}/out'...`);
     await mqttConnection.subscribe(`/v1/dev/${deviceId}/out`, mqtt.QoS.AtLeastOnce, (_, payload) => {
-      this.processMqttMessage(payload);
+      this._processMqttMessage(payload);
     });
 
     this._logger.debug(`Sending request to start publishing device status for '${deviceId}'...`);
@@ -452,7 +466,7 @@ export class MysaApiClient {
       Timestamp: dayjs().unix(),
       Timeout: RealtimeKeepAliveInterval.asSeconds()
     });
-    await mqttConnection.publish(`/v1/dev/${deviceId}/in`, payload, mqtt.QoS.AtLeastOnce);
+    await this._publishWithRetry(mqttConnection, `/v1/dev/${deviceId}/in`, payload, mqtt.QoS.AtLeastOnce);
 
     const timer = setInterval(async () => {
       this._logger.debug(`Sending request to keep-alive publishing device status for '${deviceId}'...`);
@@ -462,7 +476,7 @@ export class MysaApiClient {
         Timestamp: dayjs().unix(),
         Timeout: RealtimeKeepAliveInterval.asSeconds()
       });
-      await mqttConnection.publish(`/v1/dev/${deviceId}/in`, payload, mqtt.QoS.AtLeastOnce);
+      await this._publishWithRetry(mqttConnection, `/v1/dev/${deviceId}/in`, payload, mqtt.QoS.AtLeastOnce);
     }, RealtimeKeepAliveInterval.subtract(10, 'seconds').asMilliseconds());
 
     this._realtimeDeviceIds.set(deviceId, timer);
@@ -487,7 +501,7 @@ export class MysaApiClient {
     }
 
     this._logger.debug(`Initializing MQTT connection...`);
-    const mqttConnection = await this.getMqttConnection();
+    const mqttConnection = await this._getMqttConnection();
 
     this._logger.debug(`Unsubscribing to MQTT topic '/v1/dev/${deviceId}/out'...`);
     await mqttConnection.unsubscribe(`/v1/dev/${deviceId}/out`);
@@ -505,7 +519,7 @@ export class MysaApiClient {
    * @returns A promise that resolves to a valid CognitoUserSession.
    * @throws {@link UnauthenticatedError} When no session exists or refresh fails.
    */
-  private async getFreshSession(): Promise<CognitoUserSession> {
+  private async _getFreshSession(): Promise<CognitoUserSession> {
     if (!this._cognitoUser || !this._cognitoUserSession) {
       throw new UnauthenticatedError('An attempt was made to access a resource without a valid session.');
     }
@@ -543,9 +557,9 @@ export class MysaApiClient {
    * @returns A promise that resolves to an active MQTT connection.
    * @throws {@link Error} When connection establishment fails.
    */
-  private getMqttConnection(): Promise<mqtt.MqttClientConnection> {
+  private _getMqttConnection(): Promise<mqtt.MqttClientConnection> {
     if (!this._mqttConnectionPromise) {
-      this._mqttConnectionPromise = this.createMqttConnection().catch((err) => {
+      this._mqttConnectionPromise = this._createMqttConnection().catch((err) => {
         this._mqttConnectionPromise = undefined;
         throw err;
       });
@@ -554,14 +568,106 @@ export class MysaApiClient {
     return this._mqttConnectionPromise;
   }
 
+  /**
+   * Determines whether an MQTT-related error is considered transient and worth retrying.
+   *
+   * Transient errors include timeouts, cancelled operations due to clean sessions, temporary connectivity loss, and
+   * other recoverable network issues. Fatal errors (auth, permission, configuration) should not be retried at this
+   * layer.
+   *
+   * @param err - The error object thrown by the underlying MQTT operation.
+   * @returns True if the error appears transient and a retry should be attempted; false otherwise.
+   */
+  private _isTransientMqttError(err: unknown): boolean {
+    if (!err || typeof err !== 'object') {
+      return false;
+    }
+
+    const anyErr = err as { error_code?: unknown; error_name?: unknown; error?: unknown; message?: unknown };
+    const code = anyErr.error_code || anyErr.error_name || anyErr.error;
+    const msg = (anyErr.message || anyErr.error || '').toString();
+
+    const transientMarkers = [
+      'AWS_ERROR_MQTT_TIMEOUT',
+      'AWS_ERROR_MQTT_NO_CONNECTION',
+      'Time limit between request and response',
+      'timeout'
+    ];
+
+    return transientMarkers.some((m) => (code && String(code).includes(m)) || msg.includes(m));
+  }
+
+  /**
+   * Publishes an MQTT message with exponential backoff retries for transient failures.
+   *
+   * Retries occur for errors classified by `_isTransientMqttError`. Between attempts the delay grows exponentially with
+   * jitter to avoid thundering herds after broker recovery. If the connection is not currently marked as connected, a
+   * reconnect is attempted; if that fails, the connection is rebuilt (fresh credentials) before the next retry.
+   *
+   * On final failure (after maxAttempts) a {@link MqttPublishError} is thrown including the number of attempts and
+   * original error for higher-level handling.
+   *
+   * @remarks
+   * Retry options fields:
+   *
+   * - MaxAttempts: Maximum number of publish attempts before failing (default: 5).
+   * - BaseDelayMs: Base delay in milliseconds used for exponential backoff calculation (default: 500).
+   *
+   * @param connection - The active MQTT client connection used to send the publish.
+   * @param topic - The MQTT topic to publish to.
+   * @param payload - The serialized payload (binary buffer or Uint8Array).
+   * @param qos - The desired MQTT QoS level for the publish.
+   * @param opts - Retry options (defaults: maxAttempts=5, baseDelayMs=500).
+   * @returns A promise that resolves when the publish succeeds, or rejects with {@link MqttPublishError}.
+   */
+  private async _publishWithRetry(
+    connection: mqtt.MqttClientConnection,
+    topic: string,
+    payload: ArrayBuffer | Uint8Array,
+    qos: mqtt.QoS,
+    opts: MqttPublishOptions = {}
+  ): Promise<void> {
+    const maxAttempts = opts.maxAttempts ?? 5;
+    const baseDelayMs = opts.baseDelayMs ?? 500;
+
+    let attempt = 0;
+
+    while (true) {
+      attempt++;
+      try {
+        await connection.publish(topic, payload, qos);
+        return;
+      } catch (err) {
+        const isTransient = this._isTransientMqttError(err);
+
+        if (!isTransient || attempt >= maxAttempts) {
+          throw new MqttPublishError(`MQTT publish failed after ${attempt} attempts`, attempt, err);
+        }
+
+        // Apply jitter: delay is randomized between 75% and 125% of the base exponential backoff
+        const JITTER_MIN_FACTOR = 0.75;
+        const JITTER_RANGE = 0.5;
+        const delay = baseDelayMs * Math.pow(2, attempt - 1) * (JITTER_MIN_FACTOR + Math.random() * JITTER_RANGE);
+
+        this._logger.warn(
+          `Transient MQTT publish error on '${topic}' (attempt ${attempt}/${maxAttempts}). Retrying in ${Math.round(
+            delay
+          )}ms`
+        );
+
+        await new Promise((r) => setTimeout(r, delay));
+      }
+    }
+  }
+
   /**
    * Creates a new MQTT connection using AWS IoT WebSocket connections with Cognito credentials.
    *
    * @returns A promise that resolves to an active MQTT connection.
    * @throws {@link Error} When connection establishment fails.
    */
-  private async createMqttConnection(): Promise<mqtt.MqttClientConnection> {
+  private async _createMqttConnection(): Promise<mqtt.MqttClientConnection> {
-    const session = await this.getFreshSession();
+    const session = await this._getFreshSession();
     const credentialsProvider = fromCognitoIdentityPool({
       clientConfig: {
         region: AwsRegion
@@ -574,11 +680,14 @@ export class MysaApiClient {
     });
     const credentials = await credentialsProvider();
 
+    // Stable client id + persistent session to retain QoS1 queue & subscriptions across reconnects.
+    const stableClientId = `mysa-js-sdk-${this.session?.username ?? ''}`;
+
     const builder = iot.AwsIotMqttConnectionConfigBuilder.new_with_websockets()
       .with_credentials(AwsRegion, credentials.accessKeyId, credentials.secretAccessKey, credentials.sessionToken)
       .with_endpoint(MqttEndpoint)
-      .with_client_id(`mysa-js-sdk-${dayjs().unix()}`) // Unique client ID
+      .with_client_id(stableClientId)
-      .with_clean_session(true)
+      .with_clean_session(false)
       .with_keep_alive_seconds(30)
       .with_ping_timeout_ms(3000)
       .with_protocol_operation_timeout_ms(60000);
@@ -587,14 +696,44 @@ export class MysaApiClient {
     const client = new mqtt.MqttClient();
     const connection = client.new_connection(config);
 
-    connection.on('connect', () => this._logger.debug('MQTT connect'));
+    connection.on('connect', () => {
-    connection.on('connection_success', () => this._logger.debug('MQTT connection_success'));
+      this._logger.debug('MQTT connect');
-    connection.on('connection_failure', (e) => this._logger.error('MQTT connection_failure', e));
+    });
-    connection.on('interrupt', (e) => this._logger.warn('MQTT interrupt', e));
+
-    connection.on('resume', (returnCode, sessionPresent) =>
+    connection.on('connection_success', () => {
-      this._logger.info(`MQTT resume returnCode=${returnCode} sessionPresent=${sessionPresent}`)
+      this._logger.debug('MQTT connection_success');
-    );
+    });
-    connection.on('error', (e) => this._logger.error('MQTT error', e));
+
+    connection.on('connection_failure', (e) => {
+      this._logger.error('MQTT connection_failure', e);
+    });
+
+    connection.on('interrupt', (e) => {
+      this._logger.warn('MQTT interrupt', e);
+    });
+
+    connection.on('resume', async (returnCode, sessionPresent) => {
+      this._logger.info(`MQTT resume returnCode=${returnCode} sessionPresent=${sessionPresent}`);
+
+      if (!sessionPresent) {
+        this._logger.info('No session present, re-subscribing each device');
+        try {
+          for (const deviceId of Array.from(this._realtimeDeviceIds.keys())) {
+            const topic = `/v1/dev/${deviceId}/out`;
+            this._logger.debug(`Re-subscribing to ${topic}`);
+            await connection.subscribe(topic, mqtt.QoS.AtLeastOnce, (_topic, payload) => {
+              this._processMqttMessage(payload);
+            });
+          }
+        } catch (err) {
+          this._logger.error('Failed to re-subscribe after resume', err);
+        }
+      }
+    });
+
+    connection.on('error', (e) => {
+      this._logger.error('MQTT error', e);
+    });
 
     connection.on('closed', () => {
       this._logger.info('MQTT connection closed');
@@ -615,7 +754,7 @@ export class MysaApiClient {
    *
    * @param payload - The raw MQTT message payload to process.
    */
-  private processMqttMessage(payload: ArrayBuffer) {
+  private _processMqttMessage(payload: ArrayBuffer) {
     try {
       const parsedPayload = parseMqttPayload(payload);