feat(sync): Reconnect the websocket on failures.

When the websocket is disconnected, try to redo an authentication
roundtrip. This commit does the following changes:

- Change the way the SyncEngine is instanciated, passing it a server
  object and the urls.
- Add a ping/pong mechanism. This is required because otherwise we have
  no certainty that the connection is still alive.
- Try to reconnect when the connection didn't work out, increasing the
  wait time a bit more each time.

umap/static/umap/js/modules/sync/engine.js

@@ -3,6 +3,12 @@ import { HybridLogicalClock } from './hlc.js'
 import { DataLayerUpdater, FeatureUpdater, MapUpdater } from './updaters.js'
 import { WebSocketTransport } from './websocket.js'
 
+// Start reconnecting after 2 seconds, then double the delay each time
+// maxing out at 32 seconds.
+const RECONNECT_DELAY = 2000;
+const RECONNECT_DELAY_FACTOR = 2;
+const MAX_RECONNECT_DELAY = 32000;
+
 /**
  * The syncEngine exposes an API to sync messages between peers over the network.
  *
@@ -42,7 +48,7 @@ import { WebSocketTransport } from './websocket.js'
  * ```
  */
 export class SyncEngine {
-  constructor(map) {
+  constructor(map, urls, server) {
     this.updaters = {
       map: new MapUpdater(map),
       feature: new FeatureUpdater(map),
@@ -50,24 +56,54 @@ export class SyncEngine {
     }
     this.transport = undefined
     this._operations = new Operations()
+    this._server = server
+
+    // Store URIs to avoid persisting the map
+    // mainly to ensure separation of concerns.
+    this._websocketTokenURI = urls.get('map_websocket_auth_token', {
+      map_id: map.options.umap_id,
+    })
+    this._websocketURI = map.options.websocketURI
+    this._reconnectTimeout = null;
+    this._reconnectDelay = RECONNECT_DELAY;
   }
 
-  async authenticate(tokenURI, webSocketURI, server) {
-    const [response, _, error] = await server.get(tokenURI)
+  /**
+   * Authenticate with the server and start the transport layer.
+   */
+  async authenticate() {
+    const [response, _, error] = await this._server.get(this._websocketTokenURI)
     if (!error) {
-      this.start(webSocketURI, response.token)
+      this.start(response.token)
     }
   }
 
-  start(webSocketURI, authToken) {
-    this.transport = new WebSocketTransport(webSocketURI, authToken, this)
+  start(authToken) {
+    this.transport = new WebSocketTransport(this._websocketURI, authToken, this)
   }
 
   stop() {
-    if (this.transport) this.transport.close()
+    if (this.transport){
+      this.transport.close()
+    }
     this.transport = undefined
   }
 
+  onConnection() {
+    this._reconnectTimeout = null;
+    this._reconnectDelay = RECONNECT_DELAY;
+  }
+
+  reconnect() {
+    console.log("reconnecting in ", this._reconnectDelay, " ms")
+    this._reconnectTimeout = setTimeout(() => {
+      if (this._reconnectDelay < MAX_RECONNECT_DELAY) {
+        this._reconnectDelay = this._reconnectDelay * RECONNECT_DELAY_FACTOR
+      }
+      this.authenticate()
+    }, this._reconnectDelay);
+  }
+
   upsert(subject, metadata, value) {
     this._send({ verb: 'upsert', subject, metadata, value })
   }
@@ -448,3 +484,4 @@ export class Operations {
 function debug(...args) {
   console.debug('SYNC ⇆', ...args)
 }
+

umap/static/umap/js/modules/sync/websocket.js

@@ -1,16 +1,48 @@
+const PONG_TIMEOUT = 5000;
+const PING_INTERVAL = 30000;
+
 export class WebSocketTransport {
   constructor(webSocketURI, authToken, messagesReceiver) {
+    this.receiver = messagesReceiver
+
     this.websocket = new WebSocket(webSocketURI)
+
     this.websocket.onopen = () => {
       this.send('JoinRequest', { token: authToken })
+      this.receiver.onConnection()
     }
     this.websocket.addEventListener('message', this.onMessage.bind(this))
-    this.receiver = messagesReceiver
+    this.websocket.onclose = () => {
+      console.log("websocket closed")
+      this.receiver.reconnect()
+    }
+
+    // To ensure the connection is still alive, we send ping and expect pong back.
+    // Websocket provides a `ping` method to keep the connection alive, but it's
+    // unfortunately not possible to access it from the WebSocket object.
+    // See https://making.close.com/posts/reliable-websockets/ for more details.
+    this.pingInterval = setInterval(() => {
+      if (this.websocket.readyState === WebSocket.OPEN) {
+        this.websocket.send('ping');
+        this.pongReceived = false;
+        setTimeout(() => {
+          if (!this.pongReceived) {
+            console.warn('No pong received, reconnecting...');
+            this.websocket.close()
+            clearInterval(this.pingInterval)
+          }
+        }, PONG_TIMEOUT);
+      }
+    }, PING_INTERVAL);
   }
 
   onMessage(wsMessage) {
+    if (wsMessage.data === 'pong') {
+      this.pongReceived = true;
+    } else {
       this.receiver.receive(JSON.parse(wsMessage.data))
     }
+  }
 
   send(kind, payload) {
     const message = { ...payload }
@@ -18,8 +50,4 @@ export class WebSocketTransport {
     const encoded = JSON.stringify(message)
     this.websocket.send(encoded)
   }
-
-  close() {
-    this.websocket.close()
-  }
 }

umap/static/umap/js/umap.js

@@ -30,8 +30,6 @@ U.Map = L.Map.extend({
   includes: [ControlsMixin],
 
   initialize: async function (el, geojson) {
-    this.sync_engine = new U.SyncEngine(this)
-    this.sync = this.sync_engine.proxy(this)
     // Locale name (pt_PT, en_US…)
     // To be used for Django localization
     if (geojson.properties.locale) L.setLocale(geojson.properties.locale)
@@ -70,6 +68,9 @@ U.Map = L.Map.extend({
     this.server = new U.ServerRequest()
     this.request = new U.Request()
 
+    this.sync_engine = new U.SyncEngine(this, this.urls, this.server)
+    this.sync = this.sync_engine.proxy(this)
+
     this.initLoader()
     this.name = this.options.name
     this.description = this.options.description
@@ -209,10 +210,7 @@ U.Map = L.Map.extend({
     if (this.options.syncEnabled !== true) {
       this.sync.stop()
     } else {
-      const ws_token_uri = this.urls.get('map_websocket_auth_token', {
-        map_id: this.options.umap_id,
-      })
-      await this.sync.authenticate(ws_token_uri, this.options.websocketURI, this.server)
+      await this.sync.authenticate()
     }
   },
 

umap/websocket_server.py

@@ -126,6 +126,10 @@ async def join_and_listen(
 
     try:
         async for raw_message in websocket:
+            if raw_message == "ping":
+                await websocket.send("pong")
+                continue
+
             # recompute the peers list at the time of message-sending.
             # as doing so beforehand would miss new connections
             other_peers = connections.get_other_peers(websocket)