fix: always allow selecting a new node for cluster mode subscriptions when the current one fails

lib/cluster/ClusterSubscriber.ts

@@ -15,6 +15,14 @@ export default class ClusterSubscriber {
     private connectionPool: ConnectionPool,
     private emitter: EventEmitter
   ) {
+    // If the current node we're using as the subscriber disappears
+    // from the node pool for some reason, we will select a new one
+    // to connect to.
+    // Note that this event is only triggered if the connection to
+    // the node has been used; cluster subscriptions are setup with
+    // lazyConnect = true. It's possible for the subscriber node to
+    // disappear without this method being called!
+    // See https://github.com/luin/ioredis/pull/1589
     this.connectionPool.on("-node", (_, key: string) => {
       if (!this.started || !this.subscriber) {
         return;
@@ -54,16 +62,30 @@ export default class ClusterSubscriber {
     debug("stopped");
   }
 
+  private onSubscriberEnd = () => {
+    if (!this.started) {
+      debug("subscriber has disconnected, but ClusterSubscriber is not started, so not reconnecting.");
+      return;
+    }
+    // If the subscriber closes whilst it's still the active connection,
+    // we might as well try to connecting to a new node if possible to
+    // minimise the number of missed publishes.
+    debug("subscriber has disconnected, selecting a new one...");
+    this.selectSubscriber();
+  }
+
   private selectSubscriber() {
     const lastActiveSubscriber = this.lastActiveSubscriber;
 
     // Disconnect the previous subscriber even if there
     // will not be a new one.
     if (lastActiveSubscriber) {
+      lastActiveSubscriber.off("end", this.onSubscriberEnd);
       lastActiveSubscriber.disconnect();
     }
 
     if (this.subscriber) {
+      this.subscriber.off("end", this.onSubscriberEnd);
       this.subscriber.disconnect();
     }
 
@@ -97,11 +119,22 @@ export default class ClusterSubscriber {
       connectionName: getConnectionName("subscriber", options.connectionName),
       lazyConnect: true,
       tls: options.tls,
+      // Don't try to reconnect the subscriber connection. If the connection fails
+      // we will get an end event (handled below), at which point we'll pick a new
+      // node from the pool and try to connect to that as the subscriber connection.
+      retryStrategy: null
     });
 
     // Ignore the errors since they're handled in the connection pool.
     this.subscriber.on("error", noop);
 
+    // The node we lost connection to may not come back up in a
+    // reasonable amount of time (e.g. a slave that's taken down
+    // for maintainence), we could potentially miss many published
+    // messages so we should reconnect as quickly as possible, to
+    // a different node if needed.
+    this.subscriber.once("end", this.onSubscriberEnd);
+
     // Re-subscribe previous channels
     const previousChannels = { subscribe: [], psubscribe: [] };
     if (lastActiveSubscriber) {

test/functional/cluster/pub_sub.ts

@@ -49,6 +49,7 @@ describe("cluster:pub/sub", function () {
     sub.subscribe("test cluster", function () {
       sub.set("foo", "bar").then((res) => {
         expect(res).to.eql("OK");
+        sub.disconnect()
         done();
       });
     });
@@ -73,6 +74,7 @@ describe("cluster:pub/sub", function () {
     const sub = new Cluster([{ port: "30001", password: "abc" }]);
 
     sub.subscribe("test cluster", function () {
+      sub.disconnect()
       done();
     });
   });