Default reconnect strategy uses exponential backoff and jitter (#2736)

* Default reconnect strategy uses exponential backoff and jitter

Both are recommended parts of client reconnect strategies to prevent
thundering herd problems when many clients lose their connection at once
(for example, during a Redis upgrade).

* Move default retry strategy to constant

* Plain english explanation of default 'socket.reconnectStrategy'

* Extract default connect strategy into helper function

Author: qsymmachus

docs/client-configuration.md

@@ -13,7 +13,7 @@
 | socket.keepAlive             | `true`                                   | Toggle [`keep-alive`](https://nodejs.org/api/net.html#socketsetkeepaliveenable-initialdelay) functionality                                                                                                                                          |
 | socket.keepAliveInitialDelay | `5000`                                   | If set to a positive number, it sets the initial delay before the first keepalive probe is sent on an idle socket                                                                                                                                   |
 | socket.tls                   |                                          | See explanation and examples [below](#TLS)                                                                                                                                                                                                          |
-| socket.reconnectStrategy     | `retries => Math.min(retries * 50, 500)` | A function containing the [Reconnect Strategy](#reconnect-strategy) logic                                                                                                                                                                           |
+| socket.reconnectStrategy     | Exponential backoff with a maximum of 2000 ms; plus 0-200 ms random jitter.       | A function containing the [Reconnect Strategy](#reconnect-strategy) logic                                                                                                                                                                           |
 | username                     |                                          | ACL username ([see ACL guide](https://redis.io/topics/acl))                                                                                                                                                                                         |
 | password                     |                                          | ACL password or the old "--requirepass" password                                                                                                                                                                                                    |
 | name                         |                                          | Client name ([see `CLIENT SETNAME`](https://redis.io/commands/client-setname))                                                                                                                                                                      |
@@ -35,12 +35,19 @@ When the socket closes unexpectedly (without calling `.quit()`/`.disconnect()`),
 2. `number` -> wait for `X` milliseconds before reconnecting.
 3. `(retries: number, cause: Error) => false | number | Error` -> `number` is the same as configuring a `number` directly, `Error` is the same as `false`, but with a custom error.
 
-By default the strategy is `Math.min(retries * 50, 500)`, but it can be overwritten like so:
+By default the strategy uses exponential backoff, but it can be overwritten like so:
 
 ```javascript
 createClient({
   socket: {
-    reconnectStrategy: retries => Math.min(retries * 50, 1000)
+    reconnectStrategy: retries => {
+        // Generate a random jitter between 0 – 200 ms:
+        const jitter = Math.floor(Math.random() * 200);
+        // Delay is an exponential back off, (times^2) * 50 ms, with a maximum value of 2000 ms:
+        const delay = Math.min(Math.pow(2, retries) * 50, 2000);
+
+        return delay + jitter;
+    }
   }
 });
 ```

packages/client/lib/client/socket.ts

@@ -97,12 +97,12 @@ export default class RedisSocket extends EventEmitter {
           return retryIn;
         } catch (err) {
           this.emit('error', err);
-          return Math.min(retries * 50, 500);
+          return this.defaultReconnectStrategy(retries);
         }
       };
     }
 
-    return retries => Math.min(retries * 50, 500);
+    return this.defaultReconnectStrategy;
   }
 
   #createSocketFactory(options?: RedisSocketOptions) {
@@ -333,4 +333,13 @@ export default class RedisSocket extends EventEmitter {
     this.#isSocketUnrefed = true;
     this.#socket?.unref();
   }
+
+  defaultReconnectStrategy(retries: number) {
+    // Generate a random jitter between 0 – 200 ms:
+    const jitter = Math.floor(Math.random() * 200);
+    // Delay is an exponential back off, (times^2) * 50 ms, with a maximum value of 2000 ms:
+    const delay = Math.min(Math.pow(2, retries) * 50, 2000);
+  
+    return delay + jitter;
+  }
 }