How to Handle Reconnection in Real-Time Apps

Reconnection is the part of real-time that everyone forgets until production teaches them. Connections drop constantly on real networks, a phone changes towers, wifi hiccups, a laptop sleeps, and a real-time app that does not handle the reconnect gracefully feels broken even when every other part works. Handling it well means three things: reconnect without stampeding your servers, resume the session instead of starting over, and replay the messages that were missed. Miss any one and users see stale state, lost messages, or an outage that was not really an outage.

Reconnect with backoff and jitter

The first mistake is reconnecting naively. A connection drops, the client immediately retries, it fails, it retries again, tight loop. Now multiply that across every client when a node restarts. Every one of them slams back at the same instant and you have built a thundering herd that knocks over the node that was about to recover.

The fix is exponential backoff with jitter. After a drop, wait a short random interval, then a longer one, then longer still, up to a cap. The randomness is the important part. Without jitter, all your clients back off in lockstep and retry in synchronized waves. With it, they spread out and the recovering cluster absorbs them smoothly. This is the same disposable-node thinking behind scaling WebSockets without it falling over: the system should tolerate mass reconnection as a normal event, not a crisis.

Resume the session, do not restart it

A reconnect that starts from zero is barely better than an error. If the user reconnects and their session is gone, their subscriptions are gone, and their place in the conversation is gone, the socket working perfectly does not help them. They experienced an outage.

Session resume means the client holds a session identifier and presents it on reconnect. The backend recognizes it and restores the subscriptions and state that belonged to it, rather than treating the client as brand new. That requires the server to keep session state alive for a grace period after a drop, so a client that returns within, say, thirty seconds picks up exactly where it left off. State that matters has to outlive the connection, which is why session, presence, and delivery have to be designed as one system rather than bolted together, a point I keep returning to in the backend real-time apps actually need.

Replay what was missed

Even with a resumed session, there is a gap: the messages that happened while the client was disconnected. Handling reconnection properly means the client comes back and receives exactly what it missed, no more and no less.

That requires the backend to track per-client delivery position, what each connection has acknowledged, so on reconnect it can replay from that point instead of dumping the entire history or, worse, skipping the gap silently. This is where reconnection meets delivery guarantees. Messages that must not be lost need to be buffered for a disconnected client and replayed on return, which is the at-least-once machinery I lay out in message delivery guarantees for real-time apps. Ephemeral messages like cursor positions can be skipped, because the next update corrects any gap. Knowing which is which is the design work.

Tell the user what is happening

One human detail that separates polished real-time apps from frustrating ones: show connection state. A quiet indicator that says reconnecting, then connected, turns an invisible failure into an understood pause. Users forgive a two-second reconnect they can see. They do not forgive an app that silently shows them stale data with no signal that anything is wrong. This is cheap to build and it changes how reliable your app feels.

Why I put this in the backend

Backoff, session resume, delivery-position tracking, and replay are identical across every real-time product. The messages differ, the mechanics do not. Rebuilding this per app is how you get three reconnection implementations, two of which quietly lose messages. So I would rather own it once, correctly, in the infrastructure itself. AltoHost is built to handle reconnection, session resume, and replay as first-class guarantees, so the application assumes a client can always come back cleanly instead of engineering that promise from scratch. It is the same instinct behind wanting infrastructure I actually control: solve the hard, generic part once and stop paying for it.

Closing

Connections will drop. That is not a failure to prevent, it is a condition to design for. Reconnect with backoff and jitter so you do not stampede, resume sessions so users do not restart, and replay missed messages so state stays whole. Do those three and a dropped connection becomes a two-second pause nobody remembers, instead of the bug report you read every Monday.