Message Acknowledgement and Delivery Guarantees
Ensure messages are not silently lost by adding acknowledgements, retries, and idempotency to reach reliable delivery guarantees.
Message Acknowledgement and Delivery Guarantees is a free WebSockets & Realtime Systems Programming lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the WebSockets & Realtime Systems Programming learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Messages Can Vanish
A message can be lost if the socket drops mid-send, the server crashes, or the client reloads before processing. Without acknowledgements, neither side knows it happened.
Delivery Guarantee Levels
Three common guarantees:
- At-most-once: may lose, never duplicate
- At-least-once: never lose, may duplicate
- Exactly-once: never lose, never duplicate (hardest)
Application-Level Acks
WebSockets only confirm transport delivery, not processing. To know a message was handled, the receiver must send an explicit ack back.
Adding a Message Id
Tag each message with a unique id so acks can reference it.
const msg = { id: crypto.randomUUID(), type: 'order', data };
ws.send(JSON.stringify(msg));Sending the Ack
The receiver replies with an ack carrying the original id once it has safely processed the message.
ws.send(JSON.stringify({ type: 'ack', id: msg.id }));Tracking Unacked Messages
The sender keeps a map of in-flight messages until their ack arrives.
const pending = new Map();
pending.set(msg.id, msg);Retrying on Timeout
If no ack arrives within a timeout, resend the message. This is how at-least-once delivery is achieved.
setTimeout(() => {
if (pending.has(msg.id)) ws.send(JSON.stringify(msg));
}, 3000);The Duplicate Problem
Retries can deliver the same message twice (e.g. the ack was lost, not the message). At-least-once therefore implies the receiver may see duplicates.
Idempotency to the Rescue
Make processing idempotent: handling the same id twice has the same effect as once. Track seen ids and ignore repeats.
const seen = new Set();
if (seen.has(msg.id)) return;
seen.add(msg.id);
process(msg);Approaching Exactly-Once
True exactly-once is impossible to guarantee end-to-end, but at-least-once delivery plus idempotent handling gives the same practical result, which is what most systems use.
Best Practices
Build reliable delivery:
- Tag messages with unique ids
- Require application-level acks
- Retry unacked messages with a timeout
- Make processing idempotent to absorb duplicates
Quick Check
Test your delivery knowledge.
Recap
You added delivery guarantees to realtime messaging:
- Understand at-most/at-least/exactly-once
- Use message ids and application-level acks
- Retry unacked messages
- Combine at-least-once with idempotency
Your system no longer silently loses messages.
Frequently asked questions
Is the “Message Acknowledgement and Delivery Guarantees” lesson free?
Yes — the full text of “Message Acknowledgement and Delivery Guarantees” is free to read here on the web, and the WebSockets & Realtime Systems Programming course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the WebSockets & Realtime Systems Programming course, upgrade to CoddyKit PRO.
What will I learn in “Message Acknowledgement and Delivery Guarantees”?
Ensure messages are not silently lost by adding acknowledgements, retries, and idempotency to reach reliable delivery guarantees. You practise WebSockets & Realtime Systems Programming with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start WebSockets & Realtime Systems Programming?
No prior experience is required. WebSockets & Realtime Systems Programming on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Message Acknowledgement and Delivery Guarantees” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this WebSockets & Realtime Systems Programming lesson?
Yes. Every WebSockets & Realtime Systems Programming lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Handling Disconnections and Reconnects
- Robust Error Propagation and Recovery
- Heartbeats and Keep-Alives
- Message Acknowledgement and Delivery Guarantees