Resequencer

You're receiving a fax, page by page. But the fax machine hiccups, and the pages arrive as: 3, 1, 4, 2, 5. You can't read the document until you sort them back into order: 1, 2, 3, 4, 5.

In messaging systems, messages can arrive out of order for many reasons, parallel processing, network routing differences, retries, competing consumers. If your downstream logic requires messages in sequence (event sourcing replay, financial transactions, step-by-step workflows), you need a resequencer.

A resequencer collects out-of-order messages, buffers them, and releases them in the correct sequence number order. It's like a sorting buffer, messages go in out of order and come out in order.

Each message carries a sequence number. The resequencer tracks the next expected sequence number and holds messages that arrive too early:

Expected: 1

Arrives: 3 → buffer (waiting for 1, 2)
Arrives: 1 → release 1, expected = 2
Arrives: 4 → buffer (waiting for 2)
Arrives: 2 → release 2, then release 3 (buffered), then release 4 (buffered)
 expected = 5
Arrives: 5 → release 5, expected = 6

Implementation:

class Resequencer {
 private nextExpected = 1;
 private buffer = new Map<number, Message>();
 
 receive(msg: Message) {
 if (msg.sequenceNumber === this.nextExpected) {
 this.release(msg);
 this.nextExpected++;
 // Release any buffered messages that are now in sequence
 while (this.buffer.has(this.nextExpected)) {
 this.release(this.buffer.get(this.nextExpected)!);
 this.buffer.delete(this.nextExpected);
 this.nextExpected++;
 }
 } else if (msg.sequenceNumber > this.nextExpected) {
 this.buffer.set(msg.sequenceNumber, msg);
 }
 // Ignore messages with sequence < nextExpected (duplicates)
 }
}

The gap problem: What if message 2 is lost entirely? The resequencer holds messages 3, 4, 5, ... Forever, waiting for 2. Solutions:

• Timeout: After N seconds, skip the missing message and release the rest
• Gap detection: After buffering too many messages, alert and skip
• Request retransmission: Ask the sender to resend the missing message