Message Broker

Ownership and transitions

One retained message log can serve many independent consumer-group cursors. Delivery attempt and in-flight state differ: repeated poll returns the same attempt; nack clears in-flight ownership and only redelivery increments. Acknowledgment may advance only the exact current delivery. Final nack appends once to that group's DLQ. Log operations are amortized O(1); retention grows with published messages and group failures.

Java boundaries

Use the supplied nested records as immutable value carriers. A record is only shallowly immutable: copy every retained mutable input and every outward list/map, including lists inside records. Absence is null; validation failures use IllegalArgumentException, missing-key purchase errors use NoSuchElementException, and invalid transaction boundaries use IllegalStateException. Java primitive types already exclude booleans from numeric inputs. Numeric values use long where signatures say long and all computations must fit signed 64-bit values. Calls in these service exercises are serialized.

An in-flight delivery and a retryable rejected delivery are different states, even while the group stays at the same log position:

import java.util.*;

class Example {
    public static void main(String[] args) {
        int position = 0, attempt = 1;
        boolean inFlight = true;
        inFlight = false; // a retryable nack clears only delivery ownership
        assert position == 0;
        assert attempt == 1;
        assert !inFlight;
    }
}

Another poll at that position creates attempt 2. A repeated poll while inFlight was true would instead repeat attempt 1. Acknowledgment between nack and redelivery must fail because no delivery is currently owned.

Design boundary

The retained log is shared, but cursors, attempts, in-flight state, and DLQs belong to each group. This makes one group failure independent of another progress. There is no timeout or automatic retry clock; adding one would require new delivery-ownership rules.