Summary

Tier 4 overload control S25 — opt-in per-queue LIFO dispatch order, the last of the tier-4 knobs (after S26 max_pending #449 and S27 CoDel #450). fifo (default) runs same-priority jobs oldest-first — the fair ordering a durable queue is expected to keep; lifo runs newest-first, a freshness lever for workloads that would rather run recent work than clear a stale backlog in order under overload. Priority always dominates — this only breaks same-priority ties. Off by default; nothing changes unless a queue opts in.

• Core: DispatchOrder{Fifo,Lifo} enum + order_for helper; orders: &HashMap<String,DispatchOrder> threaded through dequeue_from/dequeue_batch_from across the trait, diesel_common, SQLite, Postgres, Redis, and the delegate! macro. Diesel flips scheduled_at asc↔desc for LIFO ties (priority stays desc) with an id tie-break for determinism. Scheduler.dispatch_orders map + register_queue_dispatch_order; poller passes it to dequeue.
• Redis is FIFO-only (documented): its single ZSET score packs priority + scheduled_at, so per-priority LIFO would need a second score-inverted set. lifo is accepted but ignored there, mirroring the list_jobs_after Redis note.
• Python: set_queue_dispatch_order(queue, order); folded into the _queue_configs wire JSON.
• Node: QueueLimits.dispatchOrder; wired through config.rs/worker.rs.
• Java: Taskito.dispatchOrder(queue, order); encodeQueueConfigs now emits CoDel + dispatch order per queue via the late-bound supplier.

Testing

• cargo test --workspace — 183 core tests (LIFO ties dispatch newest-first, FIFO default oldest-first, priority dominates, id determinism); cargo check default/postgres/redis; clippy --all-targets --all-features + doc -D warnings clean
• Python: test_dispatch_order + coexistence with test_admission/test_codel — 16 passed; ruff + mypy clean
• Node: dispatch-order/codel/admission — 12 passed; tsc + biome clean
• Java: DispatchOrderTest 3 + CodelTest 3 + AdmissionTest 6 — all green