Cycle 6: Per-Actor Mailbox Backpressure — Development History

Commit: 265992c · 6 files · 163 insertions, 8 deletions

Motivation

Before this change, swactor mailboxes were unbounded — a fast producer could flood a slow consumer's mailbox without limit, eventually exhausting memory. Every production actor framework provides some form of backpressure. This was identified as a key weakness in the competitor analysis.

Competitor Analysis

Framework	Default Capacity	Overflow Policy	Backpressure Model
Erlang/OTP	Unbounded	N/A (pobox for opt-in bounding)	Process isolation limits blast radius
Actix	16	`do_send()` bypasses for internal msgs	Tiny default, force callers to handle
Kameo	64	Bounded tokio mpsc (sender blocks)	Blocking backpressure
Tokio mpsc	User-specified	Bounded (sender blocks or permit pattern)	Blocking or try_send
Go channels	User-specified	Blocking send / non-blocking select	Blocking backpressure
Swactor (before)	Unbounded	None	None

Key observation: Actix's default capacity of 16 is aggressive — it forces callers to think about message flow. Kameo's 64 matches swactor's message budget. The consensus across frameworks: bounded by default, with configurable overflow policy.

Implementation

Added MailboxOverflow enum: DropNewest (discard incoming when full) and DropOldest (evict oldest to make room)
Added default_mailbox_capacity and mailbox_overflow to RuntimeConfig
Default: capacity=0 (unbounded) — 100% backward compatible
ActorSlot stores per-actor capacity and policy (initialized from runtime defaults at spawn time)
deliver() in worker enforces bounds; dropped messages tracked via drops_this_tick counter
messages_dropped: AtomicU64 added to WorkerStats and WorkerInfo

Key files modified: src/config.rs, src/worker.rs, src/runtime.rs, src/stats.rs, tests/runtime_api.rs

Design Decisions

DropNewest vs DropOldest (not blocking) — swactor's synchronous tick model can't block the sender (it would deadlock the entire worker). Drop policies are the only viable option for a sync runtime.
Default unbounded — backward compatibility. Users opt into backpressure by setting capacity > 0.
Per-runtime defaults, not per-actor — simpler configuration. Per-actor overrides could be added later via a builder pattern on spawn.
Drop counting — critical for observability. Without it, users can't tell if their system is losing messages.
No DropRandom — the two policies cover the common cases. DropNewest protects against producer floods (newest messages are redundant). DropOldest keeps the freshest state (useful for sensor/status actors).

Tests Added

4 new tests (60 → 64 total):

bounded_mailbox_drop_newest_caps_at_capacity — 50 msgs sent, capacity 10 → only 10 delivered (oldest 10)
bounded_mailbox_drop_oldest_keeps_newest — 10 msgs sent, capacity 5 → newest 5 kept
unbounded_mailbox_delivers_all_messages — backward compatibility: capacity=0 delivers everything
bounded_mailbox_refills_after_processing — capacity 5, process batch, refill works correctly

Result

64 tests pass
All workspace crates compile
Swactor weakness "no backpressure" resolved

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