Module limits 

Concurrency and rate limits for steps.

engine.limit("stripe", LimitSpec { concurrent: 10, per_second: 100.0 }) defines a limit; a step run with StepOptions::limit("stripe") takes a permit before its function runs and returns it when the attempt ends. A step waiting for a permit holds nothing else (its gate is already durable). After a crash a new attempt requests a fresh permit.

Waiting steps form a WaitQueue: the highest Priority::level is served first; within a level, steps are grouped by Priority::fairness_key and the groups take turns, each taking up to Priority::fairness_weight permits per turn, so one key with a deep backlog cannot starve the others. Within a key, arrival order. With the default priority every step is in one group at one level: plain arrival order.

The priority is not journaled. It comes from the step’s options, which the code passes again when it reaches the step after a crash, so the request is made again with the same priority and key.

In one process every shard shares one LocalLimits: a counting semaphore and a token bucket per limit in front of one wait queue. In server mode the limit lives on the server and permits are leases (see fusee_server::RemoteLimits), so a crashed holder’s permits come back when its lease expires; the server keeps the wait queue and decides with the same WaitQueue.

Modules

answer

The replicated limit ledger answers an acquire with one number: 0 queued, 1 the asked ticket granted, 2 + delay ask again after delay ms (the rate limit refills then), or the top bit plus the zigzag distance to another ticket of the same session that was granted instead (the one the wait queue serves next).

Structs

LimitInfo

A limit and its permits right now (crate::Engine::limits).

LimitSpec

A limit’s size.

LocalLimits

Limits shared by every shard of one process.

Priority

Where a step waiting for a permit stands. The default (level 0, no key, weight 1) is plain arrival order.

WaitQueue

Waiters for one limit’s permits: highest level first, then the levels’ fairness keys in turn (weighted round-robin), then arrival order within a key. Deterministic, so a replicated server can apply it as a state machine. A queue of default-priority waiters only saves as the plain arrival-order list queues always saved as; any other as its levels.

Enums

Acquire

The answer to an acquire.

Traits

LimitBackend

Where limits live.