# Workload Admission Contracts

**Use this when:** adding a workload class or snapshot provider, consuming admission state from a background job, or deciding whether a job can "join" admission (it cannot; see Observation Surface Only).
**Source of truth:** `WorkloadClass`, `AdmissionState`, `WorkloadAdmissionSnapshot`, `WorkloadAdmissionRegistrySnapshot`, `WorkloadAdmissionSnapshotProvider`, and `foreground_pressure` in `crates/concurrency/src/workload.rs`; the provider and consumer files named below.

## Contract Shapes

`rustfs-concurrency` owns the read-only shapes. `WorkloadClass` enumerates the admission categories (the variants are the source of truth); `AdmissionState`, `WorkloadAdmissionSnapshot`, and `WorkloadAdmissionRegistrySnapshot` are status shapes for runtime owners to fill. They do not replace the scheduler, request guards, scanner, heal, replication, or ECStore placement behavior. `GetObjectQueueSnapshot` permit semantics (saturated, over-available, zero-total) and worker-slot over-release clamping are pinned by `rustfs-concurrency` tests; scheduler buffer/priority behavior is pinned by `rustfs-io-core` and `rustfs/src/storage/concurrency/` tests.

## Class To Provider Table

| Class | Provider (`impl WorkloadAdmissionSnapshotProvider`) | `active` / `queued` / `limit` source | Reports `Unknown` when |
|---|---|---|---|
| `ForegroundRead` | `ConcurrencyManager` in `rustfs/src/storage/concurrency/manager.rs` (source of truth); re-exposed unchanged by the RustFS runtime provider | disk-read permits in use / `None` (the semaphore exposes no waiter count) / configured max concurrent disk reads | the storage registry has no entry |
| `ForegroundWrite` | `ConcurrencyManager` in `rustfs/src/storage/concurrency/manager.rs` (source of truth); re-exposed unchanged by the RustFS runtime provider | foreground-write permits in use or legacy active-write counter / `None` / configured or derived write-admission limit | the storage registry has no entry |
| `Metadata` | `RustFsWorkloadAdmissionSnapshotProvider` in `rustfs/src/workload_admission.rs` | `Open` once the bucket metadata runtime handle exists; no counts | bucket metadata runtime not initialized |
| `Scanner` | same | scanner active work-unit counter / none / configured set-scan limit when nonzero | scanner runtime not initialized |
| `Repair` | same | heal active tasks / heal queue length / `None` (limits live behind the async heal manager state) | heal manager not initialized |
| `Replication` | same | active regular + large-object + MRF workers / site replication queue count / `None` (limits owned by the async pool and resize policy) | replication runtime not initialized, or queue stats currently locked |

## Observation Surface Only

This is an observation surface only. Permit acquisition, priority assignment, buffer sizing, storage media detection, request guards, queue capacity, heal admission and priority merge/drop policy, replication worker resize and MRF handling, scanner cycle scheduling, bucket metadata loading and locks, and object write paths are unchanged by any provider. There is no runtime admission API for a background job to join; a job that needs bounded contention must bound it itself (see [kms-bulk-rekey-contract.md](kms-bulk-rekey-contract.md)).

Consumers that read the snapshot to self-throttle exist, and they do not change the owners' decisions:

| Consumer | File | Behavior |
|---|---|---|
| Data-movement backpressure (decommission, rebalance) | `crates/ecstore/src/data_movement/backpressure.rs` (`wait_for_data_movement_admission`, `foreground_pressure`) | Delays the next data-movement step while `ForegroundRead` or `ForegroundWrite` usage exceeds the configured high-water percent. ECStore receives the provider through `set_workload_admission_snapshot_provider` (`crates/ecstore/src/lib.rs`), published from `rustfs/src/startup_background.rs`; with no provider the step is admitted immediately. |
| Heal manager mainline throttle | `crates/heal/src/heal/manager.rs` (`new_with_workload_provider`) | When `mainline_throttle_enable` is set, defers heal work while `ForegroundRead` or `ForegroundWrite` utilization exceeds the configured high-water percents; with no provider or the throttle disabled, heal pacing is unchanged. |
| Scanner sleeper and scan fan-out | `crates/scanner/src/workload_admission.rs`, `crates/scanner/src/sleeper.rs`, and `crates/scanner/src/scanner_io/guards.rs` | Reads the same provider published from `rustfs/src/startup_background.rs` and combines it with scanner-local foreground read guards. Foreground activity increases scanner sleeps and reduces set/disk scan fan-out to one; cycle budgets still own object, directory, and duration limits. With no provider, scanner keeps the legacy local foreground-read behavior. |

## Boundary Rules

- `rustfs-concurrency` owns the contract surface and does not depend on `rustfs-ecstore` or RustFS binary runtime state.
- Adding a class or provider changes no scheduler decision logic, queue capacity, Tokio runtime default, scanner/heal/replication admission, placement, membership, or NUMA behavior.
- Providers report `Unknown` rather than blocking or guessing when their owner is uninitialized or its stats are not immediately observable.

## Provider Composition

`WorkloadAdmissionRegistrySnapshot::overlay` composes provider-owned registries without mutating runtime owners: the storage concurrency provider is the source of truth for `ForegroundRead`; the RustFS runtime owner provider overlays metadata, scanner, repair, replication, and foreground-write status on top; matching classes are replaced by the later snapshot and new classes are appended without reordering. `workload_admission_registry_snapshot` in `rustfs/src/workload_admission.rs` is the single composed registry consumers read.
