Module state 

Persistence for watch resume state.

StateStore is an async trait for persisting and retrieving Checkpoints keyed by ResumeKey, consumed by the aviso watch supervisor for at-least-once delivery across reconnects and process restarts. Implementations are Send + Sync and serialise concurrent writes internally so the in-memory and (where applicable) on-disk states stay consistent.

A successful StateStore::put is committed-before-visible: a subsequent StateStore::get returns the resolved value. For durable implementations (such as JsonFileStore) the same guarantee is durable-before-visible: the disk write returns success before the new value becomes visible to readers. A failed put leaves all state unchanged.

put is strictly monotonic in last_committed_sequence: a put with a sequence less than or equal to the existing value is a silent no-op. Both implementations enforce this so consumers see identical behaviour regardless of the backing store. Callers needing to reset a checkpoint to a lower sequence must delete first, then put.

Two implementations are provided:

• MemoryStore: in-process. State dies with the program. Good for tests and short-lived consumers.
• JsonFileStore: backed by a JSON file with crash-safe atomic writes via the atomicwrites crate plus a cross-process advisory lock on a sidecar lockfile. Safe for cooperating processes on local filesystems; see the type docs for the precondition list (no NFS/CIFS, no external deletion of the lockfile).

Resume keys are derived from a base URL, an event type, the watch filter body, and an optional schema fingerprint. The hash deliberately excludes any server-side resume position; the same logical subscription always computes the same key.

Structs

Checkpoint

A committed position in a watch stream, keyed by ResumeKey.

JsonFileStore

File-backed state store, multi-process safe for cooperating processes on local filesystems.

MemoryStore

In-process StateStore backed by a HashMap behind a tokio RwLock.

ResumeKey

A logical identifier for a watch subscription.

Enums

ResumeKeyError

Errors specific to resume-key construction.

StoreError

Errors a StateStore implementation can return.

Traits

StateStore

Persistent storage for Checkpoint keyed by ResumeKey.