Module session_backend 

Pluggable session storage backend.

What changed vs the original

Before	After
Only save(PersistedSession) + load(): full round-trip for every change	New update_dc, set_home_dc, update_state allow granular writes
All methods sync (io::Result)	New methods are async (optional; default impls fall back to save/load)
No way to update a single DC key without touching everything else	update_dc only rewrites what changed

Backward compatibility

The existing SessionBackend trait is unchanged. The new methods have default implementations that call load → mutate → save, so existing backends (BinaryFileBackend, InMemoryBackend, SqliteBackend, LibSqlBackend) continue to compile and work without modification.

High-performance backends (e.g. a future Redis backend) can override the granular methods to avoid the load/save round-trip.

Ported from

’ Session trait (in -session/src/session.rs) exposes:

• home_dc_id() -> i32 : cheap sync read
• set_home_dc_id(dc_id) -> BoxFuture<'_, ()> : async write
• dc_option(dc_id) -> Option<DcOption> : cheap sync read
• set_dc_option(&DcOption) -> BoxFuture<'_, ()> : async write
• updates_state() -> BoxFuture<UpdatesState> : async read
• set_update_state(UpdateState) -> BoxFuture<()>: fine-grained async write

We adopt the same pattern while keeping layer’s PersistedSession struct.

Structs

BinaryFileBackend

Stores the session in a compact binary file (v2 format).

InMemoryBackend

Ephemeral in-process session: nothing persisted to disk.

SqliteBackendsqlite-session

SQLite-backed session (via rusqlite).

StringSessionBackend

Portable base64 string session backend.

Enums

UpdateStateChange

A single update-sequence change, applied via SessionBackend::apply_update_state.

Traits

SessionBackend

Synchronous snapshot backend: saves and loads the full session at once.