Crate mcp_usage_store 

Distributed durable-task attribution for UsageKit for MCP.

Backends store SHA-256 digests of tenant and task identifiers, never their plaintext values. Values contain only a fixed method category and the price resolved when the task was created, never a name, URI, or extension method string. Inserts are first-writer-wins and claims are atomic, so a completed task can be accounted for by at most one application instance.

Choosing a TTL

The TTL must outlive the longest task the upstream server can run. It is the deadline on the whole task, not on the polling interval: the attribution is written when the task is created and read when it completes, so a task that takes longer than the TTL loses the price resolved at creation.

Nothing fails loudly when that happens. The completing poll finds no attribution, classifies as FreeReason::MissingTaskAttribution, and the call bills zero. From the customer’s side the work was delivered and never charged, so the symptom is silent under-billing that scales with how long tasks run. Watch mcp_usage_free_deliveries_by_reason_total{reason= "missing_task_attribution"}: a nonzero rate on a server that issues durable tasks is the signal that this TTL is too short.

Set it from the upstream’s own task timeout with headroom, not from a guess about client behaviour. Over-provisioning is cheap - rows are 10 bytes of value behind two 32-byte digests, and expiry reclaims them - while under-provisioning loses revenue.

A per-task TTL would remove the guess, since the creating call knows what it commissioned. It is not available yet: TaskAttribution carries only the origin category and the resolved units, and TaskAttributionStore::insert takes no deadline, so expressing one is a breaking change to that trait rather than a change inside these backends.

Structs

PostgresTaskStore

PostgreSQL durable-task attribution with atomic, once-only claims.

RedisTaskStore

Redis or Valkey durable-task attribution with atomic, once-only claims.

Enums

StoreConfigError

Configuration or connection failure while constructing a store.