StorageBusyException

A store is up but already runs as much as it takes, and did not admit this call: a query store's bulkhead, a server out of connections or rejecting work (PostgreSQL's 53300, Elasticsearch's 429, ClickHouse's TOO_MANY_SIMULTANEOUS_QUERIES). The same call may pass once the load drops, so it is retried with backoff, never right away: a subscription throttles its partition and retries the entry within its backpressure budget, without pausing the partition.

It is a StorageUnavailableException for every other caller: the store cannot take the call now, so a command or a route answers wow.storage.unavailable with Retry-After. A backend throws it with the driver's exception as cause; a bound on the store's load throws it as the cause of the error it answers with (wow.query.busy, say).