Module store_locks
Expand description
Collection of small helpers that implement store-based locks.
This is a per-process lock that may be used only for very specific use cases, where multiple processes might concurrently write to the same database at the same time; this would invalidate store caches, so that should be done mindfully. Such a lock can be acquired multiple times by the same process, and it remains active as long as there’s at least one user in a given process.
The lock is implemented using time-based leases to values inserted in a
store. The store maintains the lock identifier (key), who’s the
current holder (value), and an expiration timestamp on the side; see also
CryptoStore::try_take_leased_lock
for more details.
The lock is initially acquired for a certain period of time (namely, the
duration of a lease, aka LEASE_DURATION_MS
), and then a “heartbeat” task
renews the lease to extend its duration, every so often (namely, every
EXTEND_LEASE_EVERY_MS
). Since the tokio scheduler might be busy, the
extension request should happen way more frequently than the duration of a
lease, in case a deadline is missed. The current values have been chosen to
reflect that, with a ratio of 1:10 as of 2023-06-23.
Releasing the lock happens naturally, by not renewing a lease. It happens automatically after the duration of the last lease, at most.
Modules§
- Some code that is shared by almost all
MemoryStore
implementations out there.
Structs§
- A store-based lock for a
Store
. - A guard on the store lock.
Enums§
- Error related to the locking API of the store.
Constants§
- Period of time between two attempts to extend the lease. We’ll re-request a lease for an entire duration of
LEASE_DURATION_MS
milliseconds, everyEXTEND_LEASE_EVERY_MS
, so this has to be an amount safely low compared toLEASE_DURATION_MS
, to make sure that we can miss a deadline without compromising the lock. - Amount of time a lease of the lock should last, in milliseconds.
- Maximal backoff, in milliseconds. This is the maximum amount of time we’ll wait for the lock, between two attempts.
Traits§
- Backing store for a cross-process lock.