relstorage.adapters.postgresql.locker – Locker implementations

Locker implementations.

class PostgreSQLLocker(options, driver, batcher_factory)[source]

Bases: AbstractLocker

On PostgreSQL, for locking individual objects, we use advisory locks, not row locks.

This is because row locks perform disk I/O, which can “bloat” tables; this is especially an issue for shared (readCurrent) locks, which perform I/O to pages that might not otherwise need it. Using many row locks requires a good (auto)vacuuming setup to prevent this bloat from becoming a problem.

Also, advisory locks are much faster than row locks.

We take advisory locks based on OID. OIDs begin at 0 and increase. To avoid conflicting with those, any advisory locks used for “administrative” purposes, such as the pack lock, need to use negative numbers.

Locks include:

  • -1: The commit lock

  • -2: The pack lock

hold_pack_lock(cursor)[source]

Try to acquire the pack lock.

Raise an exception if packing or undo is already in progress.

release_pack_lock(cursor)[source]

Release the pack lock.