The writings of Merlin Moncure, professional database developer, about work, life, family, and everything else.

Saturday, December 09, 2006

PostgreSQL 8.2 Advisory Locks

Recently released PostgreSQL 8.2 is a treasure trove of helpful features for the database developer. Dubbed a performance release by the advocacy team, the most powerful open source database engine has once again shown that it is greater than the sum of its parts, and the breadth and scope of the project is greater than its developers. Many internal structural improvements were made, and several user level features were sprinkled in for good measure. One such feature was a overhaul of one of the more useful (and overlooked) contrib modules. Previously known as userlocks, the module was re-branded 'advisory' locks and brought into the core core PostgreSQL architecture.

Advisory locks are 'long-term, cooperative' locks. They are long-term in the sense that they do not follow mvcc rules, and are in fact completely orthogonal to transactional locks. This is a slightly long-winded way of saying that advisory locks are not released upon transaction commit/rollback and can be held outside of a transaction. This is noteworthy because advisory locks are one of the very few things that can be used to maintain session state without using persistent storage (prepared statements, temporary tables, and listen/notify also qualify here), and especially noteworthy because they are the only mechanism in PostgreSQL that I know that can affect the behavior of another session without consideration of either session's transactional state. Advisory locks are cooperative in that all users of them are on the honor system -- this is quite different from standard mvcc locks which follow very strict rules. Anything can be locked at any time. Advisory locks can be thought of as a low level interface to the internal locking engine and will generally need some scaffolding to make them useful.

These properties make advisory locks practical for a number of useful things:
  • Emulation of 'pessimistic locks' common in non-MVCC databases
  • High-performance simple session IPC (like setting a boolean in a table, but faster and MVCC penalty free)
  • Allow cooperation locking of resources without using long duration transactions (which are evil!)

Advisory locks are very simple to use:

Administrator=# select pg_advisory_lock(1);
pg_advisory_lock
------------------
(1 row)

Administrator=# select locktype, classid, objid, pid, mode, granted from pg_locks where locktype = 'advisory';
locktype | classid | objid | pid | mode | granted
----------+---------+-------+------+---------------+---------
advisory | 0 | 1 | 3896 | ExclusiveLock | t
(1 row)

Administrator=# select pg_advisory_unlock(1);
pg_advisory_unlock
--------------------
t
(1 row)



The above commands acquired an advisory lock, demonstrated it in the pg_locks view, and released it using the built in interface. The meaning of the number '1' is completely application defined. It could represent the ID of a table, a special application mutex, a session boolean flag, or just about anything. Keep in mind that if all sessions do not agree on what the number one represents, problems are bound to follow. Regardless of how the session terminates, the locks are released immediately upon disconnect.

If you want to lock an ID of a table, the lock command can be inlined into the select statement:

Administrator=# select pg_advisory_lock(id), * from foo where id = 1;
pg_advisory_lock | id | f
------------------+----+-----
| 1 | abc
(1 row)


And the unlock is just as easy. Advisory locks are extremely fast:

Administrator=# explain analyze select pg_advisory_lock(v) from generate_series(1,1000) v;
QUERY PLAN ----------------------------------------------------------------------------------------------------------------------- Function Scan on generate_series v (cost=0.00..17.50 rows=1000 width=4) (actual time=0.304..4.365 rows=1000 loops=1) Total runtime: 6.244 ms
(2 rows)


If I read that right the time on my iMac to acquire a lock is about 6 microseconds....not bad! A note of caution here: advisory locks are drawn out of shared memory and if they are abused the results can be catastrophic:

Administrator=# select pg_advisory_lock(v) from generate_series(1,10000) v;

WARNING: out of shared memory
ERROR: out of shared memory

HINT: You may need to increase max_locks_per_transaction.


Administrator=# select * from pg_locks;
WARNING: out of shared memory

When shared memory is used up, the server basically cannot do anything at all. Quitting the session returned everything to normal however. All the same, extreme care must be taken not to leak locks. Also, they are an extremely obvious Denial of Service target following an SQL injection attack. If this is an issue you may want to consider locking them down:

Administrator=# revoke execute on function pg_advisory_lock(bigint) from public;
REVOKE

Advisory locks provide a 64 bit space for locks. You have the choice of locking two 32 bit integers or a single 64 bit integer (these are drawn from the same lock pool and are thus only superficially different). In addition, there are the 'shared' flavors of the advisory locks which do not block other shared lockers but block and are blocked by exclusive lockers. Best of all, there are 'try' flavors of the functions which do not wait for the lock to be held if another session hold it -- this is similar to the NOWAIT clause of some other SQL statements that can hold locks. The full listing of all the advisory lock functions is here.

Merlin

5 comments:

depesz said...

it' miliseconds, not microseconds. anyway - not bad :)

Merlin Moncure said...

nope, it's microseconds, the explain analyze time was for acquiring 1000 locks (generate_series).

Merlin Moncure said...

In my listing of things that are outisde the transactional engine, I forgot sequences...just thought I'd mention that (transaction rollbacks do not affect a sequence fetch).

Shaun said...

Hey merlin. Nice post. If you are interested in some contract work we have a couple of postgres databases that need to be reviewed. Feel free to contact me at mailto:shaun@methodmaker.co.nz

Cheers
S

Chris Angelico said...

"Advisory locks provide a 64 bit space for locks. You have the choice of locking two 32 bit integers or a single 64 bit integer (these are drawn from the same lock pool and are thus only superficially different)."

According to 9.1 docs, BIGINT and INT,INT locks are quite separate. Don't know if that's a change or not.

Handy article, is good to have these things blogged about!