mirror of https://github.com/postgres/postgres
57 lines
2.6 KiB
Plaintext
57 lines
2.6 KiB
Plaintext
User locks, by Massimo Dal Zotto <dz@cs.unitn.it>
|
|
Copyright (C) 1999, Massimo Dal Zotto <dz@cs.unitn.it>
|
|
|
|
This software is distributed under the GNU General Public License
|
|
either version 2, or (at your option) any later version.
|
|
|
|
|
|
This loadable module provides support for user-level long-term cooperative
|
|
locks. For example one can write:
|
|
|
|
select some_fields, user_write_lock_oid(oid) from table where id='key';
|
|
|
|
Now if the returned user_write_lock_oid field is 1 you have acquired an
|
|
user lock on the oid of the selected tuple and can now do some long operation
|
|
on it, like let the data being edited by the user.
|
|
If it is 0 it means that the lock has been already acquired by some other
|
|
process and you should not use that item until the other has finished.
|
|
Note that in this case the query returns 0 immediately without waiting on
|
|
the lock. This is good if the lock is held for long time.
|
|
After you have finished your work on that item you can do:
|
|
|
|
update table set some_fields where id='key';
|
|
select user_write_unlock_oid(oid) from table where id='key';
|
|
|
|
You can also ignore the failure and go ahead but this could produce conflicts
|
|
or inconsistent data in your application. User locks require a cooperative
|
|
behavior between users. User locks don't interfere with the normal locks
|
|
used by Postgres for transaction processing.
|
|
|
|
This could also be done by setting a flag in the record itself but in
|
|
this case you have the overhead of the updates to the records and there
|
|
could be some locks not released if the backend or the application crashes
|
|
before resetting the lock flag.
|
|
It could also be done with a begin/end block but in this case the entire
|
|
table would be locked by Postgres and it is not acceptable to do this for
|
|
a long period because other transactions would block completely.
|
|
|
|
The generic user locks use two values, group and id, to identify a lock.
|
|
Each of these are 32-bit integers.
|
|
|
|
The oid user lock functions, which take only an OID as argument, store the
|
|
OID as "id" with a group equal to 0.
|
|
|
|
The meaning of group and id is defined by the application. The user
|
|
lock code just takes two numbers and tells you if the corresponding
|
|
entity has been successfully locked. What this means is up to you.
|
|
|
|
My suggestion is that you use the group to identify an area of your
|
|
application and the id to identify an object in this area.
|
|
|
|
In all cases, user locks are local to individual databases within an
|
|
installation.
|
|
|
|
Note also that a process can acquire more than one lock on the same entity
|
|
and it must release the lock the corresponding number of times. This can
|
|
be done by calling the unlock function until it returns 0.
|