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Add support to lock manager for conditionally locking a lock (ie,

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return without waiting if we can't get the lock immediately).
Not used yet, but will be needed for concurrent VACUUM.
This commit is contained in:
Tom Lane
2001-06-22 00:04:59 +00:00
parent 986915c181
commit d8d9ed931e
6 changed files with 112 additions and 86 deletions
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15
contrib/userlock/README.user_locks
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@ -5,9 +5,8 @@ This software is distributed under the GNU General Public License
either version 2, or (at your option) any later version.
This loadable module, together with my user-lock.patch applied to the
backend, provides support for user-level long-term cooperative locks.
For example one can write:
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';
@ -26,14 +25,14 @@ After you have finished your work on that item you can do:
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.
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
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,
@ -44,12 +43,12 @@ use 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 succesfully locked. What this mean is up to you.
entity has been successfully locked. What this means is up to you.
My succestion is that you use the group to identify an area of your
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.
Or you can just lock the oid of the tuples which are by definition unique.
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 calling the unlock funtion until it returns 0.
be done by calling the unlock function until it returns 0.