Can pessimistic locking be emulated?

I am trying to emulate a pessimistic locking system you would find in

an

old school database file system, for example cobol. Generally, when a

cobol program tries to read a record that is locked by somebody else,
the read fails and either a message is displayed by the user or a

error

handling procedure is executed. I would like to emulate this behavior

for legacy code while using mvcc for newer procedures I write.

4 questions:
1. Can you query if a tuple is locked by another transaction (the
documentation unclearly suggests this can't be done via the pg_lock
view) before executing select for update...?
2. If so, is this reasonable efficient to do, i.e. straight join on
oid/xid?
3. If so, is this possible to query without a race condition regarding

the lock status?
4. If so, is this likely to be possible in future versions of postgres

without non-trivial changes?

In other words, if User B attempts to select for update a record that
user A has selected for update, it would be nice if User B's query

would

fail with a NOTICE to act upon.

No idea if this is of any help, but you may have a look into
PostgreSQL 7.3 Documentation
3.4. Run-time Configuration
STATEMENT_TIMEOUT (integer)
Aborts any statement that takes over the specified number of
milliseconds. A value of zero turns off the timer.

Regards, Christoph

That's my fallback position. Obviously, this will lead to false
positives depending on server load. In my case, I'm targeting between
30-50 users so its likely to throw timeouts for various reasons other
than locks even though my queries of interest are generally select a
from b where id = c type of thing. This is a kludgy solution but its
still better than writing cobol.

The bigger issue is that a timeout will not return the reason the query
timed out. There are cases where I would like to run a select for
update over a range of records and handle the locked records and
unlocked records differently. A query that could match locked oids vs
the oids I am interested in would be super. I could then aggregate my
select for updates into larger queries and reap massive performance
gains.

Another way of putting it is this: waiting for your select to timeout is
kind of like parking in Manhattan: you back your car up until you hit
the next car. I would sort of like to, uh, look in the rear view mirror
first.

Merlin

In other words, if User B attempts to select for update a record

that

That's my fallback position. Obviously, this will lead to false
positives depending on server load. In my case, I'm targeting between

30-50 users so its likely to throw timeouts for various reasons other
than locks even though my queries of interest are generally select a
from b where id =3D c type of thing. This is a kludgy solution but

its

still better than writing cobol.

The bigger issue is that a timeout will not return the reason the

query

timed out. There are cases where I would like to run a select for
update over a range of records and handle the locked records and
unlocked records differently. A query that could match locked oids vs

the oids I am interested in would be super. I could then aggregate my

select for updates into larger queries and reap massive performance
gains.

Another way of putting it is this: waiting for your select to timeout

is

kind of like parking in Manhattan: you back your car up until you hit
the next car. I would sort of like to, uh, look in the rear view

mirror

first.

I see your point.

1. Can you query if a tuple is locked by another transaction (the
documentation unclearly suggests this can't be done via the pg_lock
view) before executing select for update...?

Where did you find this?

Regards, Christoph

I was referring to 10.3 in the administrator's guide, regarding the
pg_lock view. According to the documentation, the view only contains
table level locks. However, the view also contains an xid for
transactions. The unclear part, at least to me, was what the role of
the xid was in the view and if it could be used to produce a list of
locked tuples somehow. The xid is referred to as a 'lockable object'.
I wasn't sure of the xid's role in the mix. I see now how it all works
together.

In my case, being able to view outstanding row level locks would be
enormously useful. I'm assuming this is not possible for structural or
performance reasons. I'm aware of the possible nasty side affects of
repeated query calls to the lock manager. I'm also aware what I'm
asking about may be folly or silly, my understanding of how mvcc and
transactions work together is not very refined.

A curious thought struck me: does the pg_lock view follow the mvcc
rules, i.e. if you query the pg_lock view inside a transaction, and an
external effect introduces new locks into the server are you able to see
those locks?

Merlin

"Merlin Moncure" <merlin.moncure@rcsonline.com> writes:

In my case, being able to view outstanding row level locks would be
enormously useful.

The only way to do that would be to grovel through every table in the
database, looking for rows that are marked locked by transactions that
are still alive.

A curious thought struck me: does the pg_lock view follow the mvcc
rules,

No, not really. If it did I don't think it'd be real useful ...

regards, tom lane

In my case, being able to view outstanding row level locks would be
enormously useful. I'm assuming this is not possible for structural or

Agreed -- but they're stored on the row themselves. You might be able
to write a function which executes dirty reads on the table and tells
you if the row is locked or not, but it's not going to be simple.

--
Rod Taylor <rbt@rbt.ca>

PGP Key: http://www.rbt.ca/rbtpub.asc

Rod Taylor <rbt@rbt.ca> writes:

Agreed -- but they're stored on the row themselves. You might be able
to write a function which executes dirty reads on the table and tells
you if the row is locked or not, but it's not going to be simple.

Actually, I don't think you need a dirty read at all. A locked row
can't be deleted as well (because there's only one xmax slot), so if you
can see it (ie, you think its xmin is committed) then you can in
principle find out whether it's locked or not. We just don't expose the
info at the moment. (You can see xmax at the user level, but you can't
easily tell if xmax is trying to delete the row or just lock it, because
you don't have access to the infomask bit that would tell you.)

regards, tom lane

On Thu, 2003-02-27 at 15:02, Tom Lane wrote:

Rod Taylor <rbt@rbt.ca> writes:

Agreed -- but they're stored on the row themselves. You might be able
to write a function which executes dirty reads on the table and tells
you if the row is locked or not, but it's not going to be simple.

Actually, I don't think you need a dirty read at all. A locked row

I see. That will make it quite a bit easier then. Perhaps I'll write a
function sometime. It would make it useful for fetching things out of a
persistent work queue. Right now I deal with userlocks -- but those can
be clumsy.

--
Rod Taylor <rbt@rbt.ca>

PGP Key: http://www.rbt.ca/rbtpub.asc

This directly answers my question (wasn't previously aware that xid
could be queried out in such a useful fashion). Not only does this
accomplish what I need, but now allows me to not use select ... for
update and stick with a transaction based locking mechanism. The 'Why'
isn't that interesting in my case: merely that the knowledge that the
record is involved in a transaction is enough.

I've felt for a while that the descriptions of transactions, mvcc, and
row level locking in the official docs could use a little bit better
treatment (selfishly motivated, I could never figure them completely
out!) but this is the wrong list for that :).

Many thanks to the hackers for helping me with my problem.
Merlin

Actually, I don't think you need a dirty read at all. A locked row
can't be deleted as well (because there's only one xmax slot), so if

you

can see it (ie, you think its xmin is committed) then you can in
principle find out whether it's locked or not. We just don't expose

the

info at the moment. (You can see xmax at the user level, but you

can't

easily tell if xmax is trying to delete the row or just lock it,

because

Merlin,

Just as a suggestion: In most of my applications, we have a security layer
which is implemented through server-side functions. These functions keep a
table updated which contains:

lock_table
record_id
lock_user
time_locked

This allows us to avoid nasty "your update cannot be processed"-type error
messages by showing the user up front which records are locked, as well as
allowing the admin to decide when locks should "time out".

I tend to find in general that database locking mechanisms are a very poor
locking strategy for a good UI.

--
Josh Berkus
josh@agliodbs.com
Aglio Database Solutions
San Francisco

Just as a suggestion: In most of my applications, we have a security

layer

which is implemented through server-side functions. These functions

keep a

table updated which contains:

lock_table
record_id
lock_user
time_locked

That's an excellent and even portable idea.

This allows us to avoid nasty "your update cannot be processed"-type

error

messages by showing the user up front which records are locked, as

well as

allowing the admin to decide when locks should "time out".

I tend to find in general that database locking mechanisms are a very

poor

locking strategy for a good UI.

True. But you circumvented it elegantly.

Regards, Christoph

Christoph,

table updated which contains:

lock_table
record_id
lock_user
time_locked

That's an excellent and even portable idea.

Thanks. It's actually part of an intranet framework that my team uses to
build all of our browser-based applications. We've been casually discussing
for the last few months whether to keep the framework to ourselves, or to
take it Open Source. Mostly we've been way too busy to make a serious
decision ....

--
Josh Berkus
Aglio Database Solutions
San Francisco