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· May 15, 2018

Remove Locks when finishing transaction

#Caché

Hi all,

Do you know if there is a way to create a Lock that is related to an existing transaction, in the sense that if transaction is finished (commit or rollback) Lock is removed. I ask this because in the following example Lock is there until process is killed.

Example :

TSTART

Lock  +^MyLock

TROLLBACK

I know, that, Caché itself is locking internally a table register  when doing an OpenId with exclusive flag during the transaction life. For example :

TSTART

Do ##class(MyTable).%OpenId(<TableID>, 4) (This internally is creating a Lock +^User.MyTable(<TableID>)

TROLLBACK (This action removes the previous lock)

I want to do the same without locking an existing table register.

Discussion (4)1
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John Murray · May 15, 2018

The TROLLBACK command doesn't automatically release LOCKs that were acquired within the transaction. The TROLLBACK documentation is a bit confusing in this regard. At one point it says this:

Then later on the same page this text seems to say that both TROLLBACK and TCOMMIT do release locks.

Jordi, in your example:

TSTART

Do ##class(MyTable).%OpenId(<TableID>, 4) (This internally is creating a Lock +^User.MyTable(<TableID>)

TROLLBACK (This action removes the previous lock)

I don't think it's the TROLLBACK that's releasing the lock, but rather the destruction of the oref that the %OpenId method created. Indeed, if tested exactly as you wrote it that oref doesn't even get stored in a local variable, so ceases to exist as soon as the method call completes.

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Robert Cemper · May 15, 2018

Jordi,
TSTART, TROLLBACK TCOMMIT is totally unrelated to LOCKing
it neither requests a LOCK nor does it release.

So to have no LOCK at all nothing prevents you from 

TSTART
Do ##class(MyTable).%OpenId(<TableID>, 0)
. . . . .
TROLLBACK 

The short time internal LOCK during %Save() is handled inside Save 

In your example:

TSTART
Lock  +^MyLock
TROLLBACK

you just miss 

Lock  -^MyLock

to unlock your transaction 

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Jordi Gomez · May 16, 2018

Trying a new example I found the answer to my question :

Doing an %OpenId with concurrent parameter to 4 is like Lock and Unlock right away, so then the Lock is in a state Exclusive->DeLock that means that is "waiting" for Transaction to finish to remove it, that is how it works.

I want to implement a MUTEX, so I cannot do the same. Then, my only way is to remove the lock explicitely and control all possible error ways.

Thanks to all,

Jordi

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