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RahulG31
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Joined: 20 Dec 2014
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Location: USA
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I am testing a CICS transaction and in the program I am using ENQ / DEQ for serialized access.
Now, I am trying to see what happens if the resource is ENQueued by User1 and at the same time a DEQ is issued by User2 with same Resource name.
(I introduced a delay of 10 seconds after ENQ so that I can issue a DEQ when the resource is held)
I am using 2 terminals.
What is currently happening is that there is No effect of issuing DEQ with same resource. So, I start the transaction on terminal 1 and wait for 5 seconds and then start it on terminal 2. The transaction takes a little over 10 seconds on terminal 1 which is what it should be But on terminal 2 it takes a little over 15 seconds meaning the transaction at terminal 2 was waiting for transaction at terminal 1 to end i.e. it was queued.
But I had issued a DEQ command for the same resource before issuing ENQ then why it was queued. Please let me know your thoughts. |
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Rohit Umarjikar
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Joined: 21 Sep 2010
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RahulG31,
| Quote: |
| So, I start the transaction on terminal 1 and wait for 5 seconds and then start it on terminal 2 |
So after 5 seconds or little more the resource should get DEQ'ed from terminal 1 and next task sould begin at terminal 2.
| Quote: |
| But on terminal 2 it takes a little over 15 seconds meaning the transaction at terminal 2 was waiting for transaction at terminal 1 to end i.e. it was queued. |
I am not sure why was it taken 15 seconds or more as understood above it should only take 5+ seconds maximum, right? as term1 should finish by issuing DEQ.
So I think it works on a token just not same resources unless it finds the same token it wouldn't release the resource.
| Code: |
When issuing the DEQ command, the resource to be dequeued from must be identified by the method used when enqueuing on the resource. If no enqueue has been issued for the resource, the dequeue is ignored.
If more than one ENQ command is issued for a resource by a task, that resource remains owned by the task until the task issues a matching number of DEQ commands. |
| Quote: |
| But I had issued a DEQ command for the same resource before issuing ENQ then why it was queued. Please let me know your thoughts. |
Have you done this at the end of trx or program processing or just before ENQ? if it just before the ENQ then then it shall ignore DEQ as happened with your case. |
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RahulG31
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Joined: 20 Dec 2014
Posts: 446
Location: USA
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I am using below for ENQ:
| Code: |
EXEC CICS ENQ
RESOURCE (RESOURCE)
LENGTH (LENGTH OF RESOURCE)
RESP (RESPONSE-CD)
NOHANDLE
END-EXEC. |
And in the program it's DEQ and then ENQ and then 10 seconds Delay. So, for terminal 1, first DEQ is ignored as there is no ENQ and then Resource is ENQueued. Delay of 10 seconds is in place now.
Then (after 5 seconds) the transaction is started at terminal 2. So, a DEQ will be issued first and then ENQ and then 10 seconds delay to program end.
It takes a total of 20 seconds but as per me it should have taken less depending on when the transaction at terminal 2 was started (15 seconds in this case).
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Rohit Umarjikar
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| Quote: |
| Then (after 5 seconds) the transaction is started at terminal 2. So, a DEQ will be issued first and then ENQ and then 10 seconds delay to program end. |
It wouldn't work that as per me as elaborated above. First ENQ then < Do all your processing> at the end issue DEQ. As I said that this works with same token and in your case when task 2 has started on term 2 it has violated the token (as token for task1 with tem1 was still in effect )and DEQ is ignore. That is what would have happened so if possible please try DEQ after ENQ. |
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enrico-sorichetti
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Joined: 14 Mar 2007
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| Quote: |
| I am trying to see what happens if the resource is ENQueued by User1 and at the same time a DEQ is issued by User2 |
First ... Just remember that en/deq works because of an agreement on the enq token
an arbitrary string up to xxx(*) chars
(*) I do not remember how many 
looks like You did not check the return code of the deq
dequeuing a resource which has not been enqueued should return an error
the same time might look like the same seen from the outside,
but if You look closer it might be pretty difficult to do it.
anyway, if done by the books, ENQ/DEQ WORKS.
tested in batch with two jobs
job 1
enq
wtor
deq
job 2
enq
wtor
deq
the ISPF enq report showed
job 1, holding the resource
job 2 waiting
as soon a reply was given for job 1
the two from job 2 appeared
CICS work the same way
transaction on term 1
enq
converse ( CONVERSATIONAL WRITE/READ )
deq
the same transaction on term 2
will display the screen as soon as enter is hit on terminal 1 |
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RahulG31
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Thanks for your comments Rohit and Enrico.
I have already read the following and I know that ENQ -> some processing -> DEQ works. But as mentioned below "A resource can be any string", I believe this string name is what you are referring to as a token. So, in my case, this "resource name" is same in DEQ and ENQ.
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A resource in the context of this command is any string of 1–255 bytes, established by in-house standards, to protect against conflicting actions between tasks, or to cause single threading within a program.
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As you see below, there are 2 methods to enqueue a resource. I am using the second method where the argument represent the resource as I am providing the length in both DEQ and ENQ.
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The resource to be enqueued on must be identified by one of the following methods:
Specifying a data area that is the resource. It is the location (address) of the data area in storage that matters, not its contents.
Specifying a data area that contains a unique argument (for example, an employee name) that represents the resource. It is the contents of the data value that matters, not its location. LENGTH is required; the presence of the LENGTH option tells CICS to enqueue on the contents of the data value. |
I hope you are getting what I am trying to say. My question is why DEQ is not ending the ENQ resource. According to what I have read, there's nothing stopping it.
*I know the purpose of ENQ/DEQ is to serialize and what I am stating will defeat the purpose But according to the theory I am not able to understand Why it doesn't work?
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enrico-sorichetti
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| Quote: |
| My question is why DEQ is not ending the ENQ resource. |
because, as You say
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... the resource is ENQueued by User1 and at the same time a DEQ is issued by User2 ...
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why do You expect that user2 would dequeue a resource enqueued by user1 ??? 
the enq/deq mechanism checks that a resource is dequeued by the same process which enqueued it |
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Rohit Umarjikar
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enq
read update
rewrite
deq
is the normal suggested method should be used.
Resource name (string name) , I don't think that is just a token it would be resource name + task #= token.
You can not issue Deq first and enq later and if you do so the Deq is ignored as we have seen in your case. so if task 1 has issued enq and it is still active (which is your case) then no other task can Deq in between the processing of task1 except only task1 issues Deq or it ends or issues syncpoint only then the resources are unlocked or released.
And this has a performance issue too if you Deq first and enq later as then you are holding up resources even after updates unnecessarily. |
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Garry Carroll
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If it were possible for task2 to DEQ a resource that had been ENQ'ed by task1, that would make the ENQ/DEQ mechanism totally useless.
 
Garry |
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RahulG31
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Ok Guys. What you all are saying makes sense But
Where is it written that we can't do it?
Where is it written that DEQ will only affect ENQ from the same task/terminal/program?
That is all I want to see as I don't find it. The text only says Resource which is a string and can be used by anyone.
If you can provide a link then that would be really helpful.
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Rohit Umarjikar
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| Quote: |
| Where is it written that we can't do it? |
You can do it but it will be ignored and I mentioned that in my second post.
| Quote: |
| Where is it written that DEQ will only affect ENQ from the same task/terminal/program? |
| Code: |
| When issuing the DEQ command, the resource to be dequeued from must be identified by the method used when enqueuing on the resource. |
Same method here means the combination of (task+Resouce) , I think that is indeed.
I will try to find,if any links which say's this in black and white of what you say. |
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RahulG31
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Thanks Rohit But I have to disagree here.
| Quote: |
| Same method here means the combination of (task+Resouce) , I think that is indeed. |
Same method means one of these 2 methods (as mentioned above as well). So, if I am using Name + Length for ENQ then I have to mention Name + Length for DEQ as well and that is what I am doing.
| Quote: |
The resource to be enqueued on must be identified by one of the following methods:
Specifying a data area that is the resource. It is the location (address) of the data area in storage that matters, not its contents.
Specifying a data area that contains a unique argument (for example, an employee name) that represents the resource. It is the contents of the data value that matters, not its location. LENGTH is required; the presence of the LENGTH option tells CICS to enqueue on the contents of the data value. |
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Rohit Umarjikar
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| Quote: |
| Thanks Rohit But I have to disagree here. |
This should help to change your mind from Disagree to Agree mode(Specially last para).
Refer here
If you want to swim litter deeper the ocean then this way here
| Quote: |
| So, if I am using Name + Length for ENQ then I have to mention Name + Length for DEQ as well and that is what I am doing. |
Yes but should be done by the same TASK , this statement is in the SAME TASK context and not any other TASK.
Missing Bill O'Boyle 's Comments . He may have more specific explanation than what we have shared so far. |
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RahulG31
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Rohit, Thanks for spending your time on this. I appreciate your efforts.
I disagreed on the statement that 'same method means task + resource'. Which I haven't seen written anywhere and which, I believe, is a completely different meaning of method. Method is how we are issuing the ENQ/DEQ commands. To me, TaskID is Not a part of method.
Moreover, the last para states
| Quote: |
| After a task has issued an ENQ RESOURCE(data-area) command, any other task that issues an ENQ RESOURCE command with the same data-area parameter is suspended until the task issues a matching DEQ RESOURCE(data-area) command, or until the unit of work ends. |
The text, which I think worth mentioning here, is
| Quote: |
| matching DEQ RESOURCE(data-area) command |
The question is: How are we matching the commands?
I believe, it is by mentioning Resource Name + Length in the ENQ/DEQ commands. And, what I believe you are saying is that, it is Resource Name + Length + TaskID. Most probably you are correct, I am only looking for a written proof of it.
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Rohit Umarjikar
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| Quote: |
| until the task issues a matching DEQ RESOURCE(data-area) command, or until the unit of work ends. |
This is indeed in the same TASK context.
Because when Enq is issued in a task CWA is protected until it finds matching Deq issued in the same task(as quoted above though it is not written as you expect) or ends the task/transaction so during this process any other task (say task2) or a transaction issues
a.Enq then it will be suspended
b.Issues Deq it will be ignored(as there is no matching Enq by that
task2 is found prior).
It is all worth learning..  |
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PeterHolland
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Nic Clouston
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| Quote: |
| After a task has issued an ENQ RESOURCE(data-area) command, any other task that issues an ENQ RESOURCE command with the same data-area parameter is suspended until the task issues a matching DEQ RESOURCE(data-area) command, or until the unit of work ends. |
TASK in the bold text in the above quote refers to the first task - not the second. |
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RahulG31
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| Thanks Rohit. You've been real help. |
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Rohit Umarjikar
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No Problem, Welcome!
This post on ENQ/DEQ would be a comprehensive ref. for others. |
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