Re: Outrigger and blitz -- lingering take?

Hi Patrick,

Greg T has basically hit the nail on the head.

When your client lodges a take with the space and blocks, a take request
is put on a "noticeboard" within the space that someone is looking for a
certain kind of entry.

If the client dies, although the connection will eventually be ripped
down (hopefully) the space impl won't necessarily hear of this.  Thus
the request is still on the "noticeboard".

Now, when a matching write comes along the "noticeboard" is checked, a
match found and an attempt is made to return it to the client which is
no longer present.  This failure tends to happen silently and thus the
space can't be smart enough to reinsert the taken entry for you.  In the
meantime, the "noticeboard" is now clear and normal behaviour resumes.

Several things to note:

(1)     If the newly written entry turns up after the expiry time for the
blocking take (assuming you didn't do Lease.FOREVER), the above won't
happen.

(2)     You can ensure this never happens by having clients wrap operations
in explicit Transactions.  Transactions are leased and can thus be
automatically aborted in face of client failure.  And when the
transaction is aborted, the space will be compelled to "reverse" the
fault take and put the newly written entry back into the available entry
population.

(3)     There are other ways to handle this issue using concepts from
various of the network protocols out there like re-posting entry's after
a period due to no ack from a client (which has died) etc.

Hope that helps,

Dan.

Logan, Patrick wrote:
> I am seeing what apears to be some sort of "lingering take" on a
> javaspace. This happens in Outrigger as well as Blitz.
>
> What I am observing… a worker process is blocked on a take with a null
> transaction. I kill the process.
>
> Using the Blitz dashboard I can see after the kill, 1 blocked take. (I'm
> not up on similar tools for Outrigger but all the other observations are
> the same when I use that.)
>
> Then I start another process running the same worker, and after doing
> some other spaces stuff, it soon blocks on a take using the same template.
>
> When another process writes an entry that matches that template, it
> appears this "lingering taker" gets the entry.
>
> I am not sure what the explanation is for this "lingering take". Running
> ps indicates the process is really gone.
>
> When I run this with a non-null transaction everything seems fine. I
> have no trouble using transactions to make sure cleanup happens, but I
> am a little surprise that the null transaction case with both space
> implementations do not get cleaned up when a process blocked on a take
> gets killed.
>
> Thanks
> -Patrick
>
>
> Patrick Logan
> Premium Innovation
> Liberty Mutual - Agency Markets IT West
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