take vs. takeIfExists

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On 18 Nov 2008, at 08:24, Asaf Lahav wrote:
> What is the difference between using take vs. takeIfExists in the  
> following cases:
>
> entry = space.takeIfExists(tmpl, null, 0);
> vs.
> entry = space.take (tmpl, null, 0);

I'd be interested to know the answer too.

My understanding is that it makes no difference unless you're in a  
transaction (which of course you're not).  From p. 245 of JavaSpaces  
Principles, Patterns  and Practice I see: "In effect, a readIfExists  
is able to 'peak inside' transactions: if a matching entry exists  
within a transaction, then readIfExists will wait up until its timeout  
value for a transaction to complete before it returns".  (surely take  
and read will behave the same way in this situation).

But I do wonder if there are some edge cases for an if exists query,  
so it'd be good to know.

Richard

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Asaf Lahav wrote: In this case there is no difference. The difference between a std. query
and an ifExists is how they deal with non-zero timeouts.

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Richard Dallaway wrote: It doesn't matter if the query is under a transaction or not - it
matters if the space is a participant in any active transactions.

All queries on a space will return without blocking if there is an
available (available == not locked by an active transaction) entry that
matches.

If there is no available matching entry then read and take will block
for up to their timeout, or until a matching entry becomes available,
which ever comes first.

The ifExists queries will always return immediately (even if the timeout
is non-zero)  if there are no matching entires in the space they will
only block if there is an entry in the space that is not available -
that is the entry is locked by a conflicting transaction - in such cases
they will wait for up to the timeout for a matching entry to become
available or until there are no matching entries in the space.

read and talk obey the ACID properties, the ifExists queries break
isolation.

Does that help?

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On 18 Nov 2008, at 14:35, John McClain wrote:
> Richard Dallaway wrote:
>> [...] My understanding is that it makes no difference unless you're  
>> in a transaction [...]
> It doesn't matter if the query is under a transaction or not - it  
> matters if the space is a participant in any active transactions.

OK, that makes sense. Thank you.

> [...]
>
> Does that help?

Absolutely. Thanks for taking the time to explain the process so  
clearly.

Richard

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