Brian Aker wrote:
Hi!
On Oct 12, 2008, at 3:01 PM, Roy Lyseng wrote:
Yes, but I guess there is usually a 1-1 connection between thread and
session. Thus, most operations on the session can be done without
synchronization. With more threads, this will change.
Nope, not in our world. There are many, and I mean many, groups who have
1K+ open sessions at a time.
You are misunderstanding me: What I mean is that when you have multiple
threads running on behalf of a session, such as when you have multiple
concurrent queries/statements running, then you need to synchronize more
heavily on the session level.
Say that one of the statements forces a transaction abort, then all
statements will need to abort. If one statement errs, then you may or
may not (depending on the strategy you choose) force all other
concurrent statements to roll back locally.
There are probably many such issues which need to be sorted out, and
they probably need some sort of synchronization and state management on
the session level.
I am not saying that decoupling Session and Thread is a bad thing, I am
actually backing that decision. Having one or several threads executing
on behalf of a session gives great opportunities for scaling, but as
always there is no free lunch...
We actually decoupled sessions and threads in the HADB product. The
reasoning behind that was that a thread might be assigned for the
duration of a server roundtrip, and the thread could also carry some
resources that were useful for the roundtrip duration, such as a
temporary heap allocator (used e.g. for query preparation) and an
execution machinery. Another reason was to accomodate parallel query
processing, but that was never implemented.
There was also a Status object. which could be shared between session
and thread in a 1-1 relationship, or there could be one Status object
per thread and accumulation of statuses on the session level when all
threads had completed their work.
Thanks,
Roy
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