I believe that the B+Tree tuple cursor is unsafe under concurrent read --
that is, updates to its internal state are not thread safe.
Why would you use more than one recman instance for a given store in the
same JVM?
-bryan
_____
From: Chas Emerick [mailto:[email protected]]
Sent: Thursday, October 22, 2009 10:45 AM
To: Alex Boisvert
Cc: JDBM General listserv
Subject: Re: [Jdbm-general] Read concurrency clarification
Right, which happens to be my situation: many concurrent reads from
different threads, and when a write needs to be done, we lock globally.
Thanks for the (extended) clarification :-)
- Chas
On Oct 22, 2009, at 10:41 AM, Alex Boisvert wrote:
No, that's not safe at all.... unless all the access is read-only.
alex
On Thu, Oct 22, 2009 at 7:36 AM, Chas Emerick <[email protected]> wrote:
Alex,
Thank you for the clarification. Just to make sure we're understanding each
other, I'm actually asking about multiple readers, each opening and using
their own RecordManagers. Your mention of synchronization leads me to think
you're talking about multiple threads reading from a single RecordManager.
Just FYI, I'm being careful here because I came across a mention of RM
changing files on disk when *reading*, which surprised me (can't find the
link right now)...
Thanks again,
- Chas
On Oct 22, 2009, at 10:27 AM, Alex Boisvert wrote:
Individual concurrent reads are safe. Pretty much everything is
synchronized at the top of level.
However, iterators / tuple browsers are unsafe under concurrent updates.
You would have to synchronize on the RecordManager instance to maintain a
consistent view while iterating.
alex
On Thu, Oct 22, 2009 at 5:10 AM, Chas Emerick <[email protected]> wrote:
I've been plumbing around the list archives for a while, but they're
something of a jumble (thanks, Sourceforge!), so I figured I'd open a
new thread to ask:
What level of read concurrency does jdbm support?
What seems clear is that, to be safe, there should only be one thread
writing to a jdbm database (although it appears that it is possible
for multiple threads to participate in a transaction, I'm perfectly
fine with serializing all write access). However, I came across at
least a number of comments that lead me to believe that concurrent
reads are unsafe as well, although they're all from pre-history (e.g.
2001).
Thanks,
- Chas
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