On 8/19/11 8:51 AM, Stefan Seelmann wrote:
On Thu, Aug 18, 2011 at 10:41 PM, Selcuk AYA<[email protected]> wrote:
Hi,
Today we had some discussion with Alex, Emmanuel and others on how we
can improve jdbm consistency semantics. I had spent sometime looking
into this issue and thought it could be useful to put a summary of my
findings here.
Currently, jdbm has issues with both concurrency and consistency:
1) jdbm table lookups, insert and remove interfaces are synchronized
methods. So even if all the directory server does is to lookups on
tables, all lookups will be serialized. Moreover, the record manager
operations are all synchronized methods too. This means, for example,
while sync of dirty pages to disk goes on, no lookup operation can go
ahead.
2) jdbm browser interface does not provide any consistency guarantees.
If there are underlying changes to the store while the browser is
open, then it might return inconsistent results. I think the situation
is even worse if the underlying record manager is CacheRecordManager
as the same page could be modified and read by a browser concurrently.
I have been working on a scheme which introduces what can be defined
as action consistency into the jdbm store.
1) Actions are lookup, insert, remove and browse. Each action is
assigned a unique version. Actions are ReadWrite or ReadOnly.
2) We allow one ReadWrite action and multiple ReadOnly actions to run
concurrently.So synchronized methods will be removed.
3)We introduce a new record manager which caches jdbm B+ pages. Each
page in the cache has a [startVersion, endVersion). When an action
with version V1 wants to read a page, its read can be satisfied
satisfied from that page's version with V1>= startVersion&& V1<
endVersion.
4) Pages' previous versions are kept in memory. A page can be purged
when the minimum version among all active actions is>= endVersion.
So say we have three pages in a chain (A0->B0->C0) and each of them
has version range [0, infinity). An write action starts and gets the
version number 1. It updates B0 and C0 to B1 and C1 in any order.
After these two updates, B0 and C0 will have version range [0,1) and
and B1 and C1 will have version range [1,infinity). Before the write
action completes, a read action comes, gets the current read version
which is 0 and reads the chain. Since B0 and C0 will be the versions
that can satisfy this read, the read only action will read the chain
A0->B0->C0. When write action completes, it posts version 1 as the new
read version. First read action completes, a second one starts with
version 1 and that one will read A0->B1->C1. Since the minimum read
version is now 1, B0 and C0 can be zapped.
Here I have a question: How can we detect that the read is finished?
In the current JDBM implementation the "browse" action can take
forever, there is no way to tell JDBM that browse is finished (i.e. a
close() method).
First, browse will last at some point. The most we can do is to read
*all* the entries from the master table using an index, but once it's
done, the browse will stop. I wondered yesterday if a persistent search
could change anything but no : the way it's handled is very different,
we just register some listeners in the EventInterceptor, and every
modification will trigger one listener. This is not a browse by all mean.
Now, I guess we will have to store the used revision somewhere (like in
the searchOperationContext), and when we don't have anymore element to
send back to the user, then we can 'close' the browse, releasing the
revision.
At least, this is how I foresee the solution...
--
Regards,
Cordialement,
Emmanuel Lécharny
www.iktek.com
