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From "Dyre Tjeldvoll (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3192) Cache session data in the client driver
Date Tue, 18 Dec 2007 12:19:46 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3192?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12552727
] 

Dyre Tjeldvoll commented on DERBY-3192:
---------------------------------------

@Knut1: I hope your interpretation of the DRDA spec is correct as
I would prefer not sending an empty string when there is no info.

@Knut2: The current version of the patch does not allow the
caching to be turned off, but this could certainly be added. Your
estimates of the overhead are accurate AFAICT. I did run some
tests (d2911) to see if there was a perf impact and I could not
see any degradation. I could re-run those tests with a final
version of the patch and attach them here.

@Dan: There is no writeup per se, only the following discussions
on derby-dev (part of which is included in the Description
field): 

http://www.nabble.com/Cache-session-data-in-the-client-driver--to13231212.html#a13231212

and

http://www.nabble.com/DRDA-confusion-to13495130.html#a13495130

I could certainly provide a writeup.  It didn't occur to me that one
would be needed, since the plan was to use existing mechanisms
that are currently used for similar
purposes (setQueryTimeout). In retrospect, there are some
unforeseen complications that mandate more explanation, but I'm
guessing that they would not have been part of an initial writeup
either...


> Cache session data in the client driver
> ---------------------------------------
>
>                 Key: DERBY-3192
>                 URL: https://issues.apache.org/jira/browse/DERBY-3192
>             Project: Derby
>          Issue Type: Improvement
>          Components: JDBC, Network Client, Network Server, Performance, SQL
>    Affects Versions: 10.3.1.4
>            Reporter: Dyre Tjeldvoll
>            Assignee: Dyre Tjeldvoll
>         Attachments: derby-3192-test.fup1.diff, derby-3192-test.fup2.diff, derby-3192-test.v1.diff,
derby-3192-test.v1.stat, derby-3192.prelim1.diff
>
>
> The reason for doing this is to avoid a rather
> substantial performance hit observed when the client driver is used
> together with an appserver that uses connection pooling. There are two
> problems:
> 1) The connection pool will compare the isolation level it has
> stored for the connection with the value returned from
> Connection.getTransactionIsolation() each and every time someone
> requests a new connection from the pool.
> 2) The users of the connection pool (ab)use it to avoid having to keep
> track of their current connection. So each time a query needs to be
> executed a call to the connection pool's getConnection() method is
> made. Getting a connection from the connection pool like this also
> means that a new PreparedStatement must be prepared each time.
> The net result is that each query results in the following sequence:
> getConnection()
> getTransactionIsolation() --> roundtrip + lookup in server's statement cache
> prepareStatment()         --> roundtrip + lookup in server's statement cache
> executeQuery()            --> roundtrip
> Arguably this is a "user error" but when suggesting this I'm kindly
> informed that this works "just fine" with other datbases (such as
> PostgreSQL and ORACLE). 
> The reason why it works is that these databases do statement caching
> in the driver. I've tried to implement a very (too) simple statement
> cache in Derby's client driver and to re-enable caching of the
> isolation level (see
> https://issues.apache.org/jira/browse/DERBY-1148). With these changes
> I observe a marked performance improvement when running with appserver
> load. 
> A proper statment cache cannot be implemented without knowing what the
> current schema is. If the current schema has changed since the
> statement was prepared, it is no longer valid and must be evicted from
> the cache.
> The problem with caching both the isolation level and the current schema in
> the driver is that both can change on the server without the client
> detecting it (through SQL and XA and possibly stored procedures).
> I think this problem can be overcome if we piggy-back the information we would 
> like to cache on messages going back to the client. This can be done by
> utilizing the EXCSQLSET DRDA command. According to the DRDA spec (v4, volume 3, 
> page 359-360) it is possible to add one or more SQLSTT objects after SQLCARD in the reply,
> I think it would be possible to cache additional session information when this becomes
relevant.  It
> would also be possible to use EXCSQLSET to batch session state changes
> going from the client to the server.

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