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From Riaan <riaan.potgie...@corefreight.com>
Subject Cache Hit-Rate Statistics
Date Thu, 07 Jan 2010 13:11:50 GMT

Hi,

I am new to Apache OpenJPA, but not new to the concept of persistence layers
in Java.
(I have played a little bit with Hibernate and TopLink in the past,
...without paying any attention to their cache schemes at the time.)

After beginning to understand the value of having an *extended* data cache
and query cache, I was drawn to Apache OpenJPA, because of the DataCache and
QueryCache features built into it.

I am busy writing my first proof-of-concept application using OpenJPA, and I
have written some Java code to interrogate the DataCache and QueryCache
instances at run-time. I find this very interesting, and the goal is to
gather *Cache Hit-Rate Statistics* from a running application.

What I would like to see, are the following four metrics:

  Data Cache Hit Count:
  Data Cache Miss Count:
  Query Cache Hit Count:
  Query Cache Miss Count:

In order to obtain values for the above four metrics, I downloaded the
OpenJPA 1.2.1 source code, and hacked in counters, and modified the source
code to update (increment) the counters when necessary.

This worked for me, and it gave me the information that I was looking for,
but my solution is (obviously) not ideal, because of the following reasons:

a.) Any newer version of OpenJPA will not contain my hacks, and I would have
to modify the source code again.

b.) Because I don't fully understand the OpenJPA source code, my
modification may not be optimal. (This is an understatement.) Although it
works, and it gives me the answers I was looking for, I really don't feel
that confident about my source code changes.

c.) I suspect my changes may not even work properly the moment there are
more than one EntityManager or Persistence Unit in the same JVM. {I say
this, because my counters are defined as static attributes on some of the
OpenJPA classes. This is bad, I know.}

Anyway, here are my questions:

1.) Is there currently a proper and correct way of obtaining values for the
above four cache performance metrics, that I am interested in, in order to
monitor a running application?

2.) If there is currently no standard way in OpenJPA to get values for these
cache performance metrics, what is the chance that the OpenJPA developers
could add these in, so that these metrics can be used by all OpenJPA users
in the future?

3.) I guess what I am asking for, is probably not part of the Java EE
Persistence API specifications. What I am suggesting, is additional features
for Apache OpenJPA, ...if it is not already there. How does other people
feel about adding these features to the OpenJPA cache classes? I would like
to hear some thoughts on this from other users as well.


Just a few comments from my side:

A.] Database roundtrips are expensive, and it is a great win whenever a
database roundtrip can be avoided. (This is obviously a great benefit of
having data cache and query cache providers.)

B.] Because cache-hits are so precious, it makes good sense to monitor
cache-hits and cache-misses in a running application. The application
developer could write functions to extract these performance metrics on
demand. 

C.] I would love to see "Cache Performance Metrics" (as listed above) as a
standard feature of OpenJPA, to be used by all OpenJPA users.

D.] I plan to use the persistence context in the *Extended* mode, as opposed
to "per transaction" or "per session". In other words, for my application, I
want the data cache and the query cache to be available throughout the full
life cycle of the application back-end. This way, the data cache and the
query cache can be *shared* by *all* the application users. This will
hopefully save a lot of database roundtrips.

E.] Apache OpenJPA looks like a high-quality, feature-rich, open source
product. My thanks go out to the OpenJPA developers!


Regards
Riaan
South Africa
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