[Mondrian] Multithreading etc

Julian Hyde julianhyde at speakeasy.net
Fri Mar 16 13:03:50 EDT 2007

If changes to mondrian are breaking your application, I sympathize. How
can we prevent that from happening? Unless we restrict ourselves to
trivial enhancements, the we obviously need to test the new
functionality against existing apps, or at least tests which exercise
existing apps' requirements.
Ideally, these tests would be in the standard regression suite. But
since some tests are too complicated to be in the standard suite, to
testing which cannot be done nightly at least has to be done once per
We already have a process in place for much of this. For example:


	Developers ensure that code changes don't break the regression

	I run the regression suite nightly, in a wide set of
configurations, and let the developers know next day if things break.

	All code changes - bug fixes, enhancements and ad hoc feature -
must be accompanied by a regression test which exercises the change.
(That means it should fail if the change is not present.)

The extra things we need to do:


	If you are using mondrian in your apps, contribute test suites
which exercise your app's functionality. LucidEra have already done
thist (ClearTestSuite) and Thomson-Medstat are working on it. Sure it's
a lot of work, but it's less work than taking a new release where things
have stopped working. It's an insurance policy.

	Would it help if we invented the notion of 'QA partner'
companies? We could run through the test suite of each QA partner before
each release, as a pre-condition for making that release.

	Write better tests for new features. Complex features require
complex tests. 

	Set up continuous integration (E.g. CruiseControl) to ensure
that the code always builds and compiles. Thiyagu is already working on

Bart, On point #3: The feature you are interested in, for working on top
of dynamic databases, is VERY complex to test. We have spent a lot of
time discussing how to implement this feature, but very little time
designing a testing infrastructure. It should be little surprise that
the feature is fragile at this point.


From: mondrian-bounces at pentaho.org [mailto:mondrian-bounces at pentaho.org]
On Behalf Of Pappyn Bart
Sent: Tuesday, March 13, 2007 1:34 AM
To: Mondrian developer mailing list
Subject: RE: [Mondrian] Multithreading etc

Hi Michael,


I don't see any problems for you to make changes to mondrian.  But I
have some concerns:


I think the changes you are about to make are quite huge and will have
an impact of how mondrian will behave.  Since this is the first source
contribution you are about to make, I urge you not to check anything
into perforce before it is actually working and passing all regression


I think most developers of mondrian have ongoing projects that are using
mondrian, I think this is

becoming more and more an important issue.


For me: it must be able to flush aggregates and member cache using the
plug-in and cubes not maintaining cache should be able to load their own
data, without messing with global cache.


And since a dynamic database cannot easily be simulated in a regression
test, I think if you are serious about tackling the read-consistency for
near-real-time data, you need a realistic (dynamic) database to test
against.  And the database must be large enough to be able to see
realistic performance.  It is also advised to test with virtual cubes,
cubes maintaining cache (with aggregate tables) in combination with
cubes not maintaining cache (without aggregate tables), shared
dimensions and so on


I released my project 4 weeks ago, not even using the latest version of
perforce, since at a given point mondrian-head was completely broken for
me.  While I know software always has some bugs that need to be patched,
things that are not tested and are breaking mondrian should not be
checked in.  All too often I had to sync with perforce to solve a bug
and this ended up in a nightmare, spending most of my time finding out
what change was causing mondrian to break.


When mondrian 2.3 will be released, it is most likely that there will be
some 2.3.x version containing some patches.  I think it must be possible
to make those patches without having to drag new huge features along.




From: mondrian-bounces at pentaho.org [mailto:mondrian-bounces at pentaho.org]
On Behalf Of michael bienstein
Sent: vrijdag 9 maart 2007 11:36
To: Mondrian developer mailing list
Subject: [Mondrian] Multithreading etc

I sent this to the list but it gets bounced because I attached the code
in a zip file.  How do I send code through without checking it in
because it is still orthogonal to the codebase?


Well, I have code that works for multi-threading infrastructure so I
would like to know if it is worth continuing with this or not.

As for ROLLUP/CUBE my thoughts are:
1) Either we keep the codebase simple by sticking to a standard
(SQL2003) even if this standard is not yet implemented widely and
certain databases have better special features than others, or we allow
a per-database SQL generation system.  The argument for the second makes
sense only if the developer resources to write and maintain each dialect
comes from the database vendor or their community.  Mondrian is probably
at a stage that such discussions can be undertaken with the database
2) Architecturally this implies loading multiple Aggregations from one
SQL query.  That requires a rethink of the way the cell cache loading is
done because at the moment an Aggregation is loaded one at a time and in
a synchronized block on the Aggregation.  Similar concerns have to be
dealt with for in-memory rollups.  I think that synchronized is too
forceful.  We need something more like a Lock from java.util.concurrent
so we can do tryLock().  Look at the TxLock idea I have in the code I'm

As for multi-threading:
I have only written most of the base infrastructure, not the cell
loading.  To integrate would require a significant amount of work in
Mondrian's code to pass all interaction with Mondrian through

Basic concerns are:

1)      Threads should be able to share data related to the request
across the threads.
2)      A Thread should be loaned to a request and returned in a way
that is well-nigh fail-safe (i.e. the thread shouldn’t keep running of
the request fails in some way).
3)      We should be able in a parameter of some sort decide to NOT use
threads at all.
4)      The number of threads should be configurable.
5)      There should be an independence from the rest of the code base.
6)      We should be able to make use of custom thread pools or use
managed thread pools from the application server.
7)      Then there is a relatively minor issue with read-consistency for
near-real-time data that turns out to be a real head-ache.  This can be
done by either: using the transaction semantics of the underlying data
store or modifying all SQL requests and cache interactions with a
timestamp and/or transaction id of some sort.  E.g. when an MDX requests
begins it asks the underlying data store for the id of the last
completed transaction that modified data and keeps this in a
request-scope available to all threads.  Then it appends “changedTxId <=
${lastTxIdWhenFirstEntered}” to each WHERE clause.  If however we use
the underlying data store’s transactions then we must keep open the JDBC
Connection for the duration of the request reusing it on the same thread
for each interaction with that data store.
Now, I think that the best way to take advantage of multiple threads in
the storage system is NOT launching multiple SQLs on the same star
schema but different aggregations but rather to use partitioning of
data.  That is to segment the cell data (and maybe dimension data) based
on values of certain columns.  For example year<2007 and year=2007 in
two different partitions.  This can be introduced slowly by simply
making a RolapStar one Partition for the moment.  Having said that
aggregation tables are also a type of Partition and hitting two of them
at once should be quite easy.
So the design I am introducing has the following features:
1) A scope for "request" or "interaction" that is larger than the Thread
that begins it.  Since this is similar to a transaction I've called it a
Tx.  See the mondrian.tx package.  Each sub-system in Mondrian can
enlist a representation of itself in the Tx.
2) Break up the different tasks performed into Task objects that can be
run potentially in parallel.  Allow a set of Tasks to be tied to the
same Thread so that the same JDBC Connection can be used for all of them
for read-consistency and cleaned up at the end of the Tx.  This is done
declaratively so the implementation can be changed easily.  The
implementation can also ensure that the J2EE context is passed onto
separate threads (JNDI, context class loader etc).
3) A system of fail-quick locks at the Tx scope rather than just Thread

If this is worth persuing as a design for the next version then good.
If not I'll stop now.



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