Jules Gosnell wrote:
Jeff Genender wrote:
Now that we have achieved the covetted J2EE Certification, we need to
start thinking about some of the things we will need to have in
Geronimo in order to be mass adopted by the Enterprise.
IMHO, I think one of the huge holes is clustering. This is a heavy
need by many companies and I believe that until we get a powerful
clustering solution into G, it will not be taken as a serious J2EE
contender.
So, with that said, I wanted to start a discussion thread on
clustering and what we need to do to get this into Geronimo. I
personally would like to be involved in this (thus the reason for me
starting this thread) - yeah, since Tomcat is done, now I am bored ;-).
I was going over the lists and emails and had some great discussion
with Jules on the WADI project he has built. This seems compelling to
me. I also noticed Active Cluster as a possibility.
So lets start from the top. Do we use an already available clusering
engine or do we roll our own? Here is a small list of choices I have
reviewed and it is by no means complete...
1) WADI
2) Active Cluster
3) Leverage the Tomcat Clustering engine
So here are some of my questions...
How complete is WADI and Active Cluster? Both look interesting to me.
My only concern with Active Cluster is it seems to be JMS based, which
I think may be slow for high performance clustering (am I incorrect on
this?). How mature is WADI?
Here is a status report on WADI.
I'm developing it full time.
A snapshot is available at wadi.codehaus.org - documentation is in the
wiki - at the moment the documentation (rather minimalist) is more up to
date than the snapshot, but I will try to get a fresh one out next week.
WADI is a plugin HttpSession Manager replacement for Tomcat-5.0/5.5 and
Jetty-5.1/60 (it can actually migrate sessions between all four in the
same cluster).
It comprises a vertical stack of pluggable caches/stores (memory, local
disc, db etc) through which sessions are demoted as they age and
promoted as and when required to service a request.
Can you please clarify the purpose of promotion/demotion of
httpsessions? Is this a mechanism to age old entries out of the cache?
How does this relate to httpsession inactivity timeouts?
Is the cache size configurable?
This stack may be connected horizontally to a cluster by inserting a
clustered store, which uses a distributed hash table (currently
un-replicated, but I am working on it) to share state around the
clusters members in a scalable manner. WADI has a working mod_jk
integration.
Does this mean that each cluster member shares it's httpsession data
with all of the other members (1-> all) or is there the notion of
limiting the httpsession replication to one (or a few) designated
partners?
WADI currently sits on top of ActiveCluster, which it uses for
membership notification and ActiveMQ which is used for transport by both
layers. ActiveMQ has pluggable protocols, including a peer:// protocol
which allows peers to talk directly to one another (this should put to
bed fears of a JMS based solution not scaling - remember, JMS is just an
API). So you do not need to choose between WADI and ActiveCluster - they
are complimentary. ActiveCluster can also (I believe) use JGroups as a
transport - I haven't tried it.
ActiveSpace is another technology in this area (distributed caching) and
it looks as if WADI and ActiveSpace will become more closely aligned. So
this may also be considered a complimentary technology.
Both Tomcat and Jetty currently have existing clustering solutions. I
looked closely at the Tomcat solutions before starting out on WADI and
knew all about the Jetty solution, because I wrote it :-). WADI is my
answer to what I see as shortcomings in all of the existing open source
approaches to this problem-space.
Can you provide a quick high level description of the advantages of WADI
over Tomcat and Jetty clustering solutions?
Some parts of WADI should soon (December) be undergoing some serious
testing. When they pass we will be able to consider them production
ready. Others, notably the distributed hash table are still under
development (although a fairly functional version is available in the
SNAPSHOT).
I think that, in the same way Tomcat clustering could be enabled easily
in Geronimo, WADI could also be added by virtue of its integration with
Tomcat/Jetty, but I have been concentrating on my distributed hash table
too hard. If anyone is interested in talking further about WADI, perhaps
trying to plug it into Geronimo (It is spring-wired and uses spring to
register its components with JMX. I guess it should be simple to hook it
into the Geronimo kernel in the same way, I just haven't had the time),
or helping out in any way at all, I would be delighted to hear from them.
I have broached the subject of a common session clustering framework
with members of the OpenEJB team and we have discussed things such as
the colocation of HttpSessions and SFSBs. I believe OpenEJB has been
moving towards JCache to facilitate the plugging in of a clustering
substrate. My distributed hash table is also moving in the same direction.
So, if I understand correctly, you are working towards some common
infrastructure with openejb.. though WADI itself, will not address
clustering beyond the Web Tier?
Thanks for the update!
I hope that gives you all a little more information to go on. If you
have any questions, just fire away,
Jules
Thoughts and opinions are welcomed.
Jeff
