fhanik 2003/12/17 13:00:40
Modified: webapps/docs cluster-howto.xml
Log:
added a little note about sync vs async replication. Sync replication is truly
working on redhat,
somehow windows is giving me problems with NIO when writing the ACK back to the
channel
Revision Changes Path
1.2 +42 -29 jakarta-tomcat-catalina/webapps/docs/cluster-howto.xml
Index: cluster-howto.xml
===================================================================
RCS file: /home/cvs/jakarta-tomcat-catalina/webapps/docs/cluster-howto.xml,v
retrieving revision 1.1
retrieving revision 1.2
diff -u -r1.1 -r1.2
--- cluster-howto.xml 15 Oct 2003 01:57:59 -0000 1.1
+++ cluster-howto.xml 17 Dec 2003 21:00:40 -0000 1.2
@@ -4,7 +4,7 @@
]>
<document url="cluster-howto.html">
- &project;
+ &project;
<properties>
<author email="[EMAIL PROTECTED]">Filip Hanik</author>
@@ -24,7 +24,7 @@
<li>Uncomment the <code>Valve(ReplicationValve)</code> element in server.xml</li>
<li>If your Tomcat instances are running on the same machine, make sure the
<code>tcpListenPort</code>
attribute is unique for each instance.</li>
-<li>Make sure your <code>web.xml</code> has the <code><distributable/></code>
element</li>
+<li>Make sure your <code>web.xml</code> has the <code><distributable/></code>
element</li>
</ul>
<p>Then all you have to do is start Tomcat, I suggest using <a
href="http://balance.sourceforge.net";>Balance</a>
as a load balancer. Apache/mod_jk will also do or hardware of course</p>
@@ -43,8 +43,8 @@
</ol>
<p>In this release of session replication, Tomcat performs an all-to-all
replication of session state.
-
- This is an algorithm that is only efficient when the clusters are small. For
large clusters, the next
+
+ This is an algorithm that is only efficient when the clusters are small. For
large clusters, the next
release will support a primary-secondary session replication where the session
will only be stored at one
or maybe two backup servers.
In order to keep the network traffic down in an all-to-all environment, you can
split your cluster
@@ -63,7 +63,7 @@
</source>
<p>What is important to mention here, is that session replication is only the
beginning of clustering.
- Another popular concept used to implement clusters is farming, ie, you deploy
your apps only to one
+ Another popular concept used to implement clusters is farming, ie, you deploy
your apps only to one
server, and the cluster will distribute the deployments across the entire
cluster.
This is all capabilities that can go into the next release.</p>
<p>In the next section will go deeper into how session replication works and how to
configure it.</p>
@@ -74,7 +74,7 @@
<p>To make it easy to understand how clustering works, I'm gonna take you through a
series of scenarios.
In the scenario I only plan to use two tomcat instances <code>TomcatA</code> and
<code>TomcatB</code>.
We will cover the following sequence of events:</p>
-
+
<ol>
<li><code>TomcatA</code> starts up</li>
<li><code>TomcatB</code> starts up</li>
@@ -105,12 +105,12 @@
<p>
When TomcatB starts up, it follows the same sequence as TomcatA did with
one exception.
The cluster is started and will establish a membership (TomcatA,TomcatB).
- TomcatB will now request the session state from a server that already
exists in the cluster,
- in this case TomcatA. TomcatA responds to the request, and before TomcatB
starts listening
+ TomcatB will now request the session state from a server that already
exists in the cluster,
+ in this case TomcatA. TomcatA responds to the request, and before TomcatB
starts listening
for HTTP requests, the state has been transferred from TomcatA to TomcatB.
- In case TomcatA doesn't respond, TomcatB will time out after 60 seconds,
and issue a log
- entry. The session state gets transferred for each web application that has
distributable in
- its web.xml. Note: To use session replication efficiently, all your tomcat
instances should be
+ In case TomcatA doesn't respond, TomcatB will time out after 60 seconds,
and issue a log
+ entry. The session state gets transferred for each web application that has
distributable in
+ its web.xml. Note: To use session replication efficiently, all your tomcat
instances should be
configured the same.
</p><p></p>
</li>
@@ -124,17 +124,17 @@
the request returns to the user, back through the valve pipeline.
For each request the entire session is replicated, this allows code that
modifies attributes
in the session without calling setAttribute or removeAttribute to be
replicated.
- a useDirtyFlag configuration parameter can be used to optimize the number
of times
- a session is replicated.
+ a useDirtyFlag configuration parameter can be used to optimize the number
of times
+ a session is replicated.
</p><p></p>
</li>
<li><b><code>TomcatA</code> crashes</b>
<p>
- When TomcatA crashes, TomcatB receives a notification that TomcatA has
dropped out
+ When TomcatA crashes, TomcatB receives a notification that TomcatA has
dropped out
of the cluster. TomcatB removes TomcatA from its membership list, and
TomcatA will no longer
be notified of any changes that occurs in TomcatB.
- The load balancer will redirect the requests from TomcatA to TomcatB and
all the sessions
+ The load balancer will redirect the requests from TomcatA to TomcatB and
all the sessions
are current.
</p><p></p>
</li>
@@ -164,9 +164,9 @@
</li>
<li><code>TomcatA</code> The session <code>S2</code> expires due to inactivity.
- <p>The invalidate is call is intercepted the same was as when a session is
invalidated by the user,
+ <p>The invalidate is call is intercepted the same was as when a session is
invalidated by the user,
and the session is queued with invalidated sessions.
- At this point, the invalidet session will not be replicated across until
+ At this point, the invalidet session will not be replicated across until
another request comes through the system and checks the invalid queue.
</p><p></p>
</li>
@@ -177,9 +177,9 @@
<p><b>Membership</b>
Clustering membership is established using very simple multicast pings.
Each Tomcat instance will periodically send out a multicast ping,
- in the ping message the instance will broad cast its IP and TCP listen port
+ in the ping message the instance will broad cast its IP and TCP listen port
for replication.
- If an instance has not received such a ping within a given timeframe, the
+ If an instance has not received such a ping within a given timeframe, the
member is considered dead. Very simple, and very effective!
Of course, you need to enable multicasting on your system.
</p>
@@ -189,13 +189,13 @@
Upon the next replication request, the sending instance will use the host and
port info and establish a TCP socket. Using this socket it sends over the
serialized data.
The reason I choose TCP sockets is because it has built in flow control and
guaranteed delivery.
- So I know, when I send some data, it will make it there :)
+ So I know, when I send some data, it will make it there :)
</p>
<p><b>Distributed locking and pages using frames</b>
Tomcat does not keep session instances in sync across the cluster.
- The implementation of such logic would be to much overhead and cause all
- kinds of problems. If your client accesses the same session
+ The implementation of such logic would be to much overhead and cause all
+ kinds of problems. If your client accesses the same session
simultanously using multiple requests, then the last request
will override the other sessions in the cluster.
</p>
@@ -208,9 +208,9 @@
<source>
Server
|
- Service
- |
- Engine
+ Service
+ |
+ Engine
/ \
Cluster ReplicationValve
|
@@ -233,14 +233,27 @@
<p>
The membership is established by all the tomcat instances are sending broadcast
messages
on the same multicast IP and port.
-
+
The TCP listen port, is the port where the session replication is received from
other members.
</p>
<p>
- The replication valve is used to find out when the request has been completed
and initiate the
+ The replication valve is used to find out when the request has been completed
and initiate the
replication.
</p>
-
+<p>
+ One of the most important performance considerations is the synchronous versus
asynchronous replication
+ mode. In a synchronous replication mode the request doesn't return until the
replicated session has been
+ sent over the wire and reinstantiated on all the other cluster nodes.
+ Using synchronous mode potentially becomes a bottleneck, but is a must if you
cant have
+ sticky sessions, what is recommended here is to increase the number of threads
that handle
+ incoming replication request. This is the tcpThreadCount property in the cluster
+ section of server.xml.
+ Asynchronous replication, should be used if you have sticky sessions until fail
over, then
+ your replicated data is not time crucial, but the request time is, at this time
leave the tcpThreadCount to
+ be number-of-nodes-1.
+ During async replication, the request is returned before the data has been
replicated. async replication yields shorter
+ request times, and synchronous replication guarantees the session to be
replicated before the request returns.
+</p>
</section>
@@ -251,7 +264,7 @@
A: Well, the other members will remove the instance from the cluster,
but when you insert the cable again, the Tomcat instance might have
completely flipped out.
- This is because the OS might start going 100% CPU when a multicast message
is sent.
+ This is because the OS might start going 100% CPU when a multicast message
is sent.
There has not yet been a good solution for this, I will let you know when I
have come up with one.
</li>
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