d8888 888 888      88888888888 888      d8b                                 888       888          888       .d8888b.           888                               
      d88888 888 888          888     888      Y8P                                 888   o   888          888      d88P  Y88b          888                               
     d88P888 888 888          888     888                                          888  d8b  888          888      Y88b.               888                               
    d88P 888 888 888          888     88888b.  888 88888b.   .d88b.  .d8888b       888 d888b 888  .d88b.  88888b.   "Y888b.   88888b.  88888b.   .d88b.  888d888 .d88b.  
   d88P  888 888 888          888     888 "88b 888 888 "88b d88P"88b 88K           888d88888b888 d8P  Y8b 888 "88b     "Y88b. 888 "88b 888 "88b d8P  Y8b 888P"  d8P  Y8b 
  d88P   888 888 888          888     888  888 888 888  888 888  888 "Y8888b.      88888P Y88888 88888888 888  888       "888 888  888 888  888 88888888 888    88888888 
 d8888888888 888 888          888     888  888 888 888  888 Y88b 888      X88      8888P   Y8888 Y8b.     888 d88P Y88b  d88P 888 d88P 888  888 Y8b.     888    Y8b.     
d88P     888 888 888          888     888  888 888 888  888  "Y88888  88888P'      888P     Y888  "Y8888  88888P"   "Y8888P"  88888P"  888  888  "Y8888  888     "Y8888  
                                                                 888                                                          888                                        
                                                            Y8b d88P                                                          888                                        
                                                             "Y88P"                                                           888   

All Things WebSphere

Concerns and issues relating to all versions of WebSphere Application Server

Wednesday, February 2, 2011

 

Should I use a database or memory-to-memory replication for session failover in WebSphere Application Server

credit Tom Alcott 



A: Performance does not differ significantly between database persistence and memory-to-memory replication. This is because 95% of the cost of replicating or persisting sessions is incurred in the serialization/deserialization of the session object -- which must occur regardless of how the session is distributed. Also, as the size of the session object increases, performance degrades -- again, about equally for both session distribution options.

Instead, the decision will be based partially on how the two technologies differ: With a database, you actually persist the data (to disk), so a highly available database server can survive a cascading failure, while using application servers as session stores and replicators for this purpose may not. In the case of a "gold standard" (two identical cells/domains), a highly available database can pretty much assure session failover between domains, while with memory to memory, there can only be a single replicator common to the two cells; hence, it becomes a single point of failure (SPOF).

Thus, for configurations where cross-cell session failover is a requirement, a highly available database is the only option for eliminating a SPOF. Note that while sharing sessions across cells is supported, this is not generally recommended. By sharing state between cells, it makes it significantly more difficult to independently upgrade components (application and WAS) in the two cells. In the end, the decision then becomes based on what technology you are most comfortable with and which delivers the required quality of service for your availability requirements.

With memory-to-memory replication, the amount of session information you can store is bounded by the JVM heap size of your application server(s). Even with the advent of 64-bit JVM support in WebSphere Application Server V6.01, the maximum application server heap size is going to be significantly smaller than the amount of disk space you have available on a database server that is serving as a session store. Therefore, I am still of the opinion that database persistence remains the best option, although I know that in many organizations it is more expedient to use memory-to-memory replication to avoid conflicts over roles and responsibilities between system and database administrators.

A common cause of serious problems is the use/abuse of the HTTP session for all kinds of garbage. It all starts when one programmer decides to throw a little something in there, then another, and another, until your poor session looks like one of those polluted ponds filled with old tires and washing machines.
Stuffing something into the session is a very common shortcut taken under duress of an impending deadline. The effect on JVM heap sizes and performance (particularly when using persistent sessions) is dramatic. Doing code reviews (see #3) can help to prevent this. Session sizes should be from 2-4K, and there are several ways to measure them; the TivoliĀ® Performance Monitor that comes with WebSphere Application Server, for example, will show you session sizes.

Also read
1. Comment lines: Erik Burckart: What you want to know about HTTP session persistence     
2. Chapter 12 of WebSphere Application Server V7 Administration and Configuration Guide
   

Comments:

Post a Comment

Subscribe to Post Comments [Atom]



Links to this post:

Create a Link



<< Home

Archives

December 2006   September 2008   January 2009   February 2009   March 2009   September 2009   October 2009   November 2009   December 2009   January 2010   February 2010   March 2010   April 2010   October 2010   January 2011   February 2011   April 2011   May 2011   June 2011   July 2011   August 2011   September 2011   October 2011   November 2011   December 2011   January 2012   February 2012   March 2012   April 2012   May 2012   June 2012   July 2012   August 2012   September 2012   October 2012   November 2012   January 2013   May 2013   June 2013   July 2013   September 2013   October 2013   June 2014   August 2014  

This page is powered by Blogger. Isn't yours?

Subscribe to Posts [Atom]