Web Application Servers

Dean Jacobs

BEA WebLogic

The Right Question

• Middleware is converging Web Application Servers = TP Monitors

• What new demands do the Internet and Java place on middleware?

Browsers

Database

Web ServersApplication Server

Cluster

System Architecture

Web Servers

• Web Servers not integrated with middleware– Adds an extra server-side hop with no benefit– Obviates the front line of session concentration

• Web Servers integrated with middleware– Standard Web Servers must be supported– Front line session handling - load balancing and

transactions - must occur in Web Server plug-ins

Relaxed Durability/Consistency Trade durability or consistency for performance:

avoid disk accesses by keeping state in memory.• Shopping Cart Stored as browser-session state

in a servlet engine. To avoid writing backups to disk, replicate in-memory on a secondary host.

• Catalog Data Changes infrequently and may be briefly out-of-date. To avoid disk reads, cache on many servers in a cluster. Update anywhere and lazily flush the caches.

Fat Processes with Threads• Radically different process model: 2-3 processes

per host, each a logical machine• Multiple resource managers per process• Multiple threads accessing each resource• Threads are not isolated from each other

Object Lifetime In-Memory

• CORBA relies on the programmer to release objects• Component models (EJB) prescribe object lifetimes• Programming language object models (RMI) require

DGC. How can that scale?