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Terracotta is Network Attached Memory (NAM) for Java VMs. It provides up to a terabyte of virtual heap for Java applications that spans hundreds of connected JVMs.

NAM is best suited for storing what they call scratch data. Scratch data is defined as object oriented data that is critical to the execution of a series of Java operations inside the JVM, but may not be critical once a business transaction is complete.

The Terracotta Architecture has three components:

1. Client Nodes - Each client node corresponds to a client node in the cluster which runs on a standard JVM
2. Server Cluster - java process that provides the clustering intelligence. The current Terracotta implementation operates in an Active/Passive mode
3. Storage used as
   • Virtual Heap storage - as objects are paged out of the client nodes, into the server, if the server heap fills up, objects are paged onto disk
   • Lock Arbiter - To ensure that there is no possibility of the classic "split-brain" problem, Terracotta relies on the disk infrastructure to provide a lock.
   • Shared Storage - to transmit the object state from the active to passive, objects are persisted to disk, which then shares the state to the passive server(s).

JVM-level clustering can turn single-node, multi-threaded apps into distributed, multi-node apps, often with no code changes. This is possible by plugging in to the Java Memory Model in order to maintain key Java semantics of pass-by-reference, thread coordination and garbage collection across the cluster. Terracotta enables this using only declarative configuration with minimal impact to existing code and provides fine-grained field-level replication which means your objects no longer need to implement Java serialization.

Ari Zilka, the founder and CTO of Terracotta had a
video session organized by Skills Matter. He will show you how it works and how you can start clustering your POJO-based Web applications (based on Spring, Struts, Wicket, RIFE, EHCache, Quartz, Lucene, DWR, Tomcat, JBoss, Jetty or Geronimo etc.).