Segment cache architecture

Each Mondrian segment cache node, regardless of which configuration it uses, loads the segments required to answer a given query into system memory. This cache space is called the query cache, and it is composed of hard Java references to the segment objects. Each individual node must have enough memory space available to answer any given query. This might seem like a big limitation, but Mondrian uses deeply optimized data structures which usually take no more than a few megabytes, even for queries returning thousands of rows.

Once the query finishes, Mondrian will usually try to keep the data locally, using a weak reference to the segment data object. A weak reference is a special type of Java object reference which doesn't force the JVM to keep this object in memory. As the Mondrian node keeps answering queries, the JVM might decide to free up that space for something more important, like answering a particularly big query. This cache is referred to as the local cache.

The local cache can be switched on or off by editing the Pentaho Analysis EE configuration file and modifying the value (set it to true or false) of the DISABLE_LOCAL_SEGMENT_CACHE property. Setting this property will not affect the query cache.

Note: The segment cache features explained in this section are for very large OLAP deployments.