10.2. Eviction Regions

The concept of regions and the Region class were Section 7.6, “Class Loading and Regions” when talking about marshaling. Regions are also used to define the eviction behavior for nodes within that region. In addition to using a region-specific configuration, you can also configure default, cache-wide eviction behavior for nodes that do not fall into predefined regions or if you do not wish to define specific regions. It is important to note that when defining regions using the configuration XML file, all elements of the Fqn that defines the region are String objects.
For each region, you can define eviction parameters.
It's possible to define regions that overlap. In other words, one region can be defined for /a/b/c, and another defined for /a/b/c/d (which is just the d subtree of the /a/b/c sub-tree). The algorithm, in order to handle scenarios like this consistently, will always choose the first region it encounters. In this way, if the algorithm needed to decide how to handle node /a/b/c/d/e, it would start from there and work its way up the tree until it hits the first defined region - in this case /a/b/c/d.

10.2.1. Resident Nodes

Nodes marked as resident (using Node.setResident() API) will be ignored by the eviction policies both when checking whether to trigger the eviction and when proceeding with the actual eviction of nodes. E.g. if a region is configured to have a maximum of 10 nodes, resident nodes will not be counted when deciding whether to evict nodes in that region. In addition, resident nodes will not be considered for eviction when the region's eviction threshold is reached.
In order to mark a node as resident the Node.setResident() API should be used. By default, the newly created nodes are not resident. The resident attribute of a node is neither replicated, persisted nor transaction-aware.
A sample use case for resident nodes would be ensuring "path" nodes do not add "noise" to an eviction policy. E.g.,: 
...
   Map lotsOfData = generateData();
   cache.put("/a/b/c", lotsOfData);
   cache.getRoot().getChild("/a").setResident(true);
   cache.getRoot().getChild("/a/b").setResident(true);
...
In this example, the nodes /a and /a/b are paths which exist solely to support the existence of node /a/b/c and do not hold any data themselves. As such, they are good candidates for being marked as resident. This would lead to better memory management as no eviction events would be generated when accessing /a and/a/b.

Note

When adding attributes to a resident node, e.g. cache.put("/a", "k", "v") in the above example, it would make sense to mark the nodes as non-resident again and let them be considered for eviction.