OrientDB uses clusters to store links to the data. A cluster is a generic way to group records. It is a concept that does not exist in the Relational world, so it is something that readers from the relational world should pay particular attention to.
You can use a cluster to group all the records of a certain type, or by a specific value. Here are some examples of how clusters may be used:
If you have a background from the RDBMS world, you may benefit to think of a cluster as a table (at least in the beginning). OrientDB uses a cluster per "class" by default, so the similarities may be striking at first. However, as you get more advanced, we strongly recommend that you spend some time understanding clustering and how it differs from RDBMS tables.
A cluster can be local (physical) or in-memory.
Note: If you used an earlier version of OrientDB. The concept of "Logical Clusters" are not supported after the introduction of version 1.0.
Also called Physical cluster, it stores data on disk.
The information stored in "In-Member clusters" is volatile (that is, it is never stored to disk). Use this cluster only to work with temporary data. If you need an In-Memory database, create it as an In-memory Database. In-memory databases have only In-memory clusters.