We've already talked about classes. A class is a logical concept in OrientDB. Clusters are also an important concept in OrientDB. Records (or documents/vertices) are stored in clusters.
A cluster is a place where a group of records are stored. Perhaps the best equivalent in the relational world would be a Table. By default, OrientDB will create one cluster per class. All the records of a class are stored in the same cluster which has the same name as the class.
Understanding the concepts of classes and clusters allows you to take advantage of the power of clusters while designing your new database.
Even though the default strategy is that each class maps to one cluster, a class can rely on multiple clusters. You can spawn records physically in multiple places, thereby creating multiple clusters. For example:
The class "Customer" relies on 2 clusters:
The default cluster (in this case, the USA_customers cluster) is used by default when the generic class "Customer" is used. Example:
When querying the "Customer" class, all the involved clusters are scanned:
If you know the location of a customer you're looking for you can query the target cluster directly. This avoids scanning the other clusters and optimizes the query:
The benefits of using different physical places to store records are:
There are two types of clusters:
For most cases physical clusters are preferred because the database must be persistent. OrientDB creates physical clusters by default so you don't have to worry too much about it for now.
To view all clusters, from the console run the clusters
command:
orientdb> clusters
CLUSTERS:
----------------------------------------------+------+---------------------+-----------+
NAME | ID | TYPE | RECORDS |
----------------------------------------------+------+---------------------+-----------+
account | 11| PHYSICAL | 1107 |
actor | 91| PHYSICAL | 3 |
address | 19| PHYSICAL | 166 |
animal | 17| PHYSICAL | 0 |
animalrace | 16| PHYSICAL | 2 |
.... | ....| .... | .... |
----------------------------------------------+------+---------------------+-----------+
TOTAL 23481 |
---------------------------------------------------------------------------------------+
Since by default each class has its own cluster, we can query the database's users by class or by cluster:
orientdb> browse cluster OUser
---+---------+--------------------+--------------------+--------------------+--------------------
#| RID |name |password |status |roles
---+---------+--------------------+--------------------+--------------------+--------------------
0| #5:0|admin |{SHA-256}8C6976E5B5410415BDE908BD4DEE15DFB167A9C873FC4BB8A81F6F2AB448A918|ACTIVE |[1]
1| #5:1|reader |{SHA-256}3D0941964AA3EBDCB00CCEF58B1BB399F9F898465E9886D5AEC7F31090A0FB30|ACTIVE |[1]
2| #5:2|writer |{SHA-256}B93006774CBDD4B299389A03AC3D88C3A76B460D538795BC12718011A909FBA5|ACTIVE |[1]
---+---------+--------------------+--------------------+--------------------+--------------------
The result is identical to browse class ouser
executed in the classes section because there is only one cluster for the OUser class in this example.