4.5. Replicating Database Tables


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4.5. Replicating Database Tables

With VoltDB, tables are either partitioned or replicated across all nodes and sites of a VoltDB database. Smaller, mostly read-only tables are good candidates for replication. Note also that if a table needs to be accessed frequently by columns other than the partitioning column, the table should be replicated instead because there is no guarantee that a particular partition includes the data that the query seeks.

The previous section describes how to partition the Reservation and Customer tables as examples, but what about the Flight table? It is possible to partition the Flight table (for example, on the FlightID column). However, not all tables benefit from partitioning.

4.5.1. Choosing Replicated Tables

Looking at the workload of the flight reservation example, the Flight table has the most frequent accesses (at 10,000 a second). However, these transactions are read-only and may involve any combination of three columns: the departure time, the point of origin, and the destination. This makes it hard to partition the table in a way that would make the transaction single-partitioned because the lookup is not restricted to one table column.

Fortunately, the number of flights available for booking at any given time is limited (estimated at 2,000) and so the size of the table is relatively small (approximately 36 megabytes). In addition, the vast majority of the transactions involving the Flight table are read-only except when new flights are added and at take-off (when the records are deleted). Therefore, Flight is a good candidate for replication.

Note that the Customer table is also largely read-only. However, because of the volume of data in the Customer table (a million records), it is not a good candidate for replication, which is why it is partitioned.

4.5.2. Specifying Replicated Tables

In VoltDB, you do not explicitly state that a table is replicated. If you do not specify a partitioning column in the database schema, the table will by default be replicated.

So, in our flight reservation example, there is no explicit action required to replicate the Flight table. However, it is very important to specify partitioning information for tables that you want to partition. If not, they will be replicated by default, significantly changing the performance characteristics of your application.