Iotellect-Based Database Replication

Iotellect Server has built-in High Availability Database Cluster engine that may perform database replication and load balancing in a generic database-independent way. This engine normally shares the same configuration on Iotellect Master and Failover servers. It is configured to:

• Monitor connections with several database servers.
• Write any data generated by Iotellect Server to all databases.
• Once certain data is requested by Iotellect Server, choose a less-loaded database to get data from. This ensures load balancing between database servers.
• Once a certain database has been recovered after temporary failure or unavailability period, synchronize the data between the failed database and other (active) databases.

Pros:

• Configuration is easy and database-independent.
• Data is replicated to all database cluster nodes to ensure reliability.
• Database servers load balancing upon read operations.

Cons:

• If an error occurred upon writing to one of cluster databases, synchronization of databases is performed by Iotellect Server, effectively pausing all other server operations. Such service interruptions may take significant time (several minutes) if one of cluster databases was not available for long time (e.g. several hours or days).
• Slight overhead of Iotellect Master server during database write operations.
• All database nodes must use the same auth credentials.

Building database cluster using Iotellect Server built-in technology provides data mirroring that results in higher system reliability. It also ensures higher system performance by load-balancing read operations. However, in case of database cluster's node unavailability Iotellect Server performs database node synchronization to ensure data consistency. This operation effectively pauses all other server operations, so using built-in database clustering is not recommended for highly-available systems, i.e. systems for which several minutes of downtime are not acceptable. Please use internal clustering technology of your preferred relational database engine or failover clustering technology of NoSQL database engines built into Iotellect Server to combine data security with high availability of the system.

Below is the list of configuration changes required to set up Iotellect database replication.

• Set up two or more identical database servers.
• Configure all servers to accept connections from IP addresses or host names of Iotellect Master and Failover servers.

• Start Server Configurator.
• Enable Database Clustering option in the Database tab.

• Edit the Cluster Databases table by adding a record for each of the database servers. Make sure that you've specified correct URL of each database.
• Make sure that Database Username, Database Password and Database Dialect settings in the Database tab match your databases.
• Save global configuration and start the server. It should now connect to the database cluster.

All databases in the cluster must use the same username/password pair for accepting Iotellect Server connections.

• Perform the same changes as on the Master server, i.e. enable Database Clustering and add records to the Cluster Databases table.

The database clustering engine performs periodic checks of each database's availability and integrity. If a certain database has failed, it is disconnected from the cluster and marked as Not Active. The server does not attempt to perform further read/write operations with this database.

The server periodically checks availability of all inactive databases. If an inactive database appears to be alive, the server will automatically reactivate it. This process involves synchronizing data between reactivated database and other databases in the cluster, and thus, may take significant time and cause high consumption of server resources.

To view the status of individual databases participating in the database cluster, open Cluster Databases table in the Iotellect Server global configuration. Active field in this table indicates whether a database is currently being used by the cluster.

Iotellect remembers what cluster databases are active and keeps this information between server restarts. The data is stored in OS-dependent way, e.g. in system registry (for Windows systems) or file (for Linux systems).

To make sure that cluster configuration file always matches remembered database state, remove cluster databases only via server configurator utility or other Iotellect Server configuration methods. Do not remove database accounts from the database cluster configuration file by editing this file directly.

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