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Backend ExecutorService

By default JanusGraph uses ExecutorService to process some queries in parallel (see storage.parallel-backend-ops configuration option).
JanusGraph allows to configure the executor service which is used via configuration options provided in storage.parallel-backend-executor-service configuration section.

Currently JanusGraph has the next ExecutorService implementations which can be used (controlled via storage.parallel-backend-executor-service.class):

  • fixed - fixed thread pool size;
  • cached - cached thread pool size;
  • Custom ExecutorService;

Custom ExecutorService

To use custom ExecutorService the configuration option storage.parallel-backend-executor-service.class must be provided which must be the full class name of the ExecutorService implementation class.

The provided class which implements ExecutorService must have either a public constructor with ExecutorServiceConfiguration argument (preferred constructor) or a public parameterless constructor. When both constructors are available the constructor with ExecutorServiceConfiguration argument will be used.

When custom ExecutorService is provided with ExecutorServiceConfiguration it isn't required to use any of the configuration options provided in ExecutorServiceConfiguration but sometimes it is convenient to use the provided configurations. The configuration options provided in ExecutorServiceConfiguration are typically those configuration options which are provided via configurations from storage.parallel-backend-executor-service.

Cassandra backend ExecutorService

Apart from the general executor service mentioned in the previous section, Cassandra backend uses an additional ExecutorService to process CQL queries by default (see storage.cql.executor-service.enabled configuration option).
The rules by which the Cassandra backend ExecutorService is built are the same as the rules which are used to build parallel backend queries ExecutorService (described above). The only difference is that the configuration for Cassandra backend ExecutorService are provided via configuration options under storage.cql.executor-service.
Disabling CQL executor service reduces overhead of thread pool but requires the user to tune maximum throughput thoroughly.
With disabled CQL executor service the parallelism will be controlled internally by the CQL driver via the next properties: storage.cql.max-requests-per-connection, storage.cql.local-max-connections-per-host, storage.cql.remote-max-connections-per-host.

Info

It is recommended to disable CQL executor service in a production environment and properly configure maximum throughput. CQL executor service does not provide any benefits other than limiting the amount of parallel requests per JanusGraph instance.

Warning

Improper tuning of maximum throughput might result in failures under heavy workloads.