NServiceBus uses defaults that ensure good performance in common cases. While this is usually the preferred mode of operation there are situations where tuning might be desired.
Examples where concurrency tuning might be relevant are:
- Non-thread-safe code that needs to run sequentially
- Databases that might deadlock when getting too many concurrent requests
Configuring concurrency limit
The default concurrency settings of an endpoint can be changed via code:
endpointConfiguration.LimitMessageProcessingConcurrencyTo(5);
The default concurrency limit is max(Number of logical processors, 2).
Limit maximum concurrency so that no more messages than the specified value are ever processed at the same time. If a maximum concurrency is not specified, the transport will choose an optimal value that is a balance between throughput and effective resource usage. The concurrency limit cannot be changed at run-time and can only be applied at endpoint instance creation and requires the instance to be restarted for concurrency changes to take effect.
Infrastructure monitoring should be set up for the environment that hosts the endpoint instance (as well as any remote resources, such as databases) to monitor CPU, RAM, network, and storage to validate if a change made to the concurrency is not negatively affecting the overall system.
The concurrency set in the endpoint configuration defines the concurrency of each endpoint instance, and not the aggregate concurrency across all endpoint instances. For example, if the endpoint configuration sets the concurrency to 4 and the endpoint is scaled-out to 3 instances, the combined concurrency will be 12 and not 4.
Parallelism
If you have long-running compute-bound code or synchronous code in your handler and want to achieve higher parallelism, refer to the calling long-running code in async handlers section of the documentation.
Sequential processing
Set the concurrency limit value to 1 to process messages sequentially. Sequential processing is not a guarantee for ordered processing. For example, processing failures and recoverability will result in out-of-order processing.
Sequential processing on the endpoint (logical) level is not possible when scaled-out.
Throttling
Throughput throttling options have been deprecated. To enable throttling on Version 6 and higher, a custom behavior should be used. The throttling sample demonstrates how such a behavior can be implemented.
TimeoutManager satellite queues
Depending on the chosen transport, additional queues (satellite queues) may be used to handle deferred messages like delayed retries or timeouts. Satellite queues use the default concurrency configuration. This concurrency setting can be configured using:
var timeoutManager = endpointConfiguration.TimeoutManager();
timeoutManager.LimitMessageProcessingConcurrencyTo(4);