Default behavior
When simultaneously handling messages, conflicts may occur. See below for examples of the exceptions which are thrown. Saga concurrency explains how these conflicts are handled, and contains guidance for high-load scenarios.
Handle method on the saga will be invoked, even though the message might be later rolled back. Hence it is important to ensure not to perform any work in saga handlers that can't roll back together with the message. This also means that should there be high levels of concurrency there will be N-1 rollbacks where N is the number of concurrent messages. This can cause throughput issues and might require design changes.Starting a saga
Example exception:
The 'OrderSagaData' saga with id '7ac4d199-6560-4d1a-b83a-b3dad94b0802' could not be created possibly due to a concurrency conflict.
Updating or deleting saga data
Due to the distributed nature of Cosmos DB optimistic concurrency control is always used when updating or deleting saga data.
Example exception:
The 'OrderSagaData' saga with id '7ac4d199-6560-4d1a-b83a-b3dad94b0802' was updated by another process or no longer exists.
Sagas concurrency control
By default NServiceBus.CosmosDB uses optimistic concurrency control. Pessimistic locking can be enabled with the following API:
var sagaPersistenceConfiguration = persistence.Sagas();
sagaPersistenceConfiguration.UsePessimisticLocking();
Pessimistic locking internals
CosmosDB does not support pessimistic locking natively. The behavior is based on a spin lock that tries to acquire a lease on a resource by performing Container. method.
Pessimistic concurrency control settings
The pessimistic locking behavior can be customized using the following options:
Pessimistic lease lock duration
By default, the persister locks a saga data document for 60 seconds. Although it is not recommended to have sagas execute long-running logic, in some scenarios it might be required to increase the lease duration. The lease duration can be adjusted using the following API:
var pessimisticLockingConfiguration = sagaPersistenceConfiguration.UsePessimisticLocking();
pessimisticLockingConfiguration.SetLeaseLockTime(TimeSpan.FromMilliseconds(500));
Pessimistic lease lock acquisition timeout
By default, the persister waits 60 seconds to acquire the lock. The value can be adjusted using the following API:
pessimisticLockingConfiguration.SetLeaseLockAcquisitionTimeout(TimeSpan.FromMilliseconds(500));
Pessimistic lease lock acquisition minimum and maximum refresh delay
To prevent request synchronization, the persister randomizes the interval between lock acquisition requests. By default, the interval has a value between 500 and 1000 milliseconds. These values can be adjusted using the following API:
pessimisticLockingConfiguration.SetLeaseLockAcquisitionMinimumRefreshDelay(TimeSpan.FromMilliseconds(50));
pessimisticLockingConfiguration.SetLeaseLockAcquisitionMaximumRefreshDelay(TimeSpan.FromMilliseconds(100));