Consistency is set in the ehcache.xml configuration file, and it has to do with how the cache you connect to is clustered on the CL Connector. A cache clustered using strong consistency will only be able to have strong operations performed on it, meaning that changes made by any node in the cluster are visible instantly. Atomic operations (for example, putIfAbsent) are always performed in the strong mode. A cache clustered using eventual consistency, on the other hand, will be able to perform both strong and eventual operations, and changes made by any node are visible to other nodes eventually. To understand consistency at the Terracotta clustering level, refer to the BigMemory Max Configuration Guide.

In the Cross-Language API, the ConsistencyType attribute sets the consistency, strong or eventual, with which you expect the CL Connector to execute an operation. All operations on a cache that can be performed using either strong or eventual consistency take ConsistencyType as an additional parameter. Other than a few operations such as atomic ops, RemoveAll, and queries, most regular cache operations accept this last parameter.

When set to STRONG, data in the cache remains consistent across the cluster at all times. This mode guarantees that a read gets an updated value only after all write operations to that value are completed, and it guarantees that each put operation is in a separate transaction. When set to EVENTUAL, reads without locks are allowed, and read/write performance is substantially boosted, but at the cost of potentially having an inconsistent cache for brief periods of time.

If you try to execute an eventually consistent operation (for example, cache.get(key, EVENTUAL)) on a strong cache, you will get an exception. Executing a strongly consistent operations on an eventual cache is allowed but will result in locks being used on the CL Connector, and your method will return only when all of the steps are finished (acquiring the appropriate lock, performing the operation, and releasing the lock).