Eviction Types and Overview
For every cache, cache size should be estimated in advance with careful consideration of data usage by your application. However, your cache has storage limitation and if data will reside in the cache forever, it will eventually become full. To handle this, NCache's Eviction feature can be utilized.
In Eviction, when your cache is full, NCache decides to evict its existing data on the basis of usage or priority to accommodate fresher incoming data. Eviction will smooth cache operations while keeping the cache size limited by removing a configured percentage of data.
NCache provides different polices for eviction. These policies decide which data would be evicted when cache is full:
Priority Based Eviction
This eviction policy lets the cache evict lesser important data first. This eviction policy is your obvious choice if you can classify the cache data into different priorities. The priority is specified with the cache item while adding it to the cache. You can specify any of the priority from the following 6 levels of priorities:
- Below Normal
- Above Normal
- Not Removable - This priority level specifies that the cache item should not be evicted.
Least Recently Used (LRU)
This eviction policy lets caches evict data which is no longer in use. Data is evicted based on its last access time. The data that was accessed a long time ago is the most suitable candidate for eviction than the data which has been accessed recently. Access time is updated when data is fetched/updated from the cache.
Least Frequently Used (LFU)
This eviction policy lets cache evict the data which is less frequently used. The data is evicted based on the frequency of its usage. For example, if an item is accessed 5 times, then it is a more suitable candidate for eviction than an item which is accessed 20 times.
Do not Evict
Along with different policies, there is another option to turn off eviction. In this case, when the cache gets full, the cache does not evict any data while all incoming data addition requests are rejected.
You can also specify eviction ratio i.e. by what percentage data should be evicted from cache. Eviction removes only the configured percentage of data from cache once eviction is triggered.
Warn user when cache is near full
When cache is about 80% full and needs eviction, NCache logs events to both Event Viewer in Windows and also in cache log files.
Bulk removal of data
The number of cache items to be removed during eviction can be huge depending on cache size and eviction ratio. That is why eviction removes data in bulks. Size of the bulk operation and delay between two bulk removal operations is configurable through cache service configuration file found in NCache installation as follows:
- .NET: Alachsioft.NCache.Service.exe.config located in %NCHOME%/bin/service
- .NET Core Windows: Alachsioft.NCache.Service.dll.config located in %NCHOME%/bin/service
- .NET Core Linux: Alachsioft.NCache.Daemon.dll.config located in /opt/ncache/bin/service
Eviction in Clustered Environment
In Partitioned and Partition of Replica topologies, data distribution is based on a hashing algorithm. Each node has divided data, thus each node is responsible to evict its own data. However, replica node in partition-of-replica does not evict the data themselves - data is automatically removed from the replica when it is removed from the active node.
In Replicated topology, every node has the same set of data, so only the cluster coordinator is responsible for eviction.
Mirror Cache has only two nodes, one active other passive. Thus, active node is responsible for carrying out eviction on the entire cache.