Caching

Caching in Morphium

Caching is very important even for such a extremely fast database as mongo. There are two kinds of caches: read cache and write cache. #Write cache The WriteCache is just a buffer, where all things to write will be stored and eventually stored to database. This is done by adding the Annotation @WriteBuffer to the class:

@Entity
 @WriteBuffer(size = 150, strategy = WriteBuffer.STRATEGY.DEL_OLD)
    public static class BufferedBySizeDelOldObject extends UncachedObject {

    }

In this case, the buffer has a maximum of 150 entries, and if the buffer has reached that maximum, the oldest entries will just be deleted from buffer and hence NOT be written! Possible strategies are:

• WriteBuffer.STRATEGY.DEL_OLD: delete oldest entries from buffer - use with caution
• WriteBuffer.STRATEGY.IGNORE_NEW: Do not write the new entry - just discard it. use with caution
• WriteBuffer.STRATEGY.JUST_WARN: just log a warning message, but store data anyway
• WriteBuffer.STRATEGY.WRITE_NEW: write the new entry synchronously and wait for it to be finished
• WriteBuffer.STRATEGY.WRITE_OLD: write some old data NOW, wait for it to be finished, than queue new entries

That's it - rest is 100% transparent - just call morphium.store(entity); - the rest is done automatically

Read Cache

Read caches are defined on type level with the annotation @Cache. There you can specify, how your cache should operate:

@Cache(clearOnWrite = true, maxEntries = 20000, strategy = Cache.ClearStrategy.LRU, syncCache = Cache.SyncCacheStrategy.CLEAR_TYPE_CACHE, timeout = 5000)
@Entity
public class MyCachedEntity {
.....
}

here a cache is defined, which has a maximum of 20000 entries. Those Entries have a lifetime of 5 seconds (timeout=5000). Which means, no element will stay longer than 5sec in cache. The strategy defines, what should happen, when you read additional object, and the cache is full:

• Cache.ClearStartegy.LRU: remove least recently used elements from cache
• Cache.ClearStrategy.FIFO:first in first out - depending time added to cache
• Cahce.ClearStrategy.RANDOM: just remove some random entries With clearOnWrite=true set, the local cache will be erased any time you write an entity of this typte to database. This prevents dirty reads. If set to false, you might end up with stale data (for as long as the timeout value) but produce less stress on mongo and be probably a bit faster.

CacheSynchronization is something important in clustered or replicated environments. In this case, the Write event is propagated to all cluster members. See Cache Syncrhonization for more details.

**Internals / Implementation details **

• Morphium uses the cache based on the search query, sort options and collection overrides given. This means that there might be doublicate cache entries. In order to minimize the memory usage, Morphium also uses an ID-Cache. So all results are just added to this id cache and those ids are added as result to the query cache. the Caches are organized per type. This means, if your entity is not marked with @Cache, queries to this type won't be cached, even if you override the collection name.
• The cache is implemented completely unblocking and completely thread safe. There is almost no synchronized block in morphium.