Oracle Coherence - Benefits and Issues

Refresh Ahead reloads a cache entry before it expires based on a configurable expiration time and refresh ahead factor. If data is accessed before expiration, an async refresh-ahead is scheduled; otherwise, it must be retrieved from the data store.

Write-Behind writes data to the data source after a configurable delay (e.g., 10 seconds, 20 minutes), coalescing multiple writes into one physical write. This reduces database load and improves app responsiveness, but requires careful handling of external updates.

Near Cache wraps two caches (Front-Cache and Back-Cache) to provide fast read access to most recently/frequently used data, combining local and clustered caching. Local Cache is a simple single-node cache without distribution.

Each server manages its own share of data with affinity to avoid hops, and knows backup locations. Adding more servers improves performance predictably, as data is logically distributed across the cluster.

Replicated Cache copies data to all nodes, increasing memory per node and causing non-linear scalability with many updates. It is best for read-heavy, small datasets.

TCP is used only for sophisticated death detection due to its overhead. Data transfer relies on UDP Multicast and Unicast, with TCMP's queue mechanism ensuring reliable delivery despite UDP's lack of ordering.