Consistency for Region Updates

GemFire ensures that all copies of a region eventually reach a consistent state on all members and clients that host the region, including GemFire members that distribute region events across a WAN.

• Consistency Checking by Region Type
  GemFire performs different consistency checks depending on the type of region you have configured.
• Configuring Consistency Checking
  GemFire enables consistency checking by default. You cannot disable consistency checking for persistent regions. For all other regions, you can explicitly enable or disable consistency checking by setting the concurrency-checks-enabled region attribute in cache.xml to "true" or "false."
• Overhead for Consistency Checks
  Consistency checking requires additional overhead for storing and distributing version and timestamp information, as well as for maintaining destroyed entries for a period of time to meet consistency requirements.
• How Consistency Checking Works for Replicated Regions
  Each region stores version and timestamp information for use in conflict detection. GemFire members use the recorded information to detect and resolve conflicts consistently before applying a distributed update.
• How Destroy and Clear Operations Are Resolved
  When consistency checking is enabled for a region, a GemFire member does not immediately remove an entry from the region when an application destroys the entry. Instead, the member retains the entry with its current version stamp for a period of time in order to detect possible conflicts with operations that have occurred. The retained entry is referred to as a tombstone. GemFire retains tombstones for partitioned regions and non-replicated regions as well as for replicated regions, in order to provide consistency.
• Transactions with Consistent Regions
  A transaction that modifies a region having consistency checking enabled generates all necessary version information for region updates when the transaction commits.
• How Consistency Is Achieved in WAN Deployments
  When two or more GemFire systems are configured to distribute events over a WAN, each system performs local consistency checking before it distributes an event to a configured gateway sender. Discarded events are not distributed across the WAN.