What is a Merge Operator in RocksDB?

A Merge Operator is custom logic an application supplies that defines how RocksDB should combine a base value with one or more pending Merge operations into a final result.

• Applied lazily, RocksDB doesn't necessarily compute the merged result immediately on every write, only when a read or compaction actually needs the final value
• Lets an application express operations like incrementing a counter or appending to a list without a read-modify-write round trip
• Must be associative and consistent, since merges can be applied in different groupings depending on when compaction happens to run

Without a properly implemented Merge Operator, the Merge operation itself has no way to know how to actually combine values, it's the piece of application-specific logic that makes Merge meaningful.

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