The user can configure the number of replicas required. Transaction isolation guarantee with client cooperationĬouchbase achieves durability by having many replicas for each vBucket. This client cooperation guarantees atomicity and isolation in the face of errors and failures. If another client marked the transaction as "Committed," the client uses the staged mutation in the record's metadata instead of the main document body. If they detect an existing transaction in the document's metadata, they read the relevant transaction's details from the ATR that holds it. To support Monotonic Atomic View, Couchbase clients check each document's metadata section as they read it. Finally, the transaction is marked as "Completed" and removed from the ATR. Then, the client updates all involved documents with the changes it previously staged in the metadata section, replacing their document body and removing the transaction metadata information.
Once the client completes all mutations, it marks the transaction as "Committed" in the ATR and updates it with the list of involved document ids. The mutated documents can reside in separate vBuckets and physical nodes. The meta section (also known as xAttrs) is an area when a client can store metadata. Next, the client adds the mutated content in each record's "Meta" section. Before the first document mutation, the client adds a new entry in the mutated record's vBucket ATR with the transaction id and the "Pending" status. Each vBucket has a single document responsible for the transaction log called an Active Transaction Record (ATR). The following image depicts the transaction coordination process.ĭistributed Transaction Example in CouchbaseĪ vBucket is a Couchbase-internal unit of sharding. In this example, the user needs to deduct a certain amount from one user's balance and increase another user's balance by the same amount. Denis Rosa, the blog post author, gives an example of a classic money transfer scenario between two users. The SDK performs this coordination under the hood. Each client is responsible for the coordination of its transactions. In Couchbase's architecture, there is no central transaction coordinator. In its blog post, Couchbase lays out how its DB engine supports the Monotonic Atomic View consistency model, which is a strengthened version of the Read Committed consistency model. Couchbase recently published a detailed explanation of its distributed multi-document ACID transaction implementation.
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