How do distributed systems ensure transaction atomicity?

Distributed systems ensure transaction atomicity through protocols like Two-Phase Commit (2PC) and Three-Phase Commit (3PC).

In a distributed system, a transaction is an operation that involves multiple nodes. To maintain consistency and reliability, these systems must ensure that a transaction is atomic, meaning it is treated as a single, indivisible operation. This means that either all changes made during a transaction are committed to the system, or none are. This is where protocols like Two-Phase Commit (2PC) and Three-Phase Commit (3PC) come into play.

The Two-Phase Commit protocol is a distributed algorithm that coordinates all the processes that participate in a distributed atomic transaction. It involves two phases: the prepare phase and the commit phase. In the prepare phase, the coordinator sends a query to commit message to all nodes and waits until it has received a reply from all nodes. In the commit phase, the coordinator sends a commit message to all nodes if all nodes replied with a 'yes' to the query. If any node replies with a 'no', the coordinator sends a rollback message to all nodes.

The Three-Phase Commit protocol is an enhancement of 2PC and is used to overcome the blocking problem that can occur in 2PC. In 3PC, an additional phase, the pre-commit phase, is added before the commit phase. In this phase, the coordinator sends a pre-commit message to all nodes and waits for their acknowledgement. Only after receiving all acknowledgements does the coordinator move to the commit phase.

Both these protocols ensure that all nodes in a distributed system agree on whether to commit or abort a transaction, thereby ensuring atomicity. However, they also have their limitations. For instance, 2PC can lead to a blocking scenario if the coordinator fails after some nodes have voted 'yes' but before a decision has been reached. On the other hand, 3PC requires more messages to be exchanged, which can increase latency.

In addition to these protocols, distributed systems can also use techniques like logging and checkpointing to ensure atomicity. Logging involves keeping a record of all changes made during a transaction, which can be used to roll back the system to a consistent state in case of a failure. Checkpointing involves saving the state of the system at regular intervals, which can be used to restart the system in case of a failure.