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• A stream of committed operations that are guaranteed to be the same on all nodes in the replication group. Each operation in the stream is provided with a unique monotonous continuous index assigned by the replication module. The stream is durable and can be re-read as many times as needed as long as operations have not been compacted by a user/system request. The committed operations can be applied to the storage asynchronously as long as read operations are properly synchronized with the storage state to make sure to read-only when the needed operations are applied.
• A readIndex() operation that returns the most up-to-date committed (durable) operation index in the replication group providing linearizability. readIndex() performance varies from protocol to protocol: for canonical Raft, this operation involves a round-trip to the majority of group members from the current leader, while in PacificA primary node can return the index locally as long as the primary lease is valid. If a node has received and applied an operation with an index at least as large as the one returned by readIndex(), the state can be safely read locally.

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The main idea behind the atomic protocol in Ignite 2.x was a performance - because people not always need a transactions transaction and associated coordination overhead.

Nevertheless, the atomic protocol implementation in Ignite 2.x haы multiple has multiple flaws - absence of atomicity for batch operations (de facto atomicity per key), best effort consistency, side effects during retries on non-idempotent operations, the presence of PartialUpdateException for manual retries when automatic is not possible. Moreover, if a cache was created in ATOMIC mode, no transaction can be run on it due to protocols incompatibility.

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Looks like we need only one cache type in Ignite 3 with strong guaranties guarantees for a better user experience.

New transaction protocol will work faster for single partition batches, allowing to commit as soon as all entries are replicated, and eliminating performance issues related to coordination overhead, making atomic protocol obsolete.

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