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This is a topic for separate IEP.

Separating replication and 2PC for transactional caches

Once we have a replication primitive in place, we can achieve the following goals:

• Single-partition transactions are committed in a single replication round
• The notion of primary and replica nodes is hidden from the upper layers of transactional logic. Upper layers operate on partitions by applying certain operations to partitions that are in turn replicated within the replication group. For example, a batch write to a partition i should be expressed as partition(i).replicate(new WriteBatch(writeMap)). The replicate operation may fail, but the write will be applied or not as a whole within the replication group
• Simplify the existing Ignite transaction modes while preserving the current guarantees of Ignite transactions: get rid of transaction isolation levels on key-value API, keeping only PESSIMISTIC and OPTIMISTIC concurrency modes, which will match to PESSIMISTIC REPEATABLE_READ and OPTIMISTIC SERIALIZABLE modes of Ignite 2.x.

The transactional protocol operates as follows:

• Each replication group leader maintains an in-memory keys lock table for pending transactions (this corresponds to the current lock candidates in GridCacheMapEntry). The lock table is not replicated and bound to the current group leader. Every transactional operation validates that the current group leader matches the locks table entries when an operation is applied.
• PESSIMISTIC transactions acquire locks on write or getForUpdate operations, OPTIMISTIC transactions acquire locks on prepare phase, similarly to Ignite 2.x
• During the transaction prepare phase, the pending updates and transaction PREPARED state is written and replicated (durably when persistence is enabled) to the partition. All possible constraints must be checked during the prepare state to guarantee that no regular flow can prevent PREPARED→COMMITTED transition
• If a replication group leader fails after the transaction becomes PREPARED, there is no need to acquire locks for the transaction keys anymore. A new replication leader must wait for the PREPARED keys to be committed or aborted before assigning new locks for the keys
• If the transaction coordinator fails before all partitions have prepared or committed the transaction, a cooperative termination protocol is initiated. To facilitate the protocol, we send the transaction participants (partitions) list along with the prepare operations. Cooperative termination assumes that if all transaction participants are in PREPARED state, then the transaction must be committed.
  • If a transaction is in PREPARED state for some partition and the transaction coordinator is suspected to be failed, the partition sends a finalize operation to all other partitions. The finalize operation returns true if such transaction was present on the node in PREPARED or COMMITTED state, but returns false if the transaction was already ABORTED or the transaction did not exist (in this case the finalize operation additionally prevents this transaction to ever enter the PREPARED state by marking it as ABORTED).
  • Cooperative termination commits the local transaction if all participants returned true, and aborts if either of the participants returned false.
  • The cooperative termination can be run by all partitions simultaneously or by a newly chosen transaction coordinator. In the latter case, once the coordinator learned the transaction outcome, it must also re-broadcast it to all pending participants
  • Cooperative termination can even be called when a transaction coordinator is alive, but hangs for some reason. In this case, the transaction outcome will be unambiguously decided by either the cooperative termination or the transaction coordinator.
• If Ignite transaction participates in an external transaction (XA), cooperative termination is not applicable, the transaction state must be resolved externally. Ignite must only provide an aggregating interface to list PREPARED, COMMITTED, and ABORTED external transactions.

Consistency guarantees

By default, both Raft and PacificA provide linearizable guarantees for reads via readIndex() procedure. This, however, puts a restriction on the reader because readIndex() can only be served from the current replica group leader and, in case of Raft, involves a network round-trip to the group quorum. To address this, we can additionally introduce a more relaxed, causally consistent, read operation.

Transactional protocol

https://github.com/apache/ignite-3/tree/main/modules/transactionsTo provide such a consistency mode, each reader maintains a last-seen index for each partition. On write operation, the written operation index is returned to the client. On read operation, the client sends the local observed index to any of the replicas. The replica then waits for at least the given operation index to be applied and returns the read value together with the applied index at the moment of the read. Therefore, the client guarantees to see it's own updates and never sees more stale value that it has already seen. The set of counters on local clients constitutes a causality token which can be attached to compute tasks, services, etc. to guarantee causal consistency across other Ignite services. Additionally, we can introduce an API to return the current token so that a client can pass it via external communication channels.

Data Structures Building Block

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