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TxLog

To be able to determine a state of TX, which has changed Tx that created a particular row, a special structure (TxLog) is introduced. TxLog is a table (can be persistent in case persistence enabled) which contains MVCC version to transaction state mappings.

TxLog is used to keep all the data consistent on cluster crush and recovery as well:

• If a particular Tx has ACTIVE or ROLLED_BACK state on at least one data node it marks as ROLLED_BACK on all nodes. 
• If a particular Tx has COMMITTED state on at least one data node it marks as COMMITTED on all nodes.
• If a particular Tx has PREPARED state on all data nodes and all involved partitions are available it marks as COMMITTED on all nodes.
• If a particular Tx has PREPARED state on all data nodes and at least one involved partition is lost (unavailable) it is left in PREPARED state, all the entries are locked for updates until state is changed manually or lost partition become available.

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