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In addition to the transaction coordinator, the transactional protocol defines remote nodes which are server nodes that keep a part of the data being accessed or updated inside of the transaction. Internally, every server node maintains a distributed hash table (DHT) for partitions it owns. The DHT helps to look up partition owners (primary and backups) efficiently from any cluster node including the transaction coordinator. Note, that the data itself is stored in pages that are arranged by B+Tree (refer to memory architecture documentation for more details).

Locking Modes and Isolation Levels


This section elaborates on locking modes and isolation levels supported in Ignite. The high-level overview is given in Ignite technical documentation.

Pessimistic Locking
 

In multi-user applications, different users can modify the same data simultaneously. To deal with reads and updates of the same data sets happening in parallel, transactional subsystems of products such as Ignite implement optimistic and pessimistic locking. In the pessimistic mode, an application will acquire locks for all the data it plans to change and will apply the changes after all the locks are owned exclusively while in the optimistic mode the locks acquisition is postponed to a later phase when a transaction is being already committed.

Locks acquisition time also depends on a type of isolation level. Let's start with the review of isolation levels in conjunction with the pessimistic mode.    

 

In pessimistic & read committed mode the locks are acquired before the changes brought by write operations such (as put or putAll) are applied as it's shown below:

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Picture 6.

However, Ignite never obtains locks for read operations (such as get or getAll) in this mode which might be inappropriate for some of the use cases. To ensure the locks are acquired prior every read or write operation, use either repeatable read or serializable isolation level in Ignite:
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Picture 7.
 

The pessimistic mode holds locks until the transaction is finished that prevents accessing locked data from other transactions. Optimistic transactions in Ignite might increase the throughput of an application by lowering contention among transactions by moving the locks acquisition to a later phase.

Optimistic Locking

In optimistic transactions, locks are acquired on primary nodes during the "prepare" phase, then promoted to backup nodes and released once the transaction is committed. Depending on an isolation level, if Ignite detects that a version of an entry has been changed since the time it was requested by a transaction then the transaction will fail at the "prepare" phase and it will be up to an application to decide whether to restart the transaction or not.


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For the isolation level Read committed and Repeatable read locks are obtained at the time of implementation of phase "prepare", at the same time check that the version has not changed since the beginning of the transaction is not executed.