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Transactions

All Cypher queries are run within transactions, which means that all modification made by a single query are held in memory by the transaction until the query is successfully executed. The changes are then committed and visible to all other transactions, users and systems. In the case of an error, the transaction is rolled back and no changes are committed.

These automatic transactions are also called implicit transactions.

Users can also create explicit transactions to execute multiple Cypher queries in sequence, then commit them or roll them back.

During transaction execution, an important property of a database is the isolation level that defines how or when the changes made by one operation become visible to others.

Explicit transactions

To start a transaction, run the BEGIN; query.

All the following queries will be executed as a part of a single transaction.

If any of the queries fails, further queries will no longer be successfully executed and it won't be possible to commit the transaction.

Commit successful transactions by executing the COMMIT; query. Roll back unsuccessful transactions by executing the ROLLBACK; query.

Isolation levels

In database systems, isolation determines how transaction integrity is visible to other users and systems.

A lower isolation level allows many users to access the same data at the same time but increases the number of concurrency effects (such as dirty reads or lost updates). A higher isolation level secures data consistency but requires more system resources and increases the chances that one transaction will block another.

Memgraph currently supports three isolation levels, from the highest to the lowest:

  • SNAPSHOT_ISOLATION (default) - guarantees that all reads made in a transaction will see a consistent snapshot of the database, and the transaction itself will successfully commit only if no updates it has made conflict with any concurrent updates made since that snapshot.
  • READ_COMMITTED - guarantees that any data read was committed at the moment it is read.
  • READ_UNCOMMITTED - one transaction may read not yet committed changes made by other transactions.

Setting the isolation level

To change the isolation level, change the --isolation-level configuration flag to any of the supported values. If you need help changing the configuration, check out the how-to guide.

The initially set isolation level can be changed during runtime using the following query:

SET <scope> TRANSACTION ISOLATION LEVEL <isolation_level>

<scope> defines the scope to which the isolation level change should apply:

  • GLOBAL - apply the new isolation level globally
  • SESSION - apply the new isolation level only for the current session
  • NEXT - apply the new isolation level only for the next transaction in the current session

<isolation_level defines the isolation level:

  • SNAPSHOT_ISOLATION
  • READ_COMMITTED
  • READ_UNCOMMITTED