Transactions

This section describes how Cypher queries work with database transactions.

All Cypher statements are explicitly run within a transaction. For read-only queries, the transaction will always succeed. For updating queries it is possible that a failure can occur for some reason, for example if the query attempts to violate a constraint, in which case the entire transaction is rolled back, and no changes are made to the graph. Every statement is executed within the context of the transaction, and nothing will be persisted to disk until that transaction is successfully committed.

In short, an updating query will always either fully succeed, or not succeed at all.

While it is not possible to run a Cypher query outside a transaction, it is possible to run multiple queries within a single transaction using the following sequence of operations:

  1. Open a transaction,

  2. Run multiple updating Cypher queries.

  3. Commit all of them in one go.

Note that the transaction will hold the changes in memory until the whole query, or whole set of queries, has finished executing. A query that makes a large number of updates will consequently use large amounts of memory. For memory configuration in Neo4j, see the Neo4j Operations Manual → Memory configuration.

For examples of the API’s used to start and commit transactions, refer to the API specific documentation:

When writing procedures or using Neo4j embedded, remember that all iterators returned from an execution result should be either fully exhausted or closed. This ensures that the resources bound to them are properly released.

1. DBMS Transactions

Beginning a transaction while connected to a DBMS will start a DBMS-level transaction. A DBMS-level transaction is a container for database transactions.

A database transaction is started when the first query to a specific database is issued. Database transactions opened inside a DBMS-level transaction are committed or rolled back when the DBMS-level transaction is committed or rolled back.

For an example of how queries to multiple databases can be issued in one transaction, see Driver Manual → Databases and execution context.

DBMS transactions have the following limitations:

  • Only one database can be written to in a DBMS transaction

  • Cypher operations fall into the following main categories:

    • Operations on graphs.

    • Schema commands.

    • Administration commands.

    It is not possible to combine any of these workloads in a single DBMS transaction.