Sessions and Transactions

Sessions

Through the Driver, we open Sessions.

It is important to remember that sessions are not the same as database connections. When the Driver connects to the database, it opens up multiple TCP connections that can be borrowed by the session. A query may be sent over multiple connections, and results may be received by the driver over multiple connections.

Instead, sessions should be considered a client-side abstraction for grouping units of work, which also handle the underlying connections. The connections themselves are managed internally by the driver and are not directly exposed to the application.

To open a new session, call the session() method on the driver.

// Open a new session
var session = driver.session();

The Session needs to be closed again, fortunately it’s an auto-closeable, so that a try-with-resources construct works well for us, where the session is automatically closed when the scope of the try ends.

// Open a new session
try (var session = driver.session()) {

  // Do something with the session...

  // Close the session automatically in try-with-resources block
}

This session method takes an optional configuration argument, which can be used to set the database to run any queries against in a multi-database setup, and the default access mode for any queries run within the transaction (either READ or WRITE).

// Create a Session for the `people` database
var sessionConfig = SessionConfig.builder()
        .withDefaultAccessMode(AccessMode.WRITE)
        .withDatabase("people")
        .build();

try (var session = driver.session(sessionConfig)) {

If no database is supplied, the default database will be used. This is configured in the dbms.default_database in neo4j.conf, the default value is neo4j. You cannot create multiple databases in Neo4j Aura or in Neo4j Community Edition.

The default access mode is set to WRITE, but this can be overwritten by explicitly calling the executeRead() or executeWrite() methods.

Transactions

Through a Session, we can run one or more Transactions.

There are three types of transaction exposed by the driver:

var query = "MATCH () RETURN count(*) AS count";
var params = Values.parameters();

// Run a query in an auto-commit transaction
var res = session.run(query, params).single().get("count").asLong();

// Run a query within a Read Transaction
var res = session.readTransaction(tx -> {
return tx.run("""
                MATCH (p:Person)-[:ACTED_IN]->(m:Movie)
                WHERE m.title = $title // (1)
                RETURN p.name AS name
                LIMIT 10
                """,
        Values.parameters("title", "Arthur") // (2)
).list(r -> r.get("name").asString());

session.writeTransaction(tx -> {
  return tx.run(
          "CREATE (p:Person {name: $name})",
          Values.parameters("name", "Michael")).consume();
});

Manually Creating Transactions

It is also possible to explicitly create a transaction object by calling the beginTransaction() method on the session.

// Open a new session
var session = driver.session(
        SessionConfig.builder()
                .withDefaultAccessMode(AccessMode.WRITE)
                .build());

// Manually create a transaction
var tx = session.beginTransaction();

This returns a Transaction object identical to the one passed in to the unit of work function when calling executeRead() or executeWrite().

This method differs from the executeRead and executeWrite() methods, in that the transaction will have to be manually committed or rolled back depending on the outcome of the unit of work.

You can commit a transaction by calling the tx.commit() method, or roll back the transaction by calling tx.rollback().

try {
  // Perform an action
  tx.run(query, params);

  // Commit the transaction
  tx.commit();
} catch (Exception e) {
  // If something went wrong, rollback the transaction
  tx.rollback();
}

Closing the Session

Usually the session is auto-closed by the try-with-resources setup

Only if you manage/pass around the session manually, you need to close it explicitly by calling the close() method to release any resources held by that session.

// Close the session
session.close();

A Working Example

private static Map<String,Object> createPerson(String name) {
    // Create a Session for the `people` database
    var sessionConfig = SessionConfig.builder()
            .withDefaultAccessMode(AccessMode.WRITE)
            .withDatabase("people")
            .build();

    try (var session = driver.session(sessionConfig)) {

        // Create a node within a write transaction
        var res = session.writeTransaction(tx ->
                tx.run("CREATE (p:Person {name: $name}) RETURN p",
                                Values.parameters("name", name))
                        .single());

        // Get the `p` value from the first record
        var p = res.get("p").asNode();

        // Return the properties of the node
        return p.asMap();
        // Autoclose the sesssion
    }
}

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