# Transactions in Neptune openCypher
<a name="access-graph-opencypher-transactions"></a>

The openCypher implementation in Amazon Neptune uses the [transaction semantics defined by Neptune](transactions-neptune.md) However, isolation levels provided by the Bolt driver have some specific implications for Bolt transaction semantics, as described in the sections below.

## Read-only Bolt transaction queries
<a name="access-graph-opencypher-transactions-ro"></a>

There are various ways that read-only queries can be processed, with different transaction models and isolation levels, as follows:

### Implicit read-only transaction queries
<a name="access-graph-opencypher-transactions-ro-implicit"></a>

Here is an example of a read-only implicit transaction:

```
public void executeReadImplicitTransaction()
{
  // end point
  final String END_POINT = "(End Point URL)";

  // read query
  final String READ_QUERY = "MATCH (n) RETURN n limit 10";

  // create the driver
  final Driver driver = GraphDatabase.driver(END_POINT, AuthTokens.none(),
          Config.builder().withEncryption()
                          .withTrustStrategy(TrustStrategy.trustSystemCertificates())
                          .build());

  // create the session config
  SessionConfig sessionConfig = SessionConfig.builder()
                                             .withFetchSize(1000)
                                             .withDefaultAccessMode(AccessMode.READ)
                                             .build();

  // run the query as access mode read
  driver.session(sessionConfig).readTransaction(new TransactionWork<String>()
    {
      final StringBuilder resultCollector = new StringBuilder();

      @Override
      public String execute(final Transaction tx)
      {
        // execute the query
        Result queryResult = tx.run(READ_QUERY);

        // Read the result
        for (Record record : queryResult.list())
        {
          for (String key : record.keys())
          {
            resultCollector.append(key)
                           .append(":")
                           .append(record.get(key).asNode().toString());
          }
        }
        return resultCollector.toString();
      }

    }
  );

  // close the driver.
  driver.close();
}
```

Because read-replicas only accept read-only queries, all queries against read-replicas execute as read-implicit transactions regardless of the access mode set in the session configuration. Neptune evaluates read-implicit transactions as [read-only queries](transactions-neptune.md#transactions-neptune-read-only) under `SNAPSHOT` isolation semantics.

In case of failure, read-implicit transactions are retried by default.

### Autocommit read-only transaction queries
<a name="access-graph-opencypher-transactions-ro-autocommit"></a>

Here is an example of a read-only autocommit transaction:

```
public void executeAutoCommitTransaction()
{
  // end point
  final String END_POINT = "(End Point URL)";

  // read query
  final String READ_QUERY = "MATCH (n) RETURN n limit 10";

  // Create the session config.
  final SessionConfig sessionConfig = SessionConfig
    .builder()
    .withFetchSize(1000)
    .withDefaultAccessMode(AccessMode.READ)
    .build();

  // create the driver
  final Driver driver = GraphDatabase.driver(END_POINT, AuthTokens.none(),
    Config.builder()
          .withEncryption()
          .withTrustStrategy(TrustStrategy.trustSystemCertificates())
          .build());

  // result collector
  final StringBuilder resultCollector = new StringBuilder();

  // create a session
  final Session session = driver.session(sessionConfig);

  // run the query
  final Result queryResult = session.run(READ_QUERY);
  for (final Record record : queryResult.list())
  {
    for (String key : record.keys())
    {
      resultCollector.append(key)
                     .append(":")
                     .append(record.get(key).asNode().toString());
    }
  }

  // close the session
  session.close();

  // close the driver
  driver.close();
}
```

If the access mode is set to `READ` in the session configuration, Neptune evaluates autocommit transaction queries as [read-only queries](transactions-neptune.md#transactions-neptune-read-only) under `SNAPSHOT` isolation semantics. Note that read-replicas only accept read-only queries.

If you don't pass in a session configuration, autocommit queries are processed by default with mutation query isolation, so it is important to pass in a session configuration that explicitly sets the access mode to `READ`.

In case of failure, read-only autocommit queries are not re-tried.

### Explicit read-only transaction queries
<a name="access-graph-opencypher-transactions-ro-explicit"></a>

Here is an example of an explicit read-only transaction:

```
public void executeReadExplicitTransaction()
{
  // end point
  final String END_POINT = "(End Point URL)";

  // read query
  final String READ_QUERY = "MATCH (n) RETURN n limit 10";

  // Create the session config.
  final SessionConfig sessionConfig = SessionConfig
    .builder()
    .withFetchSize(1000)
    .withDefaultAccessMode(AccessMode.READ)
    .build();

  // create the driver
  final Driver driver = GraphDatabase.driver(END_POINT, AuthTokens.none(),
    Config.builder()
          .withEncryption()
          .withTrustStrategy(TrustStrategy.trustSystemCertificates())
          .build());

  // result collector
  final StringBuilder resultCollector = new StringBuilder();

  // create a session
  final Session session = driver.session(sessionConfig);

  // begin transaction
  final Transaction tx = session.beginTransaction();

  // run the query on transaction
  final List<Record> list = tx.run(READ_QUERY).list();

  // read the result
  for (final Record record : list)
  {
    for (String key : record.keys())
    {
      resultCollector
        .append(key)
        .append(":")
        .append(record.get(key).asNode().toString());
    }
  }

  // commit the transaction and for rollback we can use beginTransaction.rollback();
  tx.commit();

  // close the driver
  driver.close();
}
```

If the access mode is set to `READ` in the session configuration, Neptune evaluates explicit read-only transactions as [read-only queries](transactions-neptune.md#transactions-neptune-read-only) under `SNAPSHOT` isolation semantics. Note that read-replicas only accept read-only queries.

If you don't pass in a session configuration, explicit read-only transactions are processed by default with mutation query isolation, so it is important to pass in a session configuration that explicitly sets the access mode to `READ`.

In case of failure, read-only explicit queries are retried by default.

## Mutation Bolt transaction queries
<a name="access-graph-opencypher-transactions-wr"></a>

As with read-only queries, there are various ways that mutation queries can be processed, with different transaction models and isolation levels, as follows:

### Implicit mutation transaction queries
<a name="access-graph-opencypher-transactions-wr-implicit"></a>

Here is an example of an implicit mutation transaction:

```
public void executeWriteImplicitTransaction()
{
  // end point
  final String END_POINT = "(End Point URL)";

  // create node with label as label and properties.
  final String WRITE_QUERY = "CREATE (n:label {name : 'foo'})";

  // Read the vertex created with label as label.
  final String READ_QUERY = "MATCH (n:label) RETURN n";

  // create the driver
  final Driver driver = GraphDatabase.driver(END_POINT, AuthTokens.none(),
    Config.builder()
          .withEncryption()
          .withTrustStrategy(TrustStrategy.trustSystemCertificates())
          .build());

  // create the session config
  SessionConfig sessionConfig = SessionConfig
    .builder()
    .withFetchSize(1000)
    .withDefaultAccessMode(AccessMode.WRITE)
    .build();

  final StringBuilder resultCollector = new StringBuilder();

  // run the query as access mode write
  driver.session(sessionConfig).writeTransaction(new TransactionWork<String>()
  {
    @Override
    public String execute(final Transaction tx)
    {
      // execute the write query and consume the result.
      tx.run(WRITE_QUERY).consume();

      // read the vertex written in the same transaction
      final List<Record> list = tx.run(READ_QUERY).list();

      // read the result
      for (final Record record : list)
      {
        for (String key : record.keys())
        {
          resultCollector
            .append(key)
            .append(":")
            .append(record.get(key).asNode().toString());
        }
      }
      return resultCollector.toString();
    }
  }); // at the end, the transaction is automatically committed.

  // close the driver.
  driver.close();
}
```

Reads made as part of mutation queries are executed under `READ COMMITTED` isolation with the usual guarantees for [Neptune mutation transactions](transactions-neptune.md#transactions-neptune-mutation).

Whether or not you specifically pass in a session configuration, the transaction is always treated as a write transaction.

For conflicts, see [Conflict Resolution Using Lock-Wait Timeouts](transactions-neptune.md#transactions-neptune-conflicts).

### Autocommit mutation transaction queries
<a name="access-graph-opencypher-transactions-wr-autocommit"></a>

Mutation autocommit queries inherit the same behavior as mutation implicit transactions.

If you do not pass in a session configuration, the transaction is treated as a write transaction by default.

In case of failure, mutation autocommit queries are not automatically retried.

### Explicit mutation transaction queries
<a name="access-graph-opencypher-transactions-wr-explicit"></a>

Here is an example of an explicit mutation transaction:

```
public void executeWriteExplicitTransaction()
{
  // end point
  final String END_POINT = "(End Point URL)";

  // create node with label as label and properties.
  final String WRITE_QUERY = "CREATE (n:label {name : 'foo'})";

  // Read the vertex created with label as label.
  final String READ_QUERY = "MATCH (n:label) RETURN n";

  // create the driver
  final Driver driver = GraphDatabase.driver(END_POINT, AuthTokens.none(),
    Config.builder()
          .withEncryption()
          .withTrustStrategy(TrustStrategy.trustSystemCertificates())
          .build());

  // create the session config
  SessionConfig sessionConfig = SessionConfig
    .builder()
    .withFetchSize(1000)
    .withDefaultAccessMode(AccessMode.WRITE)
    .build();

  final StringBuilder resultCollector = new StringBuilder();

  final Session session = driver.session(sessionConfig);

  // run the query as access mode write
  final Transaction tx = driver.session(sessionConfig).beginTransaction();

  // execute the write query and consume the result.
  tx.run(WRITE_QUERY).consume();

  // read the result from the previous write query in a same transaction.
  final List<Record> list = tx.run(READ_QUERY).list();

  // read the result
  for (final Record record : list)
  {
    for (String key : record.keys())
    {
      resultCollector
        .append(key)
        .append(":")
        .append(record.get(key).asNode().toString());
    }
  }

  // commit the transaction and for rollback we can use tx.rollback();
  tx.commit();

  // close the session
  session.close();

  // close the driver.
  driver.close();
}
```

Explicit mutation queries inherit the same behavior as implicit mutation transactions.

If you do not pass in a session configuration, the transaction is treated as a write transaction by default.

For conflicts, see [Conflict Resolution Using Lock-Wait Timeouts](transactions-neptune.md#transactions-neptune-conflicts).