Provide transaction example using new withTransaction API

# Start Transactions withTxn API Example 1

# For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.

# uriString = 'mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl'

# For a sharded cluster, connect to the mongos instances; e.g.

# uriString = 'mongodb://mongos0.example.com:27017,mongos1.example.com:27017/'

client = MongoClient(uriString)

my_write_concern_majority = WriteConcern('majority', wtimeout=1000)

# Prereq: Create collections. CRUD operations in transactions must be on existing collections.

client.get_database(

    'mydb1',

    write_concern=my_write_concern_majority).foo.insert_one({'abc': 0})

client.get_database(

    'mydb2',

    write_concern=my_write_concern_majority).bar.insert_one({'xyz': 0})

# Step 1: Define the callback that specifies the sequence of operations to perform inside the transactions.

def callback(my_session):

    collection_one = my_session.client.mydb1.foo

    collection_two = my_session.client.mydb2.bar

    # Important:: You must pass the session to the operations.

    collection_one.insert_one({'abc': 1}, session=my_session)

    collection_two.insert_one({'xyz': 999}, session=my_session)

#. Step 2: Start a client session.

with client.start_session() as session:

    # Step 3: Use with_transaction to start a transaction, execute the callback, and commit (or abort on error).

    session.with_transaction(callback,

                             read_concern=ReadConcern('local'),

                             write_concern=my_write_concern_majority,

                             read_preference=ReadPreference.PRIMARY)

# End Transactions withTxn API Example 1

/* Start Transactions withTxn API Example 1 */

/*

  For a replica set, include the replica set name and a seedlist of the members in the URI string; e.g.

  String uri = "mongodb://mongodb0.example.com:27017,mongodb1.example.com:27017/?replicaSet=myRepl";

  For a sharded cluster, connect to the mongos instances; e.g.

  String uri = "mongodb://mongos0.example.com:27017,mongos1.example.com:27017/";

 */

final MongoClient client = MongoClients.create(uri);

/*

    Prereq: Create collections. CRUD operations in transactions must be on existing collections.

 */

client.getDatabase("mydb1").getCollection("foo")

        .withWriteConcern(WriteConcern.MAJORITY).insertOne(new Document("abc", 0));

client.getDatabase("mydb2").getCollection("bar")

        .withWriteConcern(WriteConcern.MAJORITY).insertOne(new Document("xyz", 0));

/* Step 1: Start a client session. */

final ClientSession clientSession = client.startSession();

/* Step 2: Optional. Define options to use for the transaction. */

TransactionOptions txnOptions = TransactionOptions.builder()

        .readPreference(ReadPreference.primary())

        .readConcern(ReadConcern.LOCAL)

        .writeConcern(WriteConcern.MAJORITY)

        .build();

/* Step 3: Define the sequence of operations to perform inside the transactions. */

TransactionBody txnBody = new TransactionBody<String>() {

    public String execute() {

        MongoCollection<Document> coll1 = client.getDatabase("mydb1").getCollection("foo");

        MongoCollection<Document> coll2 = client.getDatabase("mydb2").getCollection("bar");

        /*

           Important:: You must pass the session to the operations.

         */

        coll1.insertOne(clientSession, new Document("abc", 1));

        coll2.insertOne(clientSession, new Document("xyz", 999));

        return "Inserted into collections in different databases";

    }

};

try {

    /*

       Step 4: Use .withTransaction() to start a transaction,

       execute the callback, and commit (or abort on error).

    */

    clientSession.withTransaction(txnBody, txnOptions);

} catch (RuntimeException e) {

    // some error handling

} finally {

    clientSession.close();

}

/* End Transactions withTxn API Example 1 */