# Reactive DB2 Client

The Reactive DB2 Client is a client for DB2 with a straightforward API focusing on scalability and low overhead.

The client is reactive and non blocking, allowing to handle many database connections with a single thread.

Features

- Support for DB2 on Linux, Unix, and Windows

- Limited support for DB2 on z/OS

- Event driven

- Lightweight

- Built-in connection pooling

- Prepared queries caching

- Batch and cursor

- Row streaming

- RxJava API

- Direct memory to object without unnecessary copies

- Java 8 Date and Time

- SSL/TLS

- HTTP/1.x CONNECT, SOCKS4a or SOCKS5 proxy support

Current limitations

- No stored procedures support

- Some column types (e.g. BLOB and CLOB) are not supported

## Usage

To use the Reactive DB2 Client add the following dependency to the dependencies section of your build descriptor:

- Maven (in your `pom.xml`):

    <dependency>
     <groupId>io.vertx</groupId>
     <artifactId>vertx-db2-client</artifactId>
     <version>4.2.3</version>
    </dependency>

- Gradle (in your `build.gradle` file):

    dependencies {
     compile 'io.vertx:vertx-db2-client:4.2.3'
    }

## Getting started

Here is the simplest way to connect, query and disconnect

    DB2ConnectOptions connectOptions = new DB2ConnectOptions()
      .setPort(50000)
      .setHost("the-host")
      .setDatabase("the-db")
      .setUser("user")
      .setPassword("secret");

    // Pool options
    PoolOptions poolOptions = new PoolOptions()
      .setMaxSize(5);

    // Create the client pool
    DB2Pool client = DB2Pool.pool(connectOptions, poolOptions);

    // A simple query
    client
      .query("SELECT * FROM users WHERE id='julien'")
      .execute(ar -> {
      if (ar.succeeded()) {
        RowSet<Row> result = ar.result();
        System.out.println("Got " + result.size() + " rows ");
      } else {
        System.out.println("Failure: " + ar.cause().getMessage());
      }

      // Now close the pool
      client.close();
    });

## Connecting to DB2

Most of the time you will use a pool to connect to DB2:

    DB2ConnectOptions connectOptions = new DB2ConnectOptions()
      .setPort(50000)
      .setHost("the-host")
      .setDatabase("the-db")
      .setUser("user")
      .setPassword("secret");

    // Pool options
    PoolOptions poolOptions = new PoolOptions()
      .setMaxSize(5);

    // Create the pooled client
    SqlClient client = DB2Pool.client(connectOptions, poolOptions);

The pooled client uses a connection pool and any operation will borrow a connection from the pool to execute the operation and release it to the pool.

If you are running with Vert.x you can pass it your Vertx instance:

    DB2ConnectOptions connectOptions = new DB2ConnectOptions()
      .setPort(50000)
      .setHost("the-host")
      .setDatabase("the-db")
      .setUser("user")
      .setPassword("secret");

    // Pool options
    PoolOptions poolOptions = new PoolOptions()
      .setMaxSize(5);
    // Create the pooled client
    SqlClient client = DB2Pool.client(vertx, connectOptions, poolOptions);

You need to release the client when you don’t need it anymore:

    client.close();

When you need to execute several operations on the same connection, you need to acquire a `connection` from a pool

You can easily get one from the pool:

    DB2ConnectOptions connectOptions = new DB2ConnectOptions()
      .setPort(50000)
      .setHost("the-host")
      .setDatabase("the-db")
      .setUser("user")
      .setPassword("secret");

    // Pool options
    PoolOptions poolOptions = new PoolOptions()
      .setMaxSize(5);

    // Create the pooled client
    DB2Pool client = DB2Pool.pool(vertx, connectOptions, poolOptions);

    // Get a connection from the pool
    client.getConnection().compose(conn -> {
      System.out.println("Got a connection from the pool");

      // All operations execute on the same connection
      return conn
        .query("SELECT * FROM users WHERE id='julien'")
        .execute()
        .compose(res -> conn
          .query("SELECT * FROM users WHERE id='emad'")
          .execute())
        .onComplete(ar -> {
          // Release the connection to the pool
          conn.close();
        });
    }).onComplete(ar -> {
      if (ar.succeeded()) {

        System.out.println("Done");
      } else {
        System.out.println("Something went wrong " + ar.cause().getMessage());
      }
    });

Once you are done with the connection you must close it to release it to the pool, so it can be reused.

## Pool versus pooled client

The `DB2Pool` allows you to create a pool or a pooled client

    SqlClient pooledClient = DB2Pool.client(vertx, connectOptions, poolOptions);

    // Pipelined
    Future<RowSet<Row>> res1 = pooledClient.query(sql).execute();

    // Connection pool
    DB2Pool pool = DB2Pool.pool(vertx, connectOptions, poolOptions);

    // Not pipelined
    Future<RowSet<Row>> res2 = pool.query(sql).execute();

- pool operations are not pipelined, only connection client are pipelined

- pooled client operations are pipelined

## Pool sharing

You can share an pool between multiple verticles or instances of the same verticle. Such pool should be created outside a verticle otherwise it will be closed when the verticle that created it is undeployed

    DB2Pool pool = DB2Pool.pool(database, new PoolOptions().setMaxSize(maxSize));
    vertx.deployVerticle(() -> new AbstractVerticle() {
      @Override
      public void start() throws Exception {
        // Use the pool
      }
    }, new DeploymentOptions().setInstances(4));

You can also create a shared pool in each verticle:

    vertx.deployVerticle(() -> new AbstractVerticle() {
      DB2Pool pool;
      @Override
      public void start() {
        // Get or create a shared pool
        // this actually creates a lease to the pool
        // when the verticle is undeployed, the lease will be released automaticaly
        pool = DB2Pool.pool(database, new PoolOptions()
          .setMaxSize(maxSize)
          .setShared(true)
          .setName("my-pool"));
      }
    }, new DeploymentOptions().setInstances(4));

The first time a shared pool is created it will create the resources for the pool. Subsequent calls will reuse this pool and create a lease to this pool. The resources are disposed after all leases have been closed.

By default, a pool reuses the current event-loop when it needs to create a TCP connection. The shared pool will therefore randomly use event-loops of verticles using it.

You can assign a number of event loop a pool will use independently of the context using it

    DB2Pool pool = DB2Pool.pool(database, new PoolOptions()
      .setMaxSize(maxSize)
      .setShared(true)
      .setName("my-pool")
      .setEventLoopSize(4));

## Configuration

There are several alternatives for you to configure the client.

### data object

A simple way to configure the client is to specify a `DB2ConnectOptions` data object.

    DB2ConnectOptions connectOptions = new DB2ConnectOptions()
      .setPort(50000)
      .setHost("the-host")
      .setDatabase("the-db")
      .setUser("user")
      .setPassword("secret");

    // Pool Options
    PoolOptions poolOptions = new PoolOptions().setMaxSize(5);

    // Create the pool from the data object
    DB2Pool pool = DB2Pool.pool(vertx, connectOptions, poolOptions);

    pool.getConnection(ar -> {
      // Handling your connection
    });

You can also configure the generic properties with the `setProperties` or `addProperty` methods. Note `setProperties` will override the default client properties.

### connection uri

Apart from configuring with a `DB2ConnectOptions` data object, We also provide you an alternative way to connect when you want to configure with a connection URI:

    String connectionUri = "db2://dbuser:secretpassword@database.server.com:50000/mydb";

    // Create the pool from the connection URI
    DB2Pool pool = DB2Pool.pool(connectionUri);

    // Create the connection from the connection URI
    DB2Connection.connect(vertx, connectionUri, res -> {
      // Handling your connection
    });

The URI format for a connection string is:

    db2://<USERNAME>:<PASSWORD>@<HOSTNAME>:<PORT>/<DBNAME>

Currently the client supports the following parameter key words in connection uri

- host

- port

- user

- password

- dbname

Note: configuring properties in connection URI will override the default properties.

## Connect retries

You can configure the client to retry when a connection fails to be established.

    options
      .setReconnectAttempts(2)
      .setReconnectInterval(1000);

## Running queries

When you don’t need a transaction or run single queries, you can run queries directly on the pool; the pool will use one of its connection to run the query and return the result to you.

Here is how to run simple queries:

    client
      .query("SELECT * FROM users WHERE id='andy'")
      .execute(ar -> {
      if (ar.succeeded()) {
        RowSet<Row> result = ar.result();
        System.out.println("Got " + result.size() + " rows ");
      } else {
        System.out.println("Failure: " + ar.cause().getMessage());
      }
    });

### Prepared queries

You can do the same with prepared queries.

The SQL string can refer to parameters by position, using the database syntax `?`​

    client
      .preparedQuery("SELECT * FROM users WHERE id=$1")
      .execute(Tuple.of("andy"), ar -> {
      if (ar.succeeded()) {
        RowSet<Row> rows = ar.result();
        System.out.println("Got " + rows.size() + " rows ");
      } else {
        System.out.println("Failure: " + ar.cause().getMessage());
      }
    });

Query methods provides an asynchronous `RowSet` instance that works for SELECT queries

    client
      .preparedQuery("SELECT first_name, last_name FROM users")
      .execute(ar -> {
      if (ar.succeeded()) {
        RowSet<Row> rows = ar.result();
        for (Row row : rows) {
          System.out.println("User " + row.getString(0) + " " + row.getString(1));
        }
      } else {
        System.out.println("Failure: " + ar.cause().getMessage());
      }
    });

or UPDATE/INSERT queries:

    client
      .preparedQuery("INSERT INTO users (first_name, last_name) VALUES ($1, $2)")
      .execute(Tuple.of("Andy", "Guibert"),  ar -> {
      if (ar.succeeded()) {
        RowSet<Row> rows = ar.result();
        System.out.println(rows.rowCount());
      } else {
        System.out.println("Failure: " + ar.cause().getMessage());
      }
    });

The `Row` gives you access to your data by index

    System.out.println("User " + row.getString(0) + " " + row.getString(1));

or by name

    System.out.println("User " + row.getString("first_name") + " " + row.getString("last_name"));

The client will not do any magic here and the column name is identified with the name in the table regardless of how your SQL text is.

You can access a wide variety of of types

    String firstName = row.getString("first_name");
    Boolean male = row.getBoolean("male");
    Integer age = row.getInteger("age");

You can use cached prepared statements to execute one-shot prepared queries:

    connectOptions.setCachePreparedStatements(true);
    client
      .preparedQuery("SELECT * FROM users WHERE id = ?")
      .execute(Tuple.of("julien"), ar -> {
        if (ar.succeeded()) {
          RowSet<Row> rows = ar.result();
          System.out.println("Got " + rows.size() + " rows ");
        } else {
          System.out.println("Failure: " + ar.cause().getMessage());
        }
      });

You can create a `PreparedStatement` and manage the lifecycle by yourself.

    sqlConnection
      .prepare("SELECT * FROM users WHERE id= ?", ar -> {
        if (ar.succeeded()) {
          PreparedStatement preparedStatement = ar.result();
          preparedStatement.query()
            .execute(Tuple.of("julien"), ar2 -> {
              if (ar2.succeeded()) {
                RowSet<Row> rows = ar2.result();
                System.out.println("Got " + rows.size() + " rows ");
                preparedStatement.close();
              } else {
                System.out.println("Failure: " + ar2.cause().getMessage());
              }
            });
        } else {
          System.out.println("Failure: " + ar.cause().getMessage());
        }
      });

### Batches

You can execute prepared batch

    List<Tuple> batch = new ArrayList<>();
    batch.add(Tuple.of("julien", "Julient Viet"));
    batch.add(Tuple.of("emad", "Emad Alblueshi"));
    batch.add(Tuple.of("andy", "Andy Guibert"));

    // Execute the prepared batch
    client
      .preparedQuery("INSERT INTO USERS (id, name) VALUES ($1, $2)")
      .executeBatch(batch, res -> {
      if (res.succeeded()) {

        // Process rows
        RowSet<Row> rows = res.result();
      } else {
        System.out.println("Batch failed " + res.cause());
      }
    });

You can fetch generated keys by wrapping your query in `SELECT <COLUMNS> FROM FINAL TABLE ( <SQL> )`, for example:

    client
      .preparedQuery("SELECT color_id FROM FINAL TABLE ( INSERT INTO color (color_name) VALUES (?), (?), (?) )")
      .execute(Tuple.of("white", "red", "blue"), ar -> {
      if (ar.succeeded()) {
        RowSet<Row> rows = ar.result();
        System.out.println("Inserted " + rows.rowCount() + " new rows.");
        for (Row row : rows) {
          System.out.println("generated key: " + row.getInteger("color_id"));
        }
      } else {
        System.out.println("Failure: " + ar.cause().getMessage());
      }
    });

## Using connections

### Getting a connection

When you need to execute sequential queries (without a transaction), you can create a new connection or borrow one from the pool. Remember that between acquiring the connection from the pool and returning it to the pool, you should take care of the connection because it might be closed by the server for some reason such as an idle time out.

    pool
      .getConnection()
      .compose(connection ->
        connection
          .preparedQuery("INSERT INTO Users (first_name,last_name) VALUES (?, ?)")
          .executeBatch(Arrays.asList(
            Tuple.of("Julien", "Viet"),
            Tuple.of("Andy", "Guibert")
          ))
          .compose(res -> connection
            // Do something with rows
            .query("SELECT COUNT(*) FROM Users")
            .execute()
            .map(rows -> rows.iterator().next().getInteger(0)))
          // Return the connection to the pool
          .eventually(v -> connection.close())
      ).onSuccess(count -> {
      System.out.println("Insert users, now the number of users is " + count);
    });

Prepared queries can be created:

    connection
      .prepare("SELECT * FROM users WHERE first_name LIKE $1")
      .compose(pq ->
        pq.query()
          .execute(Tuple.of("Andy"))
          .eventually(v -> pq.close())
      ).onSuccess(rows -> {
      // All rows
    });

### Simplified connection API

When you use a pool, you can call `withConnection` to pass it a function executed within a connection.

It borrows a connection from the pool and calls the function with this connection.

The function must return a future of an arbitrary result.

After the future completes, the connection is returned to the pool and the overall result is provided.

    pool.withConnection(connection ->
      connection
        .preparedQuery("INSERT INTO Users (first_name,last_name) VALUES (?, ?)")
        .executeBatch(Arrays.asList(
          Tuple.of("Julien", "Viet"),
          Tuple.of("Andy", "Guibert")
        ))
        .compose(res -> connection
          // Do something with rows
          .query("SELECT COUNT(*) FROM Users")
          .execute()
          .map(rows -> rows.iterator().next().getInteger(0)))
    ).onSuccess(count -> {
      System.out.println("Insert users, now the number of users is " + count);
    });

## Using transactions

### Transactions with connections

You can execute transaction using SQL `BEGIN`/`COMMIT`/`ROLLBACK`, if you do so you must use a `SqlConnection` and manage it yourself.

Or you can use the transaction API of `SqlConnection`:

    pool.getConnection()
      // Transaction must use a connection
      .onSuccess(conn -> {
        // Begin the transaction
        conn.begin()
          .compose(tx -> conn
            // Various statements
            .query("INSERT INTO Users (first_name,last_name) VALUES ('Julien','Viet')")
            .execute()
            .compose(res2 -> conn
              .query("INSERT INTO Users (first_name,last_name) VALUES ('Andy','Guibert')")
              .execute())
            // Commit the transaction
            .compose(res3 -> tx.commit()))
          // Return the connection to the pool
          .eventually(v -> conn.close())
          .onSuccess(v -> System.out.println("Transaction succeeded"))
          .onFailure(err -> System.out.println("Transaction failed: " + err.getMessage()));
      });

When the database server reports the current transaction is failed (e.g the infamous current transaction is aborted, commands ignored until end of transaction block), the transaction is rollbacked and the `completion` future is failed with a `TransactionRollbackException`:

    tx.completion()
      .onFailure(err -> {
        System.out.println("Transaction failed => rolled back");
      });

### Simplified transaction API

When you use a pool, you can call `withTransaction` to pass it a function executed within a transaction.

It borrows a connection from the pool, begins the transaction and calls the function with a client executing all operations in the scope of this transaction.

The function must return a future of an arbitrary result:

- when the future succeeds the client will commit the transaction

- when the future fails the client will rollback the transaction

After the transaction completes, the connection is returned to the pool and the overall result is provided.

    pool.withTransaction(client -> client
      .query("INSERT INTO Users (first_name,last_name) VALUES ('Julien','Viet')")
      .execute()
      .flatMap(res -> client
        .query("INSERT INTO Users (first_name,last_name) VALUES ('Andy','Guibert')")
        .execute()
        // Map to a message result
        .map("Users inserted")))
      .onSuccess(v -> System.out.println("Transaction succeeded"))
      .onFailure(err -> System.out.println("Transaction failed: " + err.getMessage()));

## Cursors and streaming

By default prepared query execution fetches all rows, you can use a `Cursor` to control the amount of rows you want to read:

    connection.prepare("SELECT * FROM users WHERE first_name LIKE $1", ar0 -> {
      if (ar0.succeeded()) {
        PreparedStatement pq = ar0.result();

        // Cursors require to run within a transaction
        connection.begin(ar1 -> {
          if (ar1.succeeded()) {
            Transaction tx = ar1.result();

            // Create a cursor
            Cursor cursor = pq.cursor(Tuple.of("julien"));

            // Read 50 rows
            cursor.read(50, ar2 -> {
              if (ar2.succeeded()) {
                RowSet<Row> rows = ar2.result();

                // Check for more ?
                if (cursor.hasMore()) {
                  // Repeat the process...
                } else {
                  // No more rows - commit the transaction
                  tx.commit();
                }
              }
            });
          }
        });
      }
    });

Cursors shall be closed when they are released prematurely:

    cursor.read(50, ar2 -> {
      if (ar2.succeeded()) {
        // Close the cursor
        cursor.close();
      }
    });

A stream API is also available for cursors, which can be more convenient, specially with the Rxified version.

    connection.prepare("SELECT * FROM users WHERE first_name LIKE $1", ar0 -> {
      if (ar0.succeeded()) {
        PreparedStatement pq = ar0.result();

        // Streams require to run within a transaction
        connection.begin(ar1 -> {
          if (ar1.succeeded()) {
            Transaction tx = ar1.result();

            // Fetch 50 rows at a time
            RowStream<Row> stream = pq.createStream(50, Tuple.of("julien"));

            // Use the stream
            stream.exceptionHandler(err -> {
              System.out.println("Error: " + err.getMessage());
            });
            stream.endHandler(v -> {
              tx.commit();
              System.out.println("End of stream");
            });
            stream.handler(row -> {
              System.out.println("User: " + row.getString("last_name"));
            });
          }
        });
      }
    });

The stream read the rows by batch of `50` and stream them, when the rows have been passed to the handler, a new batch of `50` is read and so on.

The stream can be resumed or paused, the loaded rows will remain in memory until they are delivered and the cursor will stop iterating.

## Tracing queries

The SQL client can trace query execution when Vert.x has tracing enabled.

The client reports the following client spans:

- `Query` operation name

- tags

- `db.user`: the database username

- `db.instance`: the database instance

- `db.statement`: the SQL query

- `db.type`: sql

The default tracing policy is `PROPAGATE`, the client will only create a span when involved in an active trace.

You can change the client policy with `setTracingPolicy`, e.g you can set `ALWAYS` to always report a span:

    options.setTracingPolicy(TracingPolicy.ALWAYS);

## DB2 type mapping

Currently the client supports the following DB2 types

- BOOLEAN (`java.lang.Boolean`) (DB2 LUW only)

- SMALLINT (`java.lang.Short`)

- INTEGER (`java.lang.Integer`)

- BIGINT (`java.lang.Long`)

- REAL (`java.lang.Float`)

- DOUBLE (`java.lang.Double`)

- DECIMAL (`io.vertx.sqlclient.data.Numeric`)

- CHAR (`java.lang.String`)

- VARCHAR (`java.lang.String`)

- ENUM (`java.lang.String`)

- DATE (`java.time.LocalDate`)

- TIME (`java.time.LocalTime`)

- TIMESTAMP (`java.time.LocalDateTime`)

- BINARY (`byte[]`)

- VARBINARY (`byte[]`)

- ROWID (`io.vertx.db2client.impl.drda.DB2RowId` or `java.sql.RowId`) (DB2 z/OS only)

Some types that are currently NOT supported are:

- XML

- BLOB

- CLOB

- DBCLOB

- GRAPHIC / VARGRAPHIC

For a further documentation on DB2 data types, see the following resources:

- DB2 for LUW 11.5 data types

- DB2 for z/OS 12.0 data types

Tuple decoding uses the above types when storing values, it also performs on the fly conversion of the actual value when possible:

    pool
      .query("SELECT an_int_column FROM exampleTable")
      .execute(ar -> {
      RowSet<Row> rowSet = ar.result();
      Row row = rowSet.iterator().next();

      // Stored as INTEGER column type and represented as java.lang.Integer
      Object value = row.getValue(0);

      // Convert to java.lang.Long
      Long longValue = row.getLong(0);
    });

## Using Java enum types

You can map Java enum types to these column types:

- Strings (VARCHAR, TEXT)

- Numbers (SMALLINT, INTEGER, BIGINT)

    client.preparedQuery("SELECT day_name FROM FINAL TABLE ( INSERT INTO days (day_name) VALUES (?), (?), (?) )")
    .execute(Tuple.of(Days.FRIDAY, Days.SATURDAY, Days.SUNDAY), ar -> {
     if (ar.succeeded()) {
      RowSet<Row> rows = ar.result();
      System.out.println("Inserted " + rows.rowCount() + " new rows");
      for (Row row : rows) {
              System.out.println("Day: " + row.get(Days.class, "day_name"));
      }
     } else {
      System.out.println("Failure: " + ar.cause().getMessage());
     }
    });
    client.preparedQuery("SELECT day_num FROM FINAL TABLE ( INSERT INTO days (day_num) VALUES (?), (?), (?) )")
       .execute(Tuple.of(Days.FRIDAY.ordinal(), Days.SATURDAY.ordinal(), Days.SUNDAY.ordinal()), ar -> {
               if (ar.succeeded()) {
                       RowSet<Row> rows = ar.result();
                       System.out.println("Inserted " + rows.rowCount() + " new rows");
                       for (Row row : rows) {
                               System.out.println("Day: " + row.get(Days.class, "day_num"));
                       }
               } else {
                       System.out.println("Failure: " + ar.cause().getMessage());
               }
       });

The String type is matched with the Java enum’s name returned by the `name()` method.

Number types are matched with the Java enum’s ordinal returned by the `ordinal()` method and the row.get() method returns the corresponding enum’s `name()` value at the ordinal position of the integer value retrieved.

## Collector queries

You can use Java collectors with the query API:

    Collector<Row, ?, Map<Long, String>> collector = Collectors.toMap(
      row -> row.getLong("id"),
      row -> row.getString("last_name"));

    // Run the query with the collector
    client.query("SELECT * FROM users")
      .collecting(collector)
      .execute(ar -> {
      if (ar.succeeded()) {
        SqlResult<Map<Long, String>> result = ar.result();

        // Get the map created by the collector
        Map<Long, String> map = result.value();
        System.out.println("Got " + map);
      } else {
        System.out.println("Failure: " + ar.cause().getMessage());
      }
    });

The collector processing must not keep a reference on the `Row` as there is a single row used for processing the entire set.

The Java `Collectors` provides many interesting predefined collectors, for example you can create easily create a string directly from the row set:

    Collector<Row, ?, String> collector = Collectors.mapping(
      row -> row.getString("last_name"),
      Collectors.joining(",", "(", ")")
    );

    // Run the query with the collector
    client.query("SELECT * FROM users").collecting(collector).execute(ar -> {
        if (ar.succeeded()) {
          SqlResult<String> result = ar.result();

          // Get the string created by the collector
          String list = result.value();
          System.out.println("Got " + list);
        } else {
          System.out.println("Failure: " + ar.cause().getMessage());
        }
      });

## Using SSL/TLS

To configure the client to use SSL connection, you can configure the `DB2ConnectOptions` like a Vert.x `NetClient`.

    DB2ConnectOptions options = new DB2ConnectOptions()
      .setPort(50001)
      .setHost("the-host")
      .setDatabase("the-db")
      .setUser("user")
      .setPassword("secret")
      .setSsl(true)
      .setTrustStoreOptions(new JksOptions()
          .setPath("/path/to/keystore.p12")
          .setPassword("keystoreSecret"));

    DB2Connection.connect(vertx, options, res -> {
      if (res.succeeded()) {
        // Connected with SSL
      } else {
        System.out.println("Could not connect " + res.cause());
      }
    });

More information can be found in the Vert.x documentation.

## Using a proxy

You can also configure the client to use an HTTP/1.x CONNECT, SOCKS4a or SOCKS5 proxy.

More information can be found in the Vert.x documentation.

## Advanced pool configuration

### Server load balancing

You can configure the pool with a list of servers instead of a single server.

    DB2Pool pool = DB2Pool.pool(Arrays.asList(server1, server2, server3), options);

The pool uses a round-robin load balancing when a connection is created to select different servers.

Note | this provides load balancing when the connection is created and not when the connection is borrowed from the pool.

### Pool connection initialization

You can use the `connectHandler` to interact with a connection after it has been created and before it is inserted in the pool.

    pool.connectHandler(conn -> {
      conn.query(sql).execute().onSuccess(res -> {
        // Release the connection to the pool, ready to be used by the application
        conn.close();
      });
    });

Once you are done with the connection, you should simply close it to signal the pool to use it.

Unresolved directive in index.adoc - include::override/rxjava3.adoc[]