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

    MySQLPool pool = MySQLPool.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() {
      MySQLPool 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 = MySQLPool.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

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