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

    Pool pool = DB2Builder.pool()
      .with(new PoolOptions().setMaxSize(maxSize))
      .connectingTo(database)
      .using(vertx)
      .build();
    vertx.deployVerticle(() -> new VerticleBase() {
      @Override
      public Future<?> start() throws Exception {
        // Use the pool
        return super.start();
      }
    }, new DeploymentOptions().setInstances(4));

You can also create a shared pool in each verticle:

    vertx.deployVerticle(() -> new VerticleBase() {
      Pool pool;
      @Override
      public Future<?> start() throws Exception {
        // 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 = DB2Builder.pool()
          .with(new PoolOptions()
            .setMaxSize(maxSize)
            .setShared(true)
            .setName("my-pool"))
          .connectingTo(database)
          .using(vertx)
          .build();
        return super.start();
      }
    }, 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

    Pool pool = DB2Builder.pool()
      .with(new PoolOptions()
        .setMaxSize(maxSize)
        .setShared(true)
        .setName("my-pool")
        .setEventLoopSize(4))
      .connectingTo(database)
      .using(vertx)
      .build();