By design, the HBase RPC client multiplexes calls to a given region server (or the master for that matter) over a single socket, access to which is managed by a connection thread defined in the HBaseClient class. While this approach may suffice for most cases, it tends to break down in the context of a real-time, multi-threaded server, where latencies need to be lower and throughputs higher.
In brief, the problem is that we dedicate one thread to handle all client-side reads and writes for a given server, which in turn forces them to share the same socket. As load increases, this is bound to serialize calls on the client-side. In particular, when the rate at which calls are submitted to the connection thread is greater than that at which the server responds, then some of those calls will inevitably end up sitting idle, just waiting their turn to go over the wire.
In general, sharing sockets across multiple client threads is a good idea, but limiting the number of such sockets to one may be overly restrictive for certain cases. Here, we propose a way of defining multiple sockets per server endpoint, access to which may be managed through either a load-balancing or thread-local pool. To that end, we define the notion of a SharedMap, which maps a key to a resource pool, and supports both of those pool types. Specifically, we will apply that map in the HBaseClient, to associate multiple connection threads with each server endpoint (denoted by a connection id).
Currently, the SharedMap supports the following types of pools:
- A ThreadLocalPool, which represents a pool that builds on the ThreadLocal class. It essentially binds the resource to the thread from which it is accessed.
- A ReusablePool, which represents a pool that builds on the LinkedList class. It essentially allows resources to be checked out, at which point it is (temporarily) removed from the pool. When the resource is no longer required, it should be returned to the pool in order to be reused.
- A RoundRobinPool, which represents a pool that stores its resources in an ArrayList. It load-balances access to its resources by returning a different resource every time a given key is looked up.
To control the type and size of the connection pools, we give the user a couple of parameters (viz. "hbase.client.ipc.pool.type" and "hbase.client.ipc.pool.size"). In case the size of the pool is set to a non-zero positive number, that is used to cap the number of resources that a pool may contain for any given key. A size of Integer#MAX_VALUE is interpreted to mean an unbounded pool.