Request execution

The Netty pipeline gives us the ability to send low-level protocol messages on a single connection.

The request execution layer builds upon that to:

  • manage multiple connections (many nodes, possibly many connections per node);
  • abstract the protocol layer behind higher-level, user-facing types.

The session is the main entry point. CqlSession is the type that users will most likely reference in their applications. It extends a more generic Session type, for the sake of extensibility; this will be explained in Request processors.

Text Diagram

DefaultSession contains the session implementation. It follows the confined inner class pattern to simplify concurrency.

Connection pooling

Text Diagram

ChannelPool handles the connections to a given node, for a given session. It follows the confined inner class pattern to simplify concurrency. There are a few differences compared to the 3.x implementation:

Fixed size

The pool has a fixed number of connections, it doesn’t grow or shrink dynamically based on current usage. In other words, there is no more “max” size, only a “core” size.

However, this size is specified in the configuration. If the value is changed at runtime, the driver will detect it, and trigger a resize of all active pools.

The rationale for removing the dynamic behavior is that it introduced a ton of complexity in the implementation and configuration, for unclear benefits: if the load fluctuates very rapidly, then you need to provision for the max size anyway, so you might as well run with all the connections all the time. If on the other hand the fluctuations are rare and predictable (e.g. peak for holiday sales), then a manual configuration change is good enough.


To get a connection to a node, client code calls This returns the less busy connection, based on the the getAvailableIds() counter exposed by InFlightHandler.

If all connections are busy, there is no queuing; the driver moves to the next node immediately. The rationale is that it’s better to try another node that might be ready to reply, instead of introducing an additional wait for each node. If the user wants queuing when all nodes are busy, it’s better to do it at the session level with a throttler, which provides more intuitive configuration.

Also, note that there is no preemptive acquisition of the stream id outside of the event loop: we select a channel based on a volatile counter, so a race condition is possible; if the channel gets full by the time we arrive in InFlightHandler, the client will simply get a BusyConnectionException and move on to the next node. We only acquire stream ids from the event loop, which makes it much easier to track the current load (in driver 3, “inflight count getting out of sync” bugs were very frequent).

The pool manages its channels with ChannelSet, a simple copy-on-write data structure.

Built-in reconnection

The pool has its own independent reconnection mechanism (based on the Reconnection utility class). The goal is to keep the pool at its expected capacity: whenever a connection is lost, the task starts and will try to reopen the missing connections at regular intervals.

Request processors

Text Diagram

The driver can execute different types of requests, in different ways. This is abstracted by the top-level Session interface, with a very generic execution method:

<RequestT extends Request, ResultT> ResultT execute(
      RequestT request, GenericType<ResultT> resultType);

It takes a request, and a type token that serves as a hint at the expected result. Each (RequestT, ResultT) combination defines an execution model, for example:

RequestT ResultT Execution
Statement ResultSet CQL, synchronous
Statement CompletionStage<AsyncResultSet> CQL, asynchronous
Statement ReactiveResultSet CQL, reactive
GraphStatement GraphResultSet DSE Graph, synchronous
GraphStatement CompletionStage<AsyncGraphResultSet> DSE Graph, asynchronous

In general, regular client code doesn’t use Session.execute directly. Instead, child interfaces expose more user-friendly shortcuts for a given result type:

public interface CqlSession extends Session {
  default ResultSet execute(Statement<?> statement) {
    return execute(statement, Statement.SYNC);

The logic for each execution model is encapsulated in a RequestProcessor<RequestT, ResultT>. Processors are stored in a RequestProcessorRegistry. For each request, the session invokes the registry to find the processor that matches the request and result types.

Text Diagram

A processor is responsible for:

  • converting the user request into protocol-level messages;
  • selecting a coordinator node, and obtaining a channel from its connection pool;
  • writing the request to the channel;
  • handling timeouts, retries and speculative executions;
  • translating the response into user-level types.

The RequestProcessor interface makes very few assumptions about the actual processing; but in general, implementations create a handler for the lifecycle of every request. For example, CqlRequestHandler is the central component for basic CQL execution.

Processors can be implemented in terms of other processors. In particular, this is the case for synchronous execution models, which are just a blocking wrapper around their asynchronous counterpart. You can observe this in CqlRequestSyncProcessor.

Note that preparing a statement is treated as just another execution model. It has its own processors, that operate on a special PrepareRequest type:

public interface CqlSession extends Session {
  default PreparedStatement prepare(SimpleStatement statement) {
    return execute(new DefaultPrepareRequest(statement), PrepareRequest.SYNC);

Extension points


You can customize the set of request processors by extending the context and overriding buildRequestProcessorRegistry.

This can be used to either:

  • add your own execution models (new request types and/or return types);
  • remove existing ones;
  • or a combination of both.

The driver codebase contains an integration test that provides a complete example: RequestProcessorIT. It shows how you can build a session that returns Guava’s ListenableFuture instead of Java’s CompletionStage (existing request type, different return type).

GuavaDriverContext is the custom context subclass. It plugs a custom registry that wraps the default async processors with GuavaRequestAsyncProcessor, to transform the returned futures.

Note that the default async processors are not present in the registry anymore; if you try to call a method that returns a CompletionStage, it fails. See the next section for how to hide those methods.

Exposing a custom session interface

If you add or remove execution models, you probably want to expose a session interface that matches the underlying capabilities of the implementation.

For example, in the RequestProcessorIT example mentioned in the previous section, we remove the ability to return CompletionStage, but add the ability to return ListenableFuture. Therefore we expose a custom GuavaSession with a different return type for async methods:

public interface GuavaSession extends Session {
  default ListenableFuture<AsyncResultSet> executeAsync(Statement<?> statement) { ... }
  default ListenableFuture<PreparedStatement> prepareAsync(SimpleStatement statement) { ... }

We need an implementation of this interface. Our new methods all have default implementations in term of the abstract Session.execute(), so the only thing we need is to delegate to an existing Session. The driver provides SessionWrapper to that effect. See DefaultGuavaSession:

public class DefaultGuavaSession extends SessionWrapper implements GuavaSession {
  public DefaultGuavaSession(Session delegate) {

Finally, we want to create an instance of this wrapper. Since we extended the context (see previous section), we already wrote a custom builder subclass; there is another protected method we can override to plug our wrapper. See GuavaSessionBuilder:

public class GuavaSessionBuilder extends SessionBuilder<GuavaSessionBuilder, GuavaSession> {

  protected DriverContext buildContext( ... ) { ... }

  protected GuavaSession wrap(CqlSession defaultSession) {
    return new DefaultGuavaSession(defaultSession);

Client code can now use the familiar pattern to create a session:

GuavaSession session = new GuavaSessionBuilder()