content: /*
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You under the Apache License, Version 2.0
   * (the "License"); you may not use this file except in compliance with
   * the License.  You may obtain a copy of the License at
   *
   *    http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.arrow.flight;
  
  import java.util.Objects;
  
  /**
   * Server-side middleware for Flight calls.
   *
   * <p>Middleware are instantiated per-call.
   *
   * <p>Methods are not guaranteed to be called on any particular thread, relative to the thread that
   * Flight requests are executed on. Do not depend on thread-local storage; instead, use state on the
   * middleware instance. Service implementations may communicate with middleware implementations
   * through {@link org.apache.arrow.flight.FlightProducer.CallContext#getMiddleware(Key)}. Methods on
   * the middleware instance are non-reentrant, that is, a particular RPC will not make multiple
   * concurrent calls to methods on a single middleware instance. However, methods on the factory
   * instance are expected to be thread-safe, and if the factory instance returns the same middleware
   * object more than once, then that middleware object must be thread-safe.
   */
  public interface FlightServerMiddleware {
  
    /**
     * A factory for Flight server middleware.
     *
     * @param <T> The middleware type.
     */
    interface Factory<T extends FlightServerMiddleware> {
      /**
       * A callback for when the call starts.
       *
       * @param info Details about the call.
       * @param incomingHeaders A mutable set of request headers.
       * @param context Context about the current request.
       * @throws FlightRuntimeException if the middleware wants to reject the call with the given
       *     status
       */
      T onCallStarted(CallInfo info, CallHeaders incomingHeaders, RequestContext context);
    }
  
    /**
     * A key for Flight server middleware. On a server, middleware instances are identified by this
     * key.
     *
     * <p>Keys use reference equality, so instances should be shared.
     *
     * @param <T> The middleware class stored in this key. This provides a compile-time check when
     *     retrieving instances.
     */
    class Key<T extends FlightServerMiddleware> {
      final String key;
  
      Key(String key) {
        this.key = Objects.requireNonNull(key, "Key must not be null.");
      }
  
      /** Create a new key for the given type. */
      public static <T extends FlightServerMiddleware> Key<T> of(String key) {
        return new Key<>(key);
      }
    }
  
    /**
     * Callback for when the underlying transport is about to send response headers.
     *
     * @param outgoingHeaders A mutable set of response headers. These can be manipulated to send
     *     different headers to the client.
     */
    void onBeforeSendingHeaders(CallHeaders outgoingHeaders);
  
    /**
     * Callback for when the underlying transport has completed a call.
     *
     * @param status Whether the call completed successfully or not.
     */
    void onCallCompleted(CallStatus status);
  
    /**
     * Callback for when an RPC method implementation throws an uncaught exception.
     *
     * <p>May be called multiple times, and may be called before or after {@link
     * #onCallCompleted(CallStatus)}. Generally, an uncaught exception will end the call with a error
     * {@link CallStatus}, and will be reported to {@link #onCallCompleted(CallStatus)}, but not
     * necessarily this method.
     *
     * @param err The exception that was thrown.
     */
    void onCallErrored(Throwable err);
  }
  