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/*
 * Copyright 2014 The gRPC Authors
 *
 * Licensed 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 io.grpc;

import javax.annotation.concurrent.ThreadSafe;

/**
 * Interface for intercepting outgoing calls before they are dispatched by a {@link Channel}.
 *
 * Implementers use this mechanism to add cross-cutting behavior to {@link Channel} and
 * stub implementations. Common examples of such behavior include:
 * 

 * Logging and monitoring call behavior
 * Adding metadata for proxies to observe
 * Request and response rewriting
 * 
 *
 * Providing authentication credentials is better served by {@link
 * CallCredentials}. But a {@code ClientInterceptor} could set the {@code
 * CallCredentials} within the {@link CallOptions}.
 *
 * 
The interceptor may be called for multiple {@link ClientCall calls} by one or more threads
 * without completing the previous ones first. Refer to the
 * {@link io.grpc.ClientCall.Listener ClientCall.Listener} docs for more details regarding thread
 * safety of the returned listener.
 */
@ThreadSafe
public interface ClientInterceptor {
  /**
   * Intercept {@link ClientCall} creation by the {@code next} {@link Channel}.
   *
   * 
Many variations of interception are possible. Complex implementations may return a wrapper
   * around the result of {@code next.newCall()}, whereas a simpler implementation may just modify
   * the header metadata prior to returning the result of {@code next.newCall()}.
   *
   * {@code next.newCall()} must not be called under a different {@link Context}
   * other than the current {@code Context}. The outcome of such usage is undefined and may cause
   * memory leak due to unbounded chain of {@code Context}s.
   *
   * @param method the remote method to be called.
   * @param callOptions the runtime options to be applied to this call.
   * @param next the channel which is being intercepted.
   * @return the call object for the remote operation, never {@code null}.
   */
   ClientCall interceptCall(
      MethodDescriptor method, CallOptions callOptions, Channel next);
}