/*
 * Licensed to Elasticsearch under one or more contributor
 * license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright
 * ownership. Elasticsearch 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.elasticsearch.transport;

import org.apache.logging.log4j.util.Supplier;
import org.elasticsearch.cluster.node.DiscoveryNode;
import org.elasticsearch.common.io.stream.StreamInput;
import org.elasticsearch.common.io.stream.StreamOutput;
import org.elasticsearch.threadpool.ThreadPool;

import java.io.IOException;
import java.io.UncheckedIOException;

/**
 * TransportActionProxy allows an arbitrary action to be executed on a defined target node while the initial request is sent to a second
 * node that acts as a request proxy to the target node. This is useful if a node is not directly connected to a target node but is
 * connected to an intermediate node that establishes a transitive connection.
 */
public final class TransportActionProxy {

    private TransportActionProxy() {} // no instance

    private static class ProxyRequestHandler<T extends ProxyRequest> implements TransportRequestHandler<T> {

        private final TransportService service;
        private final String action;
        private final Supplier<TransportResponse> responseFactory;

        ProxyRequestHandler(TransportService service, String action, Supplier<TransportResponse> responseFactory) {
            this.service = service;
            this.action = action;
            this.responseFactory = responseFactory;
        }

        @Override
        public void messageReceived(T request, TransportChannel channel) throws Exception {
            DiscoveryNode targetNode = request.targetNode;
            TransportRequest wrappedRequest = request.wrapped;
            service.sendRequest(targetNode, action, wrappedRequest, new ProxyResponseHandler<>(channel, responseFactory));
        }
    }

    private static class ProxyResponseHandler<T extends TransportResponse> implements TransportResponseHandler<T> {

        private final Supplier<T> responseFactory;
        private final TransportChannel channel;

        ProxyResponseHandler(TransportChannel channel, Supplier<T> responseFactory) {
            this.responseFactory = responseFactory;
            this.channel = channel;

        }
        @Override
        public T newInstance() {
            return responseFactory.get();
        }

        @Override
        public void handleResponse(T response) {
            try {
                channel.sendResponse(response);
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
        }

        @Override
        public void handleException(TransportException exp) {
            try {
                channel.sendResponse(exp);
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
        }

        @Override
        public String executor() {
            return ThreadPool.Names.SAME;
        }
    }

    static class ProxyRequest<T extends TransportRequest> extends TransportRequest {
        T wrapped;
        Supplier<T> supplier;
        DiscoveryNode targetNode;

        ProxyRequest(Supplier<T> supplier) {
            this.supplier = supplier;
        }

        ProxyRequest(T wrapped, DiscoveryNode targetNode) {
            this.wrapped = wrapped;
            this.targetNode = targetNode;
        }

        @Override
        public void readFrom(StreamInput in) throws IOException {
            super.readFrom(in);
            targetNode = new DiscoveryNode(in);
            wrapped = supplier.get();
            wrapped.readFrom(in);
        }

        @Override
        public void writeTo(StreamOutput out) throws IOException {
            super.writeTo(out);
            targetNode.writeTo(out);
            wrapped.writeTo(out);
        }
    }

    /**
     * Registers a proxy request handler that allows to forward requests for the given action to another node.
     */
    public static void registerProxyAction(TransportService service, String action, Supplier<TransportResponse> responseSupplier) {
        RequestHandlerRegistry requestHandler = service.getRequestHandler(action);
        service.registerRequestHandler(getProxyAction(action), () -> new ProxyRequest(requestHandler::newRequest), ThreadPool.Names.SAME,
            true, false, new ProxyRequestHandler<>(service, action, responseSupplier));
    }

    private static final String PROXY_ACTION_PREFIX = "internal:transport/proxy/";
    /**
     * Returns the corresponding proxy action for the given action
     */
    public static String getProxyAction(String action) {
        return  PROXY_ACTION_PREFIX + action;
    }

    /**
     * Wraps the actual request in a proxy request object that encodes the target node.
     */
    public static TransportRequest wrapRequest(DiscoveryNode node, TransportRequest request) {
        return new ProxyRequest<>(request, node);
    }

    /**
     * Unwraps a proxy request and returns the original request
     */
    public static TransportRequest unwrapRequest(TransportRequest request) {
        if (request instanceof ProxyRequest) {
            return ((ProxyRequest)request).wrapped;
        }
        return request;
    }
}
