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/**
 * Copyright 2016 Netflix, Inc.
 * 
 * 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 com.netflix.hystrix.contrib.sample.stream;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import rx.Observable;
import rx.Subscriber;
import rx.Subscription;
import rx.schedulers.Schedulers;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.concurrent.atomic.AtomicBoolean;

/**
 */
public abstract class HystrixSampleSseServlet extends HttpServlet {
    protected final Observable sampleStream;

    private static final Logger logger = LoggerFactory.getLogger(HystrixSampleSseServlet.class);

    //wake up occasionally and check that poller is still alive.  this value controls how often
    protected static final int DEFAULT_PAUSE_POLLER_THREAD_DELAY_IN_MS = 500;

    private final int pausePollerThreadDelayInMs;

    /* Set to true upon shutdown, so it's OK to be shared among all SampleSseServlets */
    private static volatile boolean isDestroyed = false;

    protected HystrixSampleSseServlet(Observable sampleStream) {
        this.sampleStream = sampleStream;
        this.pausePollerThreadDelayInMs = DEFAULT_PAUSE_POLLER_THREAD_DELAY_IN_MS;
    }

    protected HystrixSampleSseServlet(Observable sampleStream, int pausePollerThreadDelayInMs) {
        this.sampleStream = sampleStream;
        this.pausePollerThreadDelayInMs = pausePollerThreadDelayInMs;
    }

    protected abstract int getMaxNumberConcurrentConnectionsAllowed();

    protected abstract int getNumberCurrentConnections();

    protected abstract int incrementAndGetCurrentConcurrentConnections();

    protected abstract void decrementCurrentConcurrentConnections();

    /**
     * Handle incoming GETs
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        if (isDestroyed) {
            response.sendError(503, "Service has been shut down.");
        } else {
            handleRequest(request, response);
        }
    }

    /**
     * WebSphere won't shutdown a servlet until after a 60 second timeout if there is an instance of the servlet executing
     * a request.  Add this method to enable a hook to notify Hystrix to shutdown.  You must invoke this method at
     * shutdown, perhaps from some other servlet's destroy() method.
     */
    public static void shutdown() {
        isDestroyed = true;
    }

    @Override
    public void init() throws ServletException {
        isDestroyed = false;
    }

    /**
     * Handle servlet being undeployed by gracefully releasing connections so poller threads stop.
     */
    @Override
    public void destroy() {
        /* set marker so the loops can break out */
        isDestroyed = true;
        super.destroy();
    }

    /**
     * - maintain an open connection with the client
     * - on initial connection send latest data of each requested event type
     * - subsequently send all changes for each requested event type
     *
     * @param request  incoming HTTP Request
     * @param response outgoing HTTP Response (as a streaming response)
     * @throws javax.servlet.ServletException
     * @throws java.io.IOException
     */
    private void handleRequest(HttpServletRequest request, final HttpServletResponse response) throws ServletException, IOException {
        final AtomicBoolean moreDataWillBeSent = new AtomicBoolean(true);
        Subscription sampleSubscription = null;

        /* ensure we aren't allowing more connections than we want */
        int numberConnections = incrementAndGetCurrentConcurrentConnections();
        try {
            int maxNumberConnectionsAllowed = getMaxNumberConcurrentConnectionsAllowed(); //may change at runtime, so look this up for each request
            if (numberConnections > maxNumberConnectionsAllowed) {
                response.sendError(503, "MaxConcurrentConnections reached: " + maxNumberConnectionsAllowed);
            } else {
                /* initialize response */
                response.setHeader("Content-Type", "text/event-stream;charset=UTF-8");
                response.setHeader("Cache-Control", "no-cache, no-store, max-age=0, must-revalidate");
                response.setHeader("Pragma", "no-cache");

                final PrintWriter writer = response.getWriter();

                //since the sample stream is based on Observable.interval, events will get published on an RxComputation thread
                //since writing to the servlet response is blocking, use the Rx IO thread for the write that occurs in the onNext
                sampleSubscription = sampleStream
                        .observeOn(Schedulers.io())
                        .subscribe(new Subscriber() {
                            @Override
                            public void onCompleted() {
                                logger.error("HystrixSampleSseServlet: ({}) received unexpected OnCompleted from sample stream", getClass().getSimpleName());
                                moreDataWillBeSent.set(false);
                            }

                            @Override
                            public void onError(Throwable e) {
                                moreDataWillBeSent.set(false);
                            }

                            @Override
                            public void onNext(String sampleDataAsString) {
                                if (sampleDataAsString != null) {
                                    writer.print("data: " + sampleDataAsString + "\n\n");
                                    // explicitly check for client disconnect - PrintWriter does not throw exceptions
                                    if (writer.checkError()) {
                                        moreDataWillBeSent.set(false);
                                    }
                                    writer.flush();
                                }
                            }
                        });

                while (moreDataWillBeSent.get() && !isDestroyed) {
                    try {
                        Thread.sleep(pausePollerThreadDelayInMs);
                        //in case stream has not started emitting yet, catch any clients which connect/disconnect before emits start
                        writer.print("ping: \n\n");
                        // explicitly check for client disconnect - PrintWriter does not throw exceptions
                        if (writer.checkError()) {
                            moreDataWillBeSent.set(false);
                        }
                        writer.flush();
                    } catch (InterruptedException e) {
                        moreDataWillBeSent.set(false);
                    }
                }
            }
        } finally {
            decrementCurrentConcurrentConnections();
            if (sampleSubscription != null && !sampleSubscription.isUnsubscribed()) {
                sampleSubscription.unsubscribe();
            }
        }
    }
}