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 * Copyright 2014 Red Hat, Inc., and individual contributors
 * as indicated by the @author tags.
 *
 * 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,
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 *  See the License for the specific language governing permissions and
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package io.undertow.server.handlers;

import io.undertow.UndertowLogger;
import io.undertow.server.Connectors;
import io.undertow.server.ExchangeCompletionListener;
import io.undertow.server.HttpHandler;
import io.undertow.server.HttpServerExchange;
import io.undertow.util.SameThreadExecutor;

import java.util.Queue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;

Represents a limit on a number of running requests.
 This is basically a counter with a configured set of limits, that is used by RequestLimitingHandler. 

When the number of active requests goes over the configured max requests then requests will be suspended and queued.

If the queue is full requests will be rejected with a 503 Service Unavailable according to RFC7231 Section 6.6.4.

The reason why this is abstracted out into a separate class is so that multiple handlers can share the same state. This
allows for fine grained control of resources.
Author:
Stuart Douglas
See Also:
RequestLimitingHandler/**
 * Represents a limit on a number of running requests.
 * <p>
 * This is basically a counter with a configured set of limits, that is used by {@link RequestLimitingHandler}.
 * <p>
 * When the number of active requests goes over the configured max requests then requests will be suspended and queued.
 * <p>
 * If the queue is full requests will be rejected with a 503 Service Unavailable according to RFC7231 Section 6.6.4.
 * <p>
 * The reason why this is abstracted out into a separate class is so that multiple handlers can share the same state. This
 * allows for fine grained control of resources.
 *
 * @author Stuart Douglas
 * @see RequestLimitingHandler
 */
public class RequestLimit {
    @SuppressWarnings("unused")
    private volatile int requests;
    private volatile int max;

    private static final AtomicIntegerFieldUpdater<RequestLimit> requestsUpdater = AtomicIntegerFieldUpdater.newUpdater(RequestLimit.class, "requests");


    
The handler that will be invoked if the queue is full.
/**
     * The handler that will be invoked if the queue is full.
     */
    private volatile HttpHandler failureHandler = new ResponseCodeHandler(503);

    private final Queue<SuspendedRequest> queue;

    private final ExchangeCompletionListener COMPLETION_LISTENER = new ExchangeCompletionListener() {

        @Override
        public void exchangeEvent(final HttpServerExchange exchange, final NextListener nextListener) {
            SuspendedRequest task = null;
            boolean found = false;
            while ((task = queue.poll()) != null) {
                try {
                    task.exchange.addExchangeCompleteListener(COMPLETION_LISTENER);
                    task.exchange.dispatch(task.next);
                    found = true;
                    break;
                } catch (Throwable e) {
                    UndertowLogger.ROOT_LOGGER.error("Suspended request was skipped", e);
                }
            }

            if (!found) {
                decrementRequests();
            }

            nextListener.proceed();
        }
    };


    public RequestLimit(int maximumConcurrentRequests) {
        this(maximumConcurrentRequests, -1);
    }

    
Construct a new instance. The maximum number of concurrent requests must be at least one.
Params:
maximumConcurrentRequests – the maximum concurrent requests
queueSize –                 The maximum number of requests to queue/**
     * Construct a new instance. The maximum number of concurrent requests must be at least one.
     *
     * @param maximumConcurrentRequests the maximum concurrent requests
     * @param queueSize                 The maximum number of requests to queue
     */
    public RequestLimit(int maximumConcurrentRequests, int queueSize) {
        if (maximumConcurrentRequests < 1) {
            throw new IllegalArgumentException("Maximum concurrent requests must be at least 1");
        }
        max = maximumConcurrentRequests;

        this.queue = new LinkedBlockingQueue<>(queueSize <= 0 ? Integer.MAX_VALUE : queueSize);
    }

    public void handleRequest(final HttpServerExchange exchange, final HttpHandler next) throws Exception {
        int oldVal, newVal;
        do {
            oldVal = requests;
            if (oldVal >= max) {
                exchange.dispatch(SameThreadExecutor.INSTANCE, new Runnable() {
                    @Override
                    public void run() {
                        //we have to try again in the sync block
                        //we need to have already dispatched for thread safety reasons
                        synchronized (RequestLimit.this) {
                            int oldVal, newVal;
                            do {
                                oldVal = requests;
                                if (oldVal >= max) {
                                    if (!queue.offer(new SuspendedRequest(exchange, next))) {
                                        Connectors.executeRootHandler(failureHandler, exchange);
                                    }
                                    return;
                                }
                                newVal = oldVal + 1;
                            } while (!requestsUpdater.compareAndSet(RequestLimit.this, oldVal, newVal));
                            exchange.addExchangeCompleteListener(COMPLETION_LISTENER);
                            exchange.dispatch(next);
                        }
                    }
                });
                return;
            }
            newVal = oldVal + 1;
        } while (!requestsUpdater.compareAndSet(this, oldVal, newVal));
        exchange.addExchangeCompleteListener(COMPLETION_LISTENER);
        next.handleRequest(exchange);
    }

    
Get the maximum concurrent requests.
Returns:
the maximum concurrent requests/**
     * Get the maximum concurrent requests.
     *
     * @return the maximum concurrent requests
     */
    public int getMaximumConcurrentRequests() {
        return max;
    }

    
Set the maximum concurrent requests.  The value must be greater than or equal to one.
Params:
newMax – the maximum concurrent requests/**
     * Set the maximum concurrent requests.  The value must be greater than or equal to one.
     *
     * @param newMax the maximum concurrent requests
     */
    public int setMaximumConcurrentRequests(int newMax) {
        if (newMax < 1) {
            throw new IllegalArgumentException("Maximum concurrent requests must be at least 1");
        }
        int oldMax = this.max;
        this.max = newMax;
        if(newMax > oldMax) {
            synchronized (this) {
                while (!queue.isEmpty()) {
                    int oldVal, newVal;
                    do {
                        oldVal = requests;
                        if (oldVal >= max) {
                            return oldMax;
                        }
                        newVal = oldVal + 1;
                    } while (!requestsUpdater.compareAndSet(this, oldVal, newVal));
                    SuspendedRequest res = queue.poll();
                    res.exchange.dispatch(res.next);
                }
            }
        }
        return oldMax;
    }

    private void decrementRequests() {
        requestsUpdater.decrementAndGet(this);
    }

    public HttpHandler getFailureHandler() {
        return failureHandler;
    }

    public void setFailureHandler(HttpHandler failureHandler) {
        this.failureHandler = failureHandler;
    }

    private static final class SuspendedRequest {
        final HttpServerExchange exchange;
        final HttpHandler next;

        private SuspendedRequest(HttpServerExchange exchange, HttpHandler next) {
            this.exchange = exchange;
            this.next = next;
        }
    }
}