AIMDBackoffManager

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package org.apache.hc.client5.http.impl.classic;

import java.util.HashMap;
import java.util.Map;

import org.apache.hc.client5.http.HttpRoute;
import org.apache.hc.client5.http.classic.BackoffManager;
import org.apache.hc.core5.annotation.Experimental;
import org.apache.hc.core5.pool.ConnPoolControl;
import org.apache.hc.core5.util.Args;
import org.apache.hc.core5.util.TimeValue;

/**
 * <p>The {@code AIMDBackoffManager} applies an additive increase,
 * multiplicative decrease (AIMD) to managing a dynamic limit to
 * the number of connections allowed to a given host. You may want
 * to experiment with the settings for the cooldown periods and the
 * backoff factor to get the adaptive behavior you want.</p>
 *
 * <p>Generally speaking, shorter cooldowns will lead to more steady-state
 * variability but faster reaction times, while longer cooldowns
 * will lead to more stable equilibrium behavior but slower reaction
 * times.</p>
 *
 * <p>Similarly, higher backoff factors promote greater
 * utilization of available capacity at the expense of fairness
 * among clients. Lower backoff factors allow equal distribution of
 * capacity among clients (fairness) to happen faster, at the
 * expense of having more server capacity unused in the short term.</p>
 *
 * @since 4.2
 */
@Experimental
public class AIMDBackoffManager implements BackoffManager {

    private final ConnPoolControl<HttpRoute> connPerRoute;
    private final Clock clock;
    private final Map<HttpRoute, Long> lastRouteProbes;
    private final Map<HttpRoute, Long> lastRouteBackoffs;
    private TimeValue coolDown = TimeValue.ofSeconds(5L);
    private double backoffFactor = 0.5;
    private int cap = 2; // Per RFC 2616 sec 8.1.4

    /**
     * Creates an {@code AIMDBackoffManager} to manage
     * per-host connection pool sizes represented by the
     * given {@link ConnPoolControl}.
     * @param connPerRoute per-host routing maximums to
     *   be managed
     */
    public AIMDBackoffManager(final ConnPoolControl<HttpRoute> connPerRoute) {
        this(connPerRoute, new SystemClock());
    }

    AIMDBackoffManager(final ConnPoolControl<HttpRoute> connPerRoute, final Clock clock) {
        this.clock = clock;
        this.connPerRoute = connPerRoute;
        this.lastRouteProbes = new HashMap<>();
        this.lastRouteBackoffs = new HashMap<>();
    }

    @Override
    public void backOff(final HttpRoute route) {
        synchronized(connPerRoute) {
            final int curr = connPerRoute.getMaxPerRoute(route);
            final Long lastUpdate = getLastUpdate(lastRouteBackoffs, route);
            final long now = clock.getCurrentTime();
            if (now - lastUpdate < coolDown.toMilliseconds()) {
                return;
            }
            connPerRoute.setMaxPerRoute(route, getBackedOffPoolSize(curr));
            lastRouteBackoffs.put(route, now);
        }
    }

    private int getBackedOffPoolSize(final int curr) {
        if (curr <= 1) {
            return 1;
        }
        return (int)(Math.floor(backoffFactor * curr));
    }

    @Override
    public void probe(final HttpRoute route) {
        synchronized(connPerRoute) {
            final int curr = connPerRoute.getMaxPerRoute(route);
            final int max = (curr >= cap) ? cap : curr + 1;
            final Long lastProbe = getLastUpdate(lastRouteProbes, route);
            final Long lastBackoff = getLastUpdate(lastRouteBackoffs, route);
            final long now = clock.getCurrentTime();
            if (now - lastProbe < coolDown.toMilliseconds()
                || now - lastBackoff < coolDown.toMilliseconds()) {
                return;
            }
            connPerRoute.setMaxPerRoute(route, max);
            lastRouteProbes.put(route, now);
        }
    }

    private Long getLastUpdate(final Map<HttpRoute, Long> updates, final HttpRoute route) {
        Long lastUpdate = updates.get(route);
        if (lastUpdate == null) {
            lastUpdate = 0L;
        }
        return lastUpdate;
    }

    /**
     * Sets the factor to use when backing off; the new
     * per-host limit will be roughly the current max times
     * this factor. {@code Math.floor} is applied in the
     * case of non-integer outcomes to ensure we actually
     * decrease the pool size. Pool sizes are never decreased
     * below 1, however. Defaults to 0.5.
     * @param d must be between 0.0 and 1.0, exclusive.
     */
    public void setBackoffFactor(final double d) {
        Args.check(d > 0.0 && d < 1.0, "Backoff factor must be 0.0 < f < 1.0");
        backoffFactor = d;
    }

    /**
     * Sets the amount of time to wait between adjustments in
     * pool sizes for a given host, to allow enough time for
     * the adjustments to take effect. Defaults to 5 seconds.
     * @param coolDown must be positive
     */
    public void setCoolDown(final TimeValue coolDown) {
        Args.positive(coolDown.getDuration(), "coolDown");
        this.coolDown = coolDown;
    }

    /**
     * Sets the absolute maximum per-host connection pool size to
     * probe up to; defaults to 2 (the default per-host max).
     * @param cap must be &gt;= 1
     */
    public void setPerHostConnectionCap(final int cap) {
        Args.positive(cap, "Per host connection cap");
        this.cap = cap;
    }

}