DNSCache.java

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// to you under the Apache License, Version 2.0 (the
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//
//   http://www.apache.org/licenses/LICENSE-2.0
//
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package org.apache.doris.common;

import org.apache.doris.common.util.NetUtils;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

import java.net.UnknownHostException;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ScheduledExecutorService;

/**
 * DNSCache is a class that caches DNS lookups and periodically refreshes them.
 * It uses a ConcurrentHashMap to store the hostname to IP address mappings and a ScheduledExecutorService
 * to periodically refresh these mappings.
 */
public class DNSCache {
    private static final Logger LOG = LogManager.getLogger(DNSCache.class);

    private final ConcurrentHashMap<String, String> cache = new ConcurrentHashMap<>();
    private final Set<String> refreshPending = ConcurrentHashMap.newKeySet();
    private final ConcurrentHashMap<String, Object> refreshInFlight = new ConcurrentHashMap<>();
    private final ScheduledExecutorService executor = ThreadPoolManager.newDaemonScheduledThreadPool(1,
            "dns_cache_pool", true);

    /**
     * Check if the enable_fqdn_mode configuration is set.
     * If it is, it schedules a task to refresh the DNS cache every 60 seconds,
     * starting after an initial delay of 120 seconds.
     */
    public void start() {
        if (Config.enable_fqdn_mode) {
            executor.scheduleAtFixedRate(this::refresh, 120, 60, java.util.concurrent.TimeUnit.SECONDS);
        }
    }

    /**
     * The get method retrieves the IP address for a given hostname from the cache.
     * If the hostname is not in the cache, it resolves the hostname to an IP address and stores it in the cache.
     *
     * @param hostname The hostname for which to get the IP address.
     * @return The IP address for the given hostname.
     */
    public String get(String hostname) {
        if (!refreshPending.contains(hostname) && !refreshInFlight.containsKey(hostname)) {
            // Read the IP after the markers: a refresh may have just published a new value.
            String cachedIp = cache.get(hostname);
            if (cachedIp != null) {
                return cachedIp;
            }
        }
        Object refresh = new Object();
        try {
            return cache.compute(hostname, (host, currentIp) -> {
                // Mark in-flight before claiming pending work, so warm readers cannot
                // bypass this computation and reuse the old IP while DNS is running.
                refreshInFlight.put(host, refresh);
                boolean needsRefresh = refreshPending.remove(host);
                if (currentIp == null || needsRefresh) {
                    return resolveHostname(host);
                }
                return currentIp;
            });
        } finally {
            // compute has published the result. A following computation may already
            // own this marker, so remove only this call's token.
            refreshInFlight.remove(hostname, refresh);
        }
    }

    public void invalidate(String hostname) {
        // A successful heartbeat may coincide with a temporary DNS outage. Keep the
        // last known IP so resolveHostname can fall back instead of caching an empty IP.
        // The first lookup may still be resolving and have no published cache entry.
        refreshPending.add(hostname);
    }

    /**
     * The resolveHostname method resolves a hostname to an IP address.
     * If the hostname cannot be resolved, it returns an empty string.
     *
     * @param hostname The hostname to resolve.
     * @return The IP address for the given hostname, or an empty string if the hostname cannot be resolved.
     */
    private String resolveHostname(String hostname) {
        String cachedIp = cache.get(hostname);
        try {
            String ip = NetUtils.getIpByHost(hostname, 0);
            if (ip == null || ip.isEmpty()) {
                if (cachedIp != null && !cachedIp.isEmpty()) {
                    LOG.warn("Failed to resolve hostname {}, use cached ip: {}", hostname, cachedIp);
                    return cachedIp;
                } else {
                    LOG.warn("Failed to resolve hostname {}, no cached ip available", hostname);
                    return "";
                }
            }
            return ip;
        } catch (UnknownHostException e) {
            if (cachedIp != null && !cachedIp.isEmpty()) {
                LOG.warn("Failed to resolve hostname {}, use cached ip: {}", hostname, cachedIp);
                return cachedIp;
            } else {
                LOG.warn("Failed to resolve hostname {}, no cached ip available", hostname);
                return "";
            }
        }
    }

    /**
     * The refresh method periodically refreshes the DNS cache.
     * It iterates over each hostname in the cache, resolves the hostname to an IP address,
     * and compares it with the current IP address in the cache.
     * If they are different, it updates the cache with the new IP address and logs the change.
     */
    private void refresh() {
        for (String hostname : cache.keySet()) {
            // Use the same per-host serialization as forced lookups. An older refresh
            // must not overwrite a newer resolved address with its captured fallback.
            cache.computeIfPresent(hostname, (host, currentIp) -> {
                // Leave explicit invalidations pending for get(); a periodic refresh
                // must not let warm readers bypass a requested forced lookup.
                String resolvedIp = resolveHostname(host);
                if (!resolvedIp.equals(currentIp)) {
                    LOG.info("IP for hostname {} has changed from {} to {}", host, currentIp, resolvedIp);
                }
                return resolvedIp;
            });
        }
    }
}