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package io.rtr.alchemy.caching;

import org.joda.time.DateTime;
import org.joda.time.Duration;

import java.util.concurrent.atomic.AtomicBoolean;

/**
 * This caching strategy will periodically check whether the data in the cache is stale, and if so,
 * refresh it. The periodic checking of staleness is driven by cache reads, so, any time the cache
 * is read, if it's been a while since the last refresh and the data is stale, the cache will be
 * refreshed.
 */
public class PeriodicStaleCheckingCacheStrategy extends BasicCacheStrategy {
    private final Duration period;
    private final AtomicBoolean lock = new AtomicBoolean(false);
    private volatile DateTime lastSync;

    public PeriodicStaleCheckingCacheStrategy(Duration period) {
        this.period = period;
        this.lastSync = DateTime.now();
    }

    @Override
    public void onCacheRead(String experimentName, CachingContext context) {
        invalidateAllIfStale(context);
    }

    @Override
    public void onCacheRead(CachingContext context) {
        invalidateAllIfStale(context);
    }

    private void invalidateAllIfStale(CachingContext context) {
        if (!lock.compareAndSet(false, true)) {
            return;
        }

        try {
            final Duration elapsed = new Duration(lastSync, DateTime.now());
            if (!elapsed.isLongerThan(period)) {
                return;
            }

            lastSync = DateTime.now();
        } finally {
            lock.set(false);
        }

        if (context.checkIfAnyStale()) {
            context.invalidateAll(true);
        }
    }
}