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Keeps track of node assignment in a multi-application setup.
// Copyright Vespa.ai. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
package com.yahoo.vespa.hosted.provision.persistence;
import com.google.common.cache.AbstractCache;
import com.yahoo.config.provision.HostName;
import com.yahoo.path.Path;
import com.yahoo.transaction.NestedTransaction;
import com.yahoo.vespa.curator.Curator;
import com.yahoo.vespa.curator.Lock;
import com.yahoo.vespa.curator.recipes.CuratorCounter;
import com.yahoo.vespa.curator.transaction.CuratorTransaction;
import org.apache.zookeeper.data.Stat;
import java.time.Duration;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.OptionalInt;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Supplier;
/**
* A caching wrapper for {@link Curator}.
*
* It serves reads from an in-memory cache of the content which is invalidated when changed on another node
* using a global, shared counter. The counter is updated on all write operations, ensured by wrapping write
* operations in a try block, with the counter increment in a finally block.
*
* Locks must be used to ensure consistency.
*
* @author bratseth
* @author jonmv
*/
public class CachingCurator {
private final Curator curator;
/** A shared atomic counter which is incremented every time we write to the curator database */
private final CuratorCounter changeGenerationCounter;
/** A partial cache of the Curator database, which is only valid if generations match */
private final AtomicReference cache = new AtomicReference<>();
/** Whether we should return data from the cache or always read from ZooKeeper */
private final boolean enabled;
private final Object cacheCreationLock = new Object();
/**
* Creates a curator database
*
* @param curator the curator instance
* @param root the file system root of the db
*/
public CachingCurator(Curator curator, Path root, boolean enabled) {
this.enabled = enabled;
this.curator = curator;
changeGenerationCounter = new CuratorCounter(curator, root.append("changeCounter"));
cache.set(newCache(changeGenerationCounter.get()));
}
/** Returns all hosts configured to be part of this ZooKeeper cluster */
public List cluster() {
return Arrays.stream(curator.zooKeeperEnsembleConnectionSpec().split(","))
.filter(hostAndPort -> !hostAndPort.isEmpty())
.map(hostAndPort -> hostAndPort.split(":")[0])
.map(HostName::of)
.toList();
}
/** Create a reentrant lock */
public Lock lock(Path path, Duration timeout) {
return curator.lock(path, timeout);
}
// --------- Write operations ------------------------------------------------------------------------------
// These must either create a nested transaction ending in a counter increment or not depend on prior state
/**
* Creates a new curator transaction against this database and adds it to the given nested transaction.
* Important: It is the nested transaction which must be committed - never the curator transaction directly.
*/
public CuratorTransaction newCuratorTransactionIn(NestedTransaction transaction) {
// Wrap the curator transaction with an increment of the generation counter.
CountingCuratorTransaction curatorTransaction = new CountingCuratorTransaction(curator, changeGenerationCounter);
transaction.add(curatorTransaction);
return curatorTransaction;
}
/** Creates a path in curator and all its parents as necessary. If the path already exists this does nothing. */
void create(Path path) {
if (curator.create(path))
changeGenerationCounter.next(); // Increment counter to ensure getChildren sees any change.
}
/** Returns whether given path exists */
boolean exists(Path path) {
return curator.exists(path);
}
// --------- Read operations -------------------------------------------------------------------------------
// These can read from the memory file system, which accurately mirrors the ZooKeeper content IF
// the current generation counter is the same as it was when data was put into the cache, AND
// the data to read is protected by a lock which is held now, and during any writes of the data.
/** Returns the immediate, local names of the children under this node in any order */
List getChildren(Path path) { return getSession().getChildren(path); }
Optional getData(Path path) { return getSession().getData(path); }
/** Invalidates the current cache if outdated. */
Session getSession() {
if (changeGenerationCounter.get() != cache.get().generation)
synchronized (cacheCreationLock) {
while (changeGenerationCounter.get() != cache.get().generation)
cache.set(newCache(changeGenerationCounter.get()));
}
return cache.get();
}
CacheStats cacheStats() {
return cache.get().stats();
}
/** Caches must only be instantiated using this method */
private Cache newCache(long generation) {
return enabled ? new Cache(generation, curator) : new NoCache(generation, curator);
}
/**
* A thread safe partial snapshot of the curator database content with a given generation.
* This is merely a recording of what Curator returned at various points in time when
* it had the counter at this generation.
*/
private static class Cache implements Session {
private final long generation;
/** The curator instance used to fetch missing data */
protected final Curator curator;
// The data of this partial state mirror. The amount of curator state mirrored in this may grow
// over time by multiple threads. Growing is the only operation permitted by this.
// The content of the map is immutable.
private final Map> children = new ConcurrentHashMap<>();
private final Map> data = new ConcurrentHashMap<>();
private final Map> stats = new ConcurrentHashMap<>();
private final AbstractCache.SimpleStatsCounter statistics = new AbstractCache.SimpleStatsCounter();
/** Create an empty snapshot at a given generation (as an empty snapshot is a valid partial snapshot) */
private Cache(long generation, Curator curator) {
this.generation = generation;
this.curator = curator;
}
@Override
public List getChildren(Path path) {
return get(children, path, () -> List.copyOf(curator.getChildren(path)));
}
@Override
public Optional getData(Path path) {
return get(data, path, () -> curator.getData(path)).map(data -> Arrays.copyOf(data, data.length));
}
@Override
public Optional getStat(Path path) {
return get(stats, path, () -> curator.getStat(path).map(Stat::getVersion));
}
private T get(Map values, Path path, Supplier loader) {
return values.compute(path, (key, value) -> {
if (value == null) {
statistics.recordMisses(1);
return loader.get();
}
statistics.recordHits(1);
return value;
});
}
public CacheStats stats() {
var stats = this.statistics.snapshot();
return new CacheStats(stats.hitRate(), stats.evictionCount(), children.size() + data.size());
}
}
/** An implementation of the curator database cache which does no caching */
private static class NoCache extends Cache {
private NoCache(long generation, Curator curator) { super(generation, curator); }
@Override
public List getChildren(Path path) { return curator.getChildren(path); }
@Override
public Optional getData(Path path) { return curator.getData(path); }
@Override
public Optional getStat(Path path) {
return curator.getStat(path).map(Stat::getVersion);
}
}
interface Session {
/**
* Returns the children of this path, which may be empty.
*/
List getChildren(Path path);
/**
* Returns a copy of the content of this child - which may be empty.
*/
Optional getData(Path path);
Optional getStat(Path path);
}
}
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