[Tiered Caching] Gate CacheStatsHolder logic behind FeatureFlags.PLUGGABLE_CACHE setting (opensearch-project#13238)

Stats rework step 2 of 4

---------

Signed-off-by: Peter Alfonsi <petealft@amazon.com>
Co-authored-by: Peter Alfonsi <petealft@amazon.com>

Author: peteralfonsi

CHANGELOG.md

@@ -24,6 +24,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Add an individual setting of rate limiter for segment replication ([#12959](https://github.com/opensearch-project/OpenSearch/pull/12959))
 - [Streaming Indexing] Ensure support of the new transport by security plugin ([#13174](https://github.com/opensearch-project/OpenSearch/pull/13174))
 - Add cluster setting to dynamically configure the buckets for filter rewrite optimization. ([#13179](https://github.com/opensearch-project/OpenSearch/pull/13179))
+- [Tiered Caching] Gate new stats logic behind FeatureFlags.PLUGGABLE_CACHE ([#13238](https://github.com/opensearch-project/OpenSearch/pull/13238))
 - [Tiered Caching] Add a dynamic setting to disable/enable disk cache. ([#13373](https://github.com/opensearch-project/OpenSearch/pull/13373))
 - [Remote Store] Add capability of doing refresh as determined by the translog ([#12992](https://github.com/opensearch-project/OpenSearch/pull/12992))
 - [Tiered caching] Make Indices Request Cache Stale Key Mgmt Threshold setting dynamic ([#12941](https://github.com/opensearch-project/OpenSearch/pull/12941))

plugins/cache-ehcache/src/main/java/org/opensearch/cache/store/disk/EhcacheDiskCache.java

@@ -25,6 +25,7 @@
 import org.opensearch.common.cache.serializer.ICacheKeySerializer;
 import org.opensearch.common.cache.serializer.Serializer;
 import org.opensearch.common.cache.stats.CacheStatsHolder;
+import org.opensearch.common.cache.stats.DefaultCacheStatsHolder;
 import org.opensearch.common.cache.stats.ImmutableCacheStatsHolder;
 import org.opensearch.common.cache.store.builders.ICacheBuilder;
 import org.opensearch.common.cache.store.config.CacheConfig;
@@ -162,7 +163,8 @@ private EhcacheDiskCache(Builder<K, V> builder) {
         this.ehCacheEventListener = new EhCacheEventListener(builder.getRemovalListener(), builder.getWeigher());
         this.cache = buildCache(Duration.ofMillis(expireAfterAccess.getMillis()), builder);
         List<String> dimensionNames = Objects.requireNonNull(builder.dimensionNames, "Dimension names can't be null");
-        this.cacheStatsHolder = new CacheStatsHolder(dimensionNames);
+        // If this cache is being used, FeatureFlags.PLUGGABLE_CACHE is already on, so we can always use the DefaultCacheStatsHolder.
+        this.cacheStatsHolder = new DefaultCacheStatsHolder(dimensionNames);
     }
 
     @SuppressWarnings({ "rawtypes" })

server/src/main/java/org/opensearch/common/cache/stats/CacheStatsHolder.java

@@ -8,288 +8,31 @@
 
 package org.opensearch.common.cache.stats;
 
-import java.util.Collections;
-import java.util.HashMap;
 import java.util.List;
-import java.util.Map;
-import java.util.TreeMap;
-import java.util.concurrent.ConcurrentHashMap;
-import java.util.concurrent.locks.Lock;
-import java.util.concurrent.locks.ReentrantLock;
-import java.util.function.Consumer;
 
 /**
- * A class ICache implementations use to internally keep track of their stats across multiple dimensions.
- * Not intended to be exposed outside the cache; for this, caches use getImmutableCacheStatsHolder() to create an immutable
- * copy of the current state of the stats.
- * Currently, in the IRC, the stats tracked in a CacheStatsHolder will not appear for empty shards that have had no cache
- * operations done on them yet. This might be changed in the future, by exposing a method to add empty nodes to the
- * tree in CacheStatsHolder in the ICache interface.
- *
- * @opensearch.experimental
+ * An interface extended by DefaultCacheStatsHolder and NoopCacheStatsHolder.
  */
-public class CacheStatsHolder {
-
-    // The list of permitted dimensions. Should be ordered from "outermost" to "innermost", as you would like to
-    // aggregate them in an API response.
-    private final List<String> dimensionNames;
-    // A tree structure based on dimension values, which stores stats values in its leaf nodes.
-    // Non-leaf nodes have stats matching the sum of their children.
-    // We use a tree structure, rather than a map with concatenated keys, to save on memory usage. If there are many leaf
-    // nodes that share a parent, that parent's dimension value will only be stored once, not many times.
-    private final Node statsRoot;
-    // To avoid sync problems, obtain a lock before creating or removing nodes in the stats tree.
-    // No lock is needed to edit stats on existing nodes.
-    private final Lock lock = new ReentrantLock();
-
-    public CacheStatsHolder(List<String> dimensionNames) {
-        this.dimensionNames = Collections.unmodifiableList(dimensionNames);
-        this.statsRoot = new Node("", true); // The root node has the empty string as its dimension value
-    }
-
-    public List<String> getDimensionNames() {
-        return dimensionNames;
-    }
-
-    // For all these increment functions, the dimensions list comes from the key, and contains all dimensions present in dimensionNames.
-    // The order has to match the order given in dimensionNames.
-    public void incrementHits(List<String> dimensionValues) {
-        internalIncrement(dimensionValues, Node::incrementHits, true);
-    }
-
-    public void incrementMisses(List<String> dimensionValues) {
-        internalIncrement(dimensionValues, Node::incrementMisses, true);
-    }
-
-    public void incrementEvictions(List<String> dimensionValues) {
-        internalIncrement(dimensionValues, Node::incrementEvictions, true);
-    }
-
-    public void incrementSizeInBytes(List<String> dimensionValues, long amountBytes) {
-        internalIncrement(dimensionValues, (node) -> node.incrementSizeInBytes(amountBytes), true);
-    }
-
-    // For decrements, we should not create nodes if they are absent. This protects us from erroneously decrementing values for keys
-    // which have been entirely deleted, for example in an async removal listener.
-    public void decrementSizeInBytes(List<String> dimensionValues, long amountBytes) {
-        internalIncrement(dimensionValues, (node) -> node.decrementSizeInBytes(amountBytes), false);
-    }
-
-    public void incrementEntries(List<String> dimensionValues) {
-        internalIncrement(dimensionValues, Node::incrementEntries, true);
-    }
-
-    public void decrementEntries(List<String> dimensionValues) {
-        internalIncrement(dimensionValues, Node::decrementEntries, false);
-    }
-
-    /**
-     * Reset number of entries and memory size when all keys leave the cache, but don't reset hit/miss/eviction numbers.
-     * This is in line with the behavior of the existing API when caches are cleared.
-     */
-    public void reset() {
-        resetHelper(statsRoot);
-    }
-
-    private void resetHelper(Node current) {
-        current.resetSizeAndEntries();
-        for (Node child : current.children.values()) {
-            resetHelper(child);
-        }
-    }
-
-    public long count() {
-        // Include this here so caches don't have to create an entire CacheStats object to run count().
-        return statsRoot.getEntries();
-    }
-
-    private void internalIncrement(List<String> dimensionValues, Consumer<Node> adder, boolean createNodesIfAbsent) {
-        assert dimensionValues.size() == dimensionNames.size();
-        // First try to increment without creating nodes
-        boolean didIncrement = internalIncrementHelper(dimensionValues, statsRoot, 0, adder, false);
-        // If we failed to increment, because nodes had to be created, obtain the lock and run again while creating nodes if needed
-        if (!didIncrement && createNodesIfAbsent) {
-            try {
-                lock.lock();
-                internalIncrementHelper(dimensionValues, statsRoot, 0, adder, true);
-            } finally {
-                lock.unlock();
-            }
-        }
-    }
-
-    /**
-     * Use the incrementer function to increment/decrement a value in the stats for a set of dimensions.
-     * If createNodesIfAbsent is true, and there is no stats for this set of dimensions, create one.
-     * Returns true if the increment was applied, false if not.
-     */
-    private boolean internalIncrementHelper(
-        List<String> dimensionValues,
-        Node node,
-        int depth, // Pass in the depth to avoid having to slice the list for each node.
-        Consumer<Node> adder,
-        boolean createNodesIfAbsent
-    ) {
-        if (depth == dimensionValues.size()) {
-            // This is the leaf node we are trying to reach
-            adder.accept(node);
-            return true;
-        }
-
-        Node child = node.getChild(dimensionValues.get(depth));
-        if (child == null) {
-            if (createNodesIfAbsent) {
-                boolean createMapInChild = depth < dimensionValues.size() - 1;
-                child = node.createChild(dimensionValues.get(depth), createMapInChild);
-            } else {
-                return false;
-            }
-        }
-        if (internalIncrementHelper(dimensionValues, child, depth + 1, adder, createNodesIfAbsent)) {
-            // Function returns true if the next node down was incremented
-            adder.accept(node);
-            return true;
-        }
-        return false;
-    }
-
-    /**
-     * Produce an immutable version of these stats.
-     */
-    public ImmutableCacheStatsHolder getImmutableCacheStatsHolder() {
-        return new ImmutableCacheStatsHolder(statsRoot.snapshot(), dimensionNames);
-    }
-
-    public void removeDimensions(List<String> dimensionValues) {
-        assert dimensionValues.size() == dimensionNames.size() : "Must specify a value for every dimension when removing from StatsHolder";
-        // As we are removing nodes from the tree, obtain the lock
-        lock.lock();
-        try {
-            removeDimensionsHelper(dimensionValues, statsRoot, 0);
-        } finally {
-            lock.unlock();
-        }
-    }
-
-    // Returns a CacheStatsCounterSnapshot object for the stats to decrement if the removal happened, null otherwise.
-    private ImmutableCacheStats removeDimensionsHelper(List<String> dimensionValues, Node node, int depth) {
-        if (depth == dimensionValues.size()) {
-            // Pass up a snapshot of the original stats to avoid issues when the original is decremented by other fn invocations
-            return node.getImmutableStats();
-        }
-        Node child = node.getChild(dimensionValues.get(depth));
-        if (child == null) {
-            return null;
-        }
-        ImmutableCacheStats statsToDecrement = removeDimensionsHelper(dimensionValues, child, depth + 1);
-        if (statsToDecrement != null) {
-            // The removal took place, decrement values and remove this node from its parent if it's now empty
-            node.decrementBySnapshot(statsToDecrement);
-            if (child.getChildren().isEmpty()) {
-                node.children.remove(child.getDimensionValue());
-            }
-        }
-        return statsToDecrement;
-    }
-
-    // pkg-private for testing
-    Node getStatsRoot() {
-        return statsRoot;
-    }
-
-    static class Node {
-        private final String dimensionValue;
-        // Map from dimensionValue to the DimensionNode for that dimension value.
-        final Map<String, Node> children;
-        // The stats for this node. If a leaf node, corresponds to the stats for this combination of dimensions; if not,
-        // contains the sum of its children's stats.
-        private CacheStats stats;
-
-        // Used for leaf nodes to avoid allocating many unnecessary maps
-        private static final Map<String, Node> EMPTY_CHILDREN_MAP = new HashMap<>();
-
-        Node(String dimensionValue, boolean createChildrenMap) {
-            this.dimensionValue = dimensionValue;
-            if (createChildrenMap) {
-                this.children = new ConcurrentHashMap<>();
-            } else {
-                this.children = EMPTY_CHILDREN_MAP;
-            }
-            this.stats = new CacheStats();
-        }
-
-        public String getDimensionValue() {
-            return dimensionValue;
-        }
-
-        protected Map<String, Node> getChildren() {
-            // We can safely iterate over ConcurrentHashMap without worrying about thread issues.
-            return children;
-        }
-
-        // Functions for modifying internal CacheStatsCounter without callers having to be aware of CacheStatsCounter
-
-        void incrementHits() {
-            this.stats.incrementHits();
-        }
-
-        void incrementMisses() {
-            this.stats.incrementMisses();
-        }
-
-        void incrementEvictions() {
-            this.stats.incrementEvictions();
-        }
-
-        void incrementSizeInBytes(long amountBytes) {
-            this.stats.incrementSizeInBytes(amountBytes);
-        }
+public interface CacheStatsHolder {
+    void incrementHits(List<String> dimensionValues);
 
-        void decrementSizeInBytes(long amountBytes) {
-            this.stats.decrementSizeInBytes(amountBytes);
-        }
+    void incrementMisses(List<String> dimensionValues);
 
-        void incrementEntries() {
-            this.stats.incrementEntries();
-        }
+    void incrementEvictions(List<String> dimensionValues);
 
-        void decrementEntries() {
-            this.stats.decrementEntries();
-        }
+    void incrementSizeInBytes(List<String> dimensionValues, long amountBytes);
 
-        long getEntries() {
-            return this.stats.getEntries();
-        }
+    void decrementSizeInBytes(List<String> dimensionValues, long amountBytes);
 
-        ImmutableCacheStats getImmutableStats() {
-            return this.stats.immutableSnapshot();
-        }
+    void incrementEntries(List<String> dimensionValues);
 
-        void decrementBySnapshot(ImmutableCacheStats snapshot) {
-            this.stats.subtract(snapshot);
-        }
+    void decrementEntries(List<String> dimensionValues);
 
-        void resetSizeAndEntries() {
-            this.stats.resetSizeAndEntries();
-        }
+    void reset();
 
-        Node getChild(String dimensionValue) {
-            return children.get(dimensionValue);
-        }
+    long count();
 
-        Node createChild(String dimensionValue, boolean createMapInChild) {
-            return children.computeIfAbsent(dimensionValue, (key) -> new Node(dimensionValue, createMapInChild));
-        }
+    void removeDimensions(List<String> dimensionValues);
 
-        ImmutableCacheStatsHolder.Node snapshot() {
-            TreeMap<String, ImmutableCacheStatsHolder.Node> snapshotChildren = null;
-            if (!children.isEmpty()) {
-                snapshotChildren = new TreeMap<>();
-                for (Node child : children.values()) {
-                    snapshotChildren.put(child.getDimensionValue(), child.snapshot());
-                }
-            }
-            return new ImmutableCacheStatsHolder.Node(dimensionValue, snapshotChildren, getImmutableStats());
-        }
-    }
+    ImmutableCacheStatsHolder getImmutableCacheStatsHolder();
 }