Use FrozenEngine for shared_cache searchable snapshot shards (#69277)

Searchable snapshots that cache data using the shared_cache are expect to only have a small footprint on a node. While
shared_cache helps with reducing the disk footprint of such shards, there is still the challenge of memory overhead
associated with each open Lucene index. FrozenEngine helps with that by only opening up the Lucene index on-demand,
and is the better default for searchable snapshot shards in the frozen tier.

Author: ywelsch

…ticsearch/index/engine/FrozenEngine.java …ticsearch/index/engine/FrozenEngine.javax-pack/plugin/frozen-indices/src/main/java/org/elasticsearch/index/engine/FrozenEngine.java renamed to x-pack/plugin/core/src/main/java/org/elasticsearch/index/engine/FrozenEngine.java x-pack/plugin/frozen-indices/src/main/java/org/elasticsearch/index/engine/FrozenEngine.java renamed to x-pack/plugin/core/src/main/java/org/elasticsearch/index/engine/FrozenEngine.java

@@ -20,8 +20,10 @@
 import org.elasticsearch.common.lucene.index.ElasticsearchDirectoryReader;
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.core.internal.io.IOUtils;
+import org.elasticsearch.index.seqno.SeqNoStats;
 import org.elasticsearch.index.shard.DocsStats;
 import org.elasticsearch.index.store.Store;
+import org.elasticsearch.index.translog.TranslogStats;
 
 import java.io.IOException;
 import java.io.UncheckedIOException;
@@ -48,7 +50,12 @@ public final class FrozenEngine extends ReadOnlyEngine {
     private final ElasticsearchDirectoryReader canMatchReader;
 
     public FrozenEngine(EngineConfig config, boolean requireCompleteHistory) {
-        super(config, null, null, true, Function.identity(), requireCompleteHistory);
+        this(config, null, null, true, Function.identity(), requireCompleteHistory);
+    }
+
+    public FrozenEngine(EngineConfig config, SeqNoStats seqNoStats, TranslogStats translogStats, boolean obtainLock,
+                        Function<DirectoryReader, DirectoryReader> readerWrapperFunction, boolean requireCompleteHistory) {
+        super(config, seqNoStats, translogStats, obtainLock, readerWrapperFunction, requireCompleteHistory);
 
         boolean success = false;
         Directory directory = store.directory();

…ngine/RewriteCachingDirectoryReader.java …ngine/RewriteCachingDirectoryReader.javax-pack/plugin/frozen-indices/src/main/java/org/elasticsearch/index/engine/RewriteCachingDirectoryReader.java renamed to x-pack/plugin/core/src/main/java/org/elasticsearch/index/engine/RewriteCachingDirectoryReader.java x-pack/plugin/frozen-indices/src/main/java/org/elasticsearch/index/engine/RewriteCachingDirectoryReader.java renamed to x-pack/plugin/core/src/main/java/org/elasticsearch/index/engine/RewriteCachingDirectoryReader.java

@@ -42,9 +42,8 @@ public class FrozenIndices extends Plugin implements ActionPlugin, EnginePlugin
 
     @Override
     public Optional<EngineFactory> getEngineFactory(IndexSettings indexSettings) {
-        if (indexSettings.getValue(FrozenEngine.INDEX_FROZEN)) {
-            final boolean requireCompleteHistory = isSearchableSnapshotStore(indexSettings.getSettings()) == false;
-            return Optional.of(config -> new FrozenEngine(config, requireCompleteHistory));
+        if (indexSettings.getValue(FrozenEngine.INDEX_FROZEN) && isSearchableSnapshotStore(indexSettings.getSettings()) == false) {
+            return Optional.of(config -> new FrozenEngine(config, true));
         } else {
             return Optional.empty();
         }

x-pack/plugin/frozen-indices/src/main/java/org/elasticsearch/xpack/frozen/FrozenIndices.java

@@ -38,6 +38,7 @@
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.engine.Engine;
 import org.elasticsearch.index.engine.EngineFactory;
+import org.elasticsearch.index.engine.FrozenEngine;
 import org.elasticsearch.index.engine.ReadOnlyEngine;
 import org.elasticsearch.index.store.SearchableSnapshotDirectory;
 import org.elasticsearch.index.store.Store;
@@ -388,20 +389,38 @@ public Map<String, DirectoryFactory> getDirectoryFactories() {
 
     @Override
     public Optional<EngineFactory> getEngineFactory(IndexSettings indexSettings) {
-        if (SearchableSnapshotsConstants.isSearchableSnapshotStore(indexSettings.getSettings())
-            && indexSettings.getSettings().getAsBoolean("index.frozen", false) == false) {
-            return Optional.of(
-                engineConfig -> new ReadOnlyEngine(
-                    engineConfig,
-                    null,
-                    new TranslogStats(),
-                    false,
-                    indexSettings.getValue(SourceOnlySnapshotRepository.SOURCE_ONLY)
-                        ? SourceOnlySnapshotRepository.readerWrapper(engineConfig)
-                        : Function.identity(),
-                    false
-                )
-            );
+        if (SearchableSnapshotsConstants.isSearchableSnapshotStore(indexSettings.getSettings())) {
+            final Boolean frozen = indexSettings.getSettings().getAsBoolean("index.frozen", null);
+            final boolean useFrozenEngine = SearchableSnapshots.SNAPSHOT_PARTIAL_SETTING.get(indexSettings.getSettings())
+                && (frozen == null || frozen.equals(Boolean.TRUE));
+
+            if (useFrozenEngine) {
+                return Optional.of(
+                    engineConfig -> new FrozenEngine(
+                        engineConfig,
+                        null,
+                        new TranslogStats(),
+                        false,
+                        indexSettings.getValue(SourceOnlySnapshotRepository.SOURCE_ONLY)
+                            ? SourceOnlySnapshotRepository.readerWrapper(engineConfig)
+                            : Function.identity(),
+                        false
+                    )
+                );
+            } else {
+                return Optional.of(
+                    engineConfig -> new ReadOnlyEngine(
+                        engineConfig,
+                        null,
+                        new TranslogStats(),
+                        false,
+                        indexSettings.getValue(SourceOnlySnapshotRepository.SOURCE_ONLY)
+                            ? SourceOnlySnapshotRepository.readerWrapper(engineConfig)
+                            : Function.identity(),
+                        false
+                    )
+                );
+            }
         }
         return Optional.empty();
     }