Allow _update and upsert to read from the transaction log (#29264)

We historically removed reading from the transaction log to get consistent
results from _GET calls. There was also the motivation that the read-modify-update
principle we apply should not be hidden from the user. We still agree on the fact
that we should not hide these aspects but the impact on updates is quite significant
especially if the same documents is updated before it's written to disk and made serachable.

This change adds back the ability to read from the transaction log but only for update calls.
Calls to the _GET API will always do a refresh if necessary to return consistent results ie.
if stored fields or DocValues Fields are requested.

Closes #26802

server/src/main/java/org/elasticsearch/action/explain/TransportExplainAction.java

@@ -112,13 +112,13 @@ protected ExplainResponse shardOperation(ExplainRequest request, ShardId shardId
             if (uidTerm == null) {
                 return new ExplainResponse(shardId.getIndexName(), request.type(), request.id(), false);
             }
-            result = context.indexShard().get(new Engine.Get(false, request.type(), request.id(), uidTerm));
+            result = context.indexShard().get(new Engine.Get(false, false, request.type(), request.id(), uidTerm));
             if (!result.exists()) {
                 return new ExplainResponse(shardId.getIndexName(), request.type(), request.id(), false);
             }
             context.parsedQuery(context.getQueryShardContext().toQuery(request.query()));
             context.preProcess(true);
-            int topLevelDocId = result.docIdAndVersion().docId + result.docIdAndVersion().context.docBase;
+            int topLevelDocId = result.docIdAndVersion().docId + result.docIdAndVersion().docBase;
             Explanation explanation = context.searcher().explain(context.query(), topLevelDocId);
             for (RescoreContext ctx : context.rescore()) {
                 Rescorer rescorer = ctx.rescorer();

server/src/main/java/org/elasticsearch/action/update/UpdateHelper.java

@@ -47,7 +47,6 @@
 import org.elasticsearch.script.ExecutableScript;
 import org.elasticsearch.script.Script;
 import org.elasticsearch.script.ScriptService;
-import org.elasticsearch.search.fetch.subphase.FetchSourceContext;
 import org.elasticsearch.search.lookup.SourceLookup;
 
 import java.io.IOException;
@@ -71,9 +70,8 @@ public UpdateHelper(Settings settings, ScriptService scriptService) {
      * Prepares an update request by converting it into an index or delete request or an update response (no action).
      */
     public Result prepare(UpdateRequest request, IndexShard indexShard, LongSupplier nowInMillis) {
-        final GetResult getResult = indexShard.getService().get(request.type(), request.id(),
-                new String[]{RoutingFieldMapper.NAME, ParentFieldMapper.NAME},
-                true, request.version(), request.versionType(), FetchSourceContext.FETCH_SOURCE);
+        final GetResult getResult = indexShard.getService().getForUpdate(request.type(), request.id(), request.version(),
+            request.versionType());
         return prepare(indexShard.shardId(), request, getResult, nowInMillis);
     }
 

server/src/main/java/org/elasticsearch/common/lucene/uid/PerThreadIDVersionAndSeqNoLookup.java

@@ -100,7 +100,7 @@ public DocIdAndVersion lookupVersion(BytesRef id, LeafReaderContext context)
             if (versions.advanceExact(docID) == false) {
                 throw new IllegalArgumentException("Document [" + docID + "] misses the [" + VersionFieldMapper.NAME + "] field");
             }
-            return new DocIdAndVersion(docID, versions.longValue(), context);
+            return new DocIdAndVersion(docID, versions.longValue(), context.reader(), context.docBase);
         } else {
             return null;
         }

server/src/main/java/org/elasticsearch/common/lucene/uid/VersionsAndSeqNoResolver.java

@@ -20,6 +20,7 @@
 package org.elasticsearch.common.lucene.uid;
 
 import org.apache.lucene.index.IndexReader;
+import org.apache.lucene.index.LeafReader;
 import org.apache.lucene.index.LeafReaderContext;
 import org.apache.lucene.index.NumericDocValues;
 import org.apache.lucene.index.Term;
@@ -97,12 +98,14 @@ private VersionsAndSeqNoResolver() {
     public static class DocIdAndVersion {
         public final int docId;
         public final long version;
-        public final LeafReaderContext context;
+        public final LeafReader reader;
+        public final int docBase;
 
-        DocIdAndVersion(int docId, long version, LeafReaderContext context) {
+        public DocIdAndVersion(int docId, long version, LeafReader reader, int docBase) {
             this.docId = docId;
             this.version = version;
-            this.context = context;
+            this.reader = reader;
+            this.docBase = docBase;
         }
     }
 

server/src/main/java/org/elasticsearch/index/engine/Engine.java

@@ -1232,14 +1232,16 @@ public static class Get {
         private final boolean realtime;
         private final Term uid;
         private final String type, id;
+        private final boolean readFromTranslog;
         private long version = Versions.MATCH_ANY;
         private VersionType versionType = VersionType.INTERNAL;
 
-        public Get(boolean realtime, String type, String id, Term uid) {
+        public Get(boolean realtime, boolean readFromTranslog, String type, String id, Term uid) {
             this.realtime = realtime;
             this.type = type;
             this.id = id;
             this.uid = uid;
+            this.readFromTranslog = readFromTranslog;
         }
 
         public boolean realtime() {
@@ -1275,6 +1277,10 @@ public Get versionType(VersionType versionType) {
             this.versionType = versionType;
             return this;
         }
+
+        public boolean isReadFromTranslog() {
+            return readFromTranslog;
+        }
     }
 
     public static class GetResult implements Releasable {

server/src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -78,6 +78,7 @@
 import org.elasticsearch.threadpool.ThreadPool;
 
 import java.io.IOException;
+import java.io.UncheckedIOException;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
@@ -145,6 +146,7 @@ public class InternalEngine extends Engine {
      * being indexed/deleted.
      */
     private final AtomicLong writingBytes = new AtomicLong();
+    private final AtomicBoolean trackTranslogLocation = new AtomicBoolean(false);
 
     @Nullable
     private final String historyUUID;
@@ -558,6 +560,27 @@ public GetResult get(Get get, BiFunction<String, SearcherScope, Searcher> search
                         throw new VersionConflictEngineException(shardId, get.type(), get.id(),
                             get.versionType().explainConflictForReads(versionValue.version, get.version()));
                     }
+                    if (get.isReadFromTranslog()) {
+                        // this is only used for updates - API _GET calls will always read form a reader for consistency
+                        // the update call doesn't need the consistency since it's source only + _parent but parent can go away in 7.0
+                        if (versionValue.getLocation() != null) {
+                            try {
+                                Translog.Operation operation = translog.readOperation(versionValue.getLocation());
+                                if (operation != null) {
+                                    // in the case of a already pruned translog generation we might get null here - yet very unlikely
+                                    TranslogLeafReader reader = new TranslogLeafReader((Translog.Index) operation, engineConfig
+                                        .getIndexSettings().getIndexVersionCreated());
+                                    return new GetResult(new Searcher("realtime_get", new IndexSearcher(reader)),
+                                        new VersionsAndSeqNoResolver.DocIdAndVersion(0, ((Translog.Index) operation).version(), reader, 0));
+                                }
+                            } catch (IOException e) {
+                                maybeFailEngine("realtime_get", e); // lets check if the translog has failed with a tragic event
+                                throw new EngineException(shardId, "failed to read operation from translog", e);
+                            }
+                        } else {
+                            trackTranslogLocation.set(true);
+                        }
+                    }
                     refresh("realtime_get", SearcherScope.INTERNAL);
                 }
                 scope = SearcherScope.INTERNAL;
@@ -790,6 +813,10 @@ public IndexResult index(Index index) throws IOException {
                     }
                     indexResult.setTranslogLocation(location);
                 }
+                if (plan.indexIntoLucene && indexResult.hasFailure() == false) {
+                    versionMap.maybePutUnderLock(index.uid().bytes(),
+                        getVersionValue(plan.versionForIndexing, plan.seqNoForIndexing, index.primaryTerm(), indexResult.getTranslogLocation()));
+                }
                 if (indexResult.getSeqNo() != SequenceNumbers.UNASSIGNED_SEQ_NO) {
                     localCheckpointTracker.markSeqNoAsCompleted(indexResult.getSeqNo());
                 }
@@ -916,8 +943,6 @@ private IndexResult indexIntoLucene(Index index, IndexingStrategy plan)
                 assert assertDocDoesNotExist(index, canOptimizeAddDocument(index) == false);
                 index(index.docs(), indexWriter);
             }
-            versionMap.maybePutUnderLock(index.uid().bytes(),
-                new VersionValue(plan.versionForIndexing, plan.seqNoForIndexing, index.primaryTerm()));
             return new IndexResult(plan.versionForIndexing, plan.seqNoForIndexing, plan.currentNotFoundOrDeleted);
         } catch (Exception ex) {
             if (indexWriter.getTragicException() == null) {
@@ -941,6 +966,13 @@ private IndexResult indexIntoLucene(Index index, IndexingStrategy plan)
         }
     }
 
+    private VersionValue getVersionValue(long version, long seqNo, long term, Translog.Location location) {
+        if (location != null && trackTranslogLocation.get()) {
+            return new TranslogVersionValue(location, version, seqNo, term);
+        }
+        return new VersionValue(version, seqNo, term);
+    }
+
     /**
      * returns true if the indexing operation may have already be processed by this engine.
      * Note that it is OK to rarely return true even if this is not the case. However a `false`