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[GitHub] drcrallen commented on a change in pull request #5913: Move Caching Cluster Client to java streams and allow parallel intermediate merges


drcrallen commented on a change in pull request #5913: Move Caching Cluster Client to java streams and allow parallel intermediate merges
URL: https://github.com/apache/incubator-druid/pull/5913#discussion_r202748487
 
 

 ##########
 File path: server/src/main/java/io/druid/client/CachingClusteredClient.java
 ##########
 @@ -249,74 +296,88 @@ public CachingClusteredClient(
         contextBuilder.put(CacheConfig.POPULATE_CACHE, false);
         contextBuilder.put("bySegment", true);
       }
-      return contextBuilder.build();
+      return Collections.unmodifiableMap(contextBuilder);
     }
 
-    Sequence<T> run(final UnaryOperator<TimelineLookup<String, ServerSelector>> timelineConverter)
+    Stream<Sequence<T>> run(final UnaryOperator<TimelineLookup<String, ServerSelector>> timelineConverter)
     {
       @Nullable
       TimelineLookup<String, ServerSelector> timeline = serverView.getTimeline(query.getDataSource());
       if (timeline == null) {
-        return Sequences.empty();
+        return Stream.empty();
       }
       timeline = timelineConverter.apply(timeline);
       if (uncoveredIntervalsLimit > 0) {
         computeUncoveredIntervals(timeline);
       }
 
-      final Set<ServerToSegment> segments = computeSegmentsToQuery(timeline);
+      Stream<ServerToSegment> segments = computeSegmentsToQuery(timeline);
       @Nullable
       final byte[] queryCacheKey = computeQueryCacheKey();
       if (query.getContext().get(QueryResource.HEADER_IF_NONE_MATCH) != null) {
+        // Materialize then re-stream
+        List<ServerToSegment> materializedSegments = segments.collect(Collectors.toList());
+        segments = materializedSegments.stream();
+
         @Nullable
         final String prevEtag = (String) query.getContext().get(QueryResource.HEADER_IF_NONE_MATCH);
         @Nullable
-        final String currentEtag = computeCurrentEtag(segments, queryCacheKey);
+        final String currentEtag = computeCurrentEtag(materializedSegments, queryCacheKey);
         if (currentEtag != null && currentEtag.equals(prevEtag)) {
-          return Sequences.empty();
+          return Stream.empty();
         }
       }
 
-      final List<Pair<Interval, byte[]>> alreadyCachedResults = pruneSegmentsWithCachedResults(queryCacheKey, segments);
-      final SortedMap<DruidServer, List<SegmentDescriptor>> segmentsByServer = groupSegmentsByServer(segments);
-      return new LazySequence<>(() -> {
-        List<Sequence<T>> sequencesByInterval = new ArrayList<>(alreadyCachedResults.size() + segmentsByServer.size());
-        addSequencesFromCache(sequencesByInterval, alreadyCachedResults);
-        addSequencesFromServer(sequencesByInterval, segmentsByServer);
-        return Sequences
-            .simple(sequencesByInterval)
-            .flatMerge(seq -> seq, query.getResultOrdering());
-      });
+      // This pipeline follows a few general steps:
+      // 1. Fetch cache results - Unfortunately this is an eager operation so that the non cached items can
+      // be batched per server. Cached results are assigned to a mock server ALREADY_CACHED_SERVER
+      // 2. Group the segment information by server
+      // 3. Per server (including the ALREADY_CACHED_SERVER) create the appropriate Sequence results - cached results
+      // are handled in their own merge
+      final Stream<SerializablePair<ServerToSegment, Optional<T>>> cacheResolvedResults = deserializeFromCache(
+          maybeFetchCacheResults(
+              queryCacheKey,
+              segments
+          )
+      );
+      return groupCachedResultsByServer(
 
 Review comment:
   I can do this in some place. In others, specifically with lambdas this kind of style becomes much harder to read. So just to keep things in the same format I used this style in all places

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