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_r203791629
##########
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(
+ cacheResolvedResults
+ ).map(
+ this::runOnServer
+ ).collect(
+ // We do a hard materialization here so that the resulting
spliterators have properties that we want
+ // Otherwise the stream's spliterator is of a hash map entry
spliterator from the group-by-server operation
+ // This also causes eager initialization of the **sequences**, aka
forking off the direct druid client requests
+ // Sequence result accumulation should still be lazy
+ Collectors.toList()
+ ).stream();
}
- private Set<ServerToSegment> computeSegmentsToQuery(TimelineLookup<String,
ServerSelector> timeline)
+ private Stream<ServerToSegment>
computeSegmentsToQuery(TimelineLookup<String, ServerSelector> timeline)
Review comment:
I find the Stream workflow easier to follow since the steps taken are
explicitly called out. Any areas that *need* eager materialization of the
stream should do so. And they do in some places (like in the caching part).
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