/**
* Makes a quick request to the aggregation manager to see whether the cell value required by a
* particular cell request is in external cache.
*
* <p>'Quick' is relative. It is an asynchronous request (due to the aggregation manager being an
* actor) and therefore somewhat slow. If the segment is in cache, will save batching up future
* requests and re-executing the query. Win should be particularly noticeable for queries running
* on a populated cache. Without this feature, every query would require at least two iterations.
*
* <p>Request does not issue SQL to populate the segment. Nor does it try to find existing
* segments for rollup. Those operations can wait until next phase.
*
* <p>Client is responsible for adding the segment to its private cache.
*
* @param request Cell request
* @return Segment with data, or null if not in cache
*/
public SegmentWithData peek(final CellRequest request) {
final SegmentCacheManager.PeekResponse response =
execute(new PeekCommand(request, Locus.peek()));
for (SegmentHeader header : response.headerMap.keySet()) {
final SegmentBody body = compositeCache.get(header);
if (body != null) {
final SegmentBuilder.SegmentConverter converter =
response.converterMap.get(SegmentCacheIndexImpl.makeConverterKey(header));
if (converter != null) {
return converter.convert(header, body);
}
}
}
for (Map.Entry<SegmentHeader, Future<SegmentBody>> entry : response.headerMap.entrySet()) {
final Future<SegmentBody> bodyFuture = entry.getValue();
if (bodyFuture != null) {
final SegmentBody body = Util.safeGet(bodyFuture, "Waiting for segment to load");
final SegmentHeader header = entry.getKey();
final SegmentBuilder.SegmentConverter converter =
response.converterMap.get(SegmentCacheIndexImpl.makeConverterKey(header));
if (converter != null) {
return converter.convert(header, body);
}
}
}
return null;
}
