sijie commented on a change in pull request #1808: Allow to configure sticky
reads
URL: https://github.com/apache/bookkeeper/pull/1808#discussion_r232909063
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File path:
bookkeeper-server/src/main/java/org/apache/bookkeeper/client/PendingReadOp.java
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@@ -96,13 +96,38 @@
this.ensemble = ensemble;
this.eId = eId;
+ long entryIdToConsiderForWriteSet = eId;
+
+ if (clientCtx.getConf().enableStickyReads
+ && lh.getLedgerMetadata().getWriteQuorumSize() ==
lh.getLedgerMetadata().getEnsembleSize()) {
+ // When sticky reads are enabled we want to make sure to take
+ // advantage of read-ahead (or, anyway, from effciencies in
+ // reading sequential data from disk through the page cache).
+ // For this, all the entries that a given bookie prefetches,
+ // should read from that bookie.
+ // For example, with e=2, w=2, a=2 we would have
+ // B-1 B-2
+ // e-0 X X
+ // e-1 X X
+ // e-2 X X
+ //
+ // In this case we want all the requests to be issued to B-1
(by
+ // preference), so that cache hits will be maximized.
+ //
+ // We can only enable sticky reads if the ensemble==writeQuorum
+ // otherwise the same bookie will not have all the entries
+ // stored
+ entryIdToConsiderForWriteSet = 0;
Review comment:
hmm I don't think it is a good idea to sticky to ensemble 0. if the first
bookie of ensemble 0 went down, all the reads will have huge penalties. I think
"sitckiness" here should be at ledger handle level, when a ledger handle is
constructed, one bookie is chosen as the sticky bookie, most of the requests
would be sent to this sticky bookie. until failure occurs, a new sticky bookie
would be choose. that is being said, ledger handle should probably implement a
`Supplier<WriteSet>` which supplies the `WriteSet` for pending read ops to
read.
so the pending read ops only talks to this supplier to get write set to read
entries.
- for normal settings, the supplier will return
`lh.distributionSchedule.getWriteSet(entryId)`.
- if reorder is enabled, the supplier will return reordered write set.
- if sticky read is enabled, the supplier will return a fix writeset most of
the time, if errors occurs, the supplier will update the write set, so the
subsequent reads will choose a different sticky bookie to read.
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