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https://issues.apache.org/jira/browse/BOOKKEEPER-1019?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15945253#comment-15945253
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Enrico Olivelli commented on BOOKKEEPER-1019:
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This issue relates to BOOKKEEPER-874 in that we do not need to force LAC if
there is an out-of-band way of telling the readers that the entry as been acked
to the writer
> Support for reading entries after LAC (causal consistency driven by
> out-of-band communications)
> -----------------------------------------------------------------------------------------------
>
> Key: BOOKKEEPER-1019
> URL: https://issues.apache.org/jira/browse/BOOKKEEPER-1019
> Project: Bookkeeper
> Issue Type: New Feature
> Components: bookkeeper-client
> Affects Versions: 4.4.0
> Reporter: Enrico Olivelli
> Assignee: Enrico Olivelli
> Fix For: 4.5.0
>
>
> Currently we check in asyncReadEntries that the range of entries is within
> the range 0....LastAddConfirmed.
> This is because the LAC guarantees that the client can read only entries that
> have been acked from the writer.
> The LAC protocol is very useful when there is not direct communication
> between "writers" and "readers".
> I have an use case in which the "writer" blocks until the write is acked
> (like addEntry) and then it takes the returned id (ledgerId + entryid) and
> passes it to a "reader" which in turn tries to read the entry.
> This communication is done out-of-band in respect to BookKeeper and we can
> assume that the entries has been stored in a durable way (the write as been
> acked by a quorum of bookies).
> As the 'reader' as received a confirmation the the writer as successifully
> written the entry it can read it without waiting for the piggyback of the LAC
> of the standard bookkeeper protocol.
> This is the normal way of working with transactional databases or with
> filesystems.
> This is kind of "causal consistency".
> The idea is to add a configuration option to relax the check in
> asyncreadEntries
> this is 4.4 version:
> {code}
> if (lastEntry > lastAddConfirmed) {
> LOG.error("ReadException on ledgerId:{} firstEntry:{}
> lastEntry:{}",
> new Object[] { ledgerId, firstEntry, lastEntry });
> cb.readComplete(BKException.Code.ReadException, this, null, ctx);
> return;
> }
> {code}
> this is my proposal:
> {code}
> if (lastEntry > lastAddConfirmed &&
> !allowReadingAfterLastAddConfirmed) {
> LOG.error("ReadException on ledgerId:{} firstEntry:{}
> lastEntry:{}",
> new Object[] { ledgerId, firstEntry, lastEntry });
> cb.readComplete(BKException.Code.ReadException, this, null, ctx);
> return;
> }
> {code}
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