[jira] [Created] (BOOKKEEPER-1019) Support for reading entries after LAC (causal consistency driven by out-of-band comunications)

[Enrico Olivelli (JIRA)]

Enrico Olivelli created BOOKKEEPER-1019:
-------------------------------------------

             Summary: Support for reading entries after LAC (causal consistency 
driven by out-of-band comunications)
                 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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