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

• Type: New Feature
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 4.4.0
• Fix Version/s: 4.5.0
• Component/s: bookkeeper-client
• Assignee: Enrico Olivelli
• Reporter: Enrico Olivelli
• Created: 28/Mar/17 14:21
• Updated: 22/May/17 14:54
• Resolved: 22/May/17 14:54

Description

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:

        if (lastEntry > lastAddConfirmed) {
            LOG.error("ReadException on ledgerId:{} firstEntry:{} lastEntry:{}",
                    new Object[] { ledgerId, firstEntry, lastEntry });
            cb.readComplete(BKException.Code.ReadException, this, null, ctx);
            return;
        }

this is my proposal:

        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;
        }

Issue Links

• relates to: BOOKKEEPER-874 Explict LAC from Writer to Bookies
• links to: GitHub Pull Request #121
• mentioned in: Page Loading...