Problem

Using BookKeeper as a low-latency distributed data storage achiving read-your-writes consistency outside the LAC protocol

The primary purpose of BookKeeper is to be used as distributed transaction log, but for its internal design it can be used as a distributed storage of binary data.

The idea is to store binary objects (BLOBS) as entries inside ledgers. In this case there is no need for "tailing reads" or "fencing".

This way of working can be drafted with this every simple scenario:

• Writers create ledgers, writes BLOBS to them and store to an external database the ID of each BLOB a pair (ledgerid, entryid) as BLOB_ID
• Once a write as been acked by the configured quorum of bookies the BLOB can be considered to be stored in a durable storage
• Readers can use the BLOB_IDs to access data reading directly from any of the the bookies which contain a copy of the BLOB

Design

In BookKeeper 4.4.0 the asyncReadEntries method checks that the range of entries to be retrieved is within the safe range of entry ids according to the LAC protocol:

public void asyncReadEntries(long firstEntry, long lastEntry, ReadCallback cb, Object ctx) {
        // Little sanity check
        if (firstEntry < 0 || firstEntry > lastEntry) {
            LOG.error("IncorrectParameterException on ledgerId:{} firstEntry:{} lastEntry:{}",
                    new Object[] { ledgerId, firstEntry, lastEntry });
            cb.readComplete(BKException.Code.IncorrectParameterException, this, null, ctx);
            return;
        }
        if (lastEntry > lastAddConfirmed) {
            LOG.error("ReadException on ledgerId:{} firstEntry:{} lastEntry:{}",
                    new Object[] { ledgerId, firstEntry, lastEntry });
            cb.readComplete(BKException.Code.ReadException, this, null, ctx);
            return;
        }
        asyncReadEntriesInternal(firstEntry, lastEntry, cb, ctx);
    }

The Idea is to add a new BookKeeper client side option "allowUnsafeReads" to skip the check

if (lastEntry > lastAddConfirmed)

This way the reader will be able to read data from Bookies, assuming its own responsability on the consistency of data.

We can say that there is an out-of-band contract between the writer and the readers, we only want to achieve "read-your-writes consistency"

Actions

• File a patch and discuss with the community
• Decide a better name for the allowUnsafeReads option