[jira] [Created] (HDFS-6777) Supporting consistent edit log reads when in-progress edit log segments are included

["James Thomas (JIRA)" <ji...@apache.org>]

James Thomas created HDFS-6777:
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             Summary: Supporting consistent edit log reads when in-progress edit log segments are included
                 Key: HDFS-6777
                 URL: https://issues.apache.org/jira/browse/HDFS-6777
             Project: Hadoop HDFS
          Issue Type: Sub-task
          Components: qjm
            Reporter: James Thomas
            Assignee: James Thomas


For inotify, we want to be able to read transactions from in-progress edit log segments so we can serve transactions to listeners soon after they are committed.

Inotify clients ask the active NN for transactions, and the NN opens EditLogInputStreams that pull transactions from its various edit repositories (local directories, individual JournalNodes, etc.). The goal is to send back only successfully sync’ed transactions. What constitutes a successful sync varies among the edit repositories. In the case of the JournalNodes, a successful sync requires a write to a quorum of the JournalNodes. Properly sync’ed transactions are always applied to the namesystem, and unsync’ed transactions are never applied. So it is natural that listeners are only interested in sync’ed transactions.

To achieve this goal, the NN returns to clients only transactions up to the last txid it has sync'ed.
Some infrastructural changes are required for this policy to prevent any unsync’ed transactions from reaching the client, and we tackle those in this JIRA.

The first issue is that the current implementation of selectInputStreams (inProgressOk = true, fromTxId = X) in FSEditLog (which the NN uses to read transactions) essentially asks all available edit log repositories for all in-progress or finalized log segments with transactions after and including X, and combines log segments starting at the same transaction ID in a RedundantEditLogInputStream. The problem here is that the RedundantEditLogInputStream may combine finalized and in-progress segments starting at the same txid, and may serve unsync’ed transactions from the in-progress segments. So we simply discard in-progress segments if we have any finalized segments starting at the same txid -- the modifcation is to JournalSet.chainAndMakeRedundantStreams.

The second change we need to make is for the case where we only have in-progress segments starting at a particular txid. In this case, we know we are at the log segment the NN currently has open, since the various journals ensure that if there are finalized segments available for a particular starting txid, we are able to see them (e.g. reads from QJM require a quorum of JournalNodes to respond, and a finalized segment must be present on at least one JN in the quorum). Our goal in this case is to return only the in-progress segments being written by the current NN. This happens trivially in the local edits directory case and the NFS shared edits case, since there is only a single “replica” and if the NN is writing to it, it is fully up-to-date. So we focus on QJM here. The key is that JournalNodes maintain a lastWriterEpoch, which increases monotonically as new writers arrive. So we keep track of the lastWriterEpochs of the segments we receive from the JNs and discard any segments with lastWriterEpochs less than the maximum one seen. We know that when a new writer contacts the JournalNodes, it finalizes all sync’ed transactions, so in-progress segments with out-of-date lastWriterEpochs may contain unsync’ed transactions and any sync’ed transactions they contain will also be contained in finalized segments on a quorum on JNs. So we can safely discard them. The segments with the largest lastWriterEpochs may also contain unsync’ed transactions from a previous writer if the current NN has not yet sync’ed any transactions, but in this case we will make sure to read only from finalized segments. The main modification for this change is to QuorumJournalManager.selectInputStreams.



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