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https://issues.apache.org/jira/browse/CASSANDRA-6134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13800870#comment-13800870
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Aleksey Yeschenko commented on CASSANDRA-6134:
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For some context: early versions of batchlog manager used to rely strictly on
hints, so making replay parallel made zero sense. This is also why the
implementation was loosely modeled against HHOM. Now things have changed,
though.
> More efficient BatchlogManager
> ------------------------------
>
> Key: CASSANDRA-6134
> URL: https://issues.apache.org/jira/browse/CASSANDRA-6134
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Oleg Anastasyev
> Priority: Minor
> Attachments: BatchlogManager.txt
>
>
> As we discussed earlier in CASSANDRA-6079 this is the new BatchManager.
> It stores batch records in
> {code}
> CREATE TABLE batchlog (
> id_partition int,
> id timeuuid,
> data blob,
> PRIMARY KEY (id_partition, id)
> ) WITH COMPACT STORAGE AND
> CLUSTERING ORDER BY (id DESC)
> {code}
> where id_partition is minute-since-epoch of id uuid.
> So when it scans for batches to replay ot scans within a single partition for
> a slice of ids since last processed date till now minus write timeout.
> So no full batchlog CF scan and lot of randrom reads are made on normal
> cycle.
> Other improvements:
> 1. It runs every 1/2 of write timeout and replays all batches written within
> 0.9 * write timeout from now. This way we ensure, that batched updates will
> be replayed to th moment client times out from coordinator.
> 2. It submits all mutations from single batch in parallel (Like StorageProxy
> do). Old implementation played them one-by-one, so client can see half
> applied batches in CF for a long time (depending on size of batch).
> 3. It fixes a subtle racing bug with incorrect hint ttl calculation
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