Details
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New Feature
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Status: Open
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Normal
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Resolution: Unresolved
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None
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None
Description
Cassandra currently lacks a sophisticated back pressure mechanism to prevent clients ingesting data at too high throughput. One of the reasons why it hasn't done so is because of its SEDA (Staged Event Driven Architecture) design. With SEDA, an overloaded thread pool can drop those droppable messages (in this case, MutationStage can drop mutation or counter mutation messages) when they exceed the 2-second timeout. This can save the JVM from running out of memory and crash. However, one downside from this kind of load-shedding based backpressure approach is that increased number of dropped mutations will increase the chance of inconsistency among replicas and will likely require more repair (hints can help to some extent, but it's not designed to cover all inconsistencies); another downside is that excessive writes will also introduce much more pressure on compaction (especially LCS), and backlogged compaction will increase read latency and cause more frequent GC pauses, and depending on the type of compaction, some backlog can take a long time to clear up even after the write is removed. It seems that the current load-shedding mechanism is not adequate to address a common bulk loading scenario, where clients are trying to ingest data at highest throughput possible. We need a more direct way to tell the client drivers to slow down.
It appears that HBase had suffered similar situation as discussed in HBASE-5162, and they introduced some special exception type to tell the client to slow down when a certain "overloaded" criteria is met. If we can leverage a similar mechanism, our dropped mutation event can be used to trigger such exceptions to push back on the client; at the same time, backlogged compaction (when the number of pending compactions exceeds a certain threshold) can also be used for the push back and this can prevent vicious cycle mentioned in https://issues.apache.org/jira/browse/CASSANDRA-11366?focusedCommentId=15198786&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15198786.