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https://issues.apache.org/jira/browse/CASSANDRA-8844?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15246467#comment-15246467
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Joshua McKenzie commented on CASSANDRA-8844:
--------------------------------------------
bq. Descriptor parsed id mismatch error doesn't look right. The replay position
specifies from which id (and position within that id) we should replay. In
addition to (parts of) the file with the same id, this includes all files with
higher ids. Mismatch is normal.
The logical flow should be identical to trunk. It only returns at that point if
{{CommitLogReadHandler.shouldStopOnError}} returns true, which
{{CommitLogReplayer.shouldStopOnError}} doesn't ever do. It's either
permissable(not in this case), ignored, or we throw from CLR.
bq. Mutation before offset check compares file position with logical segment
position and is only valid for uncompressed files.
My understanding of this code - the before offset check is against
[reader.getFilePointer|https://github.com/apache/cassandra/compare/trunk...josh-mckenzie:8844_review#diff-9fe0bd988c4fc47a022f589f5ad72b09R232],
which in the compressed and encrypted case is from
[syncSegment.input|https://github.com/apache/cassandra/compare/trunk...josh-mckenzie:8844_review#diff-9fe0bd988c4fc47a022f589f5ad72b09R196].
That consists of a
[FileSegmentInputStream|https://github.com/josh-mckenzie/cassandra/blob/8844_review/src/java/org/apache/cassandra/db/commitlog/CommitLogSegmentReader.java#L290]
wrapped around the uncompressed buffer in the compressed case, so the raw
sentinel checking in Mutation skipping logic should correctly apply as though
it were a normal uncompressed file as the {{getFilePointer}} calls in
{{FileSegmentInputStream}} (and {{EncryptedFileSegmentInputStream}} for that
matter) take their offsets into account. As
CommitLogReaderTest.testReadFromMidpoint passes on test-compression and
validates that the replayed indexes after the passed in offset are the correct
numeric value, I'd be surprised if that sentinel check didn't work as I tried
to explicitly test for that. Added comments surrounding this fact to the
comparison for Mutation skipping. Also - please let me know if there's
something I'm missing or if I've misread this code.
bq. shouldSkipSegment JavaDoc should make it clear which kind of position it
needs (it is used correctly).
Added clarification that it's a logical position
bq. prepReader is only called for pre-2.1 segments. JavaDoc does not say so. I
don't think we want this in the handler interface, inline it at its one use
site.
Documented in javadoc. Since globalPosition exists inside CommitLogReplayer,
I've left it in the interface for now as I don't see a need to move
globalPosition into the reader.
bq. statusTracker.flagError isn't a very fitting name for what is actually a
termination request.
Renamed to requestTermination.
bq. flagError and return is inconsistent with the rest in readSection. It
should also return regardless of the shouldStop result as there's nothing
meaningful to be done with the rest of the section...
I'd actually prefer to revert this delta and consider opening a follow-up
ticket with this change if we consider it an improvement. There's enough going
into this CDC refactor that my primary goal is to keep the reading logic itself
untouched.
bq. segmentId confuses that it would be the one used later. We should rename
this to segmentIdFromFilename.
Good call. Changed.
bq. tolerateErrorsInSection &=: I don't think it was intended for the value to
depend on previous iterations.
Given the implementation of
[tolerateSegmentErrors|https://github.com/josh-mckenzie/cassandra/blob/8844_review/src/java/org/apache/cassandra/db/commitlog/CommitLogSegmentReader.java#L229]
I'm inclined to agree. That being said, this is also another change I'd prefer
to create as a follow-up ticket rather than changing behavior of non-CDC
related things on this ticket.
I'll address your further feedback in another update. The above changes are
pushed.
> Change Data Capture (CDC)
> -------------------------
>
> Key: CASSANDRA-8844
> URL: https://issues.apache.org/jira/browse/CASSANDRA-8844
> Project: Cassandra
> Issue Type: New Feature
> Components: Coordination, Local Write-Read Paths
> Reporter: Tupshin Harper
> Assignee: Joshua McKenzie
> Priority: Critical
> Fix For: 3.x
>
>
> "In databases, change data capture (CDC) is a set of software design patterns
> used to determine (and track) the data that has changed so that action can be
> taken using the changed data. Also, Change data capture (CDC) is an approach
> to data integration that is based on the identification, capture and delivery
> of the changes made to enterprise data sources."
> -Wikipedia
> As Cassandra is increasingly being used as the Source of Record (SoR) for
> mission critical data in large enterprises, it is increasingly being called
> upon to act as the central hub of traffic and data flow to other systems. In
> order to try to address the general need, we (cc [~brianmhess]), propose
> implementing a simple data logging mechanism to enable per-table CDC patterns.
> h2. The goals:
> # Use CQL as the primary ingestion mechanism, in order to leverage its
> Consistency Level semantics, and in order to treat it as the single
> reliable/durable SoR for the data.
> # To provide a mechanism for implementing good and reliable
> (deliver-at-least-once with possible mechanisms for deliver-exactly-once )
> continuous semi-realtime feeds of mutations going into a Cassandra cluster.
> # To eliminate the developmental and operational burden of users so that they
> don't have to do dual writes to other systems.
> # For users that are currently doing batch export from a Cassandra system,
> give them the opportunity to make that realtime with a minimum of coding.
> h2. The mechanism:
> We propose a durable logging mechanism that functions similar to a commitlog,
> with the following nuances:
> - Takes place on every node, not just the coordinator, so RF number of copies
> are logged.
> - Separate log per table.
> - Per-table configuration. Only tables that are specified as CDC_LOG would do
> any logging.
> - Per DC. We are trying to keep the complexity to a minimum to make this an
> easy enhancement, but most likely use cases would prefer to only implement
> CDC logging in one (or a subset) of the DCs that are being replicated to
> - In the critical path of ConsistencyLevel acknowledgment. Just as with the
> commitlog, failure to write to the CDC log should fail that node's write. If
> that means the requested consistency level was not met, then clients *should*
> experience UnavailableExceptions.
> - Be written in a Row-centric manner such that it is easy for consumers to
> reconstitute rows atomically.
> - Written in a simple format designed to be consumed *directly* by daemons
> written in non JVM languages
> h2. Nice-to-haves
> I strongly suspect that the following features will be asked for, but I also
> believe that they can be deferred for a subsequent release, and to guage
> actual interest.
> - Multiple logs per table. This would make it easy to have multiple
> "subscribers" to a single table's changes. A workaround would be to create a
> forking daemon listener, but that's not a great answer.
> - Log filtering. Being able to apply filters, including UDF-based filters
> would make Casandra a much more versatile feeder into other systems, and
> again, reduce complexity that would otherwise need to be built into the
> daemons.
> h2. Format and Consumption
> - Cassandra would only write to the CDC log, and never delete from it.
> - Cleaning up consumed logfiles would be the client daemon's responibility
> - Logfile size should probably be configurable.
> - Logfiles should be named with a predictable naming schema, making it
> triivial to process them in order.
> - Daemons should be able to checkpoint their work, and resume from where they
> left off. This means they would have to leave some file artifact in the CDC
> log's directory.
> - A sophisticated daemon should be able to be written that could
> -- Catch up, in written-order, even when it is multiple logfiles behind in
> processing
> -- Be able to continuously "tail" the most recent logfile and get
> low-latency(ms?) access to the data as it is written.
> h2. Alternate approach
> In order to make consuming a change log easy and efficient to do with low
> latency, the following could supplement the approach outlined above
> - Instead of writing to a logfile, by default, Cassandra could expose a
> socket for a daemon to connect to, and from which it could pull each row.
> - Cassandra would have a limited buffer for storing rows, should the listener
> become backlogged, but it would immediately spill to disk in that case, never
> incurring large in-memory costs.
> h2. Additional consumption possibility
> With all of the above, still relevant:
> - instead (or in addition to) using the other logging mechanisms, use CQL
> transport itself as a logger.
> - Extend the CQL protoocol slightly so that rows of data can be return to a
> listener that didn't explicit make a query, but instead registered itself
> with Cassandra as a listener for a particular event type, and in this case,
> the event type would be anything that would otherwise go to a CDC log.
> - If there is no listener for the event type associated with that log, or if
> that listener gets backlogged, the rows will again spill to the persistent
> storage.
> h2. Possible Syntax
> {code:sql}
> CREATE TABLE ... WITH CDC LOG
> {code}
> Pros: No syntax extesions
> Cons: doesn't make it easy to capture the various permutations (i'm happy to
> be proven wrong) of per-dc logging. also, the hypothetical multiple logs per
> table would break this
> {code:sql}
> CREATE CDC_LOG mylog ON mytable WHERE MyUdf(mycol1, mycol2) = 5 with
> DCs={'dc1','dc3'}
> {code}
> Pros: Expressive and allows for easy DDL management of all aspects of CDC
> Cons: Syntax additions. Added complexity, partly for features that might not
> be implemented
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