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https://issues.apache.org/jira/browse/CASSANDRA-8844?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15237363#comment-15237363
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Carl Yeksigian commented on CASSANDRA-8844:
-------------------------------------------
I've finished my first pass of reviewing this ticket. Some of these are some
cleanups that came about because of the renames.
- On recovery, we are going to delete the CDC Commit Logs instead of moving
them to the CDC Overflow folder; we use
{{ACLSM#deleteUntrackedCommitLogSegment}}, which isn't overwritten for the CDC
case
- Right now, there is no way to avoid getting a failed allocation even if the
consumer is matching the speed of the CDC overflow logic. CDC keyspaces will
have at least 250ms of failure after it has written up to capacity files, even
though the space could have been reclaimed. I suggest that in
{{CommitLogSegmentManagerCDC#discard}}, we also call
{{maybeUpdateCDCSizeCounterAsync}} so that we update the size, but not too
quickly
- The reset of {{recalculating}} in {{CLSMCDC#updateCDCDirectorySize}} should
happen inside of a finally; for example, if we get an IOException, we will
never be able to recalculate the CDC directory size. If this is intentional, we
should make sure that we explicitly flag that decision
- We aren't actually splitting the space between the regular Commit Log and the
CDC log, so I'd think we should use the same space for the commit log and the
CDC log
- In {{DropKeyspaceStatement#announceMigration}}, we should keep the catching
of the exception as we had before; this check is not sufficient, as it is the
same as in the validate step. Even though we've passed validation, we could
still get an exception when we try to update the schema
- The {{FileUtils.createDirectory}} calls should be in the checks for cdc being
empty; right now, it only works if saved_caches hasn't been specified
- In {{Parser.g}}, do we need to use anything in the value of the map? or can
we just use a null value?
- In {{Config.java}}, the change in name from
{{commitlog_max_compression_buffers_in_pool}} to
{{commitlog_max_compression_buffers_per_pool}} isn't compatible for users who
used that option; we need a NEWS entry for it
- In {{PropertyDefinitions#getSet}}, can we just use the {{keySet}} instead of
creating a new {{HashSet}} for it?
- In {{AbstractCommitLogSegmentManager#start}}, we should include the type in
the name of the Thread so that we can tell whether the thread is for the
standard CL or the CDC CL
- Would be good to add a flag to {{CommitLogReadErrorReason}} to tell whether
the error is recoverable or not; this would explain whether we will check the
return value or not in {{CR#readMutation}}
- Not sure if there is a reason to keep {{MutationInitiator}}, it serves a
similar role to the new {{ICommitLogReadHandler}}
- Don't understand the immediate use case in the comment above
{{ICommitLogReadHandler#prepReader}}
- In {{KeyspaceParams}}, we should combine those two constructors and just use
the {{create()}} calls where the 2 parameter case is used
- The {{KeyspaceParams#validate}} is a "best-effort", since we can do things
like change the topology on one side of a split and change the CDC DC's on the
other
- The comment in {{CommitLogSegmentManagerCDCTest#testCDCFunctionality}} about
the directory structure should also be in the test .yaml
nits:
- In {{cassandra.yaml}} above cdc_overflow_directory, should be
/data/cdc_overflow
- potentailly -> potentially in cassandra.yaml
- {{CommitLogTest#testRecovery(byte[])}} doesn't look used
- In {{AbstractCommitLogSegmentManager#awaitManagementTasksCompletion}} could
add a new job, and use a signal to signal the current thread. Since they will
be done, all previous jobs will have been completed
- Comment at the end of {{ACLSM#forceRecycleAll}} mention that the method has a
return value, but it is void
- In {{CommitLog#recoverSegmentManager}}, the comment above
{{manager.allocatingFrom()}} refers to Standard, works for both Standard and CDC
- We should annotate {{CommitLog#resetUnsafe}}, {{stopUnsafe}}, and
{{restartUnsafe}} as {{VisibleForTesting}}
- {{CommitLogReader#ALL_MUTATIONS}} should be public, possibly with
{{VisibleForTesting}} since we refer to it in the javadocs for the public
method {{CommitLogReader#readCommitLogSegment}}
- {{CommitLogReader#readCommitLogSegment}} should be {{VisibleForTesting}}
- There are a lot of params in javadocs that aren't properly defined, we should
remove them
- In {{CommitLogReader#readMutation}}, can replace the catch statement with
{{invalidMutations.computeIfAbsent(ex.cfId, id -> new
AtomicInteger()).incrementAndGet();}}
- Split the {{CommitLogReplayer}} constructor on multiple lines
- Comments in {{CommitLogReplayer#construct}} refer to replay position by
abbeviation, should be update to CLSP
- There are a lot of variable names {{rp}} which should be updated because of
the {{ReplayPosition}} rename
- There is a comment above {{CommitLogSegment.Allocation}} that no longer
applies; the logic was removed in CASSANDRA-7515, but the comment was not
updated
- In {{CommitLogSegmentManagerCDC#updateCDCDirectorySize}}, {{IEXception}}
should be {{IOException}}
- Above {{CLSMCDC#atCapacity}}, typo: submit should submitted
- {{CommitLogSegmentReader#EncryptedSegmenter}} and
{{CommitLogSegmentReader#CompressedSegmenter}} should share the same
constructor signature
- Interfaces don't normally start with 'I': {{ICommitLogReadHandler}}
- Typo in {{DirectorySizeBench#setUp}}; "div size" should be "dir size"
- In {{CDCStatementTest}}, both in {{testAlterAddBadCDCNts}} and in
{{testDropWithCDCFails}}, we need to make sure that we call {{fail}} if we
reach the end of the {{try}}, since we're expecting an exception
- In {{CommitLogReaderTest}} and {{CommitLogSegmentManagerCDCTest}}, we should
be using the junit {{Assert}}
- In {{CommitLogSegmentManagerCDCTest#testCDCWriteTimeout}}, we can just use
{{fail}} at the end of the {{ try { for}} block, and removing the {{pass =
true}} in {{catch}}
These are some of the stray observations that I had while looking at this code:
- Not sure what the implications of CDC is on using {{Keyspace.writeOrder}} in
{{ACLSM#forceRecycleAll}}
- Seems like we are shoehorning WTE to work when we can't allocate a new CDC
segment; it should be something between WTE and UE. WTE is fine for now, but we
should consider adding a new exception to protocol 5
I'm not sure that I understand the workflow that will allow users to be able to
read out of the CDC Log while it is still being written.
> 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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