DataStax Academy FAQ

DataStax Academy migrated to a new learning management system (LMS) in July 2020. We are also moving to a new Cassandra Certification process so there are changes to exam bookings, voucher system and issuing of certificates.

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question

jarimatti_193147 avatar image
jarimatti_193147 asked ·

The DS201 Read Path Quiz question about key cache seems to be a mistake

The DS201 Read Path Quiz question "The key cache.." seems to expect correct answer as "stores the actual partitions". This seems to be a mistake in the quiz: I think the right answer is "stores partition byte offsets in the SSTable".


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1 Answer

Erick Ramirez avatar image
Erick Ramirez answered ·

Quiz

The options for the key cache question in the Academy DS201 Cassandra Foundations course Read Path Quiz are:

  • stores the byte offset of the most recently accessed records
  • stores partition byte offsets in the SSTable
  • stores byte offsets into the partition index
  • stores the actual partitions

Definitions

SSTables are the files that store data on disk. The data component file (*-Data.db) contains the actual partitions.

The partition summary (*-Summary.db) stores a sampling of the partition index, that is samples of every N keys in the index.

The partition index (*-Index.db) is an index mapping of all the partition keys against their respective byte offsets in the SSTable data files.

The partition key cache is an off-heap cache that stores the index of a partition after it has been read so requires "warming" to improve performance.

Conclusion

To respond to your query directly, I agree that the key cache does not store the actual partitions but it isn't the correct answer in the quiz. But what you think is the right answer is not the correct one either.

I hope you can work out the correct answer based on the definitions I provided above. If not, let me know and I can provide it in a follow-up comment so as not to spoil it for everyone else. :)

For details, see How data is read in Cassandra. Cheers!

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Thanks, I got it by looking at the descriptions and docs more closely.

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Good. Cheers!

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