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question

papishetty.praneeth_19978 avatar image
papishetty.praneeth_19978 asked ·

Does LWT pick a master DC when we have multi DC deployment?

In a 2 DC deployment, When we use the consistency serial for Light Weight Transactions, does Cassandra pick a DC as a master DC to serialize the transactions?

cassandralightweight transactions
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1 Answer

Erick Ramirez avatar image
Erick Ramirez answered ·

@papishetty.praneeth_19978 There is no concept of master/slave in Cassandra. Cassandra also doesn't "serialize" LWTs.

The SERIAL consistency is used in the read part of the read-before-write to satisfy the condition for the lightweight transaction. It lets C* read the current version of the data and if it encounters a write for the data which is still in progress, it commits the data as part of the read before returning a result.

At SERIAL consistency, it does not enlist nodes from a "master" DC. Instead it reads from a QUORUM of nodes (regardless of the DC they belong to) in order to satisfy the request. Cheers!

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That was my understanding too. I was confused when I saw high ClientRequestLatencies only in one region. If a write comes in to DC "A" with serial as a consistency instead of local_serial, it might hit the cross DC "B" and the same happens when a write comes to DC "B" . Ok let me pull some more metrics from the service side .

Thanks you very much for the reply .

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