CODEWITHSUNDEEP
nosqlcassandra
Ivan Longin
Ivan Longin

Reputation: 3353

Cassandra write performance regarding consistency level

Here is quote from cassandra documentation about writes (LINK)

If all replicas for the affected row key are down, it is still possible for a write to succeed if using a write consistency level of ANY. Under this scenario, the hint and written data are stored on the coordinator node, but will not be available to reads until the hint gets written to the actual replicas that own the row. The ANY consistency level provides absolute write availability at the cost of consistency, as there is no guarantee as to when written data will be available to reads (depending how long the replicas are down). Using the ANY consistency level can also potentially increase load on the cluster, as coordinator nodes must temporarily store extra rows whenever a replica is not available to accept a write.

My question is: is writing to cassandra slower if we use consistency level of ANY than writes when we use consistency level of ONE ?

Upvotes: 2

Views: 806

Answers (1)

Daniel S.
Daniel S.

Reputation: 3514

Hints are generated when appropriate replica nodes are inaccessible at write time. Write requests are then serialized locally on the request coordinator node. Once a valid replica node becomes available and the coordinator node learns of it, the request is passed along to the newly available replica.

With that background, there are two write-time scenarios to consider:

1) At least one replica is up for the affected row. In this case, there is no difference between consistency levels of ANY and ONE. The write just goes to the replica(s), and hinted handoff is not triggered. No performance difference.

2) All replicas are down for the affected row. This is where hints enter the picture. With consistency ANY there is extra work to be done on the coordinator node at request time, as the hint is written to a local system table for later replay. With consistency ONE, you would simply get a refused write in the same circumstances. ONE will expose write failures to the client, and will be faster than ANY.

Essentially, the tradeoff is refusing requests vs. pushing work onto remaining nodes, but only when nodes responsible for storing that row are down.

Upvotes: 2

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