CODEWITHSUNDEEP
cassandra
Maciej Miklas
Maciej Miklas

Reputation: 3330

Cassandra - write with CL.ALL with multiple data centers

I have two Data Centers, each one with replication factor 3.

Will write with CL.ALL block until data is stored in both DCs (6 nodes, or 3 + 1)? I would assume, that it blocks until all 3 replicas in local DC has acknowledged successful write.

I would like to have something like CL.ALL_LOCAL, which stores data on all replicas in single DC, so I can read with CL.ONE. The idea is, that write blocks until all replicas in single DC has persisted data, and following read will have high probability to read fresh data

Upvotes: 2

Views: 502

Answers (2)

Maciej Miklas
Maciej Miklas

Reputation: 3330

I've checked Cassandra 1.1 code and noticed interesting behavior when writing with CL.ALL in multi DC deployment. Probably I've interpreted code wrong.... anyway:

on the beginning they are collecting IP addresses of nodes to send row mutation - this is independent from consistency level provided by the client. In 1.0 it were all nodes from all DCs, from 1.1 they get all nodes from local DC plus one node from each remote DC (the remaining nodes are as "forward to" in the message). Each mutation will be send by separate thread, so the requests can run in parallel. Each such mutation is being handled as a message by messaging service. When node in remote DC receives message, it forwards it to remaining nodes, which are provided in "forward to".

The consistency level provided by the client, defines number of nodes which must acknowledge received message. In case of CL.ALL this number is determined by replication factor - now is getting interesting: since we've send message to all nodes from local DC and to nodes from remote DCs, we will get also acknowledgement from those remove nodes too - yes this is still the number which is defined by replication factor, but depending on notwork latency, we can not be sure which nodes has conformed received message - could be mix from nodes from local and remote DC, but could be also only nodes from local DC. In the worst case, it could happen, that none of the local nodes got the message, and confirmation come from remote DCs (if you have many). This means - writing with CL.ALL does not grantee, that you can immediately read message from your local DC.

Upvotes: 0

nickmbailey
nickmbailey

Reputation: 3684

There isn't currently a consistency level that provides what you are describing. The closest is LOCAL_QUORUM which will return after a quorum of nodes in the local datacenter respond.

You can file a ticket on jira to add this functionality if you would like.

https://issues.apache.org/jira/browse/CASSANDRA

Upvotes: 2

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