Cassandra LOCAL_QUORUM

Question

I'm having trouble understanding / finding information about how various quorums are calculated in cassandra.

Let's say I have a 16 node cluster using Network Topology Strategy across 2 data centers. The replication factor is 2 in each datacenter (DC1: 2, DC2: 2).

In this example, if I write using a LOCAL_QUORUM, I will write the data to 4 nodes (2 in each data center) but when will the acknowledgement happen? After 2 nodes in 1 data center are written?

In addition, to maintain strong read consistency, I need Write nodes + read nodes > replication factor. In the above example, if both reads and writes were LOCAL_QUORUM, I would have 2 + 2 which would not guarantee strong read consistency. Am I understanding this correctly? What level would I need then to ensure strong read consistency?

The goal here is to ensure that if a data center fails, reads/writes can continue while minimizing latency.

Delfinadelfine

Answer 1

The write will be successful after the coordinator received acknowledgement from 2 nodes from the same DC of the coordinator.

Using LOCAL_QUORUM for both reads and write will get you strong consistency, provided the same DC will be used for both reads and write, and just for this DC.

Roomer

How would I change this to be able to read from both data centers and maintain strong consistency? – Delfinadelfine

Use EACH_QUORUM instead. – Roomer

Use EACH_QUORUM on write, there is no EACH_QUORUM on read, as LOCAL_QUORUM on read is sufficient. Alternatively, in your environment, writing and reading with QUORUM does approximately the same thing, but will tolerate a single node failure (with RF=2 in each DC, if any replica is down, then you can't meet the quorum for that DC - however, cluster wide quorum is any 3 of the 4 replicas, probably does what you need). – Martynne

Answer 2

The previous answer is correct: "The write will be successful after the coordinator received acknowledgement from 2 nodes from the same DC of the coordinator." It is the same for reads.

The Quorum is always calculated by N/2+1 (N being the replication factor), having a local_quorum avoids the latency of the other data center.

As far as I understand, with a RF of 2 and LOCAL_QUORUM you have better local consistency but no availability in case of partition: if one single node drops, all writes and reads will fail for the range tokens of that node and its replica.

Therefore I recommend a RF of 3 if you intend to use Quorum. For 2 replica you should better use ONE.

Gayelord

More: People often confuse hints implication with CL and availability. example: 2 nodes alive on 3, the node down is serving tokens for 1/2 of the data (because RF of 2), so half of write queries and read queries would fail with consistency QUORUM. If you want a RF of 2 and extreme availability you should change the CL to ONE or ANY so your queries will succeed. Alternatively you may change your RF to 3 to solve the quorum issue. Cf. datastax.com/dev/blog/modern-hinted-handoff and official documentation: docs.datastax.com/en/cassandra/2.0/cassandra/dml/…. – Gayelord

Answer 3

client will get WRITE or READ acknowledgement from the corrdinator node once LOCAL_QUORUM complete its work in any one data center.

https://docs.datastax.com/en/archived/cassandra/3.0/cassandra/dml/dmlClientRequestsMultiDCWrites.html

If the write consistency level is LOCAL_ONE or LOCAL_QUORUM, only the nodes in the same datacenter as the coordinator node must respond to the client request in order for the request to succeed.

Use either LOCAL_ONE or LOCAL_QUORUM to reduce geographical latency lessen the impact on client write request response times.

Thespian