I just started learning Cassandra, wondering if there is a default consistency level and quorum setting. seems to me there are quite a few parameters (like replicator number, quorum number) are tunable to balance Consistency with performance, is there a best practice on these settings? what's the default settings?
Thank you very much.
Default READ and WRITE consistency is ONE in cassandra.
Consistency can be specified for each query. CONSISTENCY command can be used from cqlsh to check current consistency value or set new consistency value.
Replication factor is number of copies of data required.
Deciding consistency depends on factors like whether it is write heavy workload or read heavy workload, how many nodes failure can be handled at a time.
Ideally LOCAL_QUORUM READ & WRITE will give you strong consistency.
quorum = (sum_of_replication_factors / 2) + 1
For example, using a replication factor of 3, a quorum is 2 nodes ((3 / 2) + 1 = 2). The cluster can tolerate one replica down.Similar to QUORUM, the LOCAL_QUORUM level is calculated based on the replication factor of the same datacenter as the coordinator node. Even if the cluster has more than one datacenter, the quorum is calculated with only local replica nodes.
Consistency in cassandra
Following are the excellent links and should help you to understand consistency level and its configuration in Cassandra. Second link contains many pictorial diagrams.
https://docs.datastax.com/en/cassandra-oss/3.0/cassandra/dml/dmlConfigConsistency.html#dmlConfigConsistency__about-the-quorum-level
https://docs.datastax.com/en/cassandra-oss/3.0/cassandra/dml/dmlClientRequestsReadExp.html
Related
Datastax course says that Cassandra is availability/partition tolerance. However, according to this document it can be tuned to be strong consistency (i.e. CP) by setting W + R > RF, where W is the write consistency level, R is the read consistency level, and RF is the replication factor.
Tuneable to strong consistency for single partition
It can be tuned to strong consistency for documents in single partition. So if you statements belong to different partitions (note same partition key and different table is still different partition), you cannot tune it for strong consistency. So Cassandra has its upper bound to it's strong consistency unlike in RDBMS where you can update multiple records in different tables or different rows in same table atomically.
Tuning for higher consistency makes you lose some of the AvailabilityandPartition Tolerance`
When you use hinted handoff, it is almost on the AP axis as it is always available to write even with network partitions. But as soon you start tuning for higher consistency, clients have to wait for writes or reads until it is written to enough replicas /read from enough replicas to satisfy the requested consistency. So you are losing bit of availability and partition tolerance
Summary
You can configure it for maximum availability and partition tolerance but you cannot configure for much stronger consistency. So Cassandra lies in AP axis in CAP
If Replication Factor and Consistency Level are set to QUORUM then we can achieve Availability and Consistency but Performance degrade will increase as the number of nodes increases.
Is this statement correct? If yes then what is the best practice to get better result, considering Availability and Consistency as high priority and not to decrease performance as number of nodes increases.
Not necessarily. If you increase the number of nodes in your cluster, but do not alter your replication factor, the number of replicas required for single partition queries does not increase so you should therefore not expect performance to degrade.
With a 10 node cluster, replication factor 3 and CL QUORUM, only 2 replicas are required to meet quorum, the same is true for a 20 node cluster.
Things change if your query requires some kind of fan out that requires touching all replica sets. Since you have more replica sets, your client or the coordinating C* node needs to make more requests to retrieve all of your data which will impact performance.
I am new to cassandra and I wanted to understand the granular difference between consistency level and replication factor.
Scenario: If I have a replication factor of 2 and consistency level of 3, how the write operation would be performed? When consistency level is set to 3, it means the results will be acknowledged to the client after writing to the 3 nodes. If data is written to 3 nodes, then it gives me a replication factor of 3 and not 2..? Are we sacrificing the replication factor in this case?
Can someone please explain where my understanding is wrong?
Thanks!
Replication factor: How many nodes should hold the data for this keyspace.
Consistency level: How many nodes needs to respond the coordinator node in order for the request to be successful.
So you can't have a consistency level higher than the replication factor simply because you can't expect more nodes to answer to a request than the amount of nodes holding the data.
Here are some references:
Understand cassandra replication factor versus consistency level
http://docs.datastax.com/en/cassandra/2.1/cassandra/architecture/architectureDataDistributeReplication_c.html
http://docs.datastax.com/en/archived/cassandra/2.0/cassandra/dml/dml_config_consistency_c.html
You will get an error: Cannot achieve consistency level THREE.
You can do some further reading here
Consistency levels are of two types, write consistency and read consistency. Consistency levels can be one, two, three or quorum. If it's quorum, atleast half of the nodes should be available for the operation. Otherwise (for one, two, three), the name itself gives you the definition.
Replication factor is the number of copies that you are planning to maintain in the cluster. If the strategy is simple, you will have just one replication factor. If you are having network topology strategy and is using multi dc cluster, then you have to set replication factor for each data centre.
In your scenario, if you have RF as 2 and CL as 3, it will work(I am assuming there are more than 3 nodes in the cluster and atleast one seed node). In this scenario, it will check whether three nodes are up and normal to receive the data and if the CL is met, it will write two copies to two nodes.
For your second question
When consistency level is set to 3, it means the results will be
acknowledged to the client after writing to the 3 nodes. If data is written to 3 nodes, then it gives me a replication factor of 3 and not 2..?
As far as I understood cassandra, It will not be acknowledged to cassandra. It just needs the CL to be met and the number of nodes acknowledged about new data will be equal to the RF.
So, there is no question in sacrificing RF.
I have setup cassandra in 2 data centers with 4 nodes each with replication factor of 2.
Consistency level is ONE (set by default)
I was facing consistency issue when trying to read data at consistency level of ONE.
As read in DataStax documentation, Consistency level (read + write) should be greater than replication factor.
I decided to change the write consistency level to TWO and read consistency level as ONE which resolves the inconsistency problem in single data center.
But in case of multiple data center, the problem would be resolved by consistency level as LOCAL_QUORUM.
How would i achieve that write should be (LOCAL_QUORUM + TWO) so that i should write to the local data center and also on 2 nodes.
Just write using LOCAL_QUORUM in the datacenter you want. If you have a replication factor of 2 in each of your datacenter then the data you are writing in the "local" datacenter will eventually be replicated in the "other" datacenter (but you have no guaranty of when).
LOCAL_QUORUM means: "after the write operation returns, data has been effectively writen on a quorum of nodes in the local datacenter"
TWO means: "after the write operation returns, data has been writen on at least 2 nodes in any of the datacenter"
If you want to read the data you have just written with LOCAL_QUORUM in the same datacenter, you should use LOCAL_ONE consistency. If you read with ONE, then there is a chance that the closest replica is in the "remote" datacenter and therefore not yet replicated by Cassandra.
This also depends on the load balancing strategy configured at the driver level. You can read more about this here: https://datastax.github.io/java-driver/manual/load_balancing/
We have our Cassandra cluster running on AWS EC2 with 4 nodes in the ring. We have face data inconsistency issue. We changed consistency level two while using "cqlsh" shell, the data inconsistency issue has been solved.
But we dont know "How to set consistency level on Cassandra cluster?"
Consistency level can be set at per session or per statement basis. You will need to check the consistency level of writes and reads, to get a strong consistency your R + W ( read consistency + write consistency ) should be greater than your replication factor.
If you are using Java Driver, you can set default consistency at cluster level using "Cluster.Builder.withQueryOptions()" method.
http://docs.datastax.com/en/drivers/java/2.0/com/datastax/driver/core/Cluster.Builder.html#withQueryOptions-com.datastax.driver.core.QueryOptions-