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-
Related
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
Experts,
I have the following configuration 3 nodes cluster (Cassandra 2.1):
- Replication factor of 2
- Consistency level ONE
- Driver consistency level SERIAL
- SimpleStrategy
- GossipingPropertyFileSnitch
With this configuration, if I bring down one node I get the following error:
Not enough replica available for query at consistency SERIAL (2 required but only 1 alive)
Data is distributed evenly across all the nodes and nodetool status correctly shows that one node is down on the running 2 cassandra nodes
With CONSISTENCY ONE and 2 nodes ups, why does it require both the replica nodes to be up???
Also I read that with SERIAL drive consistency wrt WRITE failures:
If one of three nodes is down, the Paxos commit fails under the following conditions:
CQL query-configured consistency level of ALL
Driver-configured serial consistency level of SERIAL
Replication factor of 3
This works if I set the replication factor to 3. But I don't think there should be a need to do so.
Am I missing something here?
You have hit one of the hidden gems of the Paxos protocol in Cassandra. Under the hood, Paxos works in a way that it uses a QUORUM-like consistency level for its calls.
Note that it complains about SERIAL consistency level in your error message instead of the consistency level ONE that you have set. LWT ignores what normal consistency level is set in most cases. It follows either SERIAL or LOCAL_SERIAL consistency level, which maps almost directly to a QUORUM or a LOCAL_QUORUM of nodes.
The quorum of two nodes is: two. Therefore you are getting this error message when one node is down.
With datastax java driver to connect to Cassandra, I wish to set explicitly WRITE consistency, but seems like we can set consistency level only for queries. Below is the sample code. How do i mention write consistency from driver lever ?
Cluster cluster = Cluster
.builder()
.addContactPoint(host)
.withQueryOptions(new QueryOptions().setConsistencyLevel(ConsistencyLevel.ONE))
.withRetryPolicy(DefaultRetryPolicy.INSTANCE)
.withCredentials(userName,password)
.withLoadBalancingPolicy(
new TokenAwarePolicy(DCAwareRoundRobinPolicy.builder().build()))
.build();
We have completely different requirements for reads and writes (reads have really tight SLA regarding latency numbers and writes are not that important to us to finish fast).
We decided to split sessions, we created two Cluster objects and out of those we created two sessions, one for read and one for write. When we are writing we are using writeSession and we write with CL QUORUM while when we read we use readSession which is tuned for latency requirements, with CL ONE, speculative executions and tight socket read timeout.
Long story short, you can define session specific for all your writes and define consistency level on Cluster object. Be aware only that this will implicate some more connections from driver to Cassandra cluster.
Consistency can be set at Cluster level, in which case any queries run with session.execute will have that consistency level. You can also set the consistency level as a part of the session.execute statement itself.
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/
Hi I'm new to Cassandra. I have a 2 node Cassandra cluster. For reasons imposed by the front end I need...
Total replication of all data on each of the two nodes.
Eventual consistent writes. So the node being written to will respond with an acknowledge to the front end straight away. Not synchronized on the replication
Can anyone tell me is this possible? Is it done in the YAML file? I know there is properties there for consistency but I don't see that any of the Partitioners suit my needs. Where can I set the replication factor?
Thanks
You set the replication factor during creation of the keyspace. So if you use (and plan for the future on using) a single data center set-up, you create the keyspace using cqlsh like so
CREATE KEYSPACE "Excalibur"
WITH REPLICATION = {'class' : 'SimpleStrategy', 'replication_factor' : 3};
Check out the documentation regarding the create keyspace. How this is handled internally is related to the snitch definition of the cluster and a strategy option defined per keyspace. In the case of the SimpleStrategy above, this simply assumes a ring topology of your cluster and places the data clockwise in that ring (see this).
Regarding consistency, you can very different levels of consistency for write and read operations in your client/driver during each operation:
Cassandra extends the concept of eventual consistency by offering tunable consistency―for any given read or write operation, the client application decides how consistent the requested data should be.
Read the doc
If you use Java in your clients, and the DatatStax Java driver, you can set the consistency level using
QueryOptions.setConsistencyLevel(ConsistencyLevel consistencyLevel)
"One" is the default setting.
Hope that helps