Use follower reads to lower read latencies. With follower reads, the database has less work to do at read time, including serving the read from the tablet followers. Follower reads is similar to reading from a cache, which can give more read IOPS with low latency, but might have slightly stale yet timeline-consistent data (that is, no out of order data is possible).
Consider the following factors when using follower reads in YCQL.
You need to set the consistency level to
ONE in your application to work with follower reads or observer reads. Changing the consistency level to
ONE has no effect on write operations, only read operations. This is because writes in YCQL are always strongly consistent. Note that the default consistency level is
Using ycqlsh, you can check the consistency level using the
CONSISTENCY command with no arguments, and set the consistency level to one using
CONSISTENCY ONE. To learn about the consistency levels in YCQL, refer to CONSISTENCY.
You can specify the maximum staleness of data when reading from tablet followers. If the follower hasn't heard from the leader for 10 seconds (the default), the read request is forwarded to the leader. If the tablet follower and the tablet leader are far from each other, you might need to increase the duration. To change the duration for maximum staleness, add the
--max_stale_read_bound_time_ms flag and increase the value. For information on adding this flag when creating a cluster using
yb-ctl, refer to Creating a local cluster with custom flags.
Try it out
In this tutorial, you update a single key-value over and over, and read it from the tablet leader. While that workload is running, you start another workload to read from a follower and verify that you are able to read from a tablet follower.
Create a cluster
If you have a previously running cluster, destroy it by executing the following command:
$ ./bin/yb-ctl destroy
Start a new local cluster with three nodes and a replication factor (RF) of
3, as follows:
$ ./bin/yb-ctl --rf 3 create
Add one more node, as follows:
$ ./bin/yb-ctl add_node
Write some data
Download the YugabyteDB workload generator JAR file (
yb-sample-apps.jar) by running the following command:
wget https://github.com/yugabyte/yb-sample-apps/releases/download/v1.4.1/yb-sample-apps.jar -O yb-sample-apps.jar
By default, the YugabyteDB workload generator runs with strong read consistency, where all data is read from the tablet leader. Note that the
yb-sample-apps.jar sets the consistency level to ONE by default. You can populate exactly one key with a
10KB value into the system. Because the replication factor is
3, this key is replicated to only three of the four nodes in the cluster.
CassandraKeyValue workload application to constantly update this key-value, as well as perform reads with strong consistency against the local cluster, as follows:
$ java -jar ./yb-sample-apps.jar --workload CassandraKeyValue \ --nodes 127.0.0.1:9042 \ --nouuid \ --num_unique_keys 1 \ --num_threads_write 1 \ --num_threads_read 1 \ --value_size 10240
In the preceding command, the value of
num_unique_keys is set to
1, which means a single key
key:0 was overwritten. You can verify this using ycqlsh as follows:
$ ./bin/ycqlsh 127.0.0.1
Connected to local cluster at 127.0.0.1:9042. [ycqlsh 5.0.1 | Cassandra 3.9-SNAPSHOT | CQL spec 3.4.2 | Native protocol v4] Use HELP for help.
Run a query as follows:
ycqlsh> SELECT k FROM ybdemo_keyspace.cassandrakeyvalue;
k ------- key:0 (1 rows)
Strongly consistent reads from tablet leaders
When performing strongly consistent reads as a part of the above command, all reads are served by the tablet leader of the tablet that contains the key
key:0. Browse to the tablet-servers page, where you can see that all the requests are indeed being served by one YB-TServer, as shown in the following illustration:
Follower reads from tablet replicas
Stop the workload application above, and then run the following variant of that workload application. This command performs updates to the same key
key:0 via the tablet leader, but it reads from the replicas, as follows:
$ java -jar ./yb-sample-apps.jar --workload CassandraKeyValue \ --nodes 127.0.0.1:9042 \ --nouuid \ --num_unique_keys 1 \ --num_threads_write 1 \ --num_threads_read 1 \ --value_size 10240 \ --local_reads
You can see this by refreshing the tablet-servers page. The writes are served by a single YB-TServer that is the leader of the tablet for the key
key:0, while multiple YB-TServer replicas serve the reads.
Clean up (optional)
Optionally, you can execute the following command to shut down the local cluster.
$ ./bin/yb-ctl destroy