In this blog post, I’ll show how to build a three-node Cassandra cluster on Docker for testing. I’ll use official Cassandra images instead of creating my images, so all processes will take only a few minutes (depending on your network connection). I assume you have Docker installed on your PC, have an internet connection (I was born in 1976, so it’s normal for me to ask this kind of question), and have at least 8 GB RAM. First, we need to assign about 5 GB RAM to Docker (in case it has less RAM) because each node will require 1.5+ GB RAM to work properly.
Open the docker preferences, click the advanced tab, set the memory to 5 GB or more, and click “apply and restart” docker service. Launch a terminal window, and run the “docker pull cassandra” command to fetch the latest official Cassandra image.
I’ll use cas1, cas2, cas3 as the node names, and the name of my Cassandra cluster will be “MyCluster” (a very creative and unique name). I’ll also configure cas1 and cas2 like they are placed in datacenter1 and cas3 like it’s placed in datacenter2. So we’ll have three nodes, two of them in datacenter1 and one in datacenter2 (to test Cassandra’s multi-DC replication support). For multi-DC support, my Cassandra nodes will use “GossipingPropertyFileSnitch”. This extra information can be passed to docker containers using environment variables (with -e parameter).
Now it’s time to start the first node:
docker run --name cas1 -p 9042:9042 -e CASSANDRA_CLUSTER_NAME=MyCluster -e CASSANDRA_ENDPOINT_SNITCH=GossipingPropertyFileSnitch -e CASSANDRA_DC=datacenter1 -d cassandra
The -p parameter is for publishing the container’s port to the host, so I could connect to the Cassandra service from the outside of the docker container (for example, using DataStax Studio or DevCenter). After the first node is up, I’ll add the cas2 and cas3 nodes, but I need to tell them the IP address of cas1, so they can use it as the seed node and join the cluster. We can find the IP address of cas1 by running the following command:
docker inspect --format='' cas1
I’ll add it to docker run command strings for cas2 and cas3:
docker run --name cas2 -e CASSANDRA_SEEDS="$(docker inspect --format='' cas1)" -e CASSANDRA_CLUSTER_NAME=MyCluster -e CASSANDRA_ENDPOINT_SNITCH=GossipingPropertyFileSnitch -e CASSANDRA_DC=datacenter1 -d cassandra
docker run --name cas3 -e CASSANDRA_SEEDS="$(docker inspect --format='' cas1)" -e CASSANDRA_CLUSTER_NAME=MyCluster -e CASSANDRA_ENDPOINT_SNITCH=GossipingPropertyFileSnitch -e CASSANDRA_DC=datacenter2 -d cassandra
I gave a different datacenter name (datacenter2) while creating the cas3 node. Run them one by one, give time to the new nodes to join the cluster, and then run the “nodetool status” command from cas1 (or any other node):
docker exec -ti cas1 nodetool status
The above code, connects to cas1 node and runs the “nodetool status” command. If everything went fine, you should see something similar to the below output.
The status column of each node should show UN (node is UP and its state is Normal). If you see “UJ” that means your node is joining, just wait a while and recheck it. If your new nodes didn’t appear in the list, they probably crashed before joining the cluster. In this case, you may restart the missing nodes. For example, if cas3 (the last node) didn’t join the cluster and it’s down, you can run the “docker start cas3” command to start it. It’ll try to join the cluster automatically.
Now Let’s create a keyspace (database) that will be replicated to datacenter1 and datacenter2 and a table in this newly created keyspace. I’ll use NetworkTopologyStrategy for replicating data. Each datacenter will store one copy of data. Here is the CQL (Cassandra query language) commands to create the keyspace and table:
CREATE KEYSPACE mykeyspace
WITH replication = {
'class' : 'NetworkTopologyStrategy',
'datacenter1' : 1,
'datacenter2' : 1
};
CREATE TABLE mykeyspace.mytable (
id int primary key,
name text
);
We can execute these commands using cqlsh by connecting one of our nodes:
docker exec -ti cas1 cqlsh
Or we can execute them using a client program such as DevCenter (you need to register to the DataStax website to be able to download it). I tried to find a stable GUI for Cassandra, and DevCenter looks fine to me:
After we created the keyspace, we can run “nodetool status” to check the data distribution:
docker exec -ti cas1 nodetool status mykeyspace
As you can see, I gave the name of the keyspace as a parameter to nodetool, so it will show the distribution of our newly created keyspace.
Did you notice that the nodes at datacenter1 share data almost evenly, while the node at datacenter2 has a replication of all data? Remember the replication strategy of our keyspace: Each datacenter stores one copy. Because there are two nodes in datacenter1, the data will be evenly distributed between these two nodes.
You can shut down nodes using “docker stop cas1 cas2 cas3” and start them again with “docker start cas1 cas2 cas3”. So, we have a working Cassandra cluster that is deployed to multiple data centers.