What is Cassandra?

Apache Cassandra is an open source distributed database management system designed to handle large amounts of data across many commodity servers, providing high availability with no single point of failure. Cassandra offers robust support for clusters spanning multiple datacenters, with asynchronous masterless replication allowing low latency operations for all clients.

wikipedia.org/wiki/Apache_Cassandra

Motivation

This image is derived from the Official Cassandra image bundling Consul and Consul Template.

Consul is a service discovery tool and is used in this image to dynamically discover the other cassandra nodes in order to define those nodes as Cassandra seeds. The configuration is created at the container startup and is managed by Consul Template

Consul Template allows to change dynamically the Cassandra configuration as well, without rebundling and redeploying an image.

How to use this image

Start a cassandra server instance

Starting a Cassandra instance is simple:

$ docker run --name some-cassandra -d bandsintown/cassandra:2.1.18

... where some-cassandra is the name you want to assign to your container and tag is the tag specifying the Cassandra version you want. See the list above for relevant tags.

Connect to Cassandra from an application in another Docker container

This image exposes the standard Cassandra ports (see the Cassandra FAQ), so container linking makes the Cassandra instance available to other application containers. Start your application container like this in order to link it to the Cassandra container:

$ docker run --name some-app --link some-cassandra:cassandra -d app-that-uses-cassandra

Make a cluster

Using the environment variables documented below, there are two cluster scenarios: instances on the same machine and instances on separate machines. For the same machine, start the instance as described above. To start other instances, just tell each new node where the first is.

$ docker run --name some-cassandra2 -d -e CASSANDRA_SEEDS="$(docker inspect --format='{{ .NetworkSettings.IPAddress }}' some-cassandra)" bandsintown/cassandra:2.1.18

... where some-cassandra is the name of your original Cassandra Server container, taking advantage of docker inspect to get the IP address of the other container.

Or you may use the docker run --link option to tell the new node where the first is:

$ docker run --name some-cassandra2 -d --link some-cassandra:cassandra bandsintown/cassandra:2.1.18

For separate machines (ie, two VMs on a cloud provider), you need to tell Cassandra what IP address to advertise to the other nodes (since the address of the container is behind the docker bridge).

Assuming the first machine's IP address is 10.42.42.42 and the second's is 10.43.43.43, start the first with exposed gossip port:

$ docker run --name some-cassandra -d -e CASSANDRA_BROADCAST_ADDRESS=10.42.42.42 -p 7000:7000 bandsintown/cassandra:2.1.18

Then start a Cassandra container on the second machine, with the exposed gossip port and seed pointing to the first machine:

$ docker run --name some-cassandra -d -e CASSANDRA_BROADCAST_ADDRESS=10.43.43.43 -p 7000:7000 -e CASSANDRA_SEEDS=10.42.42.42 bandsintown/cassandra:2.1.18

Make a cluster with Consul

This image bundle Consul Template to create the Cassandra configuration at startup. The configuration can also be changed just setting keys in Consul.

This project has a docker-compose.yml file defining a Cassandra service running along Consul.

To use it just create the services:

$ docker-compose up -d

Then scale the cassandra cluster to the number of nodes desired:

$ docker-compose scale cassandra=3

When all nodes are up and running you can verify the cluster is created properly:

$ docker-compose exec cassandra nodetool status

root@/etc/cassandra > nodetool status
Datacenter: datacenter1
=======================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
--  Address     Load       Tokens  Owns (effective)  Host ID                               Rack
UN  172.21.0.6  61.24 KB   256     69.3%             1d0ea42f-199d-4b26-bb17-1a878ac16740  rack1
UN  172.21.0.5  171.4 KB   256     68.1%             4785f6bc-3c59-4f4b-8da7-14a61e2370d9  rack1
UN  172.21.0.4  82.52 KB   256     62.6%             40a505e9-bf80-498f-bc9a-39aeec5539bb  rack1

Connect to Cassandra from cqlsh

The following command starts another Cassandra container instance and runs cqlsh (Cassandra Query Language Shell) against your original Cassandra container, allowing you to execute CQL statements against your database instance:

$ docker run -it --link some-cassandra:cassandra --rm cassandra sh -c 'exec cqlsh "$CASSANDRA_PORT_9042_TCP_ADDR"'

... or (simplified to take advantage of the /etc/hosts entry Docker adds for linked containers):

$ docker run -it --link some-cassandra:cassandra --rm cassandra cqlsh cassandra

... where some-cassandra is the name of your original Cassandra Server container.

More information about the CQL can be found in the Cassandra documentation.

Container shell access and viewing Cassandra logs

The docker exec command allows you to run commands inside a Docker container. The following command line will give you a bash shell inside your cassandra container:

$ docker exec -it some-cassandra bash

The Cassandra Server log is available through Docker's container log:

$ docker logs some-cassandra

Environment Variables

When you start the cassandra image, you can adjust the configuration of the Cassandra instance by passing one or more environment variables on the docker run command line.

CASSANDRA_LISTEN_ADDRESS

This variable is for controlling which IP address to listen for incoming connections on. The default value is auto, which will set the listen_address option in cassandra.yaml to the IP address of the container as it starts. This default should work in most use cases.

CASSANDRA_BROADCAST_ADDRESS

This variable is for controlling which IP address to advertise to other nodes. The default value is the value of CASSANDRA_LISTEN_ADDRESS. It will set the broadcast_address and broadcast_rpc_address options in cassandra.yaml.

CASSANDRA_RPC_ADDRESS

This variable is for controlling which address to bind the thrift rpc server to. If you do not specify an address, the wildcard address (0.0.0.0) will be used. It will set the rpc_address option in cassandra.yaml.

CASSANDRA_START_RPC

This variable is for controlling if the thrift rpc server is started. It will set the start_rpc option in cassandra.yaml.

CASSANDRA_SEEDS

This variable is the comma-separated list of IP addresses used by gossip for bootstrapping new nodes joining a cluster. It will set the seeds value of the seed_provider option in cassandra.yaml. The CASSANDRA_BROADCAST_ADDRESS will be added the the seeds passed in so that the sever will talk to itself as well.

CASSANDRA_SEEDS_PER_DC

This variable defined the number of cassandra service to query in each Datacenter from Consul in order to defined the seeds value of the seed_provider option in cassandra.yaml. The CASSANDRA_BROADCAST_ADDRESS will be added the the seeds passed in so that the sever will talk to itself as well. Please see the docker-compose.yml for an example. Please note we never tested this configuration in a multi-datacenter environment.

CASSANDRA_CLUSTER_NAME

This variable sets the name of the cluster and must be the same for all nodes in the cluster. It will set the cluster_name option of cassandra.yaml.

CASSANDRA_NUM_TOKENS

This variable sets number of tokens for this node. It will set the num_tokens option of cassandra.yaml.

CASSANDRA_DC

This variable sets the datacenter name of this node. It will set the dc option of cassandra-rackdc.properties.

CASSANDRA_RACK

This variable sets the rack name of this node. It will set the rack option of cassandra-rackdc.properties.

CASSANDRA_ENDPOINT_SNITCH

This variable sets the snitch implementation this node will use. It will set the endpoint_snitch option of cassandra.yml.

Caveats

Where to Store Data

Important note: There are several ways to store data used by applications that run in Docker containers. We encourage users of the cassandra images to familiarize themselves with the options available, including:

• Let Docker manage the storage of your database data by writing the database files to disk on the host system using its own internal volume management. This is the default and is easy and fairly transparent to the user. The downside is that the files may be hard to locate for tools and applications that run directly on the host system, i.e. outside containers.
• Create a data directory on the host system (outside the container) and mount this to a directory visible from inside the container. This places the database files in a known location on the host system, and makes it easy for tools and applications on the host system to access the files. The downside is that the user needs to make sure that the directory exists, and that e.g. directory permissions and other security mechanisms on the host system are set up correctly.

The Docker documentation is a good starting point for understanding the different storage options and variations, and there are multiple blogs and forum postings that discuss and give advice in this area. We will simply show the basic procedure here for the latter option above:

1. Create a data directory on a suitable volume on your host system, e.g. /my/own/datadir.

2. Start your cassandra container like this:

   $ docker run --name some-cassandra -v /my/own/datadir:/var/lib/cassandra -d bandsintown/cassandra:2.1.18

The -v /my/own/datadir:/var/lib/cassandra part of the command mounts the /my/own/datadir directory from the underlying host system as /var/lib/cassandra inside the container, where Cassandra by default will write its data files.

Note that users on host systems with SELinux enabled may see issues with this. The current workaround is to assign the relevant SELinux policy type to the new data directory so that the container will be allowed to access it:

$ chcon -Rt svirt_sandbox_file_t /my/own/datadir

No connections until Cassandra init completes

If there is no database initialized when the container starts, then a default database will be created. While this is the expected behavior, this means that it will not accept incoming connections until such initialization completes. This may cause issues when using automation tools, such as docker-compose, which start several containers simultaneously.