The reason for this is that images once they are stopped the data it creates is lost, and we really do not want this to happen; specially if we are dealing with a data repository such as cassandra.
Lets consider we a have a cassandra image running like was explained in my previous post Set up Cassandra Cluster with Docker
First of all we are going to stop and remove the image (if was already created).
Lets create a directory in the host system wherever you prefer.
mkdir ~/cassandradata
And now lets start Cassandra image making it to keep its data in the directory just created
docker run -d --name cassandra1 --net=host -v ~/cassandradata:/var/lib/cassandra
We have telled our image to store its data in our directory.
Lets go inside Cassandra image and create a simpole table to see how data is keept.
docker exec -ti cassandra1 cqlsh localhost
Now we create a keyspace and a table.
CREATE KEYSPACE mykeyspace WITH REPLICATION = { 'class' : 'SimpleStrategy', 'replication_factor' : 1 };
CREATE TABLE mytable (id int PRIMARY KEY, name text, surname text);
INSERT INTO mytable (id,name,surname) values (1,'rafa','reyes');
And lets exit from Cassandra. If we check our directory we can see now something inside it.
We stop our Cassandra image
docker cassandra1 stop
And start it again
docker start cassandra1
If go to our keyspace again and select the data we can see it is still there
So like this we can keep our data safe even if our image stops. Also useful in case you need to move your image to another host system.
