Database Volumes

By the end of this exercise, you should be able to:

• Provide a docker volume as a database backing to mongodb
• Make one mongodb container's database available to other mongodb containers

Launching mongodb

1. Download a mongodb image, and inspect it to determine its default volume usage:

   PS: node-0 Administrator> docker image pull mongo:3-windowsservercore
   PS: node-0 Administrator> docker image inspect mongo:3-windowsservercore
   
   ...
   "Volumes": {
       "C:\\data\\configdb": {},
       "C:\\data\\db": {}
   },
   ...
   

   You should see a Volumes block like the above, indicating that those paths in the container filesystem will get volumes automatically mounted to them when a container is started based on this image.

2. Set up a running instance of this mongodb container:

   PS: node-0 Administrator> docker container run --name mongoserv -d `
       -v mongodata:C:\data\db `
       mongo:3-windowsservercore
   

   Notice the explicit volume mount, -v mongodata:C:\data\db; if we hadn't done this, a randomly named volume would have been mounted to the container's C:\data\db. Naming the volume explicitly is a best practice that will become useful when we start mounting this volume in multiple containers.

Writing to the Database

1. Spawn a mongo process inside your mongodb container:

   PS: node-0 Administrator> docker container exec -it mongoserv mongo
   

   You'll be presented with a mongodb terminal where you can manipulate a database directly.

2. Create an arbitrary table in the database:

   > use cats
   > db.cats.save({"coat":"calico", "icecream":"strawberry"})
   > db.cats.save({"coat":"tabby", "icecream":"lemon"})
   

   Double check you created the table you expected, and then quit this container:

   > db.cats.find()
   
   { "_id" : ObjectId("..."), "coat" : "calico", "icecream" : "strawberry" }
   { "_id" : ObjectId("..."), "coat" : "tabby", "icecream" : "lemon" }
   
   > exit
   

3. Delete the mongoserv container:

   PS: node-0 Administrator> docker container rm -f mongoserv
   

4. Create a new mongodb server container, mounting the mongodata volume just like last time:

   PS: node-0 Administrator> docker container run --name mongoserv -d `
       -v mongodata:C:\data\db `
       mongo:3-windowsservercore
   

5. Spawn another mongo process inside this new container to get a mongodb terminal, also like before:

   PS: node-0 Administrator> docker container exec -it mongoserv mongo
   

6. List the contents of the cats database:

   > use cats
   > db.cats.find()
   

   The contents of the database have survived the deletion and recreation of the database container; this would not have been true if the database was keeping its data in the writable container layer. As above, use exit to quit from the mongodb prompt.

7. Delete your mongodb container and volume:

   PS: node-0 Administrator> docker container rm -f mongoserv
   PS: node-0 Administrator> docker volume rm mongodata
   

Conclusion

Whenever data needs to live longer than the lifecycle of a container, it should be pushed out to a volume outside the container's filesystem; numerous popular databases are containerized using this pattern.