Why is data persisting in mongo docker container? - mongodb

Question

From my understanding, new containers should not persist data after they die.
Right now I have two Dockerfiles. One is for my application and the other is to seed a database.
This is my project structure
/
package.json
Dockerfile
docker-compose.yml
mongo-seeding/
Dockerfile
seedDatabase.mjs
This is what my seedDatabase looks like
import mongodb from "mongodb";
import data from "./dummyData";
// Connection URL
const url = "mongodb://mongo:27017/poc";
async function mongoSeeder() {
try {
// first check to see if the database exists
const client = await mongodb.MongoClient.connect(url);
const db = client.db("poc");
const questionCollection = await db.createCollection("question");
const answerCollection = await db.createCollection("answer");
await questionCollection.insertMany(data.questions);
await answerCollection.insertMany(data.answers);
} catch (e) {
console.log(e);
}
}
mongoSeeder();
This is what my the Dockerfile for seeding the database looks like
FROM node:10
WORKDIR /usr/src/app
COPY package*.json ./
COPY mongo-seeding mongo-seeding
RUN yarn add mongodb uuid faker
CMD ["yarn", "seed"]
This is what the other Dockerfile looks like
FROM node:10
WORKDIR /usr/src/app
COPY package*.json ./
RUN yarn install
RUN yarn add mongodb uuid faker
RUN yarn global add nodemon babel-cli babel-preset-env
COPY . .
EXPOSE 3000
CMD ["yarn", "dev-deploy"]
This is what my docker-compose.yml looks like
version: "3"
services:
mongo:
container_name: mongo
image: mongo
ports:
- "4040:27017"
expose:
- "4040"
app:
container_name: ingovguru
build: .
ports:
- "3000:3000"
depends_on:
- mongo
links:
- mongo
expose:
- "3000"
seed:
container_name: seed
build:
context: .
dockerfile: ./mongo-seeding/Dockerfile
links:
- mongo
depends_on:
- mongo
- app
Initially, I am seeding the database with 1000 questions and 200 answers.
Notice that I am not using any volumes, so nothing should be persisted.
I run
docker-compose build
docker-compose up
I jump into the mongo container, and I see that I have a 1000 questions and 100 answers.
I then Ctrl-C and re-run.
I now have 2000 questions and 100 answers.
Why does this happen?

Answer 1

Even though you are not declaring any volumes, the mongo image itself declares volumes for /data/db and /data/configdb in containers, which cover database data (see Dockerfile#L87). With Docker Compose, these anonymous volumes are re-used between invocations of docker-compose up and docker-compose down.
To remove the volumes when you bring down a stack, you need to use the -v, --volumes option. This will remove the volumes, giving you a clean database on the next up.
docker-compose down --volumes
docker-compose down documentation
-v, --volumes Remove named volumes declared in the `volumes`
section of the Compose file and anonymous volumes
attached to containers.

Answer 2

The problem that you have is because the mongo image has defined some volumes on its Dockerfile, you can see them doing:
docker history --no-trunc mongo | grep VOLUME
If you only stop the mongo container (you can check doing docker ps -a | grep mongo), the created volumes are kept. If you want to remove the container and its volumnes, you can remove the stopped mongo container with docker rm CONTAINER_ID or removing all the containers created by compose with docker-compose down.
In your case, if you want to build and run the services, you have to do:
docker-compose build && docker-compose down && docker-compose up -d