Update source code of server app inside docker container without touching the existing saved data

Question

I have the following Docker Compose project (files contents provided below):

.
|-- .dockerignore
|-- Dockerfile
|-- docker-compose.yml
|-- messages
    |-- 20221120-010625.txt
    |-- 20221120-010630.txt
    `-- 20221120-010641.txt
|-- package.json
`-- server.js

When you run the Docker Compose project with the following command:

$ docker-compose up -d

you can go to the url: http://localhost/?message=<message> and record multiple messages on the server.

Here you have an example:

So far so good, but...

My use case is: Some times I need to update the source code of the website. For example, imagine I need to prefix the page text on the screenshot above: Created file ... with: ### like:

### Created file: "/var/www/html/messages/20221120-010641.txt" with content: "this is a test".

BUT I cannot mess with the existing messages because that's valuable data for the server app.

I tried with the following commands:

$ docker-compose down --volumes
$ docker-compose up -d --force-recreate --build

My problem is: after updating the source code accordingly, even though the page text got properly updated, all the messages got lost, which is not good.

Could you please indicate me how can I achieve this?

I tried by defining a named volume inside the docker-compose.yml like:

services:
  serverapp:
    ...
    volumes:
      - messages:/var/www/html/messages

volumes:
  messages:

... expecting that if I destroy the server app the messages persist, but that didn't work because that named volume was owned by the user root and the messages are created by the user: node which doesn't have permission to create files on that directory, which causes an error.

Here is the content of the involved files:

.dockerignore

/node_modules/
/messages/
/npm-debug.log

Dockerfile

FROM node:16-alpine

RUN mkdir -p /var/www/html && chown -R node:node /var/www/html

WORKDIR /var/www/html

COPY --chown=node:node . .

USER node

RUN npm i

EXPOSE 8080

CMD [ "npm", "run", "start" ]

# ENTRYPOINT ["tail", "-f", "/dev/null"]

docker-compose.yml

version: '3'

services:
  serverapp:
    image: alpine:3.14
    build:
      dockerfile: Dockerfile
    container_name: serverapp
    restart: unless-stopped
    ports:
      - "80:80"

package.json

{
    "name": "docker-compose-tester",
    "version": "1.0.0",
    "main": "server.js",
    "scripts": {
        "start": "cross-env NODE_ENV=debug nodemon --exec babel-node server.js"
    },
    "dependencies": {
        "express": "^4.16.1",
        "moment": "^2.29.4"
    },
    "devDependencies": {
        "@babel/node": "^7.20.2",
        "cross-env": "^7.0.3",
        "nodemon": "^2.0.20"
    }
}

server.js

const express = require('express');
const moment = require('moment');
const path = require('path');
const fs = require('fs');

const PORT = 80;

const app = express();

app.use('/messages/', express.static(path.join(__dirname, 'messages')));

app.get('/', (req, res) => {
  const message = req.query.message;
  if (!message) {
    return res.send('<pre>Please use a query like: "/?message=Hello+World"</pre>');
  }
  const dirPathMessages = path.join(__dirname, 'messages');
  const date = moment(new Date()).format('YYYYMMDD-HHmmss');
  const fileNameMessage = `${date}.txt`;
  const filePathMessage = path.join(dirPathMessages, fileNameMessage);
  fs.mkdirSync(dirPathMessages, { recursive: true });
  fs.writeFileSync(filePathMessage, message);
  const filesList = fs.readdirSync(dirPathMessages);
  const filesListStr = filesList.reduce((output, fileNameMessage) => {
    const filePathMessage = path.join(dirPathMessages, fileNameMessage);
    const message = fs.readFileSync(filePathMessage);
    return output + `<div><a href="/messages/${fileNameMessage}">/messages/${fileNameMessage}</a> -> ${message}</div>` + "\n";
  }, '');
  res.send(`<pre>${filesListStr}\nCreated file: "${filePathMessage}" with content: "${message}".</pre>`);
});

app.listen(PORT, () => {
  console.log(`TCP Server is running on port: ${PORT}`);
});

Answer 1

Your approach with named volume is correct. To fix the permission problem, change the owner of the messages folder in the Dockerfile, before switching to the node user.

FROM node:16-alpine

RUN mkdir -p /var/www/html && chown -R node:node /var/www/html

WORKDIR /var/www/html

COPY --chown=node:node . .

RUN mkdir -p messages && chown node:node messages

USER node

...