> ## Documentation Index > Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt > Use this file to discover all available pages before exploring further. # Understanding multi container application > This article explores multi-container application architecture using Docker Compose to configure an API and MongoDB container for effective inter-container communication. In this lesson, we explore an application architecture that leverages multiple containers to separate concerns effectively. The architecture consists of two containers: * An API container running a Node.js/Express application. * A MongoDB container using the default Mongo image. This setup demonstrates how to configure inter-container communication using Docker Compose and how the API leverages environment variables to configure its connection to MongoDB. The Node.js application integrates with MongoDB through Mongoose and implements a basic CRUD interface to manage notes. ## Docker Compose Configuration Below is an example Docker Compose configuration outlining the multi-container deployment. Two variants for the MongoDB volume configuration are provided based on your persistence needs. ### API Container Setup The API container is built from the project source and exposes port 3000. It uses environment variables for connecting to the MongoDB container, as illustrated by the following excerpt: ```javascript theme={null} const express = require("express"); const mongoose = require("mongoose"); const cors = require("cors"); const Note = require("./models/noteModel"); const app = express(); app.use(cors()); app.use(express.json()); const mongoURL = `mongodb://${process.env.MONGO_USER}:${process.env.MONGO_PASSWORD}@${process.env.MONGO_IP}:${process.env.MONGO_PORT}/?authSource=admin`; // Alternative connection string for local testing: // const mongoURL = `mongodb://localhost:27017/?authSource=admin`; ``` ### Console Output Example A sample console output upon running the application might appear as follows: ```bash theme={null} user1 on user1 in ecs-project2 [!] is 📦 v1.0.0 via 🐳 ``` ### Docker Compose Variants #### Variant 1 – Using a Bind Mount This variant uses a bind mount for the MongoDB data directory to link the container's `/data/db` to a local folder. ```yaml theme={null} version: "3" services: api: build: . image: kodekloud/ecs-project2 environment: - MONGO_USER=mongo - MONGO_PASSWORD=password - MONGO_IP=mongo - MONGO_PORT=27017 ports: - "3000:3000" mongo: image: mongo environment: - MONGO_INITDB_ROOT_USERNAME=mongo - MONGO_INITDB_ROOT_PASSWORD=password volumes: - ./data:/data/db ``` #### Variant 2 – Using a Named Volume This variant uses a named volume (`mongo_data`) to manage the persistence of MongoDB data. ```yaml theme={null} version: "3" services: api: build: . image: kodekloud/ecs-project2 environment: - MONGO_USER=mongo - MONGO_PASSWORD=password - MONGO_IP=mongo - MONGO_PORT=27017 ports: - "3000:3000" mongo: image: mongo environment: - MONGO_INITDB_ROOT_USERNAME=mongo - MONGO_INITDB_ROOT_PASSWORD=password volumes: - mongo_data:/data/db volumes: mongo_data: ``` Ensure that the environment variables for MongoDB (MONGO\_USER, MONGO\_PASSWORD, MONGO\_IP, and MONGO\_PORT) are correctly passed to the API container. This is crucial for establishing a successful connection. ## API Endpoints The API provides a set of endpoints for performing CRUD operations on a "notes" collection. Below are details of each endpoint with corresponding code examples. | HTTP Method | Endpoint | Description | | ----------- | ---------- | ------------------------------------ | | GET | /notes | Retrieve all notes from the database | | GET | /notes/:id | Retrieve a single note by ID | | POST | /notes | Create a new note entry | | PATCH | /notes/:id | Update an existing note | | DELETE | /notes/:id | Delete a specific note | ### GET /notes Retrieves all notes stored in the database. ```javascript theme={null} app.get("/notes", async (req, res) => { try { const notes = await Note.find(); res.status(200).json({ notes }); } catch (error) { res.status(500).json({ error: error.message }); } }); ``` ### GET /notes/:id Retrieves the details of a single note based on the provided ID. (The relevant code for this endpoint is implied by the overall CRUD structure.) ### POST /notes Creates a new note entry in the database. ```javascript theme={null} app.post("/notes", async (req, res) => { try { const note = await Note.create(req.body); res.status(201).json({ note }); } catch (e) { console.log(e); res.status(400).json({ status: "fail" }); } }); ``` ### PATCH /notes/:id Updates an existing note and returns the updated note if successful. ```javascript theme={null} app.patch("/notes/:id", async (req, res) => { try { const note = await Note.findByIdAndUpdate(req.params.id, req.body, { new: true, runValidators: true, }); if (!note) { return res.status(404).json({ message: "Note not found" }); } res.status(200).json({ note }); } catch (e) { res.status(400).json({ error: e.message }); } }); ``` ### DELETE /notes/:id Deletes a note corresponding to a given ID. ```javascript theme={null} app.delete("/notes/:id", async (req, res) => { try { const note = await Note.findByIdAndDelete(req.params.id); if (!note) { return res.status(404).json({ message: "Note not found" }); } res.status(200).json({ status: "success" }); } catch (e) { console.log(e); res.status(400).json({ error: e.message }); } }); ``` A sample console output after executing these operations might be: ```plaintext theme={null} user1 on user1 in ecs-project2 [!] is 📦 v1.0.0 via 🐳 ``` ## Connecting to MongoDB The application uses Mongoose to establish a connection with the MongoDB container at startup. The connection relies on the environment variables defined earlier. Once the connection is successful, the server begins listening on port 3000. ```javascript theme={null} mongoose .connect(mongoURL, { useNewUrlParser: true, useUnifiedTopology: true, }) .then(() => { console.log("Successfully connected to DB"); app.listen(3000, () => console.log("Server is listening on PORT 3000") ); }) .catch((e) => { console.log(e); process.exit(1); }); ``` Make sure your environment variables for the MongoDB connection are properly set in your deployment configuration to avoid connectivity issues. Transcribed by [Otter.ai](https://otter.ai)