Скачать или смотреть Resolving Connection Issues with Multiple MongoDB Containers in Docker

Discover how to effectively troubleshoot connection problems when using multiple MongoDB containers in Docker. Get expert insights and solutions for your `docker-compose` setup.
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This video is based on the question https://stackoverflow.com/q/73415151/ asked by the user 'Domenico' ( https://stackoverflow.com/u/9998794/ ) and on the answer https://stackoverflow.com/a/73415215/ provided by the user 'Hoàng Huy Khánh' ( https://stackoverflow.com/u/9711476/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction

In a modern application architecture, it’s common to work with multiple MongoDB containers, especially to separate concerns between different services such as frontend and backend applications. However, when deploying applications with Docker Compose, you may encounter unexpected connection issues. This guide addresses a specific problem faced when running multiple MongoDB containers, specifically related to an inability to connect to a MongoDB instance running on a non-default port.

The Problem

In the scenario presented, a developer created a Docker Compose file that includes two MongoDB containers. One of the containers exposes its MongoDB instance on port 27017, while the other is set to 27018. Despite this setup, the backend service that connects to the database using port 27018 is experiencing a MongooseServerSelectionError, leading to connection failures.

Error Message:

[[See Video to Reveal this Text or Code Snippet]]

This error indicates that the connection was refused when attempting to reach the second MongoDB container. Let’s dive into understanding why this is happening and how to resolve it.

Understanding the Connection Issue

The underlying issue arises from how Docker handles networking between containers. Here’s a breakdown of the core concepts:

Docker Networking Basics

Internal vs External Ports:
In Docker, the ports defined for a service using the ports directive essentially maps a container's port to the host. For example, 27018:27017 means that external requests sent to port 27018 on the host will be forwarded to port 27017 of the MongoDB container.

Container Discovery:
Containers on the same network can communicate directly with each other using the container names as hostnames. Therefore, the MongoDB service can be accessed by the container name and its internal port.

The Issue

In this setup, while one backend service attempts to connect to mongo-db-showcase-db:27018, it is actually reaching out to an external port instead of the internal container port. Because Docker containers communicate over their internal network, the connection must be directed to port 27017, where MongoDB is listening internally.

The Solution

To fix the connection error, you need to modify the connection string in your backend service to target the correct internal port of the MongoDB container. Follow the steps below:

Step 1: Update the Database Connection String

Change the DATABASE_URL in the docker-compose.yml file for the backend service from port 27018 to 27017:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Validate Other Configurations

Ensure that the following are correctly set in your docker-compose.yml and application code:

Environment Variables: Confirm that ${DB_USER} and ${DB_PASSWORD} are correctly defined and accessible to your backend service.

Docker Compose Service Definitions: Verify that no typos or inaccurate configurations are present that could hinder the backend service from correctly resolving the MongoDB container.

Step 3: Test the Connection

After making the necessary changes, rebuild and restart your Docker containers to apply the updated configuration. You can use the command:

[[See Video to Reveal this Text or Code Snippet]]

Once the services are running, you should find that the backend service can successfully connect to the MongoDB instance without returning a connection error.

Conclusion

By understanding the nuances of Docker networking and internal versus external ports, you can effectively troubleshoot connection issues when working with multiple MongoDB containers. Remember, whenever connecting to a database service from another container in Docker, always use the internal port defined w