Where is the database placed in docker microservices?

With the widespread application of microservice architecture, Docker has become a very popular deployment choice. However, for those who are new to using Docker for microservice deployment and are still in the learning phase, where to put the database can be a confusing question. This article will explore where to place databases in Docker microservices and how to handle persistent storage and backup of databases.

First, let’s review some basics of microservices and Docker.

What are microservices?

Microservices are a software architecture based on small, autonomous services. These services are designed around business functions, and each service can be deployed, upgraded, expanded and maintained independently. Different services can use different technology stacks to maximize flexibility and scalability.

What is Docker?

Docker is a containerization technology that runs on the Linux operating system and can be used to package and run applications. Docker containers are self-contained, executable, and easily portable packaging formats that can contain an application and all the dependencies it requires (e.g., runtime, libraries, environment variables, etc.).

Now, let’s return to the issue of database placement in Docker microservices.

In Docker microservices, databases are usually divided into two types: relational databases and non-relational databases. For example, MySQL, PostgreSQL, and Oracle are relational databases, while MongoDB, Cassandra, and Redis are non-relational databases. Here we will focus on relational databases.

1. Running the database as a container

Running the database as a container is the most common practice in Docker microservices. For relational databases such as MySQL, we can use the official MySQL image. This way we can easily deploy and start the database, and the database container can be delivered and deployed with other containers.

However, a disadvantage of this method is that Docker containers are ephemeral and the data within the container will not be saved after the container is stopped. This means that if the container crashes or needs to be restarted, all data in the database will be lost. Therefore, we need to use a persistent storage method to avoid this situation.

2. Using volumes to persist data

One of the most common ways to persist data is to use Docker volumes. A Docker volume is an independent persistent storage device in Docker that can be used with containers. By mounting the volume into the database container, we can keep the data on disk instead of in the container. This way, the data remains in the volume even if the container is deleted or recreated.

When using Docker volumes, we need to pay attention to a few points:

• Make sure that the permissions of the volume match those of the database container.
• Back up the data in the volume regularly to ensure data security.

For MySQL database, you can use the following steps to mount a volume into a Docker container:

1. Create Volume

$ docker volume create mysql-data

2. Start the MySQL container

$ docker run --name mysql-server -e MYSQL_ROOT_PASSWORD=my-secret-pw -v mysql-data:/var/lib/mysql -d mysql:latest

In the above command, we mounted the volume to the /var/lib/mysql directory of the container, which is the default database directory for MySQL.

3. Back up the data in the volume

When using Docker volumes, if the data in the volume is very important, the data should be backed up regularly. There are many tools in Docker that can automatically back up data in volumes. For MySQL databases, we can use the mysqldump command to back up the database to a local disk or elsewhere.

$ docker exec mysql-server sh -c 'exec mysqldump --all-databases -u root -p"$MYSQL_ROOT_PASSWORD"' > /my/backup/folder/all-databases.sql

In the above command, we backed up the database to the /my/backup/folder directory.

3. Place the database on the host

Another common way to place a database is to install the database on the host instead of running the database as a container. The advantage of this method is that the database data can be stored persistently on the host machine without the need to use volumes to store the data. However, if we need to deploy on multiple hosts, it is not convenient to place the database on each host. At this time, we need to use other database clustering technologies to ensure data synchronization between multiple hosts.

Summary

In Docker microservices, database placement is a very complex issue, and the ephemeral nature of the container and the persistent storage of data need to be considered. By running the database as a container and using volumes to persist data, we can easily deploy and manage our database. However, we also need to pay attention to protecting the data in the volume and back up the data regularly. If you need to deploy on multiple hosts, you need to consider other database clustering technologies.

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