What Is Docker Swarm Mode and How Does It Work?
If you’ve been containerizing your development workflow, you’ll agree that Docker is one of the best choices for version control. However, Docker Swarm is one of Docker’s features used to orchestrate complex apps.
The Docker Swarm working mechanism can be hard to crack at first. But no worries, we’ll break it down in this article. So what is Docker Swarm? Why use it? And how does it work?
What Is Docker Swarm, and How Does It Work?
Docker Swarm refers to a group of Docker hosts (computers) networked as a cluster to deliver specified tasks. Each Docker host in this cluster is a node, also called a worker node.
To ensure the efficient distribution of tasks, you need a manager node. Ideally, a Docker Swarm mode initialization starts with the manager node, and subsequent nodes become workers.
As the operator, you only need to interact with the manager node, which passes instructions to the workers. Invariably, the worker nodes receive task allocation from the manager node and execute them accordingly.
However, the manager node can also participate in task execution (as a worker) or face management squarely. You can prevent task scheduling on the manager by switching its state fromactivetodrain. But your decision to assign this dual function might depend on several factors. Essentially, you want to be sure it has enough resources to handle several roles before doing so.
Nodes do fail. So the manager node actively monitors the state of each worker node and activates a fail-tolerant mechanism to reschedule the task from a failed node to another.
But what if the manager node also crashes? Interestingly, the swarm keeps running. The only pitfall is you won’t be able to communicate with the manager node to control the cluster anymore.
The common fail-safe approach to prevent this is to assign the manager role to many nodes (Docker recommends a maximum of seven per cluster). You can then select the primary manager node from them. When the primary manager crashes, one of the standby managers takes up the role.
However, you don’t have to worry about role switching among nodes or state maintenance in a cluster. The raft consensus algorithm (a fault-tolerant method) built into the Docker SwarmKit takes care of this.
Why Use Docker Swarm?
Docker Swarm is handy for deploying complex apps with high scalability prospects. One of its primary use cases is to decentralize microservices. Each microservice then shares a similar container with those on other worker nodes.
Another reason to use Docker Swarm is that multiple hosts run tasks concurrently in a cluster. This is in contrast to Docker Compose, which only allows you to run multiple containers on one Docker engine.
This scalable attribute of Docker Swarm allows apps to be consistently available with zero latency. It’s even one of the reasons you want tochoose Docker over other virtualization tools.
And what’s more? Unlike single Docker Containers, where a container stops when it fails, Docker Swarm automatically redistributes tasks among the available worker nodes whenever one fails.
Docker Swarm also keeps a backup of each state. So you can always revert new swarm configurations to the state of a former one. Say the manager node on a previous swarm fails; you can start a new cluster with more manager nodes and revert it to adapt the configuration of the previous one.
It’s also important to mention that the interaction between the manager node and the worker nodes is secure.
Docker has many alternatives, and one of the closest is Kubernetes. However, Docker Swarm is easy to use and more automated. For instance, while you may need to balance load manually in some other orchestration tools like Kubernetes, Docker Swarm features automatic load balancing, which makes life easy for DevOps.
The Docker Swarm Architecture
The Docker Swarm architecture revolves around services, nodes, and tasks. However, each has a role to play in running the stack successfully.
The Docker Swarm service details the configuration of the Docker image that runs all the containers in a swarm. It includes information about the tasks in a cluster. For instance, a service might describe aDockerized SQL server setup.
When you run a service, it compels the manager node to sync with its configurations. The manager node then runs the rest of the worker nodes based on the specified settings in the service.
Services in Docker Swarm can be global or replicated.
The difference between them is that while global services define only one task for all the nodes in a cluster, replicated services specify the number of tasks per node.
A node in Docker Swarm is an instance of the entire Docker runtime, also known as the Docker engine. Swarm nodes can be physical or virtual machines. Think of this as a network of computers running similar processes (containers).
Typically though, nodes span over several computers and servers running the Docker engine in real-life applications. And as mentioned earlier, a node can either be a manager or worker node, depending on the role.
The manager node listens to the swarm heartbeat and controls the worker nodes, which execute tasks assigned to them by the manager node. As earlier stated, you can have more than one manager node in a swarm. But ideally, try to limit the number to under seven, as adding too many manager nodes might reduce the swarm performance.
A task defines the work assigned to each node in a Docker Swarm. In the background, task scheduling in Docker Swarm starts when an orchestrator creates tasks and passes them to a scheduler, which instantiates a container for each task.
The manager node then uses the scheduler to assign and reassign tasks to nodes as required and specified in the Docker service.
Docker Swarm vs. Docker Compose: What Are the Differences?
People often use Docker Compose and Docker Swarm interchangeably. Although both involve running multiple containers, they’re different.
While Docker Compose lets you run multiple containers on a single host, Docker Swarm distributes them over several Docker engines in a cluster.
You use Docker Compose when you need to spin up separate containers for each service in your app. Thus, when one component crashes, it doesn’t interfere with the others. However, when the host machine fails, the entire app also crashes.
Docker Swarm, however, helps you run many containers on clustered nodes. So, each component of your app sits on several nodes. And when one node handling an app component crashes, the swarm allocates its task to another node within the cluster and reschedules the running tasks, preventing downtime.
Hence, while you might have downtime on Docker Compose, Docker Swarm ensures that your app keeps running with the help of backup servers (worker nodes). However, Docker 1.13 supports Docker Compose deployment to Swarm mode using thedocker stack deploycommand.
Docker Swarm Helps You Deploy Complex Apps
Containerization has trumped virtual machines in continuous integration and continuous delivery (CI/CD) software design. Therefore, understanding the nitty-gritty of the Docker Swarm mechanism is a plus skill if you’re looking to become an invaluable DevOps expert.
You probably know how to spin up a Docker container or even run a Docker Compose for multiple containers in one host. But Docker Swarm is handier for deploying apps with complex architecture. It breaks up processes into units, improves runtime access, and reduces or even eliminates the chances of downtime.
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