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How services work

To deploy an application image when Docker Engine is in swarm mode, you create a service. Frequently a service will be the image for a microservice within the context of some larger application. Examples of services might include an HTTP server, a database, or any other type of executable program that you wish to run in a distributed environment.

When you create a service, you specify which container image to use and which commands to execute inside running containers. You also define options for the service including:

  • the port where the swarm will make the service available outside the swarm
  • an overlay network for the service to connect to other services in the swarm
  • CPU and memory limits and reservations
  • a rolling update policy
  • the number of replicas of the image to run in the swarm

Services, tasks, and containers

When you deploy the service to the swarm, the swarm manager accepts your service definition as the desired state for the service. Then it schedules the service on nodes in the swarm as one or more replica tasks. The tasks run independently of each other on nodes in the swarm.

For example, imagine you want to load balance between three instances of an HTTP listener. The diagram below shows an HTTP listener service with three replicas. Each of the three instances of the listener is a task in the swarm.

services diagram

A container is an isolated process. In the swarm mode model, each task invokes exactly one container. A task is analogous to a “slot” where the scheduler places a container. Once the container is live, the scheduler recognizes that the task is in a running state. If the container fails health checks or terminates, the task terminates.

Tasks and scheduling

A task is the atomic unit of scheduling within a swarm. When you declare a desired service state by creating or updating a service, the orchestrator realizes the desired state by scheduling tasks. For instance, the you define a service that instructs the orchestrator to keep three instances of a HTTP listener running at all times. The orchestrator responds by creating three tasks. Each task is a slot that the scheduler fills by spawning a container. The container is the instantiation of the task. If a HTTP listener task subsequently fails its health check or crashes, the orchestrator creates a new replica task that spawns a new container.

A task is a one-directional mechanism. It progresses monotonically through a series of states: assigned, prepared, running, etc. If the task fails the scheduler removes the task and its container and then creates a new task to replace it according to the desired state specified by the service.

The underlying logic of Docker swarm mode is a general purpose scheduler and orchestrator. The service and task abstractions themselves are unaware of the containers they implement. Hypothetically, you could implement other types of tasks such as virtual machine tasks or non-containerized process tasks. The scheduler and orchestrator are agnostic about they type of task. However, the current version of Docker only supports container tasks.

The diagram below shows how swarm mode accepts service create requests and schedules tasks to worker nodes.

services flow

Replicated and global services

There are two types of service deployments, replicated and global.

For a replicated service, you specify the number of identical tasks you want to run. For example, you decide to deploy an HTTP service with three replicas, each serving the same content.

A global service is a service that runs one task on every node. There is no pre-specified number of tasks. Each time you add a node to the swarm, the orchestrator creates a task and the scheduler assigns the task to the new node. Good candidates for global services are monitoring agents, an anti-virus scanners or other types of containers that you want to run on every node in the swarm.

The diagram below shows a three-service replica in yellow and a global service in gray.

global vs replicated services