This section explains how Rancher interacts with the two fundamental technologies Rancher is built on: Docker and Kubernetes.
Docker is the container packaging and runtime standard. Developers build container images from Dockerfiles and distribute container images from Docker registries. Docker Hub is the most popular public registry. Many organizations also setup private Docker registries. Docker is primarily used to manage containers on individual nodes.
Note: Although Rancher 1.6 supported Docker Swarm clustering technology, it is no longer supported in Rancher 2.x due to the success of Kubernetes.
Kubernetes is the container cluster management standard. YAML files specify containers and other resources that form an application. Kubernetes performs functions such as scheduling, scaling, service discovery, health check, secret management, and configuration management.
A Kubernetes cluster consists of multiple nodes.
Although you can run etcd on just one node, it typically takes 3, 5 or more nodes to create an HA configuration.
Master nodes are stateless and are used to run the API server, scheduler, and controllers.
The application workload runs on worker nodes.
The majority of Rancher 2.x software runs on the Rancher Server. Rancher Server includes all the software components used to manage the entire Rancher deployment.
The figure below illustrates the high-level architecture of Rancher 2.x. The figure depicts a Rancher Server installation that manages two Kubernetes clusters: one created by RKE and another created by GKE.
In this section we describe the functionalities of each Rancher server components.
Rancher API Server
Rancher API server is built on top of an embedded Kubernetes API server and etcd database. It implements the following functionalities:
Rancher API server manages user identities that correspond to external authentication providers like Active Directory or GitHub.
Rancher API server manages access control and security policies.
A project is a group of multiple namespaces and access control policies within a cluster.
Rancher API server tracks identities of all the nodes in all clusters.
Cluster Controller and Agents
The cluster controller and cluster agents implement the business logic required to manage Kubernetes clusters.
The cluster controller implements the logic required for the global Rancher install. It performs the following actions:
Configuration of access control policies to clusters and projects.
Provisioning of clusters by calling:
- The required Docker machine drivers.
- Kubernetes engines like RKE and GKE.
A separate cluster agent instance implements the logic required for the corresponding cluster. It performs the following activities:
Workload Management, such as pod creation and deployment within each cluster.
Application of the roles and bindings defined in each cluster’s global policies.
Communication between clusters and Rancher Server: events, stats, node info, and health.
The authentication proxy forwards all Kubernetes API calls. It integrates with authentication services like local authentication, Active Directory, and GitHub. On every Kubernetes API call, the authentication proxy authenticates the caller and sets the proper Kubernetes impersonation headers before forwarding the call to Kubernetes masters. Rancher communicates with Kubernetes clusters using a service account.