In the context of the FINEXT project, we have been reviewing Rancher as a tool to support easy deployment of FIWARE components. (Our colleagues in the project have more experience with this tool – we’re still climbing the learning curve). Here are a few observations relating to Rancher.
The primary problem that Rancher solves is management of potentially disparate (sets of) IaaS resources to provide support for deploying containerized applications. Another important aspect of the Rancher vision is the application catalog – a well defined set of containerized applications that can be deployed to a container platform.
This is, of course, a very noisy area with much technology competition: the Rancher team developed their own orchestration framework – Cattle – but it was clear from some time ago that there would be many different orchestration frameworks and they intelligently decided to integrate with other platforms which were gaining traction. Specifically, they provide support for Kubernetes, Swarm and Mesos.
While playing with Rancher to understand how it works, we looked at how it supports three use cases:
- Deployment of applications on IaaS with Cattle based container management
- Deployment of applications on IaaS with Kubernetes container management
- Deployment of applications on IaaS with Swarm container management
Although the Mesos case is also interesting (and we like Mesos!), we decided not to consider it as Mesos does not currently have as much momentum as the other technologies.
The basic Rancher approach
Before discussing our initial observations, it is appropriate to give some details on key concepts in Rancher.
Rancher supports so-called Environments which are defined by a specific orchestration framework (eg Cattle, Kubernetes, Swarm) and comprise of a set of Hosts. Applications can be deployed to Environments from the Application Catalog; obviously, Applications need to be defined in a manner that is compatible with the Environment’s orchestration mechanisms – for Cattle and Swarm, applications can be defined using
docker-compose format; Kubernetes environments require a different pod-based format.
Hosts are typically VMs that run in cloud platforms – although it is possible to configure these manually, the intended use case is that these are created by
docker-machine contains drivers for many different hosting providers and Rancher leverages these to enable Hosts to be provisioned on a wide range of different providers. Rancher provisioning is quite complex, but generally it comprises of deploying
rancher-agent which enables Rancher to monitor and control the host and deployment of an overlay network which enables the Host to network with the other Hosts in the Environment.
The workflow then is one in which
rancher-server is first deployed (typically in a VM). In
rancher-server an Environment is created, Hosts are added to the environment and then Applications can be deployed from the catalog. Note that
rancher-server can – typically should – manage multiple different Environments. Rancher provides good support for monitoring the state of the system: for example, it is straightforward to see all containers running on the Hosts, their logs and if they are in an error state.
Standard Cattle Management
Standard Cattle Management is the most developed Rancher capability. In this mode, Cattle – running in
rancher-server – is responsible for orchestrating the application on the host.
rancher-agent runs on each of the Hosts in privileged mode and hence it has the power to create and destroy containers on each Host. rancher-server communicates with rancher-agent over a websockets connection to obtain the state of the host.
The Application Catalog for this mode is well developed. Rancher comes with a default Application Catalog and supports import of applications to the catalog. Further, it supports the use of private docker registries as it is clear that many applications would not be public. In the Cattle Environment, the Application Catalog is evident (a menu item at the top of the screen). Applications comprise of three files:
docker-compose.yml: contains the standard information for launching and managing a multi-container service
rancher-compose.yml: which contains the descriptor for the application in the service catalog, what parameters it requires as well as information pertaining to deploying the application over multiple VMs, scaling, health checks etc
Answers.txt: contains the default values of the parameters required to launch the application
We did not experiment much with Cattle orchestration, but documentation indicates that it is a sensible orchestration framework which deploys applications in a balanced manner across the Hosts in the Environment.
Rancher with Kubernetes
Rancher also supports Environments based on Kubernetes. As such, Rancher supports rapid and easy deployment of a Kubernetes cluster across disparate hosts: the Environment is created in Rancher and Hosts are added via
As with the Cattle deployment,
rancher-agent is deployed on all nodes in the cluster: this enables Rancher to have full visibility of each of the nodes in the Environment – what containers are running etc. It is also used in the process of deploying the Kubernetes environment.
It took us some time to understand the role of the Application Catalog in the Kubernetes context. Although Rancher has some support for a Kubernetes Application Catalog and the Catalog differs from that available for standard Cattle Environments – applications are described in terms of pods – we found that deploying these applications did not work.
The Kubernetes cluster was deployed successfully and usable. Rancher offers a web-based CLI by which applications can be deployed to the cluster (both with
helm); applications can also be deployed outside of the Kubernetes interface of course with Rancher making the Kubernetes credentials available which can be used by
Rancher with Swarm
Rancher support for Swarm is similar to that of Kubernetes in the sense that the primary focus is on managing the Hosts in the Swarm Environment. Rancher provides support for bringing up a Swarm and enabling it to be controlled via the standard Swarm toolset.
It is worth noting that we did have some confusion working with Applications in Swarm. The Swarm deployment mode has the capability to deploy applications from a catalog, although it is not so prominent in the interface. It took us some time to realize that this was not the intended deployment mode – this mechanism uses Cattle for the application deployment rather than Swarm. This was non-obvious – the applications were
docker-compose applications and, as such, we assumed that they could be deployed via Swarm. Deploying the applications appeared to work in the sense that they were visible in Rancher, but on closer inspection we found irregularities. Specifically, the application was not deployed in ‘managed’ mode, even though this was stipulated in the Application Catalog; also,
docker service ls did not show the application.
The limitations noted above most probably arise because Swarm support is still experimental and will be resolved as the solution matures.
Another noteworthy point relating to Swarm usage is that Rancher provides a very useful interface to both the containers and nodes within the Swarm: this can be used to understand current state and perform troubleshooting. Unlike the Kubernetes environment, the Swarm environment has no such standard tool and Rancher provides significant value add here.
Rancher is a useful and interesting evolving platform. It focuses primarily on the important problem of bridging between the classical VM/IaaS world to the newer container ecosystems; another important aspect of the Rancher vision is application management. As the world of container ecosystems is evolving rapidly – with some technologies offering key parts of Rancher’s vision – it will be challenging for Rancher to span all aspects of application management from VM management to container management to application deployment and management, but the technology has obtained some momentum and solves a real problem and hence it’s likely to be around for a while.