# Utilized Tools¶

JupyterHub is meant to connect with many tools in the world of cloud computing and container technology. This page describes these tools in greater detail in order to provide some more contextual information.

## Cloud Computing Providers¶

This is whatever will run the actual computation. Generally it means a company, university server, or some other organization that hosts computational resources that can be accessed remotely. JupyterHub will run on these computational resources, meaning that users will also be operating on these resources if they’re interacting with your JupyterHub.

They provide the following things:

• Computing
• Disk space
• Networking (both internal and external)
• Creating, resizing, and deleting clusters

Some of these organizations are companies (e.g., Google), though JupyterHub will work fine with university clusters or custom cluster deployments as well. For these materials, any cluster with Kubernetes installed will work with JupyterHub.

## Container Technology¶

Container technology is essentially the idea of bundling all of the necessary components to run a piece of software. There are many ways to do this, but one that we’ll focus on is called Docker. Here are the main concepts of Docker:

### Container Image¶

Container images contain the dependencies required to run your code. This includes everything, all the way down to the operating system itself. It also includes things like the filesystem on which your code runs, which might include data etc. Containers are also portable, meaning that you can exactly recreate the computational environment to run your code on almost any machine.

In Docker, images are described as layers, as in layers of dependencies. For example, say you want to build a container that runs scikit-learn. This has a dependency on Python, so you have two layers: one for python, and another that inherits the python layer and adds the extra piece of scikit-learn. Moreover, that base python layer needs an operating system to run on, so now you have three layers: ubuntu -> python -> scikit-learn. You get the idea. The beauty of this is that it means you can share base layers between images. This means that if you have many different images that all require ubuntu, you don’t need to have many copies of ubuntu lying around.

Images can be created from many things. If you’re using Docker, the basic way to do this is with a Dockerfile. This is essentially a list of instructions that tells Docker how to create an image. It might tell Docker which base layers you want to include in an image, as well as some extra dependencies that you need in the image. Think of it like a recipe that tells Docker how to create an image.

### Containers¶

You can “run” a container image, and it creates a container for you. A container is a particular instantiation of a container image. This means that it actually exists on a computer. It is a self-contained computational environment that is constructed according to the layers that are inside of the Container Image. However, because it is now running on the computer, it can do other useful things like talk to other Docker containers or communicate via the internet.

## Kubernetes¶

Kubernetes is a service that runs on cloud infrastructures. It provides a single point of contact with the machinery of your cluster deployment, and allows a user to specify the computational requirements that they need (e.g., how many machines, how many CPUs per machine, how much RAM). Then, it handles the resources on the cluster and ensures that these resources are always available. If something goes down, Kubernetes will try to automatically bring it back up.

Kubernetes can only manage the computing resources that it is given. This means that it generally can not create new resources on its own (with the exception of disk space).

The following sections describe some objects in Kubernetes that are most relevant for JupyterHub.

### Processes¶

Are any program that is running on a machine. For example, a Jupyter Notebook creates several processes that handle the execution of code and the display in the browser. This isn’t technically a Kubernetes object, since literally any computer has processes that run on it, but Kubernetes does keep track of running processes in order to ensure that they remain running if needed.

### Pods¶

Pods are essentially a collection of one or more containers that run together. You can think of them as a way of combining containers that, as a group, accomplish some goal.

For example, say you want to create a web server that is open to the world, but you also want authentication so that only a select group of users can access it. You could use a single pod with two containers.

• One that does the authentication. It would have something like Apache specified in its container image, and would be connected to the outside world.
• One that receives information from the authentication container, and does something fancy with it (maybe it runs a python process).

This is useful because it lets you compatmentalize the components of the service that you want to run, which makes things easier to manage and keeps things more stable.

### Deployments¶

A deployment is a collection of pods on Kubernetes. It is how Kubernetes knows exactly what containers and what machines need to be running at all times. For example, if you have two pods: one that does the authenticating described above, and another that manages a database, you can specify both in a deployment.

Kubernetes will ensure that both pods are active, and if one goes down then it will try to re-create it. It does this by continually checking the current state of the pods, and then comparing this with the original specification of the deployment. If there are differences between the current state vs. the specification of the deployment, Kubernetes will attempt to make changes until the current state matches the specification.

Note

Users don’t generally “create” deployments directly, they are instead generated from a set of instructions that are sent to Kubernetes. We’ll cover this in the section on “Helm”.

### Service¶

A service is simply a stable way of referring to a deployment. Kubernetes is all about intelligently handling dynamic and quickly-changing computational environments. This means that the VMs running your pods may change, IP addresses will be different, etc. However you don’t want to have to re-orient yourself every time this happens. A Kubernetes service keeps track of all these changes on the backend, and provides a single address to manage your deployment.

### Namespace¶

Finally, a namespace defines a collection of objects in Kubernetes. It is generally the most “high-level” of the groups we’ve discussed thus far. For example, a namespace could be a single class running with JupyterHub.

### Persistent Volume Claim¶

Persistent Volume Claims are a way to have persistent storage without being tied down to one specific computer or machine. Kubernetes is about that flexibility, and that means that we don’t want to lock ourselves in to a particular operating system just because our files are already on it. Persistent Volume Claims help deal with this problem by knowing how to convert files between disk types (e.g., AWS vs. Google disks).

For more information on Persistent Volume Claims, see the Kubernetes documentation on persistent volumes.

## Helm¶

Helm is a way of specifying Kubernetes objects with a standard template.

### Charts¶

The way that Helm controls Kubernetes is with templates of structured information that specify some computational requirements. These templates are called “charts”, or “helm charts”. They contain all of the necessary information for Kubernetes to generate:

• a deployment object
• a service object
• a persistent volume object for a deployment.
• collections of the above components

They can be installed into a namespace, which causes Kubernetes to begin deploying the objects above into that namespace.

Charts have both names and versions, which means that you can easily update them and build off of them. There are community maintained charts available, and we use a chart to install and upgrade JupyterHub in this guide. In our case, the helm chart is a file called config.yaml.

### Releases¶

A release is basically a specific instantiation of a helmchart inserted into a particular namespace. If you’d like to upgrade your kubernetes deployment (say, by changing the amount of RAM that each user should get), then you can change the helm chart, then re-deploy it to your Kubernetes cluster. This generates a new version of the release.

## JupyterHub¶

JupyterHub is a way of utilizing the components above in order to provide computational environments that users can access remotely. It exists as two Kubernetes deployments, Proxy and Hub, each of which has one pod. Each deployment accomplishes some task that, together, make up JupyterHub. Finally, the output of JupyterHub is a user pod, which specifies the computational environment in which a single user will operate. So essentially a JupyterHub is a collection of:

• Pods that contain the JupyterHub Machinery
• A bunch of user pods that are constantly being created or destroyed.

Below we’ll describe the primary JupyterHub pods.

### Proxy Pod¶

This is the user-facing pod. It provides the IP address that people will go to in order to access JupyterHub. When a new users goes to this pod, it will decide whether to:

• send that user to the Hub pod, which will create a container for that user, or
• if that user’s container already exists, send them directly to that container instead.

Information about the user’s identity is stored as a cookie on their computer. This is how the proxy pod knows whether a user already has a running container.

### Hub Pod¶

Receives traffic from the proxy pod. It has 3 main running processes:

1. An authenticator, which can verify a user’s account. It also contains a process.
2. A “KubeSpawner” that talks to the Kubernetes API and tells it to spawn pods for users if one doesn’t already exist. KubeSpawner will tell Kubernetes to create a pod for a new user, then it will tell the the Proxy Pod that the user’s pod has been created.
3. An admin panel that has information about who has pods created, and what kind of usage exists on the cluster.