We’ve sketched out a rough architecture at this point, time to get an estimate of expected cost, and think through how well we can expect it to meet our original design goals. Besides overall user experience, I find it’s important to carefully think through the limitations of any project - what a thing can’t do is important as what it can. (Avoid scope creep!) Lastly, it’s always important to include non-functional design requirements like security, performance, etc, as these can make or break the project.

Expected Cost

Given that this is a project intended primary for personal use, I’ve been quite careful in making selections that minimize cost wherever possible. As an example, a default installation of some of the software can make use of Virtual Private Clouds with private networks, which require a special interface (“NAT”) for computers on that network to reach the outside. That’s all great for security, but that default NAT device from Amazon can cost as much as a single compute VM, and needs to be running 24/7.

The solution I’ve come up with consists of the following:

  • Base infrastructure

    • Master node: t2.micro - $ 0.0116/hr, approx $8/mo

      The master is the node that is really managing K8s and helping to determine which compute node gets which work. It needs to always be running, but doesn’t require significant memory or cpu so can be a very low powered instance.

    • Admin” node: 2 x t2.small - $ 0.023/hr, approx $16/mo * 2, $32/mo

      I designate 2 nodes to be “administration” nodes - nodes that are on 24/7 to run basic connectively functionality like the LDAP user authentication database, the SSH point of entry, and network ingresses as we launch server applications.

  • Storage

    • Elastic File System (EFS) - $ 0.30/GB/mo

      I use EFS as main storage for home directories. It’s not cheap for large data ($30/mo for 100 GB, for comparison, Dropbox is $9/mo for 1 TB), but it’s extremely convenient and reliable, a true file system (unlike Dropbox), and for configurations and environments is a great no-hassle way to get started. It’s pay for what you use, so if you keep this to just configurations it can be as low as $5/mo. The other great thing about EFS is that it’s really a managed NFS service, so all of your containers can connect to it simultaneously.

    • Elastic Block Store (EBS) SSD - $ 0.10/GB/mo

      SSD storage is required for the root file systems of all nodes. I generally set these to fairly small sizes (10-20 GB), so total cost for my 3 permanent nodes is only ~$5/mo

    • Elastic Block Store (EBS) Magnetic - $ 0.045/GB/mo

      Magnetic block storage is considerably cheaper than EFS, but also is much less convenient as only a single container can mount an EBS volume at any given time. I do use these in the intial tutorials for storing databases (e.g. LDAP data) as only a single container needs access. In future tutorials I plan to show how to launch an NFS server hosted in K8s (based on Ganesha), which will offer a low-cost alternative to EFS for larger data. I’ll also demonstrate use of rook.io for k8s-native distributed storage, backed by EBS.

    • Simple Storage Service (S3) - $ 0.023/GB/mo

      This is going to be the bread-and-butter long term storage for my environment, as it’s most appropriate for large datasets. While traditional tools like Python are only just starting to work with object storage, Spark can use it quite effectively.

  • Advanced Compute

    • Jupyter Hub

      My initial tutorials will deploy a single-instance juptyerhub that will be shared by any users of the system. The instance can be sized according to administrator preference, and depending on workload can make use of spot priced instances which can often be 80% discounted compared to reserved instances.

    • Spark

      Spark is a great candidate for using spot instances - you can have several instances designated as spark administrators that are smaller and always on, then a much larger pool of large workers that are spot instances - they will go in and out of the cluster, but will work hard when they join and will be extremely cost effective. The whole cluster only needs to be launched as needed, and can be destroyed when not in use.

    • GPU Nodes

      Like the other compute nodes, these only need to be provisioned when needed. A great advantage with this infrastructure is that all cuda/cudnn drivers are in the docker container, so setup time is on the order of minutes - compared to hours if doing everything from scratch.

All told, base cost of the system to leave running 24/7 is ~$50/month, which will allow users to login, edit code, and launch simple jobs in the jupyterhub container. Heavier computation will raise the cost, but in proportion to the size of the job, and since these are generally able to use spot instances the prices can be quite reasonable.

As an example, a t2.xlarge instance that is normally $ 0.16/hr on the reserved market can generally be had for approx $ 0.05/hr in spot - this has 4 cores and 16 GB of ram, which is plenty for many investigations. For larger work you can go to a m5.12xlarge, a monster with 48 cores and 192 GB ram - normally $2.3/hr on reserved market, but only $ 0.75 on spot.

To give an estimate for personal side projects… let’s say a user is able to really buckle down on a problem every ~3 months and spend a week straight on preliminary investigations, with a weekend of heavy duty compute to finish training, hyperparameter searches, etc. The rest of the time the administration nodes are sufficient for general coding, writing paper abstracts/posters, etc.

In this scenario our cost for the year would be something like:

  • 12 x $50 for administration ($600)

  • 4 x 7 x 24 x $ 0.05/hr for a t2.xlarge for exploratory work ($33)

  • 4 x 2 x 24 x $ 0.75/hr for a m5.12xlarge for heavy duty analysis ($144)

  • 1 TB data in S3 for the year = 1000 GB * .023/GB/mo * 12 = ($276)

Total annual cost: $1053

Even including the purchase of that Chromebook to access these resources, and adding cost to expand for spark and other things, it would still take 2-3 years to match the price of a new MacBook, and you have the advantage of being able to scale to 2-100 nodes and access GPU and other resources whenever you need.

Security

Security of these kinds of systems are always foremost on my mind. While it’s tempting to focus on just the data science aspects, it’s no fun when your account gets shut down because someone’s gotten in and been routing spam mail through your servers, mining bitcoin, or other uninvited activities.

My typical approach is to have a VPC with public and private subnets, described here. This used to be time consuming to put together, but with Amazon’s wizards it’s become quite easy. Following their wizard will result in a private cloud environment with 2 subnets - one that has direct public access where you would launch any internet-facing servers, and another that accesses websites through a proxy of some sort (e.g. a Network Address Translation (NAT) unit, or other), and that has no direct connect for outside users to access.

This type of setup is very nice from a security perspective as you dramatically reduce your “attack surface area” - the number of computers, ports, and services that are directly exposed to attack. For my purposes I generally just launch a single computer, known as a Bastion Host, that only exposes a single port for tunneling, and that I use to connect to all other internal hosts. I’ll write further posts on how I use ssh tunneling to do this, but it can be surprisingly easy to configure and work with, and it allows you to launch services like Jupyter, Spark, and other web applications inside your private cloud with little to fear as they’re not directly connected to the internet, and as all of your activity is encrypted via your SSH tunnel.

Security is always an engineering challenge, in which you try to find the optimum point that reduces risks of relevant attack vectors to an acceptible level, is feasible to implement and maintain, and feasible for end users to use. For this project I decided that this VPC private-public configuration didn’t meet the requirements, primarily due to ease of implementation for data scientists, and also due to cost with the default configurations. For example, the NAT device that is required for the computers in the private network to get outside connectivity (e.g. to download debian package) costs $ 0.05/hr, or $ 36/mo! I generally reduce costs by configuring my bastion node to also serve as a NAT device, but that gets into fairly low level linux configuration that is out of the scope of this blog.

The security approach I take here is to leverage Kubernetes itself, along with Amazon security groups, to limit access to relevant nodes and ports. Each of the kubernetes nodes has both a public and private interface, which eliminates the need for the NAT device and dramatically reduces the overhead cost. The default configuration will only expose the SSH port of the computers themselves, the API port for the master (protected by K8s authentication and https), and an SSH port for a container running inside the K8s cluster itself that will serve as our Bastion entry point. We will still be able to launch Jupyter, Spark, and other services on our cluster, and because they are not exposed with ingresses or with security groups they will be inaccessible by unauthorized users.

Limitations

Like any system, ours will not be all things to all people, and that’s ok! As currently designed, this is intended to really be a platform for a datascientist and several of his/her collaborators. As such, I’m really focusing on making a generic platform that can be expanded with the latest tools, at the expense of developing a great user interface for new users. I think this has the benefit of exposing the internals and providing a good learning tool, but it will certainly have some rough edges.

In particular, expected rough edges are:

  • No central user interface - the administrator will need to use commandline tools to create new types of instances, set the numbers of instances, launch new applications, etc. I’ve tried to make this as user friendly as possible, but it’s still a command line interface (CLI).

  • Some linux experience is helpful - the project will be run from a linux Bash command line, and SSH (Secure Shell) is used to access the services. The basics are easy to pick up and extremely beneficial for any data scientist, so I hope this doesn’t drive anyone away, but it is an aspect of the environment.

  • Limited auto scaling/automation - in a larger system with dozens/hundreds of VMs it would make sense to have all kinds of automation, e.g. monitoring the CPU usage of machines and creating new ones when capacity is being reached, or eliminating resources when there is downtime. In a system with 3-5 computers, however, this would be overkill, and due to the budget constraints would likely be unwanted - e.g. I want to know when my friends are doing something that will result in a $1k credit card charge!

Despite these rough edges, I’ve been using this platform for the last several months and already found it extremely enjoyable, and I hope you do, too!

On to the setup!


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