What does a distro provide?
The most popular docker base container image is either busybox, or scratch. This is driven by a movement that is equal parts puritanical and pragmatic. The puritan asks “Why do I need to run init(1) just to run my process?” The pragmatist asks “Why do I need a 700 meg base image to deploy my application?” And both, seeking immutable deployment units ask “Is it a good idea that I can ssh into my container?” But let’s step back for a second and look at the history of how we got to the point where questions like this are even a thing.
In the very beginnings, there were no operating systems. Programs ran one at a time with the whole machine at their disposal. While efficient, this created a problem for the keepers of these large and expensive machines. To maximise their investment, the time between one program finishing and another starting must be kept to an absolute minimum; hence monitor programs and batch processing was born.
Monitors started as barely more than watchdog timers. They knew how to load the next program off tape, then set an alarm if the program ran too long. As time went on, monitors became job control–quasi single user operating systems where the operators could schedule batch jobs with slightly more finesse than the previous model of concatenating them in the card reader.1
In response to the limitations of batch processing, and with the help of increased computing resources, interactive computing was born. Interactive computing allowing multiple users to interact with the computer directly, time slicing, or time sharing, the resources between users to present the illusion of each program having a whole computer to itself.
“The UNIX kernel is an I/O multiplexer more than a complete operating system. This is as it should be.”
Ken Thompson, BSTJ, 1978
interactive computing in raw terms was less efficient than batch, however it recognised that the potential to deliver programs faster outweighed a less than optimal utilisation of the processor; a fact borne out by the realisation that programming time was not benefiting from the same economies of scale that Moore’s law was delivering for hardware. Job control evolved to became what we know as the kernel, a supervisor program which sits above the raw hardware, portioning it out and mediating access to hardware devices.
With interactive users came the shell, a place to start programs, and return once they completed. The shell presented an environment, a virtual work space to organise your work, communicate with others, and of course customise. Customise with programs you wrote, programs you got from others, and programs that you collaborated with your coworkers on.
Interactive computing, multi user systems and then networking gave birth to the first wave of client/server computing–the server was your world, your terminal was just a pane of glass to interact with it. Thus begat userspace, a crowded bazaar of programs, written in many languages, traded, sold, swapped and sometimes stolen. A great inter breeding between the UNIX vendors produced a whole far larger than the sum of its parts.
Each server was an island, lovingly tended by operators, living for years, slowly patched and upgraded, becoming ever more unique through the tide of software updates and personnel changes.
Skip forward to Linux and the GNU generation, a kernel by itself does not serve the market, it needs a user space of tools to attract and nurture users accustomed to the full interactive environment.
But that software was hard, and messy, and spread across a million ftp, tucows, sourceforge, and cvs servers. Their installation procedures are each unique, their dependencies are unknown or unmanaged–in short, a job for an expert. Thus distributions became experts at packaging open source software to work together as a coherent interactive userspace story.
Container sprawl
We used to just have lots of servers, drawing power, running old software, old operating systems, hidden under people’s desks, and sometimes left running behind dry wall. Along came virtualisation to sweep away all the old, slow, flaky, out of warranty hardware. Yet the software remained, and multiplied.
Vmsprawl, it was called. Now free from a purchase order and a network switch port, virtual machines could spawn faster than rabbits. But their lifespan would be much longer.
Back when physical hardware existed, you could put labels on things, assign them to operators, have someone to blame, or at least ask if the operating system was up to date, but virtual machines became ephemeral, multitudinous, and increasingly, redundant,
Now that a virtual machines’ virtual bulk has given way to containers, what does that mean for the security and patching landscape? Surely it’s as bad, if not worse. Containers can multiply even faster than VMs and at such little cost compared to their bloated cousins that the problem could be magnified many times over. Or will it?
The problem is maintaining the software you didn’t write. Before containers that was everything between you and the hardware; obviously a kernel, that is inescapable, but the much larger surface area (in recent years ballooning to a DVD’s girth) was the userland. The gigabytes of software that existed to haul the machine onto the network, initialise its device drivers, scrub its /tmp partition, and so on.
But what if there was no userland? What if the network was handled for you, truly virtualised at layer 3, not layer 1. Your volumes were always mounted and your local storage was fleeting, so nothing to scrub. What would be the purpose of all those decades of lovingly crafted userland cruft?
If interactive software goes unused, was it ever installed at all?
Immutable images
Netflix tells us that immutable images are the path to enlightenment. Built it once, deploy it often. If there is a problem, an update, a software change, a patch, or a kernel fix, then build another image and roll it out. Never change something in place. This mirrors the trend towards immutability writ large by the functional programming tidal wave.
While Netflix use virtual machines, and so need software to configure their (simulated) hardware and software to plumb their (simulated) network interfaces to get to the point of being able to launch the application, containers leave all these concerns to the host. A container is spawned with any block devices or network interfaces required already mounted or plumbed a priori.
So, if you remove the requirement, and increasingly, the ability, to change the contents of the running image, and you remove the requirement to prepare the environment before starting the application, because the container is created with all its facilities already prepared, why do you need a userland inside a container?
Debugging? Possibly.
Today there are many of my generation who would feel helpless without being about to ssh to a host, run their favourite (and disparate) set of inspection tools. But a container is just a process inside a larger host operating system, so do you diagnosis there instead. Unlike virtual machines, these are not black boxes, the host operating system has far more capability to inspect and diagnose a guest than the guest itself–so leave your diagnosis tools on the host. And your ssh daemon, for good measure.
Updates, updates. Updates!
Why do we have operating system distros? In a word, outsourcing.
Sure, every admin could subscribe to the mailing lists of all the software packages installed on the servers they maintain (you do know all the software installed on the machines you are responsible for, right?) and then download, test, certify, upgrade the software promptly after being notified. Sound’s simple. Any admin worth hiring should be able to do this.
Sure, assuming you can find an admin who wants to do this grunt work, and that they can keep up with the workload, and that they can service more than a few machines before they’re hopelessly chasing their tails.
No, of course not, we outsource this to operating system vendor. In return for using outdated versions of software, distros will centralise the triage, testing and preparation of upgrades and patches.
This is the reason that a distro and its package management tool of choice are synonymous. Without a tool to automate the dissemination, installation and upgrade of packaged software, distro vendors would have no value. And without someone to marshal unique snowflake open source software into a unified form, package management software would have no value.
No wonder that the revenue model for all open source distro vendors centers around tooling that automates the distribution of update packages.
The last laugh
Ironically, the last laugh in this tale may be the closed source operating system vendors. It was Linux and open source that destroyed the proprietary UNIX market after the first dot com crash.
Linux rode Moore’s law to become the server operating system for the internet, and made kings of the operating system distributors. But it’s Linux that is driving containers, at least in their current form, and Linux containers, or more specifically a program that communicates directly with the kernel syscall api inside a specially prepared process namespace, is defining the new normal for applications.
Containers are eating the very Linux distribution market which enabled their creation.
OSX and Windows may be relegated to second class citizens–the clients in the client/server or client/container equation–but at least nobody is asking difficult questions about the role of their userspace.
Whither distros
What is the future of operating system distributions? Their services, while mature, scalable, well integrated, and expertly staffed, will unfortunately be priced out of the market. History tells us this.
In the first dot com bust, companies retreated from expensive proprietary software, not because it wasn’t good, not because it wasn’t extensible or changeable, but because it was too expensive. With no money coming in, thousands of dollars of opex walking out the door in licence fees was unsustainable.
The companies that survived the crash, or were born in its wreckage, chose open source software. Software that was arguably less mature, less refined, at the time, but with a price tag that was much more approachable. They kept their investors money in the bank, rode the wave of hardware improvements, and by pulling together in a million loosely organised software projects created a free (as in free puppy) platform to build their services on top–trading opex for some risk that they may have no-one to blame if their free software balloon sprang a leak.
Now, it is the Linux distributors who are chasing the per seat or per cpu licence fees. Offering scaled out professional services in the form of a stream of software updates and patches, well tested and well integrated.
But, with the exception of the kernel–which is actually provided by the host operating system, not the container–all those patches and updates are for software that is not used by the container, and in the case of our opening examples, busybox and scratch. not present. The temptation to go it alone, cut out the distro vendors, backed by the savings in licence fees is overwhelming.
What can distros do?
What would you do if you woke up one day to find that you owned the best butchers shop in a town that had decided to become vegetarian en mass?