Re: [Xen-devel] [stage1-xen PATCH v1 06/10] build/fedora: Add `xen-unstable-runit/*` scripts

[Rajiv Ranganath <rajiv.ranganath@xxxxxxxxxxx>]

On Thu, Sep 07 2017 at 12:10:21 AM, Stefano Stabellini <sstabellini@xxxxxxxxxx> 
wrote:

[...]

> The series is much better now thank you. One question: why did you write
> your own init scripts rather than reusing xencommons (with the caveat
> that you would have to add make sure to source_path.sh before running
> xencommons)?  Does it have something to do with systemd?

There are a few related reasons for this.

1. Using runit lets us abstract out our dependency on systemd and
upstart. We can use the same abstraction in containers [1], virtual
machines and on bare metal.

2. In Linux distributions, there is tight coupling between package
management system (rpm/deb), init systems (upstart/systemd), and service
daemons.

With containers, if the expectation is that most service daemons and
apps would be containerized, and managed by a node agent then a natural
question to ask would be what should be the role of init systems like
systemd?

By using runit (on systemd, upstart and within containers), we defer
answering this question. 

3. One of the use cases that we want to support is to have different
versions of xen co-exist on the same filesystem. Then a higher level
tool can do rolling updates and if required rollbacks.

While it is possible to accomplish this on existing init systems,
depending on how xen is packaged and deployed, it might involve using
distro package and repository management tools.

With runit, we can use regular docker tools, which is much more friendly
for mainstream developers and CI systems. We also abstract over init
systems, which is a desirable property to have.

4. I looked into xencommons script and systemd unit files when creating
runit scripts. Our runit scripts is straightforward translation of how
one would start xen manually.

Perhaps the only part of the script that might need some explanation is
in `xen-init-dom0/run`.

```
exec chpst -b xen-init-dom0 runit-pause
```

This is a pattern used to build equivalent of "oneshot" service in
systemd. It was developed in Ignite (a Arch Linux project before they
switched to systemd) and later co-opted by Void Linux [2].

I am not sure if I answered your question. Sometimes I feel, maybe we
should just let questions around init systems be like one of those
"unanswered questions" in theology. :-) [3]

Best,
Rajiv

[1] https://github.com/lambda-linux/baseimage-amzn#adding_additional_daemons

[2] 
https://github.com/voidlinux/void-runit/commit/7aecf46ec589a5bc49ae2392137bcd0e7468dd08

[3] https://en.wikipedia.org/wiki/The_unanswered_questions

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