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Thoughts on stateful services in Guix
Thoughts on stateful services in Guix
Sat, 01 Feb 2020 21:38:28 +0100
mu4e 1.2.0; emacs 26.3
As a result of FOSDEM conversations today I felt inspired to put some
thoughts on paper about an area where I think we currently run into
Not sure if it is appropriate to write this blog-post-ish contribution
here, but as I don't have a blog, and it's about Guix development, I
figured it might be OK.
Guix is amazing. A large part of why it continues to be amazing is
because it provides strong guarantees to the end-user and developer.
As such it makes reasoning about packages and deployments relatively
The functional paradigm cleary works fantastically for packages.
Unfortunately it is not quite clear that it works just as well for
services. The reason for this is that too many useful services are
inherently stateful. Their statefulness means they have side-effects,
which in turn cause issues when relying on Guix features such as
roll-back or automated deployment & guaranteed reproducability.
Disciplined developers can implement many services in such a way that
their statefulness is delegated to other dedicated services. In this
way the problem of statefulness can be isolated. However, many
existing useful services /have not/ been implemented in such a way.
These notes are an attempt to think through ways of formalising how we
mitigate stateful services in Guix.
The notes below are organised around the example of a popular PHP
content management system, Drupal. Similar problems will apply to
many other end-user services.
My intent here is to communicate my thinking in the hopes that others
can point to obvious flaws in my reasoning — or to stimulate
conversation about the topic. I hope at the very least that it is an
2 Learning by Doing: Packaging & Deploying Drupal
2.1 Problem (1): the Drupal tarball is a binary blob!
Released Drupal tarballs are shipped with a bunch of PHP dependencies,
as well as compiled JS files. A fully source-distributed installation
of Drupal would:
1) delete all shipped PHP dependencies
2) independently build all PHP dependencies and make them available to
Drupal (through it's vendor directory as symlinks?)
3) delete all compiled JS files & libraries
4) independently build the JS dependencies and make them avalable to
2.2 Packages must always be stateless!
What does this mean in practice? Let's look at Drupal again. When
you download Drupal, the resulting tarball contains the source code,
and an empty sites/ folder. The sites folder is intended to contain
/state/ files. The normal installation procedure is to simply drop
the drupal distribution in your web root directory, and for state
files to live underneath sites/ within your webroot.
In Guix, Drupal is packaged so that it is installed in the store.
The store is read-only and hence no /state/ files can live under the
sites/ directory in the store.
How do we get around this?
1) either we patch drupal to expect the sites/ directory outside of
its own folder tree.
2) or we symlink /gnu/store/…drupal…/sites/ to a different location on
the filesystem (e.g. /var/lib/drupal/sites/)
The latter solution, while easy, means that a *successful*
installation of the package Drupal in Guix results in an installation
in the store with a *broken symlink* pointing outside the store.
But the package itself has been rendered stateless!
2.3 Drupal: a stateful service
Services in Guix are rich and multidimensional entities. At core they
are promises of things that will have happened when a system is up and
running. These promises can be arbitrary, like generating a
configuration file every boot; or they can be an extension of other,
already existing services.
A particularly popular service to extend is the shepherd service,
which ensures that particular daemons are started as soon as possible
after the system has started.
What would a Drupal service look like? Essentially the software is
just a bunch of files in a webroot — so at it's heart it simply
extends a web service (e.g. nginx) with new location directives.
On the other hand it also requires that tha web server, a sql backend
and php-fpm are running, so that drupal can actually function.
These two requirements are easily met with the usual service
infrastructure: simply extend nginx with a location definition
pointing to the Drupal folder in the store as the webroot and extend
shepherd to require mysql, nginx and php-fpm.
2.3.1 Enter the state dragon
But hang-on… if Mysql service is a dependency, and we store state in a
Mysql DB, what happens when we upgrade to a newer version of Drupal,
with a different schema? Would this service instantiation change the
state of our system? Could we still roll-back?
And remember that broken symlink we introduced in the package? That
files/ subdirectory contains /at least/ a stateful configuration file
— but also Drupal modules, themes, uploaded files and caches. What
happens when we upgrade Drupal? Can we guarantee they won't be
irrevocably changed as part of the upgrade? What happens when we
The truth is that we have introduced state into our deployment and we
hence lose a lot of the beautiful hard guarantees that Guix provides
2.3.2 Fakin' it til we're makin' it
Can we get those guarantees back? I fear not. But we can add some
shims and mitigations that might take the edge off the sting.
We've elaborated, above, the extend of the state problem in Drupal.
What would a /state shim/ look like?
Working backwards, to ensure we can roll-back we need to make sure
that we have access to the state at that point in history that we want
to roll-back to. When we roll-back from version 2 of Drupal to
version 1, we must make sure we still have the state of version 1.
So therefore we need to simply make an addressable state dump just
before we activate a new service instance, so that when we roll-back,
we can flip back to that state dump. In the case of our Drupal
example, this means that we need to dump the sites/ directory and the
This state dump needs to fulfill the following criteria:
1) It needs to be reversible: a transaction into a new state needs to
make a dump before transition and then carry out state changes; a
transaction into a past state must make a dump before transition
and then load the old state's dump as it's new state.
2) It (optionally) needs to be secured: state will likely contain
personal information, and this information needs to be protected.
2.4 The stateful service shimmy
Here's the proposed general shim then:
1) We define a new type of shepherd service, a /stateful service/,
which has additional actions: state-dump-shim and state-load-shim.
2) Each stateful service (e.g. drupal) is responsible for, through
provision of imperative code snippets in the state-dump-shim and
state-load-shim fields, providing state dumpers/restorers.
3) Whenever a new service revision is installed:
1) before it is installed, state-dump-shim is called. It should
dump the service's state in a way that when state-load-shim is
called with the new service revision's store hash, it can locate
any state dumps that were stored for that particular hash.
2) after it is installed, state-load-shim is called with the store
hash of the new service revision. It should then be able to
locate and load any state associated with this hash. If no
state is associated (e.g. when we install a wholly new
revision), nothing is done.
What does this give us?
When a migration to a new service revision causes a deployed bit of
software to stop working as expected, we can do a `guix system
–roll-back`. This will then also restore the service's state to the
previous version and thus *should* restore operations.
*BEWARE*: any state changes that occured since the previous state-dump
/will/ be lost. We are restoring to a backup — so user generated
state that was added after the backup will not be present after
3 A generalisation: the Stateful-Service Service
State dumping and restoration *should* be generalisable.
It should normally consist of one or more operations of:
• running a special program to dump data, encrypting the dump, and
storing it, together with the service revision hash, in a known
• tarring up a directory tree, encrypting the archive and storing it,
together wtih the service revision hash, in a known location.
These operations should be able to be provided by a daemon, managed by
shepherd, which can be configured with a gpg key for encryption, a
mechanism for decryption (interactive or programmatic), a location for
storing/retrieving dumps (local or remote?), and a DSL for mapping
data dump / data restoration program invocations or file-system
locations to data dumps it already knows of.
- Thoughts on stateful services in Guix,
Alex Sassmannshausen <=