Service composition in GuixSD
GuixSD provides a declarative, stateless approach to operating system configuration management. In this context, the mechanism offered to select and compose system services is a crucial one. This post presents the new service framework introduced in the 0.9.0 version of GNU Guix.
Declarative Configuration Management
GuixSD is not like your parents’ distro. Instead of fiddling with configuration files all around, or running commands that do so as a side effect, the system administrator declares what the system will be like. This takes the form of an operating-system declaration, which specifies all the details: file systems, user accounts, locale, timezone, system services, etc.
If you’re familiar with it, this may remind you of what deployment tools like Ansible and Puppet provide. There is an important difference though: GuixSD takes a stateless—or “purely functional”—approach. This means that instantiating the system with guix system always produces the same result, without modifying the current system state. This is what makes it possible to test new system configurations, roll-back to previous ones, and so on. The guix system command allows system configurations to be instantiated on the bare metal, in virtual machines, or in containers, which makes it easy to test them.
In GuixSD, operating-system declarations are first-class objects in the host language. They can be inspected at the REPL:
scheme@(guile-user)> ,use (gnu) scheme@(guile-user)> (define os (load "os-config.scm")) scheme@(guile-user)> (operating-system-kernel os) $1 = #<package linux-libre-4.2.6 gnu/packages/linux.scm:279 2ea90c0> scheme@(guile-user)> (length (operating-system-user-services os)) $2 = 30 scheme@(guile-user)> (map user-account-name (operating-system-users os)) $3 = ("alice" "nobody" "root")
It is also possible to write functions that take or return OS configurations. For instance, the virtualized-operating-system function returns a variant of the given OS where the set of file systems and the initrd are changed so that the resulting OS can be used in a lightweight virtual machine environment. Likewise for containerized-operating-system.
Services Beyond Daemons
System services are specified in the services field of operating-system declarations, which is a list of service objects. As a user, we want to be able to ideally add one line specifying the system service we want to add, possibly with several instances of a service, and have GuixSD do the right thing.
Before 0.9.0, GuixSD had a narrow definition of what a “system service” is. Each service in the operating-system configuration had to map to exactly one dmd service—GNU dmd is the init system of GuixSD. This would work well in many cases: an SSH server or a log-in daemon is indeed a service that dmd has to take care of, even a file system mount is an operation that can be usefully inserted into dmd’s service dependency graph.
However, this simple mapping failed to capture more complex service composition patterns. A striking example is “super-daemons”—daemons that can spawn other daemons, such as dbus-daemon or inetd. From the user viewpoint, it does not matter whether a daemon is started by dmd, or by dbus-daemon, or by inetd; this should be transparent. If it’s a D-Bus service, then dbus-daemon’s configuration file should be told about the service; if it’s an inetd service, then inetd.conf should be augmented accordingly; if it’s a dmd service, information on how to start and stop it should go to dmd’s configuration file. Unfortunately, the pre-0.9.0 services could not express such things.
Worse, this approach did not capture the more general pattern of service extension. In the examples above, the super-daemons are effectively extended by other services that rely on them. But there are many cases where services are similarly extended: eudev can be passed new device rules, polkit can be extended with new rules and actions, the Pluggable authentication module system (PAM) can be extended with new services, and so on. At that point it was clear that GuixSD’s naive approach wouldn’t scale.
Composing System Services
The lesson learned from these observations is that system services extend each other in various way. The new service composition framework is built around this model: “system services”, broadly defined, can extend each other, and services and their “extends” relationships form a graph. The root of the graph is the operating system itself.
We can see that this pattern applies to services that are not daemons. PAM is one such example. Accounts are another example: GuixSD provides an “account service” that can be extended with new user accounts or groups; for example, the Network time protocol (NTP) daemon needs to run under the unprivileged “ntp” user, so the NTP service extends the account service with an “ntp” user account. Likewise, the “/etc” service can be extended with new files to be added to /etc; the “setuid” service can be extended with new programs to be made setuid-root. See the manual for more examples.
The nice thing is that composition of services is made explicit: extensions can only happen where explicit extension relationships have been declared. By looking at the extension graph, users can see how services fit together. The guix system extension-graph command, for instance, takes an operating-system declaration and renders the extension graph in the Graphviz format, making it easy to inspect the OS configuration structure.
The API makes it easy to see how services contributed to a specific service’s configuration. For instance, the following expression shows the PAM service as extended by other declared services:
(fold-services (operating-system-services os) #:target-type pam-root-service-type)
The result is a service object whose value is a list of pam-service objects. Likewise, the following expression returns the /etc service, whose value is a list of entries to be added to /etc:
(fold-services (operating-system-services os) #:target-type etc-service-type)
This form of ambient authority gives a lot of flexibility, but it makes it harder to reason about service composition—all a service implementation does is inspect, add, or modify attributes of the global configuration, which may or may not affect other services. The use of a loose key/value dictionary also prevents good error reporting; for instance, a typo in a service name may go undetected. Lastly, NixOS services are enabled by writing service.enable = true stanzas, which leads to complications for services that may have several instances, each with its own configuration.
The new service composition framework in GuixSD 0.9.0 addresses shortcomings found in previous versions of GuixSD. It simplifies operating-system declarations for users, and provides a highly extensible framework that clearly exposes the way services are composed.
This new framework has already allowed us to integrate Freedesktop and GNOME services in a convenient way. We hope it will prove fruitful as we address other types of services, such as Web services.
About GNU Guix
GNU Guix is a functional package manager for the GNU system. The Guix System Distribution or GuixSD is an advanced distribution of the GNU system that relies on GNU Guix and respects the user's freedom.
In addition to standard package management features, Guix supports transactional upgrades and roll-backs, unprivileged package management, per-user profiles, and garbage collection. Guix uses low-level mechanisms from the Nix package manager, except that packages are defined as native Guile modules, using extensions to the Scheme language. GuixSD offers a declarative approach to operating system configuration management, and is highly customizable and hackable.
GuixSD can be used on an i686 or x86_64 machine. It is also possible to use Guix on top of an already installed GNU/Linux system, including on mips64el and armv7.
Sujets liés :Functional programming System services
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