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A good example might be a PostgreSQL database server. Much of the complexity of setting up such a database server is encapsulated in this deceptively short service declaration:
(service postgresql-service-type
(postgresql-configuration
(postgresql postgresql-14)))
A complete operating system declaration for use with a Guix System container would look something like this:
(use-modules (gnu)) (use-package-modules databases) (use-service-modules databases) (operating-system (host-name "container") (timezone "Europe/Berlin") (file-systems (cons (file-system (device (file-system-label "does-not-matter")) (mount-point "/") (type "ext4")) %base-file-systems )) (bootloader (bootloader-configuration (bootloader grub-bootloader) (targets '("/dev/sdX")))) (services (cons* (service postgresql-service-type (postgresql-configuration (postgresql postgresql-14) (config-file (postgresql-config-file (log-destination "stderr") (hba-file (plain-file "pg_hba.conf" "\ local all all trust host all all 10.0.0.1/32 trust")) (extra-config '(("listen_addresses" "*") ("log_directory" "/var/log/postgresql"))))))) (service postgresql-role-service-type (postgresql-role-configuration (roles (list (postgresql-role (name "test") (create-database? #t)))))) %base-services )))
With postgresql-role-service-type we define a role “test” and
create a matching database, so that we can test right away without any
further manual setup. The postgresql-config-file settings allow
a client from IP address 10.0.0.1 to connect without requiring
authentication—a bad idea in production systems, but convenient for
this example.
Let’s build a script that will launch an instance of this Guix System as
a container. Write the operating-system declaration above to a
file os.scm and then use guix system container to build
the launcher. (see Invoking guix system in GNU Guix Reference
Manual).
$ guix system container os.scm The following derivations will be built: /gnu/store/…-run-container.drv … building /gnu/store/…-run-container.drv... /gnu/store/…-run-container
Now that we have a launcher script we can run it to spawn the new system
with a running PostgreSQL service. Note that due to some as yet
unresolved limitations we need to run the launcher as the root user, for
example with sudo.
$ sudo /gnu/store/…-run-container system container is running as PID 5983 …
Background the process with Ctrl-z followed by bg. Note
the process ID in the output; we will need it to connect to the
container later. You know what? Let’s try attaching to the container
right now. We will use nsenter, a tool provided by the
util-linux package:
$ guix shell util-linux $ sudo nsenter -a -t 5983 root@container /# pgrep -a postgres 49 /gnu/store/…-postgresql-14.4/bin/postgres -D /var/lib/postgresql/data --config-file=/gnu/store/…-postgresql.conf -p 5432 51 postgres: checkpointer 52 postgres: background writer 53 postgres: walwriter 54 postgres: autovacuum launcher 55 postgres: stats collector 56 postgres: logical replication launcher root@container /# exit
The PostgreSQL service is running in the container!
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