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(integration-testing-vms)=
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# Integration testing using virtual machines
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This tutorial aims to be compatible with NixOS release 22.11.
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## What will you learn?
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This tutorial introduces the functionality of Nixpkgs to write automated tests to debug NixOS configurations independent of a working NixOS installation.
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## What do you need?
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- A working installation of [Nix Package Manager ](https://nixos.org/manual/nix/stable/installation/installation.html ) or [NixOS ](https://nixos.org/manual/nixos/stable/index.html#sec-installation ).
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- Basic knowledge of the [Nix language ](<nix-language> ).
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- Basic knowledge of [NixOS configuration ](<nixos-vms> ).
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## Introduction
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Nixpkgs provides a [test environment ](https://nixos.org/manual/nixos/stable/index.html#sec-nixos-tests ) to automate integration testing for distributed systems.
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It allows defining tests based on a set of declarative NixOS configurations and using a Python shell to interact with them through [QEMU ](https://www.qemu.org/ ) as the backend.
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Those tests are widely used to ensure that NixOS works as intended, so in general they are called [NixOS Tests ](https://nixos.org/manual/nixos/stable/index.html#sec-nixos-tests ).
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They can be written and launched outside of NixOS, on any Linux machine[^darwin].
[^darwin]: Support for running NixOS VM tests on macOS is also [implemented ](https://github.com/NixOS/nixpkgs/issues/108984 ) but currently [undocumented ](https://github.com/NixOS/nixpkgs/issues/254552 ).
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Integration tests are reproducible due to the design properties of Nix, making them a valuable part of a Continuous Integration (CI) pipeline.
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## The `nixosTest` function
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NixOS VM tests are defined using the `nixosTest` function.
The pattern for NixOS VM tests looks like this:
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```nix
let
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nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarball/nixos-22.11";
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pkgs = import nixpkgs { config = {}; overlays = []; };
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in
pkgs.nixosTest {
name = "test-name";
nodes = {
machine1 = { config, pkgs, ... }: {
# ...
};
machine2 = { config, pkgs, ... }: {
# ...
};
}
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testScript = { nodes, ... }: ''
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# ...
'';
}
```
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The function `nixosTest` takes an attribute set that follows the module convention to [specify the test ](https://nixos.org/manual/nixos/stable/index.html#sec-test-options-reference ).
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Because the attribute set only defines options, one can use the abbreviated form of the [module convention ](https://nixos.org/manual/nixos/stable/#sec-writing-modules ).
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The attribute set needs to define the following options:
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- [`name` ](https://nixos.org/manual/nixos/stable/index.html#test-opt-name ) defines the name of the test.
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- [`nodes` ](https://nixos.org/manual/nixos/stable/index.html#test-opt-nodes ) contains a set of named configurations, because a test script can involve more than one virtual machine.
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Each virtual machine is setup using a NixOS configuration.
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- [`testScript` ](https://nixos.org/manual/nixos/stable/index.html#test-opt-testScript ) defines the Python test script, either as literal string or as a function that takes a `nodes` attribute.
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This Python test script can access the virtual machines via the names used for the `nodes` .
It has super user rights in the virtual machines.
In the Python script is each virtual machine is accessible via the `machine` object.
NixOS provides [various methods ](https://nixos.org/manual/nixos/stable/index.html#ssec-machine-objects ) to run tests on these configurations.
The test framework automatically starts the virtual machines and runs the Python script.
## Minimal example
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As a minimal test on the default configuration, we will check if the user `root` and `alice` can run Firefox.
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We will build the example up from scratch.
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As [recommended ](<ref-pinning-nixpkgs> ) we use an explicitly pinned version of Nixpkgs, and explicitly set configuration options and overlays to avoid them being inadvertently overridden by [global configuration ](https://nixos.org/manual/nixpkgs/stable/#chap-packageconfig ):
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```nix
let
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nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarball/nixos-22.11";
pkgs = import nixpkgs { config = {}; overlays = []; };
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in
pkgs.nixosTest {
# ...
}
```
### Options
#### Name
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Label the test with a descriptive name such as "minimal-test":
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```nix
name = "minimal-test";
```
#### Nodes
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Because this example only uses one virtual machine, the node we specify is simply called `machine` .
This name is arbitrary and can be chosen freely.
As configuration you use the relevant parts of the default configuration, [that we used in a previous tutorial ](<nixos-vms> ):
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```nix
nodes.machine = { config, pkgs, ... }: {
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users.users.alice = {
isNormalUser = true;
extraGroups = [ "wheel" ];
packages = with pkgs; [
firefox
tree
];
};
system.stateVersion = "22.11";
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};
```
#### Test script
This is the test script:
```python
machine.wait_for_unit("default.target")
machine.succeed("su -- alice -c 'which firefox'")
machine.fail("su -- root -c 'which firefox'")
```
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This Python script is referring to `machine` which is the name chosen for the virtual machine configuration used in the `nodes` attribute set.
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The script waits until systemd reaches `default.target` .
It uses the `su` command to switch between users and the `which` command to check if the user has access to `firefox` .
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It expects that the command `which firefox` to succeed for user `alice` and to fail for `root` .
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This script will be the value of the `testScript` attribute.
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### Test file
The complete `minimal-test.nix` file content looks like the following:
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```{code-block}
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let
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nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/tarball/nixos-22.11";
pkgs = import nixpkgs { config = {}; overlays = []; };
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in
pkgs.nixosTest {
name = "minimal-test";
nodes.machine = { config, pkgs, ... }: {
users.users.alice = {
isNormalUser = true;
extraGroups = [ "wheel" ];
packages = with pkgs; [
firefox
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tree
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];
};
system.stateVersion = "22.11";
};
testScript = ''
machine.wait_for_unit("default.target")
machine.succeed("su -- alice -c 'which firefox'")
machine.fail("su -- root -c 'which firefox'")
'';
}
```
## Running tests
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To set up all machines and run the test script:
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```shell-session
$ nix-build minimal-test.nix
```
...
test script finished in 10.96s
cleaning up
killing machine (pid 10)
(0.00 seconds)
/nix/store/bx7z3imvxxpwkkza10vb23czhw7873w2-vm-test-run-minimal-test
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## Interactive Python shell in the virtual machine
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When developing tests or when something breaks, it’ s useful to interactively tinker with the test or access a terminal for a machine.
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To start an interactive Python session with the testing framework:
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```shell-session
$ $(nix-build -A driverInteractive minimal-test.nix)/bin/nixos-test-driver
```
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Here you can run any of the testing operations.
Execute the `testScript` attribute from `minimal-test.nix` with the `test_script()` function.
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If a virtual machine is not yet started, the test environment takes care of it on the first call of a method on a `machine` object.
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But you can also manually trigger the start of the virtual machine with:
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```shell-session
>>> machine.start()
```
for a specific node,
or
```shell-session
>>> start_all()
```
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for all nodes.
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You can enter a interactive shell on the virtual machine using:
```shell-session
>>> machine.shell_interact()
```
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and run shell commands like:
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```shell-session
uname -a
```
Linux server 5.10.37 #1 -NixOS SMP Fri May 14 07:50:46 UTC 2021 x86_64 GNU/Linux
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< details > < summary > Re-running successful tests< / summary >
<!-- FIXME: this should be a separate recipe that can be linked to, as it's a bit of knowledge one will need now and again. -->
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Because test results are kept in the Nix store, a successful test is cached.
This means that Nix will not run the test a second time as long as the test setup (node configuration and test script) stays semantically the same.
Therefore, to run a test again, one needs to remove the result.
If you would try to delete the result using the symbolic link, you will get the following error:
```shell-session
nix-store --delete ./result
```
finding garbage collector roots...
0 store paths deleted, 0.00 MiB freed
error: Cannot delete path '/nix/store/4klj06bsilkqkn6h2sia8dcsi72wbcfl-vm-test-run-unnamed' since it is still alive. To find out why, use: nix-store --query --roots
Instead, remove the symbolic link and only then remove the cached result:
```shell-session
rm ./result
nix-store --delete /nix/store/4klj06bsilkqkn6h2sia8dcsi72wbcfl-vm-test-run-unnamed
```
This can be also done with one command:
```shell-session
result=$(readlink -f ./result) rm ./result & & nix-store --delete $result
```
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< details >
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## Tests that need multiple virtual machines
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Tests can involve multiple virtual machines.
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This example uses the use-case of a [REST ](https://en.m.wikipedia.org/wiki/REST ) interface to a [PostgreSQL ](https://www.postgresql.org/ ) database.
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The following example Nix expression is adapted from [How to use NixOS for lightweight integration tests ](https://www.haskellforall.com/2020/11/how-to-use-nixos-for-lightweight.html ).
This tutorial follows [PostgREST tutorial ](https://postgrest.org/en/stable/tutorials/tut0.html ), a generic [RESTful API ](https://restfulapi.net/ ) for PostgreSQL.
If you skim over the official tutorial, you'll notice there's quite a bit of setup in order to test if all the steps work.
The setup includes:
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- A virtual machine named `server` running PostgreSQL and PostgREST.
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- A virtual machine named `client` running HTTP client queries using `curl` .
- A `testScript` orchestrating testing logic between `client` and `server` .
The complete `postgrest.nix` file looks like the following:
```{code-block}
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let
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# Pin Nixpkgs, as some packages are broken in the 22.11 release
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nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/archive/0f8f64b54ed07966b83db2f20c888d5e035012ef.tar.gz";
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pkgs = import nixpkgs { config = {}; overlays = []; };
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# Single source of truth for all tutorial constants
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database = "postgres";
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schema = "api";
table = "todos";
username = "authenticator";
password = "mysecretpassword";
webRole = "web_anon";
postgrestPort = 3000;
# NixOS module shared between server and client
sharedModule = {
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# Since it's common for CI not to have $DISPLAY available, explicitly disable graphics support
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virtualisation.graphics = false;
};
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in
pkgs.nixosTest {
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# NixOS tests are run inside a virtual machine, and here you specify its system type
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system = "x86_64-linux";
name = "postgres-test";
nodes = {
server = { config, pkgs, ... }: {
imports = [ sharedModule ];
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networking.firewall.allowedTCPPorts = [ postgrestPort ];
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services.postgresql = {
enable = true;
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initialScript = pkgs.writeText "initialScript.sql" ''
create schema ${schema};
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create table ${schema}.${table} (
id serial primary key,
done boolean not null default false,
task text not null,
due timestamptz
);
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insert into ${schema}.${table} (task) values ('finish tutorial 0'), ('pat self on back');
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create role ${webRole} nologin;
grant usage on schema ${schema} to ${webRole};
grant select on ${schema}.${table} to ${webRole};
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create role ${username} inherit login password '${password}';
grant ${webRole} to ${username};
'';
};
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users = {
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mutableUsers = false;
users = {
# For ease of debugging the VM as the `root` user
root.password = "";
# Create a system user that matches the database user so that you
# can use peer authentication. The tutorial defines a password,
# but it's not necessary.
"${username}".isSystemUser = true;
};
};
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systemd.services.postgrest = {
wantedBy = [ "multi-user.target" ];
after = [ "postgresql.service" ];
script =
let
configuration = pkgs.writeText "tutorial.conf" ''
db-uri = "postgres://${username}:${password}@localhost:${toString config.services.postgresql.port}/${database}"
db-schema = "${schema}"
db-anon-role = "${username}"
'';
in "${pkgs.haskellPackages.postgrest}/bin/postgrest ${configuration}";
serviceConfig.User = username;
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};
};
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client = {
imports = [ sharedModule ];
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};
};
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# Disable linting for simpler debugging of the testScript
skipLint = true;
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testScript = ''
import json
import sys
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start_all()
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server.wait_for_open_port(${toString postgrestPort})
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expected = [
{"id": 1, "done": False, "task": "finish tutorial 0", "due": None},
{"id": 2, "done": False, "task": "pat self on back", "due": None},
]
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actual = json.loads(
client.succeed(
"${pkgs.curl}/bin/curl http://server:${toString postgrestPort}/${table}"
)
)
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assert expected == actual, "table query returns expected content"
'';
}
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```
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Unlike the previous example, the virtual machines need an expressive name to distinguish them.
For this example we choose `client` and `server` .
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Set up all machines and run the test script:
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```shell-session
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nix-build postgrest.nix
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```
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...
test script finished in 10.96s
cleaning up
killing client (pid 10)
killing server (pid 22)
(0.00 seconds)
/nix/store/bx7z3imvxxpwkkza10vb23czhw7873w2-vm-test-run-unnamed
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```
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## Additional information regarding NixOS tests:
- Running integration tests on CI requires hardware acceleration, which many CIs do not support.
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To run integration tests on [GitHub Actions ](<github-actions> ) see [how to disable hardware acceleration ](https://github.com/cachix/install-nix-action#how-do-i-run-nixos-tests ).
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- NixOS comes with a large set of tests that serve also as educational examples.
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A good inspiration is [Matrix bridging with an IRC ](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/matrix/appservice-irc.nix ).
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<!-- TODO: move examples from https://nixos.wiki/wiki/NixOS_Testing_library to the NixOS manual and troubleshooting tips to nix.dev -->