10 KiB
dhall-kubernetes
dhall-kubernetes
contains Dhall bindings to Kubernetes,
so you can generate Kubernetes objects definitions from Dhall expressions.
This will let you easily typecheck, template and modularize your Kubernetes definitions.
Why do I need this
Once you build a slightly non-trivial Kubernetes setup, with many objects floating around, you'll encounter several issues:
- Writing the definitions in YAML is really verbose, and the actually important things don't stand out that much
- Ok I have a bunch of objects that'll need to be configured together, how do I share data?
- I'd like to reuse an object for different environments, but I cannot make it parametric..
- In general, I'd really love to reuse parts of some definitions in other definitions
- Oh no, I typoed a key and I had to wait until I pushed to the cluster to get an error back :(
The natural tendency is to reach for a templating language + a programming language to orchestrate that + some more configuration for it... But this is just really messy (been there), and we can do better.
Dhall solves all of this, being a programming language with builtin templating, all while being non-Turing complete, strongly typed and strongly normalizing (i.e.: reduces everything to a normal form, no matter how much abstraction you build), so saving you from the "oh-noes-I-made-my-config-in-code-and-now-its-too-abstract" nightmare.
For a Dhall Tutorial, see the readme of the project, or the full tutorial.
Prerequisites
NOTE: dhall-kubernetes
requires at least version 1.14.0
of the interpreter.
You can install the latest version with the following:
stack install dhall dhall-json --resolver=nightly
For a version compatible with a previous version, check out this commit.
Quickstart - main API
We provide a simple API for the most common cases (For a list, see the api folder).
Let's say we'd like to configure a Deployment exposing an nginx
webserver.
In the following example, we:
- Define a
config
for our service, by merging a default config (with the Dhall record-merge operator//
) with a record with our parameters. - In there we define the details of the Deployment we care about (note that we do the same "merging with defaults" operation for our container as well, so we don't have to specify all the parameters)
- We call the
mkDeployment
function on ourconfig
-- examples/deployment.dhall
let config =
../api/Deployment/default
//
{ name = "nginx"
, replicas = 2
, containers =
[ ../api/Deployment/defaultContainer
//
{ name = "nginx"
, imageName = "nginx"
, imageTag = "1.15.3"
, port = [ 80 ] : Optional Natural
}
]
}
in ../api/Deployment/mkDeployment config
We then run this through dhall-to-yaml
to generate our Kubernetes definition:
dhall-to-yaml --omitNull < deployment.dhall
And we get:
## examples/out/deployment.yaml
apiVersion: apps/v1
kind: Deployment
spec:
revisionHistoryLimit: 20
selector:
matchLabels:
app: nginx
strategy:
rollingUpdate:
maxSurge: 5
maxUnavailable: 0
type: RollingUpdate
template:
spec:
containers:
- image: nginx:1.15.3
imagePullPolicy: Always
env: []
volumeMounts: []
resources:
limits:
cpu: 500m
requests:
cpu: 10m
name: nginx
ports:
- containerPort: 80
volumes: []
metadata:
name: nginx
labels:
app: nginx
replicas: 2
metadata:
name: nginx
Advanced usage - raw API
If the main API is not enough (e.g. the object you'd like to generate is not in the list), you can just fall back on using the raw Types and defaults the library provides (and Pull Request here your program afterwards!).
Let's say we want to generate an Ingress definition (for an Nginx Ingress)
that contains TLS certs and routes for every service.
For more examples of using this API see the ./examples
folder.
In the types
folder you'll find the types for the Kubernetes definitions. E.g.
here's the type for the Ingress.
Since most of the fields in all definitions are optional, for better
ergonomics while coding Dhall we also generate default values for all types, in
the default
folder. When some fields are required, the default value
is a function whose input is a record of required fields, that returns the object
with these fields set. E.g. the default for the Ingress is this
function.
Let's say we have a Service with the following configuration:
-- examples/myConfig.dhall
{ name = "foo"
, host = "foo.example.com"
, version = "1.0.1"
}
That has the following type:
-- examples/Config.dhall
{ name : Text
, host : Text
, version : Text
}
We can now expose this service out to the world with the Ingress:
-- examples/ingressRaw.dhall
-- Prelude imports
let map = https://raw.githubusercontent.com/dhall-lang/Prelude/v2.0.0/List/map
in let Some = https://raw.githubusercontent.com/dhall-lang/Prelude/v2.0.0/Optional/Some
in let None = https://raw.githubusercontent.com/dhall-lang/Prelude/v2.0.0/Optional/None
-- dhall-kubernetes types and defaults
in let TLS = ../types/io.k8s.api.extensions.v1beta1.IngressTLS.dhall
in let Rule = ../types/io.k8s.api.extensions.v1beta1.IngressRule.dhall
in let RuleVal = ../types/io.k8s.api.extensions.v1beta1.HTTPIngressRuleValue.dhall
in let Spec = ../types/io.k8s.api.extensions.v1beta1.IngressSpec.dhall
in let Ingress = ../types/io.k8s.api.extensions.v1beta1.Ingress.dhall
in let defaultIngress = ../default/io.k8s.api.extensions.v1beta1.Ingress.dhall
in let defaultMeta = ../default/io.k8s.apimachinery.pkg.apis.meta.v1.ObjectMeta.dhall
in let defaultSpec = ../default/io.k8s.api.extensions.v1beta1.IngressSpec.dhall
in let IntOrString = ../default/io.k8s.apimachinery.pkg.util.intstr.IntOrString.dhall
-- Our Service type
in let Service = ./Config.dhall
in let Config = { services : List Service }
-- A function to generate an ingress given a configuration
in let mkIngress : Config -> Ingress =
\(config : Config) ->
-- Given a service, make a TLS definition with their host and certificate
let makeTLS = \(service : Service) ->
{ hosts = Some (List Text) [ service.host ]
, secretName = Some Text "${service.name}-certificate"
}
-- Given a service, make an Ingress Rule
in let makeRule = \(service : Service) ->
{ host = Some Text service.host
, http = Some RuleVal
{ paths = [ { backend = { serviceName = service.name
, servicePort = IntOrString.Int 80
}
, path = None Text
}
]
}
}
-- Nginx ingress requires a default service as a catchall
in let defaultService =
{ name = "default"
, host = "default.example.com"
, version = " 1.0"
}
-- List of services
in let services = config.services # [ defaultService ]
-- Some metadata annotations
-- NOTE: `dhall-to-yaml` will generate a record with arbitrary keys from a list
-- of records where mapKey is the key and mapValue is the value of that key
in let genericRecord = List { mapKey : Text, mapValue : Text }
in let kv = \(k : Text) -> \(v : Text) -> { mapKey = k, mapValue = v }
in let annotations = Some genericRecord
[ kv "kubernetes.io/ingress.class" "nginx"
, kv "kubernetes.io/ingress.allow-http" "false"
]
-- Generate spec from services
in let spec = defaultSpec //
{ tls = Some (List TLS) (map Service TLS makeTLS services)
, rules = Some (List Rule) (map Service Rule makeRule services)
}
in defaultIngress
{ metadata = defaultMeta
{ name = "nginx" } //
{ annotations = annotations }
} //
{ spec = Some Spec spec }
-- Here we import our example service, and generate the ingress with it
in mkIngress { services = [ ./myConfig.dhall ] }
As before we get the yaml out by running:
dhall-to-yaml --omitNull < ingress.yaml.dhall
Result:
## examples/out/ingressRaw.yaml
apiVersion: extensions/v1beta1
kind: Ingress
spec:
rules:
- http:
paths:
- backend:
servicePort: 80
serviceName: foo
host: foo.example.com
- http:
paths:
- backend:
servicePort: 80
serviceName: default
host: default.example.com
tls:
- hosts:
- foo.example.com
secretName: foo-certificate
- hosts:
- default.example.com
secretName: default-certificate
metadata:
annotations:
kubernetes.io/ingress.class: nginx
kubernetes.io/ingress.allow-http: 'false'
name: nginx
Development
Tests
All tests are defined in release.nix
. We run these tests in CI in a Hydra
project.
You can run the tests locally with the following somewhat magic command:
nix-build --no-link -E 'with import <nixpkgs> {}; callPackage ./release.nix { src.rev = ""; }'
It is so because release.nix
expects Hydra to provide the git revision in src.rev
,
but locally we don't have/want that, so we just call the function with an empty string.
Changing the README
We build README.md
from docs/README.md.dhall
and check it into source control.
The build script ./scripts/build-readme.sh
inlines source code from the
examples
directory. If you make changes to the readme or the examples you need
to run scripts/build-readme.sh
.