Add blogpost on how I use Dhall for Kubernetes (#110)

* blog: add post on Dhall for Kubernetes

* go.sum: tidy

* Update dhall-kubernetes-2020-01-25.markdown

blog/dhall-kubernetes-2020-01-25.markdown

@@ -0,0 +1,435 @@
+---
+title: Dhall for Kubernetes
+date: 2020-01-25
+tags:
+  - dhall
+  - kubernetes
+  - witchcraft
+---
+
+# Dhall for Kubernetes
+
+Kubernetes is a surprisingly complicated software package. Arguably, it has to
+be that complicated as a result of the problems it solves being complicated; but
+managing yaml configuration files for Kubernetes is a complicated task. [YAML][yaml]
+doesn't have support for variables or type metadata. This means that the
+validity (or sensibility) of a given Kubernetes configuration file (or files)
+isn't easy to figure out without using a Kubernetes server.
+
+[yaml]: https://yaml.org
+
+In my [last post][cultk8s] about Kubernetes, I mentioned I had developed a tool
+named [dyson][dyson] in order to help me manage Terraform as well as create
+Kubernetes manifests from [a template][template]. This works for the majority of
+my apps, but it is difficult to extend at this point for a few reasons:
+
+[cultk8s]: https://christine.website/blog/the-cult-of-kubernetes-2019-09-07
+[dyson]: https://github.com/Xe/within-terraform/tree/master/dyson
+[template]: https://github.com/Xe/within-terraform/blob/master/dyson/src/dysonPkg/deployment_with_ingress.yaml
+
+- It assumes that everything passed to it are already valid yaml terms
+- It doesn't assert the type of any values passed to it
+- It is difficult to add another container to a given deployment
+- Environment variables implicitly depend on the presence of a private git repo
+- It depends on the template being correct more than the output being correct
+
+So, this won't scale. People in the community have created other solutions for
+this like [Helm][helm], but a lot of them have some of the same basic problems.
+Helm also assumes that your template is correct. [Kustomize][kustomize] does
+help with a lot of the type-safe variable replacements, but it doesn't have the
+ability to ensure your manifest is valid.
+
+[helm]: https://helm.sh
+[kustomize]: https://kustomize.io
+
+I looked around for alternate solutions for a while and eventually found
+[Dhall][dhall] thanks to a friend. Dhall is a _statically typed_ configuration
+language. This means that you can ensure that inputs are _always_ the correct
+type or the configuration file won't load. There's also a built-in
+[dhall-to-yaml][dhallyaml] tool that can be used with the [Kubernetes
+package][dhallk8s] in order to declare Kubernetes manifests in a type-safe way.
+
+[dhall]: https://dhall-lang.org
+[dhallyaml]: https://github.com/dhall-lang/dhall-haskell/tree/master/dhall-yaml#dhall-yaml
+[dhallk8s]: https://github.com/dhall-lang/dhall-kubernetes
+
+Here's a small example of Dhall and the yaml it generates:
+
+```dhall
+-- Mastodon usernames
+[ { name = "Cadey", mastodon = "@cadey@mst3k.interlinked.me" }
+, { name = "Nicole", mastodon = "@sharkgirl@mst3k.interlinked.me" }
+]
+```
+
+Which produces:
+
+```yaml
+- mastodon: "@cadey@mst3k.interlinked.me"
+  name: Cadey
+- mastodon: "@sharkgirl@mst3k.interlinked.me"
+  name: Nicole
+```
+
+Which is fine, but we still have the type-safety problem that you would have in
+normal yaml. Dhall lets us define [record types][dhallrecord] for this data like
+this:
+
+[dhallrecord]: http://www.haskellforall.com/2020/01/dhall-year-in-review-2019-2020.html
+
+```dhall
+let User =
+      { Type = { name : Text, mastodon : Optional Text }
+      , default = { name = "", mastodon = None }
+      }
+
+let users =
+      [ User::{ name = "Cadey", mastodon = Some "@cadey@mst3k.interlinked.me" }
+      , User::{
+        , name = "Nicole"
+        , mastodon = Some "@sharkgirl@mst3k.interlinked.me"
+        }
+      ]
+
+in  users
+```
+
+Which produces:
+
+```yaml
+- mastodon: "@cadey@mst3k.interlinked.me"
+  name: Cadey
+- mastodon: "@sharkgirl@mst3k.interlinked.me"
+  name: Nicole
+```
+
+This is type-safe because you cannot add arbitrary fields to User instances
+without the compiler rejecting it. Let's add an invalid "preferred_language"
+field to Cadey's instance:
+
+```
+-- ...
+let users =
+      [ User::{
+        , name = "Cadey"
+        , mastodon = Some "@cadey@mst3k.interlinked.me"
+        , preferred_language = "en-US"
+        }
+      -- ...
+      ]
+```
+
+Which gives us:
+
+```
+$ dhall-to-yaml --file example.dhall
+Error: Expression doesn't match annotation
+
+{ + preferred_language : …
+, …
+}
+
+4│         User::{ name = "Cadey", mastodon = Some "@cadey@mst3k.interlinked.me",
+5│       preferred_language = "en-US" }
+
+example.dhall:4:9
+```
+
+Or [this more detailed explanation][explanation] if you add the `--explain` flag
+to the `dhall-to-yaml` call.
+
+[explanation]: https://clbin.com/JtVWT
+
+We tried to do something that violated the contract that the type specified.
+This means that it's an invalid configuration and is therefore rejected and no
+yaml file is created.
+
+The Dhall Kubernetes package specifies record types for _every_ object available
+by default in Kubernetes. This does mean that the package is incredibly large,
+but it also makes sure that _everything_ you could possibly want to do in
+Kubernetes matches what it expects. In the [package
+documentation][k8sdhalldocs], they give an example where a
+[Deployment][k8sdeployment] is created.
+
+[k8sdhalldocs]: https://github.com/dhall-lang/dhall-kubernetes/tree/master/1.15#quickstart---a-simple-deployment
+[k8sdeployment]: https://kubernetes.io/docs/concepts/workloads/controllers/deployment/
+
+``` dhall
+-- examples/deploymentSimple.dhall
+
+-- Importing other files is done by specifying the HTTPS URL/disk location of
+-- the file. Attaching a sha256 hash (obtained with `dhall freeze`) allows
+-- the Dhall compiler to cache these files and speed up configuration loads
+-- drastically.
+let kubernetes =
+      https://raw.githubusercontent.com/dhall-lang/dhall-kubernetes/1.15/master/package.dhall
+      sha256:4bd5939adb0a5fc83d76e0d69aa3c5a30bc1a5af8f9df515f44b6fc59a0a4815
+      
+let deployment =
+      kubernetes.Deployment::{
+      , metadata = kubernetes.ObjectMeta::{ name = "nginx" }
+      , spec =
+          Some
+            kubernetes.DeploymentSpec::{
+            , replicas = Some 2
+            , template =
+                kubernetes.PodTemplateSpec::{
+                , metadata = kubernetes.ObjectMeta::{ name = "nginx" }
+                , spec =
+                    Some
+                      kubernetes.PodSpec::{
+                      , containers =
+                          [ kubernetes.Container::{
+                            , name = "nginx"
+                            , image = Some "nginx:1.15.3"
+                            , ports =
+                                [ kubernetes.ContainerPort::{
+                                  , containerPort = 80
+                                  }
+                                ]
+                            }
+                          ]
+                      }
+                }
+            }
+      }
+
+in  deployment
+```
+
+Which creates the following yaml:
+
+```
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: nginx
+spec:
+  replicas: 2
+  template:
+    metadata:
+      name: nginx
+    spec:
+      containers:
+        - image: nginx:1.15.3
+          name: nginx
+          ports:
+            - containerPort: 80
+```
+
+Dhall's lambda functions can help you break this into manageable chunks. For
+example, here's a Dhall function that helps create a docker image reference:
+
+```
+let formatImage
+    : Text -> Text -> Text
+    = \(repository : Text) -> \(tag : Text) ->
+    "${repository}:${tag}"
+
+in formatImage "xena/christinewebsite" "latest"
+```
+
+Which outputs `xena/christinewebsite:latest` when passed to `dhall text`.
+
+All of this adds up into a powerful toolset that lets you express Kubernetes
+configuration in a way that does what you want without as many headaches.
+
+Most of my apps on Kubernetes need only a few generic bits of configuration:
+
+- Their name
+- What port should be exposed
+- The domain that this service should be exposed on
+- How many replicas of the service are needed
+- Which Let's Encrypt Issuer to use (currently only `"prod"` or `"staging"`)
+- The [configuration variables of the service][12factorconfig]
+- Any other containers that may be needed for the service
+
+[12factorconfig]: https://12factor.net/config
+
+From here, I defined all of the [bits and pieces][kubermemeshttp] for the
+Kubernetes manifests that Dyson produces and then created a `Config` type that
+helps to template them out. Here's my [`Config` type
+definition][configdefinition]:
+
+[kubermemeshttp]: https://tulpa.dev/cadey/kubermemes/src/branch/master/k8s/http
+[configdefinition]: https://tulpa.dev/cadey/kubermemes/src/branch/master/k8s/app/config.dhall
+
+```dhall
+let kubernetes = ../kubernetes.dhall
+
+in  { Type =
+        { name : Text
+        , appPort : Natural
+        , image : Text
+        , domain : Text
+        , replicas : Natural
+        , leIssuer : Text
+        , envVars : List kubernetes.EnvVar.Type
+        , otherContainers : List kubernetes.Container.Type
+        }
+    , default =
+        { name = ""
+        , appPort = 5000
+        , image = ""
+        , domain = ""
+        , replicas = 1
+        , leIssuer = "staging"
+        , envVars = [] : List kubernetes.EnvVar.Type
+        , otherContainers = [] : List kubernetes.Container.Type
+        }
+    }
+```
+
+Then I defined a `makeApp` function that creates everything I need to deploy my
+stuff on Kubernetes:
+
+```dhall
+let Prelude = ../Prelude.dhall
+
+let kubernetes = ../kubernetes.dhall
+
+let typesUnion = ../typesUnion.dhall
+
+let deployment = ../http/deployment.dhall
+
+let ingress = ../http/ingress.dhall
+
+let service = ../http/service.dhall
+
+let Config = ../app/config.dhall
+
+let K8sList = ../app/list.dhall
+
+let buildService =
+        \(config : Config.Type)
+      -> let myService = service config
+
+         let myDeployment = deployment config
+
+         let myIngress = ingress config
+
+         in  K8sList::{
+             , items =
+               [ typesUnion.Service myService
+               , typesUnion.Deployment myDeployment
+               , typesUnion.Ingress myIngress
+               ]
+             }
+
+in  buildService
+```
+
+And used it to deploy the [h language website][hlang]:
+
+[hlang]: https://h.christine.website
+
+```dhall
+let makeApp = ../app/make.dhall
+
+let Config = ../app/config.dhall
+
+let cfg =
+      Config::{
+      , name = "hlang"
+      , appPort = 5000
+      , image = "xena/hlang:latest"
+      , domain = "h.christine.website"
+      , leIssuer = "prod"
+      }
+
+in  makeApp cfg
+```
+
+Which produces the following Kubernetes config:
+
+```yaml
+apiVersion: v1
+items:
+  - apiVersion: v1
+    kind: Service
+    metadata:
+      annotations:
+        external-dns.alpha.kubernetes.io/cloudflare-proxied: "false"
+        external-dns.alpha.kubernetes.io/hostname: h.christine.website
+        external-dns.alpha.kubernetes.io/ttl: "120"
+      labels:
+        app: hlang
+      name: hlang
+      namespace: apps
+    spec:
+      ports:
+        - port: 5000
+          targetPort: 5000
+      selector:
+        app: hlang
+      type: ClusterIP
+  - apiVersion: apps/v1
+    kind: Deployment
+    metadata:
+      name: hlang
+      namespace: apps
+    spec:
+      replicas: 1
+      selector:
+        matchLabels:
+          app: hlang
+      template:
+        metadata:
+          labels:
+            app: hlang
+          name: hlang
+        spec:
+          containers:
+            - image: xena/hlang:latest
+              imagePullPolicy: Always
+              name: web
+              ports:
+                - containerPort: 5000
+          imagePullSecrets:
+            - name: regcred
+  - apiVersion: networking.k8s.io/v1beta1
+    kind: Ingress
+    metadata:
+      annotations:
+        certmanager.k8s.io/cluster-issuer: letsencrypt-prod
+        kubernetes.io/ingress.class: nginx
+      labels:
+        app: hlang
+      name: hlang
+      namespace: apps
+    spec:
+      rules:
+        - host: h.christine.website
+          http:
+            paths:
+              - backend:
+                  serviceName: hlang
+                  servicePort: 5000
+      tls:
+        - hosts:
+            - h.christine.website
+          secretName: prod-certs-hlang
+kind: List
+```
+
+And when I applied it on my Kubernetes cluster, it worked the first time and had
+absolutely no effect on the existing configuration.
+
+In the future, I hope to expand this to allow for multiple deployments (IE: a
+chatbot running in a separate deployment than a web API the chatbot depends on
+or non-web projects in general) as well as supporting multiple Kubernetes
+namespaces. 
+
+Dhall is probably the most viable replacement to Helm or other Kubernetes
+templating tools I have found in recent memory. I hope that it will be used by
+more people to help with configuration management, but I can understand that
+that may not happen. At least it works for me. 
+
+If you want to learn more about Dhall, I suggest checking out the following
+links:
+
+- [The Dhall Language homepage](https://dhall-lang.org)
+- [Learn Dhall in Y Minutes](https://learnxinyminutes.com/docs/dhall/)
+- [The Dhall Language GitHub Organization](https://github.com/dhall-lang)
+
+I hope this was helpful and interesting. Be well.

go.sum

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-golang.org/x/sys v0.0.0-20191010194322-b09406accb47 h1:/XfQ9z7ib8eEJX2hdgFTZJ/ntt0swNk5oYBziWeTCvY=
+golang.org/x/sys v0.0.0-20191220142924-d4481acd189f h1:68K/z8GLUxV76xGSqwTWw2gyk/jwn79LUL43rES2g8o=
-golang.org/x/sys v0.0.0-20191010194322-b09406accb47/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20191220142924-d4481acd189f/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 gopkg.in/alecthomas/kingpin.v2 v2.2.6/go.mod h1:FMv+mEhP44yOT+4EoQTLFTRgOQ1FBLkstjWtayDeSgw=
@@ -125,10 +117,6 @@ gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8
 gopkg.in/yaml.v2 v2.2.1/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.2.2 h1:ZCJp+EgiOT7lHqUV2J862kp8Qj64Jo6az82+3Td9dZw=
 gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
-gopkg.in/yaml.v2 v2.2.4 h1:/eiJrUcujPVeJ3xlSWaiNi3uSVmDGBK1pDHUHAnao1I=
-gopkg.in/yaml.v2 v2.2.4/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
-gopkg.in/yaml.v2 v2.2.5 h1:ymVxjfMaHvXD8RqPRmzHHsB3VvucivSkIAvJFDI5O3c=
-gopkg.in/yaml.v2 v2.2.5/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.2.7 h1:VUgggvou5XRW9mHwD/yXxIYSMtY0zoKQf/v226p2nyo=
 gopkg.in/yaml.v2 v2.2.7/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.2.8 h1:obN1ZagJSUGI0Ek/LBmuj4SNLPfIny3KsKFopxRdj10=