chore(kontemplate): Prepare kontemplate for depot-merge

This merge will not yet include moving over to buildGo.nix, as support
for testing and such is not present in that library yet.

1 files changed, 77 insertions, 0 deletions

diff --git a/ops/kontemplate/docs/tips-and-tricks.md b/ops/kontemplate/docs/tips-and-tricks.md
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+Kontemplate tips & tricks
+=========================
+
+<!-- markdown-toc start - Don't edit this section. Run M-x markdown-toc-refresh-toc -->
+**Table of Contents**
+
+- [Kontemplate tips & tricks](#kontemplate-tips--tricks)
+    - [Update Deployments when ConfigMaps change](#update-deployments-when-configmaps-change)
+    - [direnv & pass](#direnv--pass)
+
+<!-- markdown-toc end -->
+
+## Update Deployments when ConfigMaps change
+
+Kubernetes does [not currently][] have the ability to perform rolling updates
+of Deployments and other resource types when `ConfigMap` or `Secret` objects
+are updated.
+
+It is possible to make use of annotations and templating functions in
+Kontemplate to force updates to these resources anyways.
+ 
+For example:
+
+```yaml
+# A ConfigMap that contains some configuration for your app
+---
+kind: ConfigMap
+metadata:
+  name: app-config
+data:
+  app.conf: |
+    name: {{ .appName }}
+    foo: bar
+```
+
+Now whenever the `appName` variable changes or we make an edit to the
+`ConfigMap` we would like to update the `Deployment` making use of it, too. We
+can do this by adding a hash of the parsed template to the annotations of the
+created `Pod` objects:
+
+```yaml
+
+---
+kind: Deployment
+metadata:
+  name: app
+spec:
+  template:
+    metadata:
+      annotations:
+        configHash: {{ insertTemplate "app-config.yaml" | sha256sum }}
+    spec:
+      containers:
+        - name: app
+          # Some details omitted ... 
+          volumeMounts:
+            - name: config
+              mountPath: /etc/app/
+      volumes:
+        - name: config
+          configMap:
+            name: app-config
+```
+
+Now any change to the `ConfigMap` - either by directly editing the yaml file or
+via a changed template variable - will cause the annotation to change,
+triggering a rolling update of all relevant pods.
+
+## direnv & pass
+
+Users of `pass` may have multiple different password stores on their machines.
+Assuming that `kontemplate` configuration exists somewhere on the filesystem
+per project, it is easy to use [direnv][] to switch to the correct
+`PASSWORD_STORE_DIR` variable when entering the folder.
+
+[not currently]: https://github.com/kubernetes/kubernetes/issues/22368
+[direnv]: https://direnv.net/