deployments.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: backend
  namespace: default
spec:
  selector:
    matchLabels:
      app: backend
  replicas: 1
  template:
    metadata:
      labels:
        app: backend
    spec:
      containers:
      - name: default
        image: user/backend
      - name: sidecar
        image: proxy
        livenessProbe:
          httpGet:
            path: /check
            port: 8081
          initialDelaySeconds: 2
          periodSeconds: 10
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: frontend
  namespace: default
spec:
  selector:
    matchLabels:
      app: frontend
  replicas: 1
  template:
    metadata:
      labels:
        app: frontend
    spec:
      containers:
      - name: default
        image: user/frontend
      - name: job-runner
        image: user/batch

ensure-probes.yaml

#@ load("@ytt:overlay", "overlay")

#@overlay/match by=overlay.subset({"kind": "Deployment"}), expects="1+"
---
spec:
  template:
    spec:
      containers:
      #! This matcher determines: of all the containers in this list,
      #!   which will we even look at... In our case, all of them.
      #@overlay/match by=overlay.all, expects="1+"
      - 
        #@overlay/match when=0
        livenessProbe:
          httpGet:
            path: /health
            port: 8080
          initialDelaySeconds: 3
          periodSeconds: 3

      #! This matcher runs independently of the one above. We're
      #!   combining two edits in one overlay document:
      #!   - one that ensures there's a livenessProbe
      #!   - a separate one that ensures there's a startupProbe
      #@overlay/match by=overlay.all, expects="1+"
      - 
        #@overlay/match when=0
        startupProbe:
          failureThreshold: 30
          periodSeconds: 10


#! line 12: note the newline before the annotation; this is needed so
#!   that the `@overlay/match ...` annotation applies to the 
#!   `livenessProbe:` map item and _not_ the array item that contains
#!   it.
#!
#! details at https://carvel.dev/ytt/docs/v0.42.0/yaml-primer/#example-annotating-an-array

#! line 13: this matcher only runs in the event there _is_ no `livenessProbe:`
#!   already defined.


#@ load("@ytt:yaml", "yaml")

#@ def copy_gets(orig, _):
#@   edit = yaml.decode(yaml.encode(orig))
#@   if "httpGet" in edit["livenessProbe"]:
#@     edit["startupProbe"]["httpGet"] = edit["livenessProbe"]["httpGet"] 
#@   end
#@   if "tcpSocket" in edit["livenessProbe"]:
#@     edit["startupProbe"]["tcpSocket"] = edit["livenessProbe"]["tcpSocket"] 
#@   end
#@   return edit
#@ end

#! line 47: we need to make an editable copy of `orig`.
#!   - `orig` is of type "YAML Fragment" and is immutable.
#!   - by encoding into a string, then decoding into Starlark-native (mutable) types
#!     we can now edit the result.

#@overlay/match by=overlay.subset({"kind": "Deployment"}), expects="1+"
---
spec:
  template:
    spec:
      containers:
      #! This matcher checks some preconditions, so that only containers
      #!   that have the requisite configuration will be targeted.
      #!   Specifically: there must be _both_ a liveness and startup probe
      #!   already.
      #@overlay/match by=lambda _,left,__: ("livenessProbe" in left) and ("startupProbe" in left), expects="1+"
      #@overlay/replace via=copy_gets
      -