Add healthchecks

Explain liveness and readiness probes.
No lab yet.

slides/k8s/healthchecks.md

@@ -0,0 +1,178 @@
+# Healthchecks
+
+- Kubernetes provides two kinds of healthchecks: liveness and readiness
+
+- Healthchecks are *probes* that apply to *containers* (not to pods)
+
+- Each container can have two (optional) probes:
+
+  - liveness = is this container dead or alive?
+
+  - readiness = is this container ready to serve traffic?
+
+- Different probes are available (HTTP, TCP, program execution)
+
+- Let's see the difference and how to use them!
+
+---
+
+## Liveness probe
+
+- Indicates if the container is dead or alive
+
+- A dead container cannot come back to life
+
+- If the liveness probe fails, the container is killed
+
+  (to make really sure that it's really dead; no zombies or undeads!)
+
+- What happens next depends on the pod's `restartPolicy`:
+
+  - `Never`: the container is not restarted
+
+  - `OnFailure` or `Always`: the container is restarted
+
+---
+
+## When to use a liveness probe
+
+- To indicate failures that can't be recovered
+
+  - deadlocks (causing all requests to time out)
+
+  - internal corruption (causing all requests to error)
+
+- If the liveness probe fails *N* consecutive times, the container is killed
+
+- *N* is the `failureThreshold` (3 by default)
+
+---
+
+## Readiness probe
+
+- Indicates if the container is ready to serve traffic
+
+- If a container becomes "unready" (let's say busy!) it might be ready again soon
+
+- If the readiness probe fails:
+
+  - the container is *not* killed
+
+  - if the pod is a member of a service, it is temporarily removed
+
+  - it is re-added as soon as the readiness probe passes again
+
+---
+
+## When to use a readiness probe
+
+- To indicate temporary failures
+
+  - the application can only service *N* parallel connections
+
+  - the runtime is busy doing garbage collection or initial data load
+
+- The container is marked as "not ready" after `failureThreshold` failed attempts
+
+  (3 by default)
+
+- It is marked again as "ready" after `successThreshold` successful attempts
+
+  (1 by default)
+
+---
+
+## Different types of probes
+
+- HTTP request
+
+  - specify URL of the request (and optional headers)
+
+  - any status code between 200 and 399 indicates success
+
+- TCP connection
+
+  - the probe succeeds if the TCP port is open
+
+- arbitrary exec
+
+  - a command is executed in the container
+
+  - exit status of zero indicates success
+
+---
+
+## Benefits of using probes
+
+- Rolling updates proceed when containers are *actually ready*
+
+  (as opposed top merely started)
+
+- Containers in a broken state gets killed and restarted
+
+  (instead of serving errors or timeouts)
+
+- Overloaded backends get removed from load balancer rotation
+
+  (thus improving response times across the board)
+
+---
+
+## Example: HTTP probe
+
+Here is a pod template for the `rng` web service of the DockerCoins app:
+
+```yaml
+apiVersion: v1
+kind: Pod
+metadata:
+  name: rng-with-liveness
+spec:
+  containers:
+  - name: rng
+    image: dockercoins/rng:v0.1
+    livenessProbe:
+      httpGet:
+        path: /
+        port: 80
+      initialDelaySeconds: 10
+      periodSeconds: 1
+```
+
+If the backend serves an error, or takes longer than 1s, 3 times in a row, it gets killed.
+
+---
+
+## Example: exec probe
+
+Here is a pod template for a Redis server:
+
+```yaml
+apiVersion: v1
+kind: Pod
+metadata:
+  name: redis-with-liveness
+spec:
+  containers:
+  - name: redis
+    image: redis
+    livenessProbe:
+      exec:
+        command: ["redis-cli", "ping"]
+```
+
+If the Redis process becomes unresponsive, it will be killed.
+
+---
+
+## Details about liveness and readiness probes
+
+- Probes are executed at intervals of `periodseconds` (default: 10)
+
+- The timeout for a probe is set with `timeoutSeconds` (default: 1)
+
+- A probe is considered successful after `successThreshold` successes (default: 1)
+
+- A probe is considered failing after `failureThreshold` failures (default: 3)
+
+- If a probe is not defined, it's as if there was an "always successful" probe

slides/new-content.yml

@@ -26,11 +26,5 @@ chapters:
 
     prometheus
 
-  - |
-    # misc todo
-
-    after build-with-docker and build-with-kaniko, show clean up orphan pods
-
-    healthchecks / liveness
-
+- - k8s/healthchecks.md