feat: pdb

dump/kubernetes/README.md

@@ -6,5 +6,5 @@ Some notes on Kubernetes.
 - [Container Storage Interface (CSI)](./csi.md)
 - [Custom Resource Definition (CRD)](./crd.md)
 - [Assigning Pods (Affinity and Anti-Affinity)](./affinity.md)
-
+- [Pod Disruption Budget (PDB)](./pdb.md)
 

dump/kubernetes/pdb.md

@@ -0,0 +1,69 @@
+## Pod Disruption Budget (PDB) 
+
+A PDB limits the number of pods of a replicated application that are 
+down simultaneously from voluntary disruptions.
+
+> A voluntary disruption is a disruption that is caused by the user.
+>
+> For example, a user might want to drain a node to perform
+> maintenance on it or to remove it from the cluster.
+
+A PDB can be used to ensure that a certain number of pods are available
+at all times.
+
+### PDB is not configured 
+
+For example, we have a deployment with 2 replicas and a PDB is not configured.
+
+```txt
+-------------------------
+| node1 | node2 | node3 |
+-------------------------
+| pod1  | pod2  |       |
+-------------------------
+```
+
+We start a drain on node1. The scheduler will move pod1 to node3.
+
+```txt
+-------------------------
+| node1 | node2 | node3 |
+-------------------------
+|       | pod2  | pod1  |
+-------------------------
+```
+
+Let's say that pod2 is in pending state.
+
+And now we start a drain on node2. The scheduler will move pod2 to node3.
+
+And we have no pods running.
+
+### PDB is configured
+
+```yaml
+apiVersion: policy/v1beta1
+kind: PodDisruptionBudget
+metadata:
+  name: your-microservice
+  namespace: your-namespace
+spec:
+  minAvailable: 50% # 50% is just to keep the example simple, it is not the recommended value
+  selector:
+# ...
+```
+
+In now case pod on node2 won't be moved to node3, because the PDB is configured.
+
+### Best practices 
+
+- Use % instead of absolute number of pods 
+- Don't use non-resolvable combinations of replicas and maxUnavailable
+> For example, you have 1 replica and maxUnavailable=20% - it will never work.
+
+### References
+
+- https://github.com/mercari/production-readiness-checklist/blob/master/docs/concepts/pod-disruption-budget.md 
+- https://kubernetes.io/docs/tasks/run-application/configure-pdb/
+
+