Pod Disruption Budgets: Why They Matter and How to Use Them to Avoid Downtime

Kubernetes pods are the smallest units you can deploy on the Kubernetes platform. Each pod represents a single running process and operates from a node or worker machine within Kubernetes, which can be either virtual or physical.

Sometimes, Kubernetes pods might get disrupted, either voluntarily or involuntarily. This is more common in highly available applications and can be a concern for cluster administrators managing automated cluster actions.

Pods will stay in Kubernetes until a user or controller removes them or a system error occurs. To keep systems running smoothly and avoid downtime, administrators can set up Kubernetes pod disruption budgets (PDBs). These budgets create a buffer, allowing for some pods to be disrupted simultaneously without affecting the overall system.

But before setting up PDBs, it's important to understand what disruptions are. What causes disruptions and what do we mean by voluntary or involuntary disruptions?

What are Disruptions?

Disruptions are events that cause your pods to stop running. They can be either voluntary or involuntary, and understanding both types is crucial for maintaining the availability of your applications.

Types of Disruptions

Involuntary Disruptions

Involuntary disruptions are unexpected and usually due to hardware or system failures. Examples include:

• Hardware failure of the physical machine backing the node
• Accidental deletion of a VM by the cluster administrator
• Cloud provider or hypervisor failures
• Kernel panics
• Node disappearance from the cluster due to network issues
• Pod eviction due to the node running out of resources

Voluntary Disruptions

Voluntary disruptions are intentional and can be initiated by application owners or cluster administrators. Examples include:

• Deleting a deployment or other controller managing the pod
• Updating a deployment's pod template, causing a restart
• Directly deleting a pod (accidentally or intentionally)
• Draining a node for repair or upgrade
• Scaling down a cluster by draining nodes
• Removing a pod to make room for others on a node

If you understand both types of disruptions, let's move on to how Pod Disruption Budgets (PDBs) can help manage these disruptions and ensure your systems remain available.

What is a Pod Disruption Budget (PDB)?

PDB is a solution to Kubernetes pod disruption managed across various controllers such as ReplicaSet, StatefulSet, ReplicationController, and Deployment. PDBs prevent server downtime/outages by shutting down too many pods at a given period. 

In simple terms, the Pod Disruption Budget (PDB) is a Kubernetes resource used to specify the minimum number of replicas of a Pod that must be available during voluntary disruptions. 

How Pod Disruption Budget (PDB) work?

PDBs consist of two main components: minAvailable and maxUnavailable. 

1. minAvailable: The minimum number of pods that must be available during a disruption.
2. maxUnavailable: The maximum number of pods that can be unavailable during a disruption

To implement a PDB, you define it in a YAML file and apply it to the Kubernetes cluster using kubectl. For example, a PDB might specify that at least two pods labeled app: my-app must be available at all times.

During voluntary disruptions, such as node maintenance, Kubernetes respects the PDB by ensuring the specified number of pods remains available before evicting any pods. However, PDBs do not prevent involuntary disruptions like node failures.

Benefits of Using PDBs

1. Controlled Disruptions: Ensures that voluntary disruptions do not reduce application availability below a specified level.
2. Improved Reliability: Maintains service levels during maintenance activities.
3. Enhanced Stability: Prevents scenarios where too many pods are taken down simultaneously.

Scenarios Where PDB Helps

1. Node Upgrades: During node upgrades, PDB ensures that critical applications remain available by limiting the number of pods that can be evicted at any time.
2. Scaling Operations: When scaling down applications, PDB prevents too many pods from being terminated simultaneously, ensuring continued service availability.
3. Automated Maintenance: In clusters with automated maintenance routines, PDBs help maintain application stability by managing pod evictions in a controlled manner.

Conclusion

Pod Disruption Budgets are a crucial tool in Kubernetes for maintaining application availability during disruptions. By understanding and effectively using PDBs, organizations can ensure high availability, improve reliability, and maintain stable application performance even during maintenance activities.