What are leases in Kubernetes?

Key takeaways:

1. Leases serve as a coordination mechanism in Kubernetes to prevent conflicts and data inconsistencies when multiple components need to access and modify shared resources. They act like locks, granting exclusive ownership to a component.

2. In Kubernetes, leases are represented by lease objects, which consist of:

   1. Lease ID: A unique identifier for the lease, allowing the Kubernetes API server and components to recognize it.

   2. Lease Duration: The time, specified in seconds, that a component wishes to hold the lease. The Kubernetes API server automatically renews the lease if the component requests renewal before expiration.

In a distributed system like Kubernetes, multiple components often need to access and modify shared resources, such as configuration files. Without proper coordination, this can result in conflicts and data inconsistencies.

Leases serve as a coordination mechanism to prevent such issues. A lease acts as a lock on a shared resource, granting exclusive ownership to a component. This ensures that the component can safely make modifications without interference from others.

Example

One use of leases is to prevent conflicts in a replication controller. When a replication controller is created, it generates a lease object to track its current state. This lease object ensures that only one instance of the replication controller can be active at any given time.

If two replication controllers are created for the same pod simultaneously, both will attempt to acquire the lease. The first replication controller to successfully obtain the lease becomes the active one, while the other is terminated.

This mechanism prevents conflicts by ensuring that only one replication controller manages the same pod at a time. With leases, Kubernetes guarantees that pod management remains consistent and orderly, with only one active replication controller.When a replication controller is created, it creates a lease object to track the current state of the replication controller. This lease object ensures that only one instance of the replication controller can be active.

Lease object

Leases are represented by lease objects in Kubernetes. A lease object in Kubernetes has two main parts:

1. Lease ID: A unique identifier for the lease. The lease ID identifies the lease to the Kubernetes API server and other components that need to interact with the lease.

2. Lease duration: The amount of time the component wants to hold the lease. The lease duration is specified in seconds. The Kubernetes API server will automatically renew the lease if the component sends a renewal request before the lease expires.

Note: In addition to these two main parts, a lease object may include other information, such as the name of the component that created the lease and the resource associated with the lease.

How do leases work in Kubernetes?

A component requests a lease from the Kubernetes API server. The API server checks if a lease already exists for the resource. If no lease exists, the API server grants it to the requesting component. If a lease is already in place, the API server only grants the lease to the new component if the current lease has expired or if the existing leaseholder is inactive.

After acquiring a lease, the component can periodically send renewal requests to the API server to extend its duration. If the component fails to renew the lease, it will expire, making the resource available for other components to acquire.

Applications of leases

Some applications that leases can be used for are as follows:

Node hearbeats

Nodes in a Kubernetes cluster use leases to send periodic heartbeats to the API server. These heartbeats are stored as lease objects in the kube-node-lease namespace.

• If a node fails to renew its lease within the specified duration, it is marked as NotReady, allowing Kubernetes to reschedule its workloads.

• This mechanism ensures efficient and scalable node health monitoring.

Leader election

Leases are critical in Kubernetes leader election processes.

• For example, in a multi-controller setup, only one controller must act as the leader to perform updates or manage resources.

• Components competing for leadership create or update a shared lease object. The component that acquires or successfully updates the lease becomes the leader.

• This guarantees a single active leader, preventing conflicts or duplicate operations.

API server identity

In high-availability clusters, multiple API servers can run simultaneously. Each API server uses leases to identify itself as active and maintain coordination across the cluster.

This ensures consistent handling of requests and prevents overlapping operations during failovers.

Quiz

Let’s test the concepts learned in this Answer with a short quiz.

Conclusion

Conclusively, leases are a valuable tool for coordinating concurrent access to shared resources in Kubernetes. They ensure that only one component can control a resource at a time, preventing conflicts and maintaining consistency. This mechanism is crucial in distributed environments like Kubernetes, where multiple components often interact with the same resources simultaneously. Leases provide a reliable way to manage these interactions, ensuring that resource management remains predictable, orderly, and free from contention.