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Enterprise multi-cluster at scale: supporting flat networks in Linkerd

William Morgan

William Morgan
July 20, 2023 • 5 min read

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Linkerd has seen a steady rise in enterprise adoption, with companies like Adidas, Microsoft, Plaid, and DB Schenker deploying Linkerd at scale to bring security, compliance, and reliability to their mission-critical production infrastructure. Based on feedback from this enterprise audience, the upcoming Linkerd 2.14 release will introduce a new set of features designed to handle types of multi-cluster Kubernetes configurations commonly found in enterprise deployments.

One of the most important new features in the next Linkerd release will be an improved ability to handle multi-cluster communication in environments with a shared flat network between clusters. In this blog post, I’ll talk about what that means and why it’s important.

How does Linkerd handle multi-cluster today?

If you’re using multiple Kubernetes clusters and want them to communicate with each other, Linkerd gives you the ability to send traffic across cluster boundaries that is:

  1. Fully secured. This means that traffic between clusters is encrypted, authenticated, and authorized using mutual TLS, workload identities (not network identities!) and Linkerd’s fine-grained, zero-trust authorization policies.
  2. Transparent to the application. This means that the application is totally decoupled from cluster topology, which allows the operator to take advantage of powerful networking capabilities such as dynamically failover traffic to other clusters.
  3. Observable and reliable. Linkerd’s powerful L7 instrospection and reliability mechanisms, including golden metrics, retries, timeouts, distributed tracing, circuit breaking, and more, are all available to cross-cluster traffic just as they are to on-cluster traffic.

Linkerd has supported multi-cluster Kubernetes deployments since the release of Linkerd 2.8 in 2020. That release introduced a simple and elegant design that involves the addition of a service mirror component to handle service discovery, and a multi-cluster gateway component to handle traffic from other clusters.

This gateway design allowed Linkerd’s multi-cluster support to be entirely independent of underlying network topology. Whether your clusters are colocated in the same datacenter; split across different cloud providers; or deployed in a hybrid fashion between on-premises and cloud deployments, Linkerd worked the same way.

This design has worked well! However, as Kubernetes adoption has grown in enterprise environments, we’ve seen a growing number of cases where clusters are deployed in a shared flat network. In this situation, we can make some significant optimizations by removing the gateway.

Multi-cluster for flat networks

In a shared flat network situation, pods in different Kubernetes clusters can route traffic directly to each other. In other words, a pod in cluster 1 can establish a TCP connection to a pod in cluster 2, just using the underlying network.

If pods are routable, why use Linkerd? For exactly the same reasons you’re using it within the cluster: to provide the security, reliability, and observability guarantees beyond what a baseline TCP connection provides.

In Linkerd 2.14, we’ll introduce an additional mode of multi-cluster communication designed for shared flat networks: direct pod-to-pod communication between clusters without the gateway intermediary.

An architectural diagram comparing hierarchical network mode with the new flat network mode

In this approach, as you might imagine, Linkerd will route communication from a pod on the source cluster directly to the destination pod on another cluster without transiting the gateway. This provides several advantages, including:

  • Improved latency of cross-cluster calls by removing the additional hop between client and server.
  • Improved security by preserving workload identity in mTLS calls across clusters, rather than overriding it with the gateway identity.
  • Reduced cloud spend by reducing the amount of traffic that is routed through the multi-cluster gateway, which is often implemented as a cloud loud balancer.

This approach still preserves two critical aspects of Linkerd’s multi-cluster design:

  1. Separation of failure domains. Each Kubernetes cluster runs its own Linkerd control plane, independently of other clusters, and the failure of a single cluster cannot take down the service mesh on other clusters.
  2. Standardized, uniform architecture.. Unlike other solutions that split L7 logic between complex proxies operating at different levels and scopes, Linkerd’s Rust-based “micro-proxy” sidecars are the sole mechanism for controlling traffic between pods and clusters, giving you a single operational surface area to monitor and manage, with clear isolation of failure and security domains.

Finally, this approach improves Linkerd’s ability to provide a uniform layer of authentication across your entire environment, and to enforce granular authorization policies, aka “micro-segmentation”. Because the gateway is no longer an intermediary, cross-cluster connections retain the workload identity of the source, and authorization policies can be crafted to take advantage of these identities directly.

(For Kubernetes experts, note that this implementation is inspired by, and loosely aligns with, the Multi-Cluster Services API proposal (KEP-1645). While strict conformance with this KEP is not currently a goal, we look forward to seeing how that proposal evolves.)

So when do we get this amazing new feature?

Linkerd 2.14 will be shipping next month. With the addition of pod-to-pod communication, we’re confident that Linkerd will continue to be the simplest way to connect multiple Kubernetes clusters, now including for deployments that can make use of flat networks.

Linkerd is for everyone

Linkerd is a graduated project of the Cloud Native Computing Foundation. Linkerd is committed to open governance. If you have feature requests, questions, or comments, we’d love to have you join our rapidly-growing community! Linkerd is hosted on GitHub, and we have a thriving community on Slack, Twitter, and the mailing lists. Come and join the fun!

(Photo by NASA on Unsplash)

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