Linkerd is built for you, the operator, SRE, architect, and platform owner. It's designed to give you power over your own fate: to provide fundamental visibility, reliability, and security capabilities at the platform level. The goal of Linkerd is to give you these powers in a way that's uniform across all code running in the entire compute environment, and totally independent of application code or developer teams.
Since Linkerd is built for operators, this also means that Linkerd has do all that while also imposing the absolute minimum operational complexity. To do this, we've designed Linkerd with three core principles in mind:
Keep it simple. Linkerd should be operationally simple with low cognitive overhead. Operators should find its components clear and its behavior understandable and predictable, with a minimum of magic.
Minimize resource requirements. Linkerd should impose as minimal a performance and resource cost as possible–especially at the data plane layer.
Just work. Linkerd should not break existing applications, nor should it require complex configuration to get started or to do something simple.
The first principle is the most important: keep it simple. Simplicity doesn't mean that Linkerd can't have powerful features, or that it has to have one-click wizards take care of everything for you. In fact, it means the opposite: every aspect of Linkerd's behavior should be explicit, clear, well-defined, bounded, understandable, and introspectable. For example, Linkerd's control plane is split into several operational components based on their functional boundaries (“web”, “api”, etc.) These components aren't just exposed directly to you in the Linkerd dashboard and CLI, they run on the same data plane as your application does, allowing you to use the same tooling to inspect their behavior.
Minimize resource requirements means that Linkerd, and especially Linkerd's data plane proxies, should consume the smallest amount of memory and CPU possible. On the control plane side, we've taken care to ensure that components scale gracefully in the presence of traffic. On the data plane side, we've build Linkerd's proxy (called simply “linkerd-proxy”) for performance, safety, and low resource consumption. Today, a single linkerd-proxy instance can proxy many thousands of requests per second in under 10mb of memory and a quarter of a core, all with a p99 tail latency of under 1ms. In the future, we can probably improve even that!
Finally, just work means that adding Linkerd to a functioning Kubernetes application shouldn't break anything, and shouldn't even require configuration. (Of course, configuration will be necessary to customize Linkerd's behavior–but it shouldn't be necessary simply to get things working.) To do this, we've invested heavily in things like automatic L7 protocol detection, and automatic re-routing of TCP traffic within a pod.
Together, these three principles give us a framework for weighing product and engineering tradeoffs in Linkerd. We hope they're also useful for understanding why Linkerd works the way it does.