Why Linkerd doesn't use Envoy

William Morgan

William Morgan
December 3, 2020 • 11 min read

Closeup of many Swiss Army knives

Why Linkerd doesn’t use Envoy

In this article I’m going to describe why Linkerd isn’t built on Envoy.

This is a bit of a weird article to write. After all, there are a million projects that Linkerd doesn’t use, and none of those decisions deserve a blog post. But the fact that Linkerd doesn’t use Envoy specifically has become a common enough topic of discussion that it probably deserves a good explanation.

Let me also state upfront that this is not an “Envoy sucks” blog post. Envoy is a great project, is clearly a popular choice for many, and we have nothing but respect for the fine folks who work on it. We recommend Envoy to Linkerd users every day in the form of ingress controllers like Ambassador, and there are production systems around the world today where you can find Envoy and Linkerd working side by side.

But we chose not to build Linkerd on top of Envoy. Instead, we built a dedicated “micro-proxy”, called simply Linkerd2-proxy, which is optimized for the service mesh sidecar use case. In the increasingly crowded field of similar-sounding service mesh projects, Linkerd is unique in this regard. But why did we go this route?

The full answer to this question is nuanced and technical at heart—exactly the kind of content that tends to get swept away in the faddish, blog-post-driven world of cloud native adoption.1 So in this article I’m going to do my best to lay out the reasons why in a frank and engineering-focused way. After all, Linkerd is built by engineers and for engineers, and if there’s one thing I’m proud of, it’s that we’ve made decisions on the basis of engineering tradeoffs rather than marketing pressure.

In short: Linkerd doesn’t use Envoy because using Envoy wouldn’t allow us to build the lightest, simplest, and most secure Kubernetes service mesh in the world.

Being the lightest, simplest, most secure Kubernetes service mesh is Linkerd’s promise to our users, and that’s also what makes Linkerd unique among service meshes: it is dramatically simpler, lighter, and more secure. And the reason we’ve been able to accomplish that is—you guessed it—because we build on Linkerd2-proxy instead of Envoy. Not because Envoy is bad, but because Linkerd2-proxy is _better_—at least, for the very specific and limited use case of being a Kubernetes sidecar proxy.

Let’s take a look at why.

What is Linkerd2-proxy?

Before we get into the details, it’s helpful to understand a bit more about Linkerd2-proxy.

Linkerd2-proxy is a “micro-proxy” designed specifically for the service mesh sidecar use case. Linkerd2-proxy is built on, and has driven many of the requirements for, the world’s most modern network programming environment circa 2020: the Rust asynchronous network ecosystem, including libraries like Tokio, Tower, and Hyper. In terms of sheer technical advancement, Linkerd2-proxy is one of the most advanced pieces of technology in the entire CNCF landscape.

Like Envoy, Linkerd2-proxy is a 100% open source Apache v2 CNCF project that features regular third-party audits, an active community, and high-scale production usage in mission-critical systems around the world. Unlike Envoy, Linkerd2-proxy is designed for only one use case: proxying requests to and from a single Kubernetes pod while receiving configuration from the Linkerd control plane. And unlike Envoy, Linkerd2-proxy is designed to be an implementation detail: it’s not user-facing, it’s not usable as a generic building block, and it has a boring name. This means Linkerd2-proxy tends to go unnoticed, though we’ve tried to shed a little more light on it recently with articles looking under the hood and at the roadmap).

So why “micro-proxy”? Loathe as we are to introduce another term into the lexicon,2 the word “proxy” doesn’t do Linkerd2-proxy justice. A proxy is something like Envoy, NGINX, Apache, or httproxy. These projects can do a huge variety of things (“send HTTP requests with a path that matches this wildcard to this backend while rewriting these headers, compressing any Javascript files, and rotating the access logs”) and they have a configuration and tuning surface area to match. Using a proxy in production requires significant operational investment: if you’re running Apache, you’re going to end up with an Apache expert somewhere in the house.

But Linkerd2-proxy is different. It’s designed to be an implementation detail that doesn’t require specialized knowledge or dedicated operational investment (though Linkerd as a whole, of course, does require it). There’s no user-facing YAML; instead, Linkerd2-proxy is configured automatically through a handful of environment variables set at injection time and by the Linkerd control plane at runtime. We’ve kept Linkerd2-proxy’s tuning surface area to a bare minimum so that end users rarely have to touch it directly. In short: Linkerd2-proxy is designed to stay behind the scenes, to be an implementation detail, and to just work.

Tl;dr: Linkerd2-proxy is dramatically different from proxies like Envoy, NGINX, and Apache, and the word “proxy” doesn’t do it justice.

Complexity

So why did we build Linkerd2-proxy rather than using Envoy? One big reason is complexity.

Envoy is a flexible and general-purpose proxy, and that’s much of the reason for its popularity. You can use Envoy as an ingress, as an egress, as a service mesh sidecar, and in many other ways. But with this flexibility comes complexity.

As a point of comparison, as of November 2020, the Envoy repo weighs in at 172 KLOC of C++ code, with a “complexity score” (measured in terms of branches and loops) of 19k.3 By contrast, Linkerd2-proxy comes in at 30 KLOC and has a complexity score of 1.5k. In other words: the Linkerd2-proxy codebase is 5 times smaller than Envoy and, by this measure, its complexity is ten times less than Envoy’s.4

This isn’t an apples-to-apples calculation, of course. It doesn’t capture the libraries or dependencies outside the repos; the complexity score numbers are not strictly portable across languages; and so on. But it should give you a general sense of the relative size of these projects: internally, Linkerd2-proxy is orders of magnitude smaller and simpler than Envoy.

Is this complexity a moral failing in Envoy? No. Again, Envoy has a lot of complex code because it can do a lot of complex things. However, this complexity is a very difficult foundation upon which to build a project that is focused on simplicity, especially operational simplicity.5

Tl;dr: Envoy is a Swiss Army knife. Linkerd2-proxy is a needle.

Resource consumption

One thing is clear with any sidecar-based service mesh: you’re going to have a lot of proxies.

That means that the aggregate CPU and memory consumed by the data plane are a critical component of the cost of running a service mesh, especially as the application scales.

Using Linkerd2-proxy allows us to keep tight reins on Linkerd’s resource consumption. In our internal benchmarks of Linkerd and Istio using Kinvolk’s open source benchmark harness, for example, at 4,000 RPS (requests per second) of ingress traffic, we see Linkerd2-proxy instances consistently between 14mb and 15mb of memory, while Istio’s Envoy ranged between 135mb and 175mb—ten times the size. Similarly, Linkerd2-proxy’s CPU usage for the test run was consistently at 15ms (CPU milliseconds) per instance, while Istio’s Envoy ranged from 22ms to 156ms—from 50% more to 10x more.

Again, this is not an entirely fair comparison. These are internal benchmarks against one particular application and one particular configuration, and undoubtedly some of Istio’s design decisions played a big role here. But Istio is built by world-class engineers, and the point is: if Linkerd were built on Envoy, we’d have to make many of those same design decisions ourselves.

Tl;dr: In practice, in the service mesh context, Linkerd2-proxy uses a fraction of the system resources that Envoy does.

Security

The last point is perhaps the most philosophical: security. The security of the data plane is a huge concern for any service mesh. Linkerd, for example, is used in production around the world to handle extraordinarily sensitive data, from health information to personally-identifiable details to financial transactions.

We have no reason to believe that Envoy is insecure. But to the extent that it is secure (especially at 170+ KLOC of C++ code), it’s secure via the manual and expensive process of getting a lot of very smart engineers using it, examining it, filing CVEs, fixing bugs, and repeating. This is the “traditional process” for software security, and it works—at least, in the fullness of time. It is also expensive, difficult, and failure prone. C++ code is notoriously difficult to secure, even for the most experienced programmers.

More fundamentally, this is not the security model we want to rely on for Linkerd and not what we believe is the future of systems programming security. Our choice of Rust for Linkerd2-proxy was intentional: Rust’s memory safety allows us to confidently write secure code in Linkerd2-proxy in a way that minimizes our reliance on humans catching the problems. Which is not to say that Linkerd2-proxy cannot have security vulnerabilities, of course! Rather, that it will have fewer; that we will need to rely less on our own humble talents to avoid them; and that we will less often need to impose on our users to upgrade their systems with critical updates in order to remain secure.

Tl;dr: Linkerd2-proxy’s Rust foundations give us confidence in the security of Linkerd’s data plane.

Could Linkerd use Envoy?

Simplicity, resource consumption, and security were the driving factors in our decision to not adopt Envoy. However, we do believe that the choice of proxy is ultimately an implementation detail. While we’ve invested tremendous amounts in Linkerd2-proxy, we do periodically re-evaluate Envoy. I can say with clarity that if the tradeoff for our users ever tips in Envoy’s favor, we will adopt it without qualms.

Our advice to would-be service mesh adopters, though, is simple: ignore the noise. Your job is not to “use a service mesh” or “adopt Envoy” or even “use only CNCF technology”. Your job is to clearly understand the problem you’re trying to solve and then to pick the solution that solves it best. And whatever you pick, you’re going to have to live with it—so make sure you’re making a decision based on concrete requirements and well-understood engineering tradeoffs, not on fashion or trends.

FAQ

So why do so many service meshes use Envoy?

Because writing your own modern, scalable, high-performance network (micro-)proxy is hard. It’s really hard. Building out Linkerd2-proxy and the Rust networking libraries that make it possible has been a tremendous effort from a many people for the past several years. Unless your project has both the technical prowess and the desire to tackle this challenge, it’s much easier to just use Envoy.

But isn’t Envoy a “standard” for service meshes?

No.6 A standard is something that is necessary for interoperability. The standard that matters for service meshes is TCP, or HTTP, or things like SMI that allow tools to be built on top of the service mesh. (E.g. this excellent example of Argo driving Linkerd via SMI for canary rollouts.)

Envoy being a popular choice of service mesh data plane proxy is not a standard, it’s simply a commonality. What would it mean for Envoy to be a “service mesh standard”? That we could keep our data plane in place, and swap out the control plane? That we can have different control planes operate the same data plane? These are far-fetched use cases at best.

But what if we have a requirement to use Envoy?

I would argue that’s not a real requirement. Your job is not to adopt a particular piece of technology. Your job is to solve a problem.

And if your problem is “we need to build a reliable, secure, and observable Kubernetes platform without paying an insane complexity cost” then I highly suggest you consider taking a look at Linkerd.

Who uses Linkerd2-proxy in production today?

Everyone who uses Linkerd uses Linkerd2-proxy. That means that you can find Linkerd2-proxy powering the critical production architecture of companies like Nordstrom, Microsoft, H-E-B, Chase, Clover Health, HP, any many more.

Could other service mesh projects use Linkerd2-proxy?

Not really. But anyone who is interested in building a high performance ultralight network proxy could certainly make use of the underlying Rust network libraries that power Linkerd.

Sounds amazing! How can I get started with Linkerd?

I never thought you’d ask. You can install Linkerd in about 5 minutes, including mutual TLS, with zero configuration required. Start with our getting started guide.

Linkerd is for everyone

Linkerd is a community project and is hosted by 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 Paul Felberbauer on Unsplash).

Footnotes


  1. And while Envoy is great, the recent crop of low-calorie vendors who attach themselves to Envoy, promote Envoy as the one true path to the service mesh, take swipes at Linkerd because it doesn’t “support” Envoy (as if that were somehow a requirement), etc, are most decidedly not great.

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  2. After all, Linkerd invented the term “service mesh”, and look where it got us.

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  3. As measured with the excellent scc tool by running scc source include inside the Envoy repo circa November 2020.

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  4. Similarly measured by running scc linkerd linkerd2-proxy inside the linkerd2-proxy repo circa November 2020.

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  5. Code complexity does not necessarily translate to user-facing complexity, of course. One might ask: can an Envoy-based service mesh successfully wrap Envoy’s complexity, so that end users can avoid the necessity of developing operational expertise? Answering this question fully is outside the scope of this article (which is about Linkerd, after all!) but a perusal of the 300+ open issues in Istio that refer to Envoy suggests that, at a minimum, it is not trivial.

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  6. We hear this particular argument put forth by vendors of Envoy-based service meshes, usually because they don’t have a good technical argument to fall back to.

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