Yaws is an HTTP/1.1 web server written in Erlang, designed especially for dynamic content. It can run as a standalone daemon or inside another Erlang application. Its distinctive page format, .yaws, combines HTML with Erlang code that generates part of the response.
What is Yaws?
The Yaws project describes it as “a webserver for dynamic content written in Erlang.” It serves static HTML and provides a built-in model for dynamic pages, alongside documented patterns for WebSockets, streaming, uploads, sessions, and other application needs. See the Yaws project README and the official documentation site.
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Yaws can be used in two ways: as a conventional standalone web-server daemon, or embedded within another Erlang application. The choice depends on whether you want to operate it as a separate server or integrate its operation with an existing Erlang system.
How does Yaws handle dynamic content?
A .yaws page combines ordinary HTML with Erlang chunks. Each Erlang chunk must provide an out/1 function. Yaws invokes that function and inserts its result into the response; the result can be regular HTML or structured EHTML. According to the Yaws manual, compiled chunks are cached so later requests can call the compiled modules.
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This puts application logic in Erlang code within the page format, rather than requiring every dynamic response to be handled by an external CGI process. It is a particular fit for teams already building and operating Erlang applications.
What can Yaws do beyond serving pages?
The official documentation includes examples for several application patterns:
- Real-time communication: WebSockets and server-sent events.
- Streaming: sending streaming data to clients.
- Application integration: appmods and AJAX/JSON RPC.
- Request and session handling: file uploads and cookie sessions.
- Page composition: server-side includes and embedding.
These examples show what the documentation covers; they do not establish that a particular configuration is production-ready for every workload. Evaluate deployment, security, and operational requirements for your application.
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What Erlang version and build tools does Yaws require?
The current project README specifies Erlang/OTP 25.0 or higher to compile Yaws. It documents both autotools and rebar3 build routes, with build dependencies that include Erlang components, build tooling, and PAM development headers. Generating optional documentation requires additional TeX and Ghostscript dependencies. Check the README for the current dependency list and instructions for your environment.
Build from a source checkout with autotools
The README notes that a GitHub source checkout does not include a ./configure script. Run autoreconf -fi first, then configure an installation prefix and build:
autoreconf -fi
./configure --prefix=/path/to/install
makeReplace /path/to/install with the prefix appropriate for your system. These are the project’s documented build steps, not a guarantee that every operating system has the same prerequisites or packaging.
Build with rebar3
The README also documents compiling with rebar3 and launching an interactive shell:
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rebar3 shellIts test-build example starts an HTTP service on 0.0.0.0:8000. Because that address listens on all network interfaces, consider where the service is reachable before using a test configuration on a networked machine.
How do you start Yaws?
For an installed build, the README recommends invoking the Yaws script interactively. It also documents daemon operation with --daemon --heart, where the heart option provides restart behavior. The README says the default configuration creates HTTP and HTTPS servers; actual availability depends on your configuration, certificates, and deployment environment.
yaws
# Daemon operation with restart behavior
yaws --daemon --heartThese commands reflect the project README’s instructions. Review the current configuration and deployment requirements before exposing a server publicly.
Is Yaws maintained, and what should you check before adopting it?
The project repository is the appropriate place to check current releases and code changes, but a repository alone does not establish a support commitment or guarantee a particular maintenance level. Before adopting Yaws, review recent release and repository activity, security guidance, and the support channels available to your team. The documentation site also provides usage and feature information.
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The Yaws project presents performance as an advantage and attributes it to handling each client with a lightweight Erlang process and Erlang’s concurrency model. That is the project’s explanation, not a comparative benchmark. The reviewed materials do not provide a measured throughput figure, latency distribution, test hardware, or cross-server workload comparison, so they do not establish that Yaws is faster than another server.
For a useful comparison, benchmark the versions and configurations you would actually deploy, using the same hardware, workload, and measurement method. Include the application behavior and operational constraints that matter to your system, not just static requests.
How does Yaws compare with Erlang’s Inets HTTP server?
Inets is part of the Erlang/OTP ecosystem. Its HTTP server documentation describes basic web-server functionality and dynamic-content facilities including CGI and ESI. Yaws, by contrast, offers its own .yaws page model and documents a broader set of application patterns, along with standalone and embedded operation. The Erlang/OTP 28 Inets documentation is a useful reference for that comparison.
| Decision point | Yaws | Inets HTTP server |
|---|---|---|
| Dynamic-content approach | .yaws pages with Erlang out/1 chunks |
Includes CGI and ESI facilities |
| Project positioning | Dedicated Erlang web-server project for dynamic content | Documentation describes basic web-server functionality |
| Additional patterns covered here | Official examples include WebSockets, streaming, server-sent events, uploads, sessions, and embedding | Not stated in the cited Inets comparison documentation |
Neither description alone determines the better choice. Compare the integration model, required features, build and runtime constraints, configuration needs, and the maintenance expectations for your application.
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