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# service-binding
[](https://github.com/wiktor-k/service-binding/actions/workflows/rust.yml)
[](https://crates.io/crates/service-binding)
Provides a way for servers and clients to describe their service bindings and client endpoints in a structured format.
This crate automates parsing and binding to TCP sockets, Unix sockets and [Windows Named Pipes][WNP].
[WNP]: https://learn.microsoft.com/en-us/windows/win32/ipc/named-pipes
By design this crate is very lean and mostly relies on what is in `std` (with an exception of macOS launchd service binding).
The URI scheme bindings have been heavily inspired by how [Docker Engine] specifies them.
[Docker Engine]: https://docs.docker.com/desktop/faqs/general/#how-do-i-connect-to-the-remote-docker-engine-api
```rust
use service_binding::{Binding, Listener};
let host = "tcp://127.0.0.1:8080"; // or "unix:///tmp/socket"
// parse the URI into a Binding
let binding: Binding = host.parse().unwrap();
// convert the binding into a Listener
match binding.try_into().unwrap() {
#[cfg(unix)]
Listener::Unix(listener) => {
// bind to a unix domain socket
},
Listener::Tcp(listener) => {
// bind to a TCP socket
}
Listener::NamedPipe(pipe) => {
// bind to a Windows Named Pipe
}
}
```
## Supported schemes
| URI format | Example | Description | Binding | Listener / Stream |
| ------------ | ---------------- | -------------------------- | ---- | --- |
| `tcp://ip:port` | `tcp://127.0.0.1:8080` | TCP IP address | `Sockets` | `Tcp` |
| `tcp://address:port` |`tcp://localhost:8080` | Hostname with address resolution [^1] | `Sockets` | `Tcp` |
| `unix://path` | `unix:///run/user/1000/test.sock` | Unix domain sockets [^2] | `FilePath` | `Unix` |
| `fd://` | `fd://` | systemd first socket activation [^3][^4] | `FileDescriptor` | `Unix` |
| `fd://<number>` | `fd://3` | exact number file descriptor | `FileDescriptor` | `Unix` |
| `fd://<socket-name>` | `fd://http` | socket activation by name [^4] | `FileDescriptor` | `Unix` |
| `npipe://<name>` | `npipe://agent` | Windows Named Pipe [^5] | `NamedPipe` | `NamedPipe` |
[^1]: binds to the first address that succeeds (see [`TcpListener::bind`](https://doc.rust-lang.org/std/net/struct.TcpListener.html#method.bind))
[^2]: not available on Windows through the `std` right now (see [#271] and [#56533])
[^3]: equivalent of `fd://3` but fails if more sockets have been passed
[^4]: *listener only*
[^5]: translates to `\\.\pipe\test`
[#271]: https://github.com/rust-lang/libs-team/issues/271
[#56533]: https://github.com/rust-lang/rust/issues/56533
## Example
The following example uses `clap` and `actix-web` and makes it possible to run the server using any combination of Unix domain sockets (including systemd socket activation) and regular TCP socket bound to a TCP port:
```rust,no_run
use actix_web::{web, App, HttpServer, Responder};
use clap::Parser;
use service_binding::{Binding, Listener};
#[derive(Parser, Debug)]
struct Args {
#[clap(
env = "HOST",
short = 'H',
long,
default_value = "tcp://127.0.0.1:8080"
)]
host: Binding,
}
async fn greet() -> impl Responder {
"Hello!"
}
#[actix_web::main]
async fn main() -> std::io::Result<()> {
let server = HttpServer::new(move || {
App::new().route("/", web::get().to(greet))
});
match Args::parse().host.try_into()? {
#[cfg(unix)]
Listener::Unix(listener) => server.listen_uds(listener),
Listener::Tcp(listener) => server.listen(listener),
_ => Err(std::io::Error::other("Unsupported listener type")),
}?.run().await
}
```
## systemd Socket Activation
This crate also supports systemd's [Socket Activation][]. If the argument to be parsed is `fd://` the `Listener` object returned will be a `Unix` variant containing the listener provided by systemd.
[Socket Activation]: https://0pointer.de/blog/projects/socket-activation.html
For example the following file defines a socket unit: `~/.config/systemd/user/app.socket`:
```ini
[Socket]
ListenStream=%t/app.sock
FileDescriptorName=service-name
[Install]
WantedBy=sockets.target
```
When enabled it will create a new socket file in `$XDG_RUNTIME_DIR` directory. When this socket is connected to systemd will start the service; `fd://` reads the correct systemd environment variable and returns the Unix domain socket.
The service unit file `~/.config/systemd/user/app.service`:
```ini
[Service]
ExecStart=/usr/bin/app -H fd://
```
Since the socket is named (`FileDescriptorName=service-name`) it can also be selected using its explicit name: `fd://service-name`.
## launchd Socket Activation
On macOS [launchd socket activation][LSA] is also available although the socket needs to be explicitly named through the `fd://socket-name` syntax.
[LSA]: https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPSystemStartup/Chapters/CreatingLaunchdJobs.html
The corresponding `plist` file (which can be placed in `~/Library/LaunchAgents` and loaded via `launchctl load ~/Library/LaunchAgents/service.plist`):
```xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>EnvironmentVariables</key>
<dict>
<key>RUST_LOG</key>
<string>debug</string>
</dict>
<key>KeepAlive</key>
<true/>
<key>Label</key>
<string>com.example.service</string>
<key>OnDemand</key>
<true/>
<key>ProgramArguments</key>
<array>
<string>/path/to/service</string>
<string>-H</string>
<string>fd://socket-name</string> <!-- activate socket by name -->
</array>
<key>RunAtLoad</key>
<true/>
<key>Sockets</key>
<dict>
<key>socket-name</key> <!-- the socket name here -->
<dict>
<key>SockPathName</key>
<string>/path/to/socket</string>
<key>SockFamily</key>
<string>Unix</string>
</dict>
</dict>
<key>StandardErrorPath</key>
<string>/Users/test/Library/Logs/service/stderr.log</string>
<key>StandardOutPath</key>
<string>/Users/test/Library/Logs/service/stdout.log</string>
</dict>
</plist>
```
## License
This project is licensed under either of:
- [Apache License, Version 2.0](https://www.apache.org/licenses/LICENSE-2.0),
- [MIT license](https://opensource.org/licenses/MIT).
at your option.
### Contribution
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.
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