tarpc/README.md

## tarpc: Tim & Adam's RPC lib
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*Disclaimer*: This is not an official Google product.

tarpc is an RPC framework for rust with a focus on ease of use. Defining a
service can be done in just a few lines of code, and most of the boilerplate of
writing a server is taken care of for you.

[Documentation](https://docs.rs/tarpc)

## What is an RPC framework?
"RPC" stands for "Remote Procedure Call," a function call where the work of
producing the return value is being done somewhere else. When an rpc function is
invoked, behind the scenes the function contacts some other process somewhere
and asks them to evaluate the function instead. The original function then
returns the value produced by the other process.

RPC frameworks are a fundamental building block of most microservices-oriented
architectures. Two well-known ones are [gRPC](http://www.grpc.io) and
[Cap'n Proto](https://capnproto.org/).

tarpc differentiates itself from other RPC frameworks by defining the schema in code,
rather than in a separate language such as .proto. This means there's no separate compilation
process, and no cognitive context switching between different languages. Additionally, it
works with the community-backed library serde: any serde-serializable type can be used as
arguments to tarpc fns.

## Usage
**NB**: *this example is for master. Are you looking for other
[versions](https://docs.rs/tarpc)?*

Add to your `Cargo.toml` dependencies:

```toml
tarpc = { git = "https://github.com/google/tarpc" }
tarpc-plugins = { git = "https://github.com/google/tarpc" }
```

## Example
```rust
// required by `FutureClient` (not used directly in this example)
#![feature(conservative_impl_trait, plugin)]
#![plugin(tarpc_plugins)]

extern crate futures;
#[macro_use]
extern crate tarpc;

use tarpc::{client, server};
use tarpc::client::sync::Connect;
use tarpc::util::Never;

service! {
    rpc hello(name: String) -> String;
}

#[derive(Clone)]
struct HelloServer;

impl SyncService for HelloServer {
    fn hello(&self, name: String) -> Result<String, Never> {
        Ok(format!("Hello, {}!", name))
    }
}

fn main() {
    let addr = "localhost:10000";
    HelloServer.listen(addr, server::Options::default()).unwrap();
    let client = SyncClient::connect(addr, client::Options::default()).unwrap();
    println!("{}", client.hello("Mom".to_string()).unwrap());
}
```

The `service!` macro expands to a collection of items that form an
rpc service. In the above example, the macro is called within the
`hello_service` module. This module will contain `SyncClient`, `AsyncClient`,
and `FutureClient` types, and `SyncService` and `AsyncService` traits.  There is
also a `ServiceExt` trait that provides starter `fn`s for services, with an
umbrella impl for all services.  These generated types make it easy and
ergonomic to write servers without dealing with sockets or serialization
directly. Simply implement one of the generated traits, and you're off to the
races! See the `tarpc_examples` package for more examples.

## Example: Futures

Here's the same service, implemented using futures.

```rust
#![feature(conservative_impl_trait, plugin)]
#![plugin(tarpc_plugins)]

extern crate futures;
#[macro_use]
extern crate tarpc;
extern crate tokio_core;

use futures::Future;
use tarpc::{client, server};
use tarpc::client::future::Connect;
use tarpc::util::{FirstSocketAddr, Never};
use tokio_core::reactor;

service! {
    rpc hello(name: String) -> String;
}

#[derive(Clone)]
struct HelloServer;

impl FutureService for HelloServer {
    type HelloFut = futures::Finished<String, Never>;

    fn hello(&self, name: String) -> Self::HelloFut {
        futures::finished(format!("Hello, {}!", name))
    }
}

fn main() {
    let addr = "localhost:10000".first_socket_addr();
    let mut core = reactor::Core::new().unwrap();
    HelloServer.listen(addr, server::Options::default().handle(core.handle())).wait().unwrap();
    let options = client::Options::default().handle(core.handle());
    core.run(FutureClient::connect(addr, options)
            .map_err(tarpc::Error::from)
            .and_then(|client| client.hello("Mom".to_string()))
            .map(|resp| println!("{}", resp)))
        .unwrap();
}
```

## Tips

### Sync vs Futures

A single `service!` invocation generates code for both synchronous and future-based applications.
It's up to the user whether they want to implement the sync API or the futures API. The sync API has
the simplest programming model, at the cost of some overhead - each RPC is handled in its own
thread. The futures API is based on tokio and can run on any tokio-compatible executor. This mean a
service that implements the futures API for a tarpc service can run on a single thread, avoiding
context switches and the memory overhead of having a thread per RPC.

### Errors

All generated tarpc RPC methods return either `tarpc::Result<T, E>` or something like `Future<T,
E>`. The error type defaults to `tarpc::util::Never` (a wrapper for `!` which implements
`std::error::Error`) if no error type is explicitly specified in the `service!` macro invocation. An
error type can be specified like so:

```rust
use tarpc::util::Message;

service! {
    rpc hello(name: String) -> String | Message
}
```

`tarpc::util::Message` is just a wrapper around string that implements `std::error::Error` provided
for service implementations that don't require complex error handling. The pipe is used as syntax
for specifying the error type in a way that's agnostic of whether the service implementation is
synchronous or future-based. Note that in the simpler examples in the readme, no pipe is used, and
the macro automatically chooses `tarpc::util::Never` as the error type.

The above declaration would produce the following synchronous service trait:

```rust
impl SyncService for HelloServer {
    fn hello(&self, name: String) -> Result<String, Message> {
        Ok(format!("Hello, {}!", name))
    }
}
```

and the following future-based trait:

```rust
impl FutureService for HelloServer {
    type HelloFut = futures::Finished<String, Message>;

    fn hello(&mut self, name: String) -> Self::HelloFut {
        futures::finished(format!("Hello, {}!", name))
    }
}
```

## Documentation

Use `cargo doc` as you normally would to see the documentation created for all
items expanded by a `service!` invocation.

## Additional Features

- Concurrent requests from a single client.
- Compatible with tokio services.
- Run any number of clients and services on a single event loop.
- Any type that `impl`s `serde`'s `Serialize` and `Deserialize` can be used in
  rpc signatures.
- Attributes can be specified on rpc methods. These will be included on both the
  services' trait methods as well as on the clients' stub methods.

## Gaps/Potential Improvements (not necessarily actively being worked on)

- Configurable server rate limiting.
- Automatic client retries with exponential backoff when server is busy.
- Load balancing
- Service discovery
- Automatically reconnect on the client side when the connection cuts out.
- Support generic serialization protocols.

## Contributing

To contribute to tarpc, please see [CONTRIBUTING](CONTRIBUTING.md).

## License

tarpc is distributed under the terms of the MIT license.

See [LICENSE](LICENSE) for details.