Conn

Before we explore the concept of a handler further, let's take a look at Conn. As mentioned above, Conn represents both the request and response, as well as any data your application associates with that request-response cycle.

🧑‍🎓 Advanced aside: Although the naming of Conn is directly borrowed from Elixir's plug and therefore also Phoenix, it does in fact also own (in the rust sense) the singular TcpStream that represents the connection with the http client, and dropping a Conn will also disconnect the client as a result.

The rustdocs for Conn contain the full details for all of the things you can do with a conn.

Returning Conn

In general, because you'll be returning Conn from handlers, it supports a chainable (fluent) interface for setting properties, like:

conn.with_status(202)
    .with_header(("content-type", "application/something-custom"))
    .with_body("this is my custom body")

Accessing http request properties

Conn also contains read-only properties like request headers, request path, and request method, each of which have getter associated functions.

Default Response

The default response for a Conn is a 404 with no response body, so it is always valid to return the Conn from a handler unmodified (|conn: Conn| async move { conn } is the simplest valid handler).

State

In addition to holding the request properties and accumulating the response your application is going to send, a Conn also serves as a data structure for any information your application needs to associate with that request. This is especially valuable for communicating between handlers, and most core handlers are implemented using conn state. One important caveat to is that each Conn can only contain exactly one of each type, so it is highly recommended that you only store types that you define in state.

🌊 Comparison with Tide: Tide has three different types of state: Server state, request state, and response state. In Trillium, server state is achieved using the trillium::State handler, which holds any type that is Clone and puts a clone of it into the state of each Conn that passes through the handler.

Extending Conn

It is a very common pattern in trillium for libraries to extend Conn in order to provide additional functionality1. The Conn interface does not provide support for sessions, cookies, route params, or many other building blocks that other frameworks build into the core types. Instead, to use sessions as an example, trillium_sessions provides a SessionConnExt trait which provides associated functions for Conn that offer session support. In general, handlers that put data into conn state also will provide convenience functions for accessing that state, and will export a [Something]ConnExt trait.

1 🧑‍🎓 see library_patterns for an example of authoring one of these