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Trillium sessions

Trillium sessions is built on top of async-session.

Sessions allows trillium to securely attach data to a browser session allowing for retrieval and modification of this data within trillium on subsequent visits. Session data is generally only retained for the duration of a browser session.

Trillium’s session implementation provides guest sessions by default, meaning that all web requests to a session-enabled trillium host will have a cookie attached, whether or not there is anything stored in that client’s session yet.


Although this crate provides two bundled session stores, it is highly recommended that trillium applications use an external-datastore-backed session storage. For a list of currently available session stores, see the documentation for async-session.


Although each session store may have different security implications, the general approach of trillium’s session system is as follows: On each request, trillium checks the cookie configurable as cookie_name on the handler.

A cryptographically random cookie value is generated. A cookie is set on the outbound response and signed with an HKDF key derived from the secret provided on creation of the SessionHandler. The configurable session store uses a SHA256 digest of the cookie value and stores the session along with a potential expiry.

The hkdf derived signing key is used to verify the cookie value’s signature. If it verifies, it is then passed to the session store to retrieve a Session. For most session stores, this will involve taking a SHA256 digest of the cookie value and retrieving a serialized Session from an external datastore based on that digest.


In addition to setting an expiry on the session cookie, trillium sessions include the same expiry in their serialization format. If an adversary were able to tamper with the expiry of a cookie, trillium sessions would still check the expiry on the contained session before using it

If anything goes wrong with the above process

If there are any failures in the above session retrieval process, a new empty session is generated for the request, which proceeds through the application as normal.

Stale/expired session cleanup

Any session store other than the cookie store will accumulate stale sessions. Although the trillium session handler ensures that they will not be used as valid sessions, For most session stores, it is the trillium application’s responsibility to call cleanup on the session store if it requires it

use trillium::Conn;
use trillium_cookies::{CookiesHandler, cookie::Cookie};
use trillium_sessions::{MemoryStore, SessionConnExt, SessionHandler};
let session_secret = std::env::var("TRILLIUM_SESSION_SECRET").unwrap();

let handler = (
    SessionHandler::new(MemoryStore::new(), session_secret.as_bytes()),
    |conn: Conn| async move {
        let count: usize = conn.session().get("count").unwrap_or_default();
        conn.with_session("count", count + 1)
            .ok(format!("count: {}", count))

use trillium_testing::prelude::*;
let mut conn = get("/").on(&handler);
assert_ok!(&mut conn, "count: 0");

let set_cookie_header = conn.headers_mut().get_str("set-cookie").unwrap();
let cookie = Cookie::parse_encoded(set_cookie_header).unwrap();

let make_request = || get("/")
    .with_request_header("cookie", format!("{}={}", cookie.name(), cookie.value()))

assert_ok!(make_request(), "count: 1");
assert_ok!(make_request(), "count: 2");
assert_ok!(make_request(), "count: 3");
assert_ok!(make_request(), "count: 4");