The Rails way, on Cloudflare's free plan.

/// Open questions first, most voted first, then answered ones.
pub async fn for_event(ctx: &Ctx, event_id: i64) -> Result<Vec<Question>> {
    query()
        .eq("event_id", event_id)
        .order_asc("answered")
        .order_desc("votes")
        .limit(200)
        .all(&ctx.db()?)
        .await
}

/// One UPDATE, so votes arriving at the same moment all count.
pub async fn vote(ctx: &Ctx, id: i64) -> Result<Option<Question>> {
    let sql = "UPDATE questions SET votes = votes + 1 WHERE id = ?1 RETURNING *";
    ctx.db()?.first(sql, params![id]).await
}

/// One vote per question and browser; voting again changes nothing.
async fn vote(State(ctx): State<Ctx>, session: Session, Htmx(is_htmx): Htmx, Path(id): Path<i64>) -> Result<Response> {
    let record = question::find(&ctx, id).await?.or_404()?;
    let event = record.event(&ctx).await?.or_404()?;
    let mut voted: Vec<i64> = session.get("voted")?.unwrap_or_default();
    if !voted.contains(&id) {
        question::vote(&ctx, id).await?;
        voted.push(id);
        session.insert("voted", &voted)?;
        broadcast(&ctx, &event).await?;
    }
    Ok(done(is_htmx, &event)) // htmx: 204, the broadcast updates the page
}

/// The whole list, rendered once per audience, swapped into every open room.
async fn broadcast(ctx: &Ctx, event: &Event) -> Result<()> {
    let questions = question::for_event(ctx, event.id).await?;
    let audience = render(&ListView { questions: &questions, host: false })?.0;
    let host = render(&ListView { questions: &questions, host: true })?.0;
    realtime::broadcast(ctx, &channel(event), &realtime::update("questions", &audience)).await.ok();
    realtime::broadcast(ctx, &host_channel(event), &realtime::update("questions", &host)).await.ok();
    Ok(())
}

<!-- templates/events/show.html: htmx connects and swaps, no custom JavaScript -->
<div hx-ext="ws" ws-connect="/realtime/event:{{ event.public_id }}">

#[test]
#[ignore = "request test: run with `ocre test --e2e`"]
fn each_browser_votes_once_per_question() {
    let (_, event) = host_with_event();
    let mut first = Client::new().htmx();     // its own cookies, like a browser
    let id = ask(&mut first, &event, "Can we get the slides?");
    first.post(&format!("/questions/{id}/vote"), &()).assert_status(204);
    first.post(&format!("/questions/{id}/vote"), &()).assert_status(204);
    assert_eq!(votes(id), 1);
    Client::new().htmx().post(&format!("/questions/{id}/vote"), &()).assert_status(204);
    assert_eq!(votes(id), 2);
}