Check access from your backend

Use the Rust library to decide, on your server, whether a customer may use a feature. The check asks Voidhash for the customer's active perk grants, so nothing a customer reaches through your API depends on a client-side decision. Set up the Rust library with a secret key first.

Check a perk

has_active_perk answers one question: does this customer hold an active grant for this perk? The handler below gates an export route on the premium perk.

async fn export(
    State(voidhash): State<Arc<VoidhashClient>>,
    Json(claims): Json<Claims>,
) -> Result<Json<Value>, ApiError> {
    let has_premium = voidhash
        .persons()
        .has_active_perk(&claims.sub, None, Some("premium"))
        .await?;

    if !has_premium {
        return Err(ApiError::PaymentRequired("premium_required"));
    }

    let export = build_export(&claims.sub).await?;
    Ok(Json(export))
}

distinct_id is the same identifier your app passed to identify(). Pass exactly one of the perk's id or slug. Passing both or neither returns voidhash::Error::Request before any request is made.

Read the grants yourself

has_active_perk only returns a boolean. When you need to render an account page, show an expiry date, or branch on where access came from, read the grants directly. They are every perk the customer can use now.

let grants = voidhash
    .persons()
    .grants_by_distinct_id("user_123")
    .await?;

Each grant has these fields.

Unlike has_active_perk, this call treats an unknown distinct_id as an error. That leaves the decision to you: whether "never seen" and "seen, but bought nothing" should mean the same thing.

The same lookup is available as two separate calls, which helps when you already hold the person's id.

let person = voidhash.persons().get_by_distinct_id("user_123").await?;

let entitlements = voidhash.persons().entitlements(&person.id).await?;

Person is intentionally thin. It carries id, distinct_id, email, name and is_identified, because it identifies the customer. It does not describe what they paid for.

Handle errors deliberately

Every failure is a voidhash::Error, an enum with three variants.

• Api { status, type_, code, message, param, request_id }: the API rejected the call with its error body. Match on type_ and code, never on message. param names the field at fault, and request_id identifies the request when you contact support.
• Transport(reqwest::Error): a DNS, TLS, or timeout failure. No response body exists.
• Request(String): anything else, such as a client setup that is invalid or cannot choose a project.

The example below treats an unknown person as "no access", treats a bad key as your own bug, propagates every other API error, and treats a transport failure as an unknown answer.

match voidhash.persons().grants_by_distinct_id("user_123").await {
    Ok(grants) => Ok(grants.iter().any(|grant| grant.perk_id == premium_perk_id)),
    Err(voidhash::Error::Api { code, .. }) if code == "person_not_found" =>
        // Never identified from a client: nothing was ever bought.
        Ok(false),
    Err(voidhash::Error::Api { type_, .. })
        if type_ == "unauthenticated" || type_ == "permission_denied" =>
        // Our key is wrong. Our bug, not the customer's. Do not lock them out.
        Err(anyhow!("Voidhash secret key is invalid or lacks access.")),
    Err(e @ voidhash::Error::Transport(_)) =>
        // Unknown, not "no access": serve the last known good value or fail the request.
        Err(e.into()),
    Err(e) => Err(e.into()),
}

The types you are most likely to see are unauthenticated (401), permission_denied (403), not_found (404, such as person_not_found) and invalid_request (400, with param naming the field).

Production and test data

The secret key decides which grants you get. Every key belongs to production or test for its whole life.

Store sandbox purchases count for production keys so that TestFlight, Google Play testers, and App Review get access in your published app. A project can turn that off. Then a production key returns no store sandbox grants. Test purchases never unlock production.

Use a test secret key in staging and tests, and a production key in production. Test grants include the simulated purchases of debug builds in development mode, which use the test publishable key.

Caching and failures

The library does not cache, retry, or de-duplicate. Every call is a live HTTP round trip.

If you check access on every request, cache the result yourself for a short window and refresh it in the background. 60 seconds is a reasonable starting point. A moka or quick-cache instance keyed by distinct_id works well. Keep the TTL short enough that a revoked grant expires from the cache quickly.

When a call fails with a transport error or a 5xx, treat the answer as unknown, not as no access. Serve the last known good value, or fail the request. Revoking a paying customer's access because of a network blip is worse than serving a slightly stale cache.

To keep the cache warm without polling, subscribe to webhooks and invalidate the cached entry when a subscription or purchase event arrives for that person.

Next steps

• Receive webhooks
• Persons in the API reference