Check access from your backend

Use the Go 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 Go library with a secret key first.

Check a perk

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

func exportHandler(client *voidhash.Client) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        hasPremium, err := client.Persons.Entitlements.HasActivePerk(r.Context(), voidhash.HasActivePerkParams{
            DistinctID: userIDFrom(r),
            PerkSlug:   "premium",
        })
        if err != nil {
            http.Error(w, "upstream error", http.StatusInternalServerError)
            return
        }

        if !hasPremium {
            w.WriteHeader(http.StatusPaymentRequired)
            json.NewEncoder(w).Encode(map[string]string{"error": "premium_required"})
            return
        }

        buildExport(w, r, userIDFrom(r))
    }
}

DistinctID is the same identifier your app passed to identify(). Set exactly one of PerkID or PerkSlug. If you set both or neither, the call returns an error before any request is made.

Read the grants yourself

HasActivePerk 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.

grants, err := client.Persons.Entitlements.GrantsByDistinctID(ctx, "user_123")

Each grant has these fields.

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

To look the person up yourself, use Persons.GetByDistinctID:

person, err := client.Persons.GetByDistinctID(ctx, "user_123")

Person is intentionally thin. It carries Id, DistinctId, Email, Name and IsIdentified, because it identifies the customer. It does not describe what they paid for.

Handle errors deliberately

When the API rejects a call, the error is a *voidhash.APIError. It carries the HTTP status and the API's error body: branch on Type and Code, never on Message. Param names the field at fault, and RequestID is the response's Request-Id header, which identifies the request when you contact support. Transport failures (DNS, TLS, timeouts) return a plain error that wraps *url.Error instead.

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

grants, err := client.Persons.Entitlements.GrantsByDistinctID(ctx, "user_123")
if err != nil {
    var apiErr *voidhash.APIError
    if errors.As(err, &apiErr) {
        switch {
        case apiErr.Code == "person_not_found":
            // Never identified from a client: nothing was ever bought.
            return false, nil
        case apiErr.Type == "unauthenticated", apiErr.Type == "permission_denied":
            // Our key is wrong. Our bug, not the customer's. Do not lock them out.
            return false, fmt.Errorf("voidhash secret key is invalid or lacks access: %w", apiErr)
        default:
            return false, err
        }
    }

    // Transport failure: unknown, not "no access".
    // Serve the last known good value or fail the request.
    return false, err
}

hasPremium := false
for _, grant := range grants {
    if grant.PerkId == premiumPerkID {
        hasPremium = true
    }
}

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).

Live and development data

Grants are scoped to an environment. The server picks the scope from the x-environment request header.

Sandbox purchases count as production so that TestFlight, Google Play testers, and App Review get access in your published app. A project can turn that off. Then no key or x-environment value returns store sandbox grants.

The library never sends this header for you, so a plain client always reads production grants. To read development purchases, create a second client that sets the header.

developmentClient, err := voidhash.New(
    os.Getenv("VOIDHASH_SECRET_KEY"),
    voidhash.WithHeader("x-environment", "development"),
)

Secret keys are not scoped to an environment. Which key you use does not change the answer. Only the header does.

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 single-flight wrapper (golang.org/x/sync/singleflight) in front of the cache collapses concurrent checks for the same person into one request.

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