Extend the Servers

The Utopia servers are built to be customized. Two interfaces are the seams where your own logic plugs in: IssuerAssistant on the issuance side, and VerifierAssistant on the verification side. Both keep your business rules separate from the protocol machinery Multipaz handles for you.

IssuerAssistant — react to issuance

Every issuer registers an IssuerAssistant. Its onIssuance hook fires after a credential is minted and delivered to the wallet — the place to record an audit event or notify a fraud pipeline:

interface IssuerAssistant {
    suspend fun onIssuance(
        systemOfRecordData: DataItem,
        credentialId: CredentialId,
        format: CredentialFormat,
    )

    object NoOp : IssuerAssistant { /* does nothing */ }
}

The DMV's implementation just logs, so the wiring is observable:

class DmvIssuerAssistant : IssuerAssistant {
    override suspend fun onIssuance(
        systemOfRecordData: DataItem,
        credentialId: CredentialId,
        format: CredentialFormat,
    ) {
        Logger.i(TAG, "Issued credential id=$credentialId format=${format.formatId}")
    }
}

Try it: open organizations/dmv/backend/.../DmvIssuerAssistant.kt, add a line that writes the credentialId and a timestamp to a file or external endpoint, rebuild, and issue an mDL again. You've added an audit trail without touching any OpenID4VCI code.

note

Because issuance has already completed when onIssuance fires, throwing here does not revoke the credential — failures are logged, not surfaced to the wallet. Treat it as a notification hook, not a gate.

VerifierAssistant — decide what a presentation means

You met this in the Brewery. A VerifierAssistant has two hooks:

• processRequest — adjust the outgoing request before it's sent (return null to leave it unchanged).
• processResponse — run your business logic after the presentation is cryptographically verified, and return the result.

class BreweryVerifierAssistant : VerifierAssistant {
    override suspend fun processRequest(request: JsonObject): JsonObject? = null

    override suspend fun processResponse(presentment: VerifierPresentment): JsonObject {
        // find the ID credential, check age, validate payment, commit the transaction,
        // and return { approved, holderName, issuerName } (or an error)
    }
}

This is the right home for policy: what counts as old enough, which credential types you accept, when to settle payment. Multipaz guarantees the response is authentic; your assistant decides what to do about it.

Try it: see the decline path. The reliable way to watch a rejection is to temporarily force the age check to fail: make checkAge return false, rebuild, and run a checkout. The Brewery shows declined and never settles the payment. Revert when you're done.

Where to go next

• Build an organization. Each org under organizations/ is a small Ktor server registered in settings.gradle.kts and wired into the deployment module. Copy the Brewery as a template for a new verifier, or the DMV for a new issuer.
• Deploy the bundle. The same image runs in the cloud — follow Deploying to Google Cloud Run to put the whole Utopia Universe behind a public HTTPS URL.

You've now run a complete digital-identity economy locally and seen where to extend it — issuers, verifiers, a records registry, and a payment processor, all cooperating through Multipaz.