Skip to content

Observable emission guards

Authorization runs once, when a subscription opens. A live query can outlive that decision: the caller’s access may lapse, or some emissions should never leave the server. An emission guard checks every rendered result, including the first snapshot, before it is delivered.

Write and register a guard

import { field } from '@cratis/fundamentals';
import {
    ArcApplication, ObservableEmissionDecision, query, readModel, serviceToken,
    type ObservableEmissionContext, type ObservableEmissionGuard
} from '@cratis/arc.core';
import { BehaviorSubject } from 'rxjs';

const prices = new BehaviorSubject<Price[]>([]);

@readModel()
export class Price {
    @field(String) symbol!: string;
    @field(Number) amount!: number;

    @query({ observable: true })
    static livePrices(): BehaviorSubject<Price[]> { return prices; }
}

export const tradingHours = serviceToken<ObservableEmissionGuard>('tradingHours');

export class TradingHoursGuard implements ObservableEmissionGuard {
    check(emission: ObservableEmissionContext): ObservableEmissionDecision {
        if (!emission.context.principal) return ObservableEmissionDecision.DenyAndTerminate;
        return Array.isArray(emission.data) && emission.data.length === 0
            ? ObservableEmissionDecision.Suppress
            : ObservableEmissionDecision.Allow;
    }
}

const builder = ArcApplication.createBuilder({ query: { observableEmissionGuards: [tradingHours] } });
builder.services.addScoped(tradingHours, () => new TradingHoursGuard());
builder.add(Price);

Without an authenticated caller, GET /api/live-prices answers 403 with isAuthorized: false. With one, an empty list is suppressed, so the snapshot answers 202 with isReady: false until a non-empty list arrives.

Decisions

Decision	Effect
`Allow`	Deliver the result
`Suppress`	Withhold it without advancing the delta baseline
`DenyAndTerminate`	Send a terminal unauthorized result or hub frame and end the subscription

The most restrictive decision across all guards wins. A guard that throws denies and ends the subscription, including while evaluating an HTTP snapshot, and the failure is logged.

What a guard sees

ObservableEmissionContext carries queryName, the bound input, the rendered data, the execution context (principal, tenant, correlation), isFirstEmission, and a signal. Guards run in the subscription’s service scope, after renderers and interceptors, on an isolated copy of the context.

Register guard tokens with the query.observableEmissionGuards option, and register each token as a service.