subscriptionHandler.ts

export const handler = async (event: APIGatewayEvent): Promise<APIGatewayResult> => {
   const body = JSON.parse(event.body);
   // ...
   if (body.type === 'subscribe') {
        const parsed = parse(body.payload.query);
        const validated = validate(schema, parsed);
        if (validated.length > 0)
            await connection.send(event.requestContext.connectionId, { errors: validated });

        await subscribe({
            schema,
            document: parsed,
            variableValues: body.payload.variables,
            contextValue: createSubscriptionContext(event),
        });

        return ok();
    }
}