@nestjs/microservices/interfaces/message-handler.interface.d.ts

import { Observable } from 'rxjs';
export interface MessageHandler<TInput = any, TContext = any, TResult = any> {
    (data: TInput, ctx?: TContext): Promise<Observable<TResult>>;
    next?: (data: TInput, ctx?: TContext) => Promise<Observable<TResult>>;
    isEventHandler?: boolean;
}