---
title: "Events, logging, promises, and asynchronous flows"
chapter: "08"
---

# Events, logging, promises, and asynchronous flows

OSGi supplies several collaboration styles; use each for its real semantics.

## Event Admin

Event Admin sends topic-based events with properties. Synchronous delivery
propagates on the caller's thread; asynchronous delivery decouples time but is
not a durable message broker.

Use Event Admin for in-process notifications. Do not claim it survives process
crashes, gives exactly-once delivery, or replaces Kafka/JMS.

## Typed Event

Typed Event provides type-safe event models while retaining OSGi dynamics.
Define ownership, ordering, error handling, and whether listeners may block.

## Log Service

The OSGi Log Service associates entries with bundles, services, and log levels.
Log Reader and Log Stream enable observation. Bridge carefully to SLF4J or a
runtime backend to avoid loops and duplicate bindings.

## Promises and Push Streams

Promise represents a future result with success/failure composition. Push
Stream models asynchronous streams with buffering and backpressure concepts.
They are useful when asynchronous APIs are genuine—not as decoration over
blocking work.

## Threading rules

- document the thread that calls consumers;
- never hold framework or component locks across slow work;
- close streams and executors with lifecycle;
- bound queues and concurrency;
- preserve diagnostic context across async boundaries.

## Feynman check

An event tells listeners something happened now. A durable message records work
that must survive failure. Explain which one an invoice settlement requires.