Day 19: Asynchronous Operations & Callbacks

Building StreamSocial's Content Moderation Pipeline

Oct 13, 2025

∙ Paid

What We're Building Today

Today we'll architect StreamSocial's content moderation system using Kafka's asynchronous producer patterns. We'll implement non-blocking message sends, custom callback handling, and error recovery mechanisms that can process millions of user posts without blocking the main application thread.

Key Deliverables:

Asynchronous producer with custom callback patterns
Content moderation pipeline with graceful error handling
Performance monitoring and metrics collection
Failover mechanisms for moderation service outages

Youtube Video:

Core Concepts: The Async Advantage

Non-Blocking Sends: Keep the UI Responsive

When a user posts content on StreamSocial, the worst thing we could do is make them wait while we send messages to Kafka. Traditional synchronous sends block the thread until Kafka acknowledges receipt—potentially 10-50ms per message. With async sends, we hand off the message and immediately return control to the user.

python

# Blocking (BAD for user experience)
producer.send(topic, message).get()  # Waits for ACK

# Non-blocking (GOOD for user experience) 
producer.send(topic, message, callback=handle_result)  # Returns immediately

Futures: Promises for Message Delivery

Kafka returns a Future object representing the eventual result of the send operation. Think of it as a "promise" that Kafka will either successfully deliver your message or tell you why it failed. You can check the status later without blocking.

Callbacks: Your Error Handling Safety Net

Callbacks are functions that execute when the send operation completes—successfully or with errors. They're your opportunity to log metrics, retry failed sends, or alert monitoring systems. Without proper callbacks, failed messages disappear into the void.

Context in Ultra-Scalable System Design

StreamSocial's Content Flow Challenge

In StreamSocial's architecture, every user action triggers multiple events:

Post creation → content moderation queue
Moderation result → timeline updates
Spam detection → user reputation scoring
Policy violations → admin notification queue

With 100M+ daily active users, we need async patterns that can handle:

10,000+ posts per second during peak hours
Sub-100ms response times for post submissions
Zero message loss even during partial system failures
Graceful degradation when moderation services are overloaded

The Async Producer Advantage

Synchronous producers would create cascading slowdowns. If the moderation service takes 200ms to respond, every user post would feel sluggish. Async producers let us: