🧠 Real-World Scenario

You’re building a live podcast platform where:

A host starts a live podcast session.
Multiple guests are invited to join.
The podcast must not start until all required guests have joined.
Guests may join at different times.
Everything happens concurrently (over the network, different threads).

📌 What Exactly Is the Problem?

We have dependencies between tasks:

Each guest joining is an independent task.
The host starting the podcast depends on all guests being present.

👉 This creates a synchronization problem:

One thread must wait until several other threads finish their work.

🧩 Mapping to Concurrency Concepts

Real World	Concurrency
Guest joining	Worker thread
Host waiting	Main / coordinator thread
“All guests joined”	Synchronization condition
Live podcast starts	Final action after coordination

🕒 Timeline Example

Imagine:

Podcast needs 3 guests.
Guests join at different times.

Time →
Guest 1 joins ───────────┐
Guest 2 joins ──────┐    │
Guest 3 joins ───────────────┐
                              ↓
                 🎤 Podcast Starts

The host cannot start early, even if:

1 guest joins
2 guests join

The host must wait for all 3.

🚨 Why This Is a Concurrency Problem

❌ Naive Thinking (Wrong)

“Just start the podcast when guests arrive.”

Problems:

Host might start before everyone joins.
Host might check “are guests ready?” while guests are still joining.
Leads to race conditions:
- Host checks → sees only 2 guests → waits
- Guest 3 joins immediately after → host never notices

⚠️ Core Challenges

Ordering
- Guests can join in any order.
Timing
- Guests join at unpredictable times.
Coordination
- Host must wait, but not forever.
Correctness
- Podcast must start exactly once.
Scalability
- Works for 2 guests, 10 guests, or 100 guests.

🧠 What Makes This Problem Special?

This is not about sharing data (like counters or money).
This is about waiting for conditions.

It belongs to a special class of concurrency problems:

🔹 “Wait until N things are done”

Examples:

App startup waits for DB + Cache + Config
Game waits for all players to be ready
Deployment waits for all services to be healthy
Podcast waits for all guests to join 🎤

🧪 Key Question the System Must Answer

“How does one thread know that all other threads are done?”

That’s the heart of the problem.

🎤 Live Podcast with Guests — Concurrency Problem