Design patterns are not meant to be memorized—they are meant to be chosen.
Two patterns that are often mentioned together, yet solve very different problems, are the Singleton Pattern and the Observer Pattern. Beginners sometimes confuse them because both are commonly used in global or shared contexts.
In this article, we’ll clearly compare these two patterns, understand what problem each one solves, and learn when to use which.
If you haven’t read them yet, you may want to start here:
Why Comparing Singleton and Observer Makes Sense
At first glance, Singleton and Observer may seem unrelated:
- One controls how many instances exist
- The other controls how objects communicate
However, in real-world systems, these two patterns are often used together, which is why understanding their differences is important.
The key distinction lies in what they are responsible for.
The Core Problem Each Pattern Solves
Singleton Pattern: Object Lifecycle
The Singleton Pattern answers the question:
How many instances of this object should exist?
Its responsibility is:
- Controlling object creation
- Ensuring a single shared instance
- Providing a global access point
It focuses on existence and ownership.
Observer Pattern: Object Communication
The Observer Pattern answers a different question:
How do multiple parts of the system react when something changes?
Its responsibility is:
- Managing subscriptions
- Notifying observers
- Keeping components loosely coupled
It focuses on events and reactions, not creation.
Lifecycle vs Communication (The Key Difference)
This single comparison explains almost everything:
| Aspect | Singleton | Observer |
|---|---|---|
| Primary concern | Object creation | Object communication |
| Core question | “How many instances?” | “Who should react?” |
| Focus | Lifecycle | Events |
| Relationship | One instance | One-to-many |
| Coupling | Global access | Loose coupling |
If you remember just this table, you’ll rarely confuse the two again.
Code Responsibility Comparison
Singleton Responsibility
A Singleton:
- Decides when and how an object is created
- Ensures the same instance is reused
- Does not handle communication logic by itself
Example responsibilities:
- Logger instance
- Database connection
- Configuration loader
Observer Responsibility
An Observer system:
- Manages subscriptions
- Decides who gets notified
- Does not care how observers are created
Example responsibilities:
- Event listeners
- UI updates
- Notification systems
Can Singleton and Observer Be Used Together?
Yes—very often.
In fact, many real systems combine them naturally.
Example Scenario
- A NotificationService exists as a Singleton
- Multiple components subscribe to it as Observers
- The service publishes events
- Observers react independently
Here:
- Singleton ensures one notification hub
- Observer ensures many listeners
They solve different layers of the problem and complement each other.
When to Use Singleton vs Observer
Use Singleton When:
- You need exactly one instance
- The object represents a shared resource
- Multiple instances would cause conflicts
- The concern is global consistency
Use Observer When:
- Multiple parts of the system must react to changes
- You want to avoid tight coupling
- Events should propagate automatically
- The system is event-driven
Use Both When:
- You have a central service (Singleton)
- That service emits events
- Multiple independent consumers need updates
This combination is extremely common in real-world applications.
Common Misconceptions
❌ “Singleton notifies observers”
No. Singleton controls instance creation, not notifications.
❌ “Observer is global”
No. Observers are scoped to a subject, not global by default.
❌ “They are alternatives”
They are orthogonal patterns, not replacements for each other.
Decision Cheat Sheet
Ask yourself:
Am I solving an object creation problem?
→ SingletonAm I solving a notification or event problem?
→ ObserverDo I need one shared service that many things react to?
→ Singleton + Observer
Final Thoughts
The Singleton and Observer patterns solve completely different problems, but they often appear together in well-designed systems.
- Singleton brings control
- Observer brings flexibility
Understanding what each pattern is responsible for is far more important than memorizing their definitions.
If you choose patterns based on problems, not popularity, your system design will naturally improve.
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