Local Cache vs Distributed Cache: When Does Distributed Caching Become Necessary?

[geekCoder99]

What is the significance of distributed caching in modern backend systems, and when should we prefer it over local in-memory caching?

In modern backend architectures, caching is often introduced to improve performance and reduce database load, but the choice between local caching and distributed caching is not always straightforward.

I’d like to understand:

• In what scenarios does distributed caching (e.g., Redis, Memcached) become necessary compared to local in-memory caching inside application instances?
• What are the main architectural problems it solves at scale (consistency, shared state, horizontal scaling, etc.)?
• What trade-offs or complexities does it introduce (latency, cache invalidation, failure modes, operational overhead)?
• Are there common anti-patterns or mistakes teams make when introducing distributed caching too early or unnecessarily?

From your experience, what are the key factors that determine whether a system should move from local cache to a distributed cache layer?

[nujaadeen]

Distributed caching provides a shared in-memory cache like Redis across multiple application instances to improve performance and reduce database load.

Significance

• Shared state across instances: avoid inconsistent local caches
• Scales with horizontal architecture: all nodes can read/write same cache
• Reduces DB load: serve hot data from memory
• Enables centralised invalidation / TTL control

Use distributed caching when:

You have multiple app instances running

• Cached data needs to be consistent across services
• Data is frequently accessed across users/requests
• You need centralised eviction or invalidation

When local cache is enough

• Single instance or small scale system
• Data is instance-specific or not shared much
• You need very low latency (no network hop)
• Some inconsistency across nodes is okay

Trade-offs

• Adds network latency compared to local cache
• More operational complexity (cluster setup, monitoring etc)
• Cache invalidation becomes harder to manage
• External dependency can become a point of failure

[geekCoder99]

Thank you for the explanation. can you share some resources like youtube videos or medium articles regarding this.

[nujaadeen]

https://www.youtube.com/watch?v=Gdfj-544AkA

You can refer any tech topics in this channel, he makes graphically pleasing videos with great explanation.
@geekCoder99