What is the difference between an in-memory cache and a distributed cache?

Compare in-memory cache and distributed cache

Explains when local process memory is enough and when shared Redis-style caching is worth the extra dependency.

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An in-memory cache lives inside one application instance. It is fast and simple, but each instance has its own copy. A distributed cache like Redis is shared by multiple app instances, so it is useful when scaled-out servers need a common cached view. The trade-off is that Redis adds network calls, serialization, deployment, monitoring, and outage behavior.

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Process-local state, scale-out behavior, shared cache state, network cost, consistency, and failure trade-offs.

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Why this question matters
An in-memory cache belongs to one process, while a distributed cache is shared across application instances. That boundary determines consistency, network cost, serialization, scale-out behavior, and what happens when the cache dependency fails.
An in-memory cache lives inside one application instance. It is fast and simple, but each instance has its own copy. A distributed cache like Redis is shared by multiple app instances, so it is useful when scaled-out servers need a common cached view. The trade-off is that Redis adds network calls, serialization, deployment, monitoring, and outage behavior.
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I would use local memory for small, cheap, disposable data that can differ per instance without hurting correctness. I would use Redis when the app is scaled out and the cached value needs to be shared, such as feature flags, lookup data, session state, rate-limit counters, or expensive read models. I still treat the database or source system as the source of truth unless the design explicitly makes Redis authoritative for that data.

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Calling Redis faster without mentioning the network hop.
Using local memory for state that must be shared across instances.
Treating cached data as automatically correct.
Adding distributed cache before identifying the slow or repeated work.

What is the difference between an in-memory cache and a distributed cache?

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