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Erlang / Erlang Basics Interview questions

When would you choose Mnesia over ETS?

Reach for Mnesia when data needs to survive beyond a single node, or beyond a single BEAM restart, or must be updated as part of a transaction spanning multiple tables. Reach for plain ETS when the data is node-local, disposable if the node restarts, and speed matters more than durability or cross-node consistency.

ETSMnesia
In-memory only, single node.Optional disk persistence, replicated across cluster nodes.
No built-in transactions across keys/tables.ACID-style transactions across multiple tables.
Slightly lower overhead per operation.Extra overhead for transaction coordination and replication.

A common pattern is actually using both: ETS as a fast local cache in front of Mnesia's authoritative, replicated copy, refreshing the cache when the underlying data changes.

Which scenario clearly favors Mnesia over ETS?
What common pattern combines ETS and Mnesia together?

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