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ETS (Erlang Term Storage) is a built-in in-memory key-value store with constant-time access. Each table can hold millions of entries with negligible overhead.

-module(main).
-export([main/1]).

main(_) ->
    %% Create a table
    Tab = ets:new(my_table, [set, public, named_table]),

    %% Insert
    ets:insert(my_table, {alice, 30}),
    ets:insert(my_table, {bob, 25}),
    ets:insert(my_table, {carol, 40}),

    %% Lookup
    [{_, Age}] = ets:lookup(my_table, alice),
    io:format("alice: ~w~n", [Age]),

    %% Iterate
    ets:foldl(fun({K, V}, Acc) -> [{K, V} | Acc] end, [], my_table),

    %% Delete
    ets:delete(my_table, bob),

    %% Cleanup
    ets:delete(my_table).

Table types:

• set — unique keys (default)
• ordered_set — keys in sorted order, can be iterated in order
• bag — multiple values per key allowed (no duplicate {K,V})
• duplicate_bag — even duplicate {K,V} allowed

Access modes:

• private — only owner can read/write (default)
• protected — owner writes, anyone reads
• public — anyone reads/writes

named_table — refer to it by atom name across processes (instead of by Tid).

ETS is concurrent-safe — multiple processes can read/write without locking (atomic per-row).

Use cases:

• Caches
• In-memory state shared across processes (without GenServer overhead)
• Session stores
• Configuration / lookup tables

Limits:

• All in RAM — for persistence, use Mnesia (its big sister) or external DB
• Tables die when their owner process dies (unless heir option set)
• Write-heavy contention can degrade — measure with ets:info

Mnesia is built on ETS+DETS — distributed, persistent, transactional. The Erlang DB.