ETS tables are absolutely mutable, they even have specific functions to iterate over them while being mutated (https://www.erlang.org/doc/man/ets#safe_fixtable-2). I use them extensively to share data in a "lock-free" fashion with other processes (a `gen_server` that gets all messages and aggregates data in ETS tables, retrieval via direct reads on a known table name instead of gen_server:call). Mnesia is also (usually) ETS down below.