How do you dynamically add a new node to a running Mnesia cluster?

Bringing a new node into an already-running Mnesia cluster involves connecting it at the Erlang distribution level first, then explicitly telling Mnesia's schema about it via mnesia:change_config(extra_db_nodes, [NewNode]), which merges the new node into the existing schema rather than creating a separate one.

%% on the new node, after connecting to the cluster:
mnesia:start(),
mnesia:change_config(extra_db_nodes, ['existing_node@host']).

After this, the new node knows about the cluster's existing tables but doesn't automatically hold a copy of any of them — you still need mnesia:add_table_copy/3 for each table you want it to replicate. This two-step process (join the schema, then opt into specific tables) is deliberate: it lets a new node join as, say, a lightweight participant that only replicates a subset of tables relevant to its role.

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