Database / Mnesia intermediate to advanced Interview questions

What is the where_to_read table_info item used for?

mnesia:table_info(Table, where_to_read) returns which node the local Mnesia instance would currently read that table from — useful for diagnosing read-routing behavior directly, rather than inferring it indirectly from latency measurements.

mnesia:table_info(session, where_to_read).
%% -> node() | 'nonode@nohost' (no copy currently reachable) | some_remote_node@host

If it returns the local node itself, reads are being served locally with no network involved; if it returns a remote node, every read for that table is crossing the network to reach it, which is worth knowing when investigating unexpectedly high read latency on a specific table. It's mainly a diagnostic tool for understanding cluster behavior at a point in time, not something application logic typically branches on directly.

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What does mnesia:table_info(Table, where_to_read) reveal? The table's storage type

✗ Not quite. Try again.

Which node the local instance would currently read that table from

✓ Correct! Well done.

The table's primary key field

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Why would you check where_to_read when investigating high read latency on a table? It has no relevance to read latency at all

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A remote answer means every read is crossing the network to reach that node

✓ Correct! Well done.

It only matters for write latency, never reads

✗ Not quite. Try again.

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Database / Mnesia intermediate to advanced Interview questions

What is the where_to_read table_info item used for?

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