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Database / Mnesia basics Interview questions

How do you check if Mnesia is running?

The most direct check is mnesia:system_info(is_running), which returns an atom describing the current state rather than a plain boolean, since Mnesia can be in intermediate states like starting up or stopping rather than strictly on or off.

mnesia:system_info(is_running).
%% -> yes | no | starting | stopping

Checking for yes specifically (rather than just "not no") is important in startup code that needs to know Mnesia is fully operational before proceeding — a value of starting means the application is initializing but not yet ready to serve reads/writes reliably.

What does mnesia:system_info(is_running) return?
Why does startup code typically check specifically for the atom `yes`?

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More Related questions...

What is Mnesia? What is a Mnesia table? What are the storage types available for a Mnesia table? What is a Mnesia schema? How do you create a Mnesia schema? How do you create a Mnesia table? What is a Mnesia transaction? How do you write a record to a Mnesia table? How do you read a record from a Mnesia table? What are dirty operations in Mnesia? What is mnesia:dirty_read/1? What is a Mnesia record? How do you delete a record from a Mnesia table? What is the purpose of mnesia:start/0? How do you stop Mnesia? What are the Mnesia table types (set, ordered_set, bag)? What is a Mnesia index? How do you add a secondary index to a Mnesia table? What is mnesia:info/0 used for? How do you check if Mnesia is running? What is the Mnesia directory (Mnesia.dir)? What is the primary key in a Mnesia table? What is mnesia:select/2 used for? What is record_info used for when creating a Mnesia table? What are the attributes of a Mnesia table? What is a Mnesia node? How do you replicate a table across multiple Mnesia nodes? What is mnesia:table_info/2 used for? Describe the difference between a Mnesia table and an ETS table? Why should Mnesia operations generally run inside a transaction? When should you use dirty operations instead of a transaction? What is the difference between ram_copies and disc_copies? What is disc_only_copies and when would you use it? What happens to a disc_copies table when its node restarts? How do you change a table's storage type at runtime? What is a Mnesia table lock and how does it work? How does Mnesia keep replicated tables consistent across nodes? How do you migrate a Mnesia record definition when its fields change? What is mnesia:transform_table/3 used for? When should you choose bag over set for a Mnesia table? How do you back up a Mnesia database? How do you restore a Mnesia database from a backup? Explain the lifecycle of a Mnesia transaction from start to commit? How do you troubleshoot a Mnesia table stuck in the "loading" state? What is mnesia:force_load_table/1 and when should you use it? What is the difference between mnesia:read/1 and mnesia:wread/1? What is a Mnesia checkpoint used for? How does Mnesia resolve conflicts after a network partition heals?
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