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Database / Mnesia basics Interview questions

When should you use dirty operations instead of a transaction?

Dirty operations fit specifically when an operation genuinely stands alone — a single read or write that doesn't depend on any other operation succeeding alongside it, and where occasional inconsistency during a race or a node failure mid-write is acceptable for that particular piece of data.

Good candidates: a hit counter, a best-effort cache entry, telemetry data, or high-frequency writes to a table where losing or slightly misordering an entry under heavy concurrent load isn't a correctness problem. Poor candidates: anything involving multiple related writes (transferring a value between two records), anything where a stale read could cause a real bug (checking a balance before debiting it), or any table where replication consistency genuinely matters to correctness.

The deciding question is usually: "if this specific operation raced with another one, or half-completed on a crash, would that actually break something a user or another part of the system depends on?" If yes, use a transaction; if a shrug is the honest answer, dirty operations are a reasonable, faster choice.

What kind of operation is a good candidate for dirty operations?
What question helps decide between dirty ops and a transaction?

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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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