How does mnesia:activity/4 let you customize the access context of an operation?

mnesia:activity(AccessContext, Fun, Args, Module) is the general-purpose entry point underlying transaction/1, sync_transaction/1, and the dirty variants — those are really just convenience wrappers around activity/4 with a specific context pre-selected.

mnesia:activity(transaction, fun update_balance/2, [AccountId, Amount], mnesia).
mnesia:activity(async_dirty, fun log_event/1, [Event], mnesia).

Calling activity/4 directly is useful when you want to select the access context dynamically — for example, choosing between a transactional and a dirty path for the same piece of business logic based on a runtime flag, rather than hardcoding which wrapper function you call. The last argument (Module) lets you plug in a completely custom access-context implementation, which is also how table fragmentation's mnesia_frag module hooks into the same mechanism.

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