I'm working on Flutter and Firebase Application.
I saw some documents about Firebase that Firebase need transaction control to process lot's of firestore documents update.(==> many users)
However I'm wondering about just document reading and writing.
Is transaction also needed for just lot's of trafic of reading or just lot's of writing new documents?
Also when just one application(admin app) is updating or deleting document and lot's of other users are reading that documents, is transaction also needed?
A transaction needed when multiple users may be updated the same document at (almost) the same time in a way that may product conflicting updated. In such scenarios you use a transaction to prevent the concurrent writes from producing a conflict.
A slight variant of this is Firestore's batched write, which you can use when you want to update multiple documents atomically, but don't to first read any data to determine the new value in those documents.
If there is no chance of conflicts in your writes, you don't need to use a transaction or batched write, and using them will likely actually hurt performance.
So in your scenario, if there's only a single concurrent write coming from the admin app, you won't need transactions. If you have a specific use-case where you are not sure though, it's always best to share that specific use-case and the implementation code with us.