Sql Server transactions - usage recommendations

I saw this sentence not only in one place:

"A transaction should be kept as short as possible to avoid concurrency issues and to enable maximum number of positive commits."

What does this really mean?

It puzzles me now because I want to use transactions for my app which in normal use will deal with inserting of hundreds of rows from many clients, concurrently.

For example, I have a service which exposes a method: AddObjects(List<Objects>) and of course these object contain other nested different objects.

I was thinking to start a transaction for each call from the client performing the appropriate actions (bunch of insert/update/delete for each object with their nested objects). EDIT1: I meant a transaction for entire "AddObjects" call in order to prevent undefined states/behaviour.

Am I going in the wrong direction? If yes, how would you do that and what are your recommendations?

EDIT2: Also, I understood that transactions are fast for bulk oeprations, but it contradicts somehow with the quoted开发者_如何学运维 sentence. What is the conclusion?

Thanks in advance!

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A transaction has to cover a business specific unit of work. It has nothing to do with generic 'objects', it must always be expressed in domain specific terms: 'debit of account X and credit of account Y must be in a transaction', 'subtract of inventory item and sale must be in a transaction' etc etc. Everything that must either succeed together or fail together must be in a transaction. If you are down an abstract path of 'adding objects to a list is a transaction' then yes, you are on a wrong path. The fact that all inserts/updates/deletes triggered by a an object save are in a transaction is not a purpose, but a side effect. The correct semantics should be 'update of object X and update of object Y must be in a transaction'. Even a degenerate case of a single 'object' being updated should still be regarded in terms of domain specific terms.

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That recommendation is best understood as Do not allow user interaction in a transaction. If you need to ask the user during a transaction, roll back, ask and run again.

Other than that, do use transaction whenever you need to ensure atomicity.
It is not a transactions' problem that they may cause "concurrency issues", it is the fact that the database might need some more thought, a better set of indices or a more standardized data access order.

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"A transaction should be kept as short as possible to avoid concurrency issues and to enable maximum number of positive commits."

The longer a transaction is kept open the more likely it will lock resources that are needed by other transactions. This blocking will cause other concurrent transactions to wait for the resources (or fail depending on the design).

Sql Server is usually setup in Auto Commit mode. This means that every sql statement is a distinct transaction. Many times you want to use a multi-statement transaction so you can commit or rollback multiple updates. The longer the updates take, the more likely other transactions will conflict.