Would it add overhead to put a DB transactions around every single service method in our application?
We currently only use DB transactions where it's an explicit/obvious necessity. I have recently suggested transactions around all service methods, but some other developers asked the prudent question: will this add overhead?
My feeling is not - auto commit is the same as a transaction from the DB perspective. But is this accurate?
DB: MySQL
You are right, with autocommit every statement is wrapped in transaction. If your service methods are executing multiple sql statements, it would be good to wrap them into a transaction. Take a look at this answer for more details, and here is a nice blog post on the subject.
And to answer your question, yes, transactions do add performance overhead, but in your specific case, you will not notice the difference since you already have autocommit enabled, unless you have long running statements in service methods, which will cause longer locks on tables participating in transactions. If you just wrap your multiple statements inside a transaction, you will get one transaction (instead of transaction for every individual statement), as pointed here ("A session that has autocommit enabled can perform a multiple-statement transaction by starting it with an explicit START TRANSACTION or BEGIN statement and ending it with a COMMIT or ROLLBACK statement") and you will achieve atomicity on a service method level...
At the end, I would go with your solution, if that makes sense from the perspective of achieving atomicity on a service method level (which I think that you want to achieve), but there are + and - effects on performance, depending on your queries, requests/s etc...
Yes, they can add overhead. The extra "bookkeeping" required to isolate transactions from each other can become significant, especially if the transactions are held open for a long time.
The short answer is that it depends on your table type. If you're using MyISAM, the default, there are no transactions really, so there should be no effect on performance.
But you should use them anyway. Without transactions, there is no demarcation of work. If you upgrade to InnoDB or a real database like PostgreSQL, you'll want to add these transactions to your service methods anyway, so you may as well make it a habit now while it isn't costing you anything.
Besides, you should already be using a transactional store. How do you clean up if a service method fails currently? If you write some information to the database and then your service method throws an exception, how do you clean out that incomplete or erroneous information? If you were using transactions, you wouldn't have to—the database would throw away rolled back data for you. Or what do you do if I'm halfway through a method and another request comes in and finds my half-written data? Is it going to blow up when it goes looking for the other half that isn't there yet? A transactional data store would handle this for you: your transactions would be isolated from each other, so nobody else could see a partially written transaction.
Like everything with databases, the only definitive answer will come from testing with realistic data and realistic loads. I recommend that you do this always, no matter what you suspect, because when it comes to databases very different code paths get activated when the data are large versus when they are not. But I strongly suspect the cost of using transactions even with InnoDB is not great. After all, these systems are heavily used constantly, every day, by organizations large and small that depend on transactions performing well. MVCC adds very little overhead. The benefits are vast, the costs are low—use them!
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