Transaction Example

[Jonathan M.]

Hello,

I am using korma with mysql and InnoDB and I am having trouble understanding the example shown here.

Are the two pieces of code different examples or are they used together? 

Can someone post a simple example of doing two inserts(or updates), checking if they succeeded and rolling back if not?

Thank you for your time

[Dennis Roberts]

Hi Jonathan,

The transaction macro automatically rolls back if it catches any Throwable, so you don't have to explicitly roll back if there's a problem as long as you don't handle any exceptions that should cause a rollback from within the transaction macro (or you throw another exception after handling an exception that should cause a rollback).

If you need to force a rollback, then you can call (rollback).

Here are some examples of transactions that will be rolled back:

(transaction

  (insert :foo (values {:level 1}))

  (insert :bar (values {:level 42}))

  (throw (Exception.)))

(transaction

  (insert :foo (values {:level 1}))

  (insert :bar (values {:level 42}))

  (rollback))

Let me know if you have any questions.

Thanks,

Dennis

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[Jonathan M.]

Hi Dennis,

Thanks for your reply. Much better now.

I do have one more question. What if I want to run some code before or after the rollback has been executed? Can you please provide a small example for this case as well? I assume that the code needs to be wrapped in try-catch block or is there another way with korma?

Jonathan

[Dennis Roberts]

Hi Jonathan,

If you want to perform some action after the rollback has completed, you can do something like this:

(transaction

  (insert :foo (values {:level 1}))

  (insert :bar (values {:level 42}))

  (rollback))

(when (is-rollback?)

  (do-something))

I don't currently know of a way to do something before the rollback is performed other than to both catch exceptions and check is-rollback? within the transaction:

(tranaction

  (try

    (insert :foo (values {:level 1}))

    (insert :bar (values {:level 42}))

    (catch Throwable t

      (rollback)))

    (if (is-rollback?)

      (do-something))))

Disclaimer: I haven't tried this yet.

Dennis

[Jonathan M.]

Thanks Dennis.

The "after rollback" case which I've asked is probably pointless but still good to know about the try-catch :)

Jonathan

[Jonathan M.]

Sorry for bringing this back up but, this example:

(transaction
  (insert :foo (values {:level 1}))
  (insert :bar (values {:level 42}))
  (rollback))
(when (is-rollback?)
  (do-something))

throws:

java.lang.NullPointerException: null
 at clojure.core$deref.invoke (core.clj:2080)
    clojure.java.jdbc$rollback.invoke (jdbc.clj:340)
    clojure.java.jdbc$is_rollback_only.invoke (jdbc.clj:405)
    korma.db$is_rollback_QMARK_.invoke (db.clj:186) 

Are you sure the example is correct?

Jonathan 

[Jonathan M.]

I think the correct is:

(transaction
  (insert :foo (values {:level 1}))
  (insert :bar (values {:level 42}))
  (rollback)

  (when (is-rollback?)
    (do-something)))

Jonathan