Question

I have a long-running operation which I perform in a background thread. As it is important for the operation to either complete successfully or not at all, I am wrapping the entire operation in a transaction.

Aspects of the UI need read-only access to the database during this time. To avoid blocking the UI, I am experimenting with inserting calls to db.yieldIfContendedSafely() in the main loop of the background operation.

This does what I want in that the UI is no longer blocked, but it’s not completely clear to me if this is risking a loss of data integrity.

The javadoc for yieldIfContendedSafely() says:

Temporarily end the transaction to let other threads run. The
transaction is assumed to be successful so far. Do not call
setTransactionSuccessful before calling this. When this returns a new
transaction will have been created but not marked as successful. This
assumes that there are no nested transactions (beginTransaction has
only been called once) and will throw an exception if that is not the
case.

Does this mean that my long-running operation is actually being committed to the database in separate chunks, or is the overall transaction maintaining enough state to commit the whole lot in one go at the end, thus preserving data-integrity?

Answer 1

Does this mean that my long-running operation is actually being committed to the database in separate chunks

Yes. Within yieldIfContendedSafely(), Android calls setTransactionSuccessful(), endTransaction(), and begins a new transaction — committing your statements in the process. There is no mechanism to rollback the “real” transaction after it ends.

This behavior only occurs if there is another thread waiting on the database, otherwise yieldIfContendedSafely() does nothing.

I checked this with the following scenario. I started two threads: one inserted data into a table using a transaction, another read data out of the same table. The transaction didn’t call setTransactionSuccessful() so normally everything is rolled back at the end, leaving the table empty. I added a call to yieldIfContendedSafely(), and afterwards the table was not empty and had data from the transaction.