Isolation level side effects
UltraLite operates by default at an isolation level of 0. This means that the following side effects are possible:
- No locking operations are required when executing a SELECT statement.
- Applications can access uncommitted data (also known as dirty reads). In this scenario, an application may access rows in
the database that are not committed and consequently may still get rolled back by another transaction. This phenomena can
result in phantom rows (rows that get added after the original query, making the result set returned in a repeated, duplicate
query different).
For a tutorial that demonstrates the effects of dirty reads, see Tutorial: Dirty reads. For a tutorial that demonstrates a phantom row, see Tutorial: Phantom rows.
- Applications can perform non-repeatable reads. In this scenario, an application reads a row from the database, and then goes
on to perform other operations. Then a second application updates/deletes the row and commits the change. If the first application
attempts to re-read the original row, it receives either the updated information or discovers that the original row was deleted.
For a tutorial that demonstrates the effects of non-repeatable reads, see Tutorial: Non-repeatable reads.
Example
Consider two connections, A and B, each with their own transactions.
- As connection A works with the result set of a query, UltraLite fetches a copy of the current row into a buffer.
Note
Reading or fetching a row does not lock the row. If connection A fetches but does not modify a row, connection B can still
modify the row.
- As A modifies the current row, it changes the copy in the buffer. The copy in the buffer is written back into the database
when connection A calls an Update method or closes the result set.
- A write lock is placed on the row to prevent other transactions from modifying it. This modification is uncommitted, until
connection A performs a commit.
- Depending on the modification, if connection B fetches the current row, it may experience the following:
1 Queries used by Connection A and B do not contain temporary tables. Temporary tables can cause other side effects.
See also
