In the previous episode: Alice and Bob were trying to simultaneously update the title of the following row of the film table:
id | title | release_year -------+----------------+-------------- 47478 | Seven Samurai | 1956
User Transaction Correctness
In the first episode, Carol also wanted to concurrently change the release_year to 1954. Let us now consider her update in conjunction with one of the others.
If Bob modifies the title and Carol changes the year —provided our SQL statements only include the updated fields— there will be no problem. In other words, the changes can be merged, just as when Alice updates one section of a Wikipedia page and Bob changes another section of the same page (Note to self: have to talk to them about this moonlighting).
So the problem with Alice’s and Bob’s updates to the title (one to remove the extra space, the other to change it to Japanese) is that they cannot be merged. How can we implement a correct user transaction without incurring the penalty of locking the row in question?
Optimistic Concurrency Control
Richard Stephan was on the right track when he asked in a comment to the previous post: Should the application be using a pessimistic or an optimistic locking technique?
Optimistic concurrency control (aka optimistic “locking”) is a transaction design approach that avoids the overhead of locking. As stated in the Wikipedia article, the “stateless nature of HTTP makes locking infeasible for web user interfaces,” so it’s almost a given that we should use OCC for our interface.
OCC involves validating that the record we updated was the correct one. A simple way to do this is to add a qualification to the original UPDATE that does the validation for us. For example, the DB-API cursor execute call could be coded as follows:
cursor.execute("UPDATE film SET title = %s " "WHERE id = %s AND title = %s", (newrec.title, oldrec.id, oldrec.title))
Above, newrec refers to the new record instance and oldrec to the old one. So both Alice and Bob would send UPDATEs with “AND title = ‘Seven Samurai’”. Naturally, since they’re both modifying the title, only one of the statements can succeed.
The downside of having to add the extra qualification —in the general case— is that the user could’ve changed several fields so each UPDATE would have to be customized accordingly. This will have a negative impact, among other things, on the query planner. In addition, the web interface would have to deal with two values for each field.
Alternatives to the extra qualification are to add either a version number for each row or a sufficiently granular timestamp that also acts as a versioning scheme. However, since we’re using PostgreSQL, perhaps we can use something else …