Chris Travers recently responded to Tony Marston’s critique of an earlier post where Chris advocated “intelligent databases”1. Chris’ response is well reasoned, particularly his point that once a database is accessed by more than a single application or via third-party tools, it’s almost a given that one should attempt to push “intelligence” and business logic into the database if possible.
However, there is a paragraph in Tony’s post that merits further scrutiny:
The database has always been a dumb data store, with all the business logic held separately within the application. This is an old idea which is now reinforced by the single responsibility principle. It is the application code which is responsible for the application logic while the database is responsible for the storing and retrieval of data. Modern databases also have the ability to enforce data integrity, manage concurrency control, backup, recovery and replication while also controlling data access and maintaining database security.
If a database (actually, a shortening of DBMS—a combination of the data and the software that manages it) has always been dumb, then presumably one would never specify UNIQUE indexes. It is a business requirement that invoice or employee numbers be unique, so if all the business logic should reside in the application, then the DBA should only create a regular index and the application —all the applications and tools!— should enforce uniqueness.
Tony’s mention of “data integrity” is somewhat ambiguous because different people have varied understandings of what that covers. As C. J. Date points out, “integrity … is the part [of the relational model] that has changed [or evolved] the most over the years.”2 Perhaps Tony believes that primary keys, and unique and referential constraints should be enforced by the DBMS, but apparently an integrity constraint such as “No supplier with status less than 20 supplies any part in a quantity greater than 500”3 should instead only be code in an application (make that all applications that access that database).
As for me, as I pointed out earlier, “constraints should be implemented, preferably declaratively, in the database schema” while “type constraints … should also be implemented in the user interface” (emphasis added). Ideally, the user interface should derive its code directly from the schema.
Update: Many thanks to everyone who participated in the discussion. I think we’ve covered just about every angle pro or con incorporating business logic in the database, so I’ve closed comments now.
Update 2: Chris has an update post that may be of interest.
1 Interestingly, the first time I heard the term “intelligent database” it was from Dave Kellogg giving a marketing presentation for Ingres 6.3, which had incorporated some of the features in the UC Berkeley POSTGRES project.
2 Date, C. J. An Introduction to Database Systems, 8th Edition. 2004, p. 253.
3 Ibid., p. 254.