In practice (and in general) is a transaction log a part of the database server or is it a part of the database itself? I've not been able to locate information in this domain anywhere. I need the information for a model that I'm putting together...
If this is extremely dependent upon implementation, I'm looking at SQL Server 2005/2008, MySQL and SQLite. But I'd prefer to stay as general as possible.
I imagine that if the transaction log is a part of the server, then there would be one (maybe two) transaction log in the entire server. This is opposed to each database having one transaction log which would give us many transaction logs in the single server.
To add to Joe's comment, when a change to a database is transmitted to a database server, it is "aimed" at one specific database. However, the change does not take place to the database directly. It is stored in memory (RAM) and then written to a transaction log. The transaction logs are then written (committed) to the actual databases. When that committal takes place varies based on the database software (MySQL, SQL Server, etc.), the database engine settings and other variables.
All that to say: transaction logs are, generally speaking, a component of the individual databases. You could theoretically have hundreds (thousands?!) of transaction logs on a single server. Some exceptions exist, such as Exchange servers where the transaction logs are a component of the Storage Group which may have multiple databases... but I won't mention that since it would muddy the waters. =)
In Microsoft SQL Server the transaction log is a component of the database.