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> the transaction log of a typical RDBMS is the canonical form

Do databases keep the whole transaction log forever? It seems like that could keep growing forever even when the tables stay at constant size.

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Usually you'll keep the transaction logs for a certain period, say for two weeks. That gives you point in time recovery for any moment in that two weeks (assuming you've set up everything correctly!). Logs past that are usually archived or just dumped, but you'll have full backups that go back further.

For your question about the logs growing forever, there are usually points in the process where a transaction log is saved off somewhere else and then can be overwritten, but on a transactional system the logs over a period of a week or two can sometimes be many times larger than the actual data stored at any given time, yes.

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