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In section 2.1 of the OAuth2.0 RFC, it states the following regarding the client identifier:

The authorization server SHOULD document the size of any identifier it issues.

What is the point of doing this?

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Some identifiers may be stored in a database or another format which specifies the size of fields.

It would be a pretty common case where a developer wanted to store authorization tokens in their database. Here's a real world example.

In the case I linked above, the token used to be under 255 characters, which would fit in a VARCHAR(255), but Facebook has been sending access tokens with more than 255 characters.

This may not seem like much of an issue, because a developer could set their varchar to 8000 or whatever their DB max size is, but in some MySql instances, having a UTF-8 VARCHAR over 255 can lead to issues indexing the tables.

There's a lot of nuances that could inform design decisions, if the developers know what they're going to be getting back from a authorization server, and if it changes, it's better to have that documented somewhere, rather than just have it fail one day and have no idea what happened.

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