This is actually quite a subjective topic.

Blacklisting may still be more efficient in some cases:

• If you use Opaque access tokens, then depending on how you store them and its data, you may need to acquire certain locks while reading/writing to them. So on each API call, there is a chance where your authentication read to the db, might have to be synchronised with other reads/writes. In the case of storing blacklisted JWTs, you would never use it for anything else apart from just checking if a JWT has been revoked — which would never require any sort of locks or synchronisation in any db. So reads to this would be more efficient from a theory point of view.
• You could use some sort of probabilistic checking method, where you have a bloom filter like data structure that you query first to check for blacklisting — this would mostly be quite efficient to read from, and if this returns that it is not blacklisted, then you can continue, but if it returns that the token is blacklisted, then you may have to actually query the db to confirm that (that’s one of the properties of a bloom filter). And since most JWTs will not be blacklisted, this could be a good solution. However, the actual efficiency totally depends on your setup.
• In Opaque access token method, you have to store each and every token in the db. Whereas in JWT with blacklisting, you need to store just a few tokens in db (in fact, most JWTs will not be stored since most are not blacklisted). So in terms of space complexity, JWT with blacklisting wins.

So given the arguments above, using JWT with blacklisting is still a better option since it saves you space.

The problems with JWT is that it relies on one shared key for all users — which can be a security bottleneck (you can keep changing it.. but still). The other problem compared to Opaque access token is that higher bandwidth is consumed, since JWTs are generally way longer than Opaque access tokens. If these are a concern to you, then stick with Opaque access tokens, else go with JWT.

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