Since Sunday night, I've been battling Redis (like a boss). There's nothing wrong with Redis itself - the problem was in our code; but, the solution involved writing a migration script that scans over tens of millions of keys looking for specific ones to delete (based on corresponding keys). As I was writing this migration script, I discovered something unexpected about Redis: it doesn't store empty sets or empty hashes. And, furthermore, it will delete a set or a hash at the time the last member or key is removed, respectively. While this isn't necessarily a bad thing, if you don't know that this happens, it can lead to expected behavior if you are providing your keys with a TTL (time to live).
First, let's demonstrate that empty sets and empty hashes are automatically purged from the Redis store. In the following terminal, I'm going to look at the existence of a set:
As you can see, the set exists until the last member is removed. At that point, the "type" command returns "none" and the "exists" command confirms that the key has actually been removed from the store.
The same thing can be demonstrated with a hash:
As you can see, the hash exists until the last key is removed. At that point, the "type" command returns "none" and the "exists" command confirms that the key has actually been removed from the store.
Now, in the vast majority of cases, this behavior is totally fine because a non-existent set or hash basically acts like an empty set or hash. Meaning, if you were to ask for the cardinality (set size) of a non-existent set, for example, Redis returns zero instead of throwing an error:
127.0.0.1:6379> scard my-non-existent-set
127.0.0.1:6379> hlen my-non-existent-hash
Which is cool.
But, the real edge-case with this behavior, at least in my mind, is when TTL (time to live) comes into play. Imagine that you have a set with a TTL on it. As you are adding and removing members, you may inadvertently delete the set. And, when that happens, the existing TTL is lost. Then, when new members are added to the set, the just-in-time set is created without a TTL.
This is easy to demonstrate:
As you can see, we start out with a TTL (time to live) on the set. However, during our series of operations, we coincidentally remove all of the set members before we add the newer ones. And, when we remove the last member, the set - along with its existing TTL - is deleted. At this point, any new members will precipitate the creation of a new set (on the fly) without a TTL.
NOTE: This behavior is true even if you are executing all of this inside a MULTI / EXEC transaction block.
I don't necessarily consider this buggy behavior. I assume this approach is beneficial for memory management. However, I do think that this behavior can be surprising, especially since the fact that deleting empty sets and hashes isn't well articulated in the Redis documentation (at least not with the relevant commands). So, just keep this in the back of your mind when you're using Redis.
Want to use code from this post? Check out the license.