Removing records from kv that exceed the limit

[SergioWhite]

Hello!

Please tell me if there is a way to remove records from kv that exceed the set limit? For example, I want to store 100 records in kv. As soon as the 101st record is added, it should displace the oldest one, as it is done in the LRU cache. I think this is a useful option.

I could find outdated records myself, but in this case I cannot guarantee that the oldest record will be removed, since after receiving it, it can be immediately updated by another user. In this unpleasant case, I will remove the current record with useful information.

I could also remove based on revision. However, you can't remove from kv based on revision, only from the stream. But I also can't remove records from the stream created with kv, since, apparently, you can't remove records directly from system streams.

At the moment, I don't see a beautiful solution to this problem. Please tell me if you have any advice. Thank you!

[derekcollison]

MaxMsgsPerSubject will do what you want and that is the default when creating a KV. You can use history to ask for N number of records per subject, e.g. 100.

[SergioWhite]

My apologies for the lack of attention! Streams do have more options, and MaxMsgsPerSubject is exactly what I need. Thank you very much!

[SergioWhite]

Maybe I was a bit hasty. Besides MaxMsgsPerSubject I saw the MaxMsgs option, and it seemed right to me. But then I realized that, for example, a message about deleting a non-existent record would displace an existing record, and I would lose useful data.

Then I tried MaxMsgsPerSubject, but as far as I understand, this option sets the limit within one key, not KV.

Just an example to demonstrate:

    const kv = await nats.jetstream().views.kv('my_kv')
    const kvStatus = await kv.status()
    const streamName = kvStatus.streamInfo.config.name

    const jsm = await this.nats.jetstreamManager()
    await jsm.streams.update(streamName, { max_msgs_per_subject: 1 }) // Here I set the limits for the stream

    await kv.put('1', '1')
    await kv.put('2', '2') // Here I expect the message '1' to be deleted from KV due to the limit.
    const value = (await kv.get('1'))?.string()
    console.log(value) // 1 <- But the message is not deleted. We can get its value

    const stream = await jsm.streams.get(streamName)
    const message1 = await stream.getMessage({ seq: 1 })
    const message2 = await stream.getMessage({ seq: 2 })
    // Here we can see that each message has different subjects, so limiting MaxMsgsPerSubject does not solve my original problem
    console.log(message1.subject) // $KV.my_kv.1
    console.log(message2.subject) // $KV.my_kv.2

Could you give me an example of how MaxMsgsPerSubject could solve my problem?