LRU remove by key

[yosiat]

Hi,

I am trying to use LRUCache class with DataLoader - https://github.com/graphql/dataloader, and I need to implement removal by key.

The only option I have is to set(key, undefined) which is an ugly hack and I have a risk where this call can evict something else from the cache.

Is that possible to add support for (I can submit a PR)?

[Yomguithereal]

Hello @yosiat. The issue with key deletion regarding this particular and very efficient implementation of a lru cache in JavaScript is that it would incur a performance & memory cost to support it. The reason why it would incur such a cost is that if I want to be able to support arbitrary deletions in the cache, I need to maintain a stack of pointers made available when unsetting keys, so I can use them subsequently when setting new keys. This said, I am not opposed to the idea of adding a variant of the structure in the lib which enables you to delete keys if you know you want to pay this cost (which you will pay one way or another if you need to support deletion in any lru cache anyway).

What do you think? Do you want to check the implementation's code and tell me if you see another way to implement deletions (to keep it working in constant time, while adding a very marginal O(n) memory cost)? Or would it be cool to have a version of the structure allowing deletions?

[yosiat]

Hi @Yomguithereal !

Sorry for a long time without any reply :)

I tried implementing deletes and it was harder than I thought, but then I looked on how many deletes I have in my apps and I saw it causes my existing lru-cache to not be effective as I wished for.

So we removed our lru-cache and our other caches I am moving them to mnemonist because they don't need deletes.

TL;DR - I don't delete now :)

[Yomguithereal]

The upper performance bound for delete in the cache is indeed the very abysmal performance of the delete keyword and of the ES6 Map #.delete method, in any case yes :). Anyway, if you find you still need deletes in the future, note we can still implement it in mnemonist if required. I will just create a different structure for that not to hamper the default lru cache exposed by the lib.

[trivikr]

I want to be able to support arbitrary deletions in the cache, I need to maintain a stack of pointers made available when unsetting keys, so I can use them subsequently when setting new keys.

I've posted a PR to introduce remove functionality in LRUCache and LRUMap in #154

@Yomguithereal I would like to get your feedback on the implementation.

[trivikr]

I will just create a different structure for that not to hamper the default lru cache exposed by the lib.

The time complexity of the default lru cache implementation is not affected as it just gets the pointer from this.removed array while doing set or setpop operations. The additional memory used is O(n) to store this.removed array.

[Yomguithereal]

Hello @trivikr. I will answer in the related PR then :)