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autocache
A cache that cleans itself.
Installation
This is not published to npm. Install from Git tags.
Concept
When using caching to speed up a build, whether a given cache entry is outdated and safe to be removed is often as simple as whether it was unused during the previous build.
However, there are often multiple build modes (prod vs. dev, etc.), and if a particular cache entry has never in the past been used in a particular mode, it not being used in it now is no reason for it to be eliminated.
However, certain cache entries are used in multiple build modes, and these should be shared across modes, so simply using different caches for different build modes is inefficient.
HOWEVER, having to declare ahead of time what the build modes are or which build modes a given cache entry will be used in is inconvenient. The caching library should be able to note which build modes each cache entry has been previously used in, and once an entry has been not used under builds of each mode that it previously had been used in, it should be automatically removed from the cache.
This was precisely the situation I found myself in, and autocache
claims to solve all of these problems.
Usage
import { autocache } from '@conduitry/autocache';
const cache = autocache('/path/to/cache.dat', 'mode name');
This library has one named export, autocache
, which is a function that accepts two arguments, path
and mode
.
path
is a string of the path of the file that is to be used to persist the cache. It should either be an absolute path, or the current working directory of the process shouldn't change throughout its lifetime, since this path is passed directly toreadFile
/writeFile
when loading and saving the cache datamode
is any primitive, immutable value representing the current build mode that the program is running in and which will affect when cache entries are eventually removed
const result = await cache.cache('some key', async () => { /* compute the value */ });
// ...
cache.close();
The value returned by autocache
is an object containing two functions, cache
and close
. This object is created by a closure and is not a class instance, so it is safe to destructure its methods.
cache
is a function to look up or to compute. It returns aPromise
(resolving to the cached or computed value), and accepts two arguments,key
andcompute_value
key
is a string uniquely identifying in some way the operation whose result you want to cache. This is hashed before it's saved or compared, so it can be long without bloating the cache filecompute_value
is a function that is passed no arguments and returns aPromise
resolving to the desired value. It will only be called if a value corresponding tokey
is not found
close
is a function that saves the current state of the cache back to disk, removing any entries it is safe to do this. This is implemented synchronously, specifically so that it's safe to run in aprocess.on('exit', () => { ... })
callback.
Details
The cache is persisted to disk as an object sent through v8.serialize
, so anything you try to cache will need to be serializable by that function.
When the compute_value
function is called, the Promise
that it returns will also be returned for any future calls issued with the same key
before the Promise
resolves.
The response is only persisted to the cache if the Promise
resolves successfully. Calling cache.close()
will not save, update, or mark as used any results that were still pending at the time or that threw or returned a Promise
that rejected.
Along with each cache entry is stored a list of the mode
s that entry has been used in. Whenever an entry is used (either through a cache hit or a cache miss), the current mode
is added to that entry's list. The current mode
is removed from all other entries' lists, and those entries whose list of mode
s is now empty are not written to disk when cache.close()
is called.