DiskCaches

Basic Julia implementation of a cache mapped to the filesystem, for persistence across multiple Julia sessions. This implementation is intended to be thread and process safe, and files are formatted with Serialization.

A DiskCache can associate keys with values, but cannot modify existing associations. Reading entries already on chip is fast, but writing values to disk is quite slow. The cache follows the AbstractDict interface, but methods which modify existing associations in the dictionary will error.

This package was designed with memoization in mind via MemoizedMethods.jl.

julia> using DiskCaches

julia> c = DiskCache("path_to_cache.jls")
DiskCache{Any,Any,Dict{Any,Any}}()

julia> c[1] = 2
2

julia> c[2] = 3
3

julia> c[1]
2

julia> c[1] = 0
ERROR: To ensure the validity of the on-chip cache, DiskCaches do not support value modification.

Again, these caches don't support modifications to existing key-value pairs. It's easiest to atomically "add a value if the key doesn't exist yet" with the get! functions.

julia> get!(c, 1, 42)
2

julia> get!(c, 3) do 4 end
4

Multiple caches pointed at the same file will shadow each other.

julia> c_shadow = DiskCache("path_to_cache.jls")
DiskCache{Any,Any,Dict{Any,Any}} with 3 entries:
  2 => 3
  3 => 4
  1 => 2

julia> c_shadow[4] = 5
5

julia> c
DiskCache{Any,Any,Dict{Any,Any}} with 3 entries:
  2 => 3
  3 => 4
  4 => 5
  1 => 2

... in a separate Julia session ...

julia> using DiskCaches

julia> c = DiskCache("path_to_cache.jls")
DiskCache{Any,Any,Dict{Any,Any}} with 3 entries:
  2 => 3
  3 => 4
  4 => 5
  1 => 2

DiskCaches use an on-chip cache which also defines the behavior (e.g. == vs. ===) of the associative collection. By default, DiskCaches use Dict (not IdDict) as the on-chip cache. You may provide a specialized AbstractDict type to be used by DiskCaches, as long as it may be serialized, does not delete its values, and supports a no-argument constructor. Nesting DiskCaches with the same file will result in deadlock.

julia> c = DiskCache{Int,Int,IdDict{Int,Int}}("path_to_special_cache.jls")
DiskCache{Int64,Int64,IdDict{Int64,Int64}}()

If you want to delete a DiskCache and are sure that no other caches with the same path will be used in the future, rm the file and make a new DiskCache with the same filename. This is a relatively unsafe operation.

julia> c = DiskCache("path_to_cache.jls")
DiskCache{Any,Any,Dict{Any,Any}} with 3 entries:
  2 => 3
  3 => 4
  4 => 5
  1 => 2

julia> rm("path_to_cache.jls")

julia> c = DiskCache("path_to_cache.jls")
DiskCache{Any,Any,Dict{Any,Any}}()