Cache method that accommodates environments, S4 methods, Rasters, & nested caching

The special assign operator %<% is equivalent to Cache. See examples at the end.

Cache(FUN, ..., notOlderThan = NULL, objects = NULL, outputObjects = NULL,
  algo = "xxhash64", cacheRepo = NULL, length = 1e+06,
  compareRasterFileLength, userTags = c(), digestPathContent,
  omitArgs = NULL, classOptions = list(), debugCache = character(),
  sideEffect = FALSE, makeCopy = FALSE,
  quick = getOption("reproducible.quick", FALSE),
  verbose = getOption("reproducible.verbose", FALSE), cacheId = NULL,
  useCache = getOption("reproducible.useCache", TRUE), showSimilar = NULL)

# S4 method for ANY
Cache(FUN, ..., notOlderThan = NULL, objects = NULL,
  outputObjects = NULL, algo = "xxhash64", cacheRepo = NULL,
  length = 1e+06, compareRasterFileLength, userTags = c(),
  digestPathContent, omitArgs = NULL, classOptions = list(),
  debugCache = character(), sideEffect = FALSE, makeCopy = FALSE,
  quick = getOption("reproducible.quick", FALSE),
  verbose = getOption("reproducible.verbose", FALSE), cacheId = NULL,
  useCache = getOption("reproducible.useCache", TRUE), showSimilar = NULL)

lhs %<% rhs

Arguments

FUN	Either a function or an unevaluated function call (e.g., using `quote`.
...	Arguments of `FUN` function .
notOlderThan	load an artifact from the database only if it was created after notOlderThan.
objects	Character vector of objects to be digested. This is only applicable if there is a list, environment or simList with named objects within it. Only this/these objects will be considered for caching, i.e., only use a subset of the list, environment or simList objects.
outputObjects	Optional character vector indicating which objects to return. This is only relevant for `simList` objects
algo	The algorithms to be used; currently available choices are `md5`, which is also the default, `sha1`, `crc32`, `sha256`, `sha512`, `xxhash32`, `xxhash64` and `murmur32`.
cacheRepo	A repository used for storing cached objects. This is optional if `Cache` is used inside a SpaDES module.
length	Numeric. If the element passed to Cache is a `Path` class object (from e.g., `asPath(filename)`) or it is a `Raster` with file-backing, then this will be passed to `digest::digest`, essentially limiting the number of bytes to digest (for speed). This will only be used if `quick = FALSE`.
compareRasterFileLength	Being deprecated; use `length`.
userTags	A character vector with Tags. These Tags will be added to the repository along with the artifact.
digestPathContent	Being deprecated. Use `quick`.
omitArgs	Optional character string of arguments in the FUN to omit from the digest.
classOptions	Optional list. This will pass into `.robustDigest` for specific classes. Should be options that the `.robustDigest` knows what to do with.
debugCache	Character or Logical. Either `"complete"` or `"quick"` (uses partial matching, so "c" or "q" work). `TRUE` is equivalent to `"complete"`. If `"complete"`, then the returned object from the Cache function will have two attributes, `debugCache1` and `debugCache2`, which are the entire `list(...)` and that same object, but after all `.robustDigest` calls, at the moment that it is digested using `fastdigest`, respectively. This `attr(mySimOut, "debugCache2")` can then be compared to a subsequent call and individual items within the object `attr(mySimOut, "debugCache1")` can be compared. If `"quick"`, then it will return the same two objects directly, without evalutating the `FUN(...)`.
sideEffect	Logical or path. Determines where the function will look for new files following function completion. See Details. NOTE: this argument is experimental and may change in future releases.
makeCopy	Logical. If `sideEffect = TRUE`, and `makeCopy = TRUE`, a copy of the downloaded files will be made and stored in the `cacheRepo` to speed up subsequent file recovery in the case where the original copy of the downloaded files are corrupted or missing. Currently only works when set to `TRUE` during the first run of `Cache`. Default is `FALSE`. NOTE: this argument is experimental and may change in future releases.
quick	Logical. If `TRUE`, little or no disk-based information will be assessed, i.e., mostly its memory content. This is relevant for objects of class `Path` and `Raster` currently. For class `Path` objects, the file's metadata (i.e., filename and file size) will be hashed instead of the file contents. If set to `FALSE` (default), the contents of the file(s) are hashed. If `quick = TRUE`, `length` is ignored. NOTE: this argument is experimental and may change in future releases.
verbose	Logical. This will output much more information about the internals of Caching, which may help diagnose Caching challenges.
cacheId	Character string. If passed, this will override the calculated hash of the inputs, and return the result from this cacheId in the cacheRepo. In general, this is not used; however, in some particularly finicky situations where Cache is not correctly detecting unchanged inputs, this can stabilize the return value.
useCache	Logical. If `FALSE`, then the entire Caching mechanism is bypassed and the function is evaluated as if it was not being Cached. Default is `getOption("reproducible.useCache")`), which is `FALSE` by default, meaning use the Cache mechanism. This may be useful to turn all Caching on or off in very complex scripts and nested functions.
showSimilar	A logical or numeric. Useful for debugging. If `TRUE` or `1`, then if the Cache does not find an identical archive in the cacheRepo, it will report (via message) the next most similar archive, and indicate which argument(s) is/are different. If a number larger than `1`, then it will report the N most similar archived objects.
lhs	A name to assign to.
rhs	A function call

Value

As with cache, returns the value of the function call or the cached version (i.e., the result from a previous call to this same cached function with identical arguments).

Details

Caching R objects using cache has five important limitations:

the archivist package detects different environments as different;
it also does not detect S4 methods correctly due to method inheritance;
it does not detect objects that have file-base storage of information (specifically RasterLayer-class objects);
the default hashing algorithm is relatively slow.
heavily nested function calls may want Cache arguments to propagate through

This version of the Cache function accommodates those four special, though quite common, cases by:

converting any environments into list equivalents;
identifying the dispatched S4 method (including those made through inheritance) before hashing so the correct method is being cached;
by hashing the linked file, rather than the Raster object. Currently, only file-backed Raster* objects are digested (e.g., not ff objects, or any other R object where the data are on disk instead of in RAM);
using fastdigest internally when the object is in RAM, which can be up to ten times faster than digest. Note that file-backed objects are still hashed using digest.
Cache will save arguments passed by user in a hidden environment. Any nested Cache functions will use arguments in this order 1) actual arguments passed at each Cache call, 2) any inherited arguments from an outer Cache call, 3) the default values of the Cache function. See section on Nested Caching.

If Cache is called within a SpaDES module, then the cached entry will automatically get 3 extra userTags: eventTime, eventType, and moduleName. These can then be used in clearCache to selectively remove cached objects by eventTime, eventType or moduleName.

Cache will add a tag to the artifact in the database called accessed, which will assign the time that it was accessed, either read or write. That way, artifacts can be shown (using showCache) or removed (using clearCache) selectively, based on their access dates, rather than only by their creation dates. See example in clearCache. Cache (uppercase C) is used here so that it is not confused with, and does not mask, the archivist::cache function.

Note

As indicated above, several objects require pre-treatment before caching will work as expected. The function .robustDigest accommodates this. It is an S4 generic, meaning that developers can produce their own methods for different classes of objects. Currently, there are methods for several types of classes. See .robustDigest.

See .robustDigest for other specifics for other classes.

Nested Caching

Commonly, Caching is nested, i.e., an outer function is wrapped in a Cache function call, and one or more inner functions are also wrapped in a Cache function call. A user can always specify arguments in every Cache function call, but this can get tedious and can be prone to errors. The normal way that R handles arguments is it takes the user passed arguments if any, and default arguments for all those that have no user passed arguments. We have inserted a middle step. The order or precedence for any given Cache function call is 1. user arguments, 2. inherited arguments, 3. default arguments. At this time, the top level Cache arguments will propagate to all inner functions unless each individual Cache call has other arguments specified, i.e., "middle" nested Cache function calls don't propagate their arguments to further "inner" Cache function calls. See example.

userTags is unique of all arguments: its values will be appended to the inherited userTags.

Caching Speed

Caching speed may become a critical aspect of a final product. For example, if the final product is a shiny app, rerunning the entire project may need to take less then a few seconds at most. There are 3 arguments that affect Cache speed: quick, length, and algo. quick is passed to .robustDigest, which currently only affects Path and Raster* class objects. In both cases, quick means that little or no disk-based information will be assessed.

Filepaths

If a function has a path argument, there is some ambiguity about what should be done. Possibilities include:

hash the string as is (this will be very system specific, meaning a Cache call will not work if copied between systems or directories);
hash the basename(path);
hash the contents of the file.

If paths are passed in as is (i.e,. character string), the result will not be predictable. Instead, one should use the wrapper function asPath(path), which sets the class of the string to a Path, and one should decide whether one wants to digest the content of the file (using quick = FALSE), or just the filename ((quick = TRUE)). See examples.

Stochasticity

In general, it is expected that caching will only be used when stochasticity is not relevant, or if a user has achieved sufficient stochasticity (e.g., via sufficient number of calls to experiment) such that no new explorations of stochastic outcomes are required. It will also be very useful in a reproducible workflow.

`sideEffect`

If sideEffect is not FALSE, then metadata about any files that added to sideEffect will be added as an attribute to the cached copy. Subsequent calls to this function will assess for the presence of the new files in the sideEffect location. If the files are identical (quick = FALSE) or their file size is identical (quick = TRUE), then the cached copy of the function will be returned (and no files changed). If there are missing or incorrect files, then the function will re-run. This will accommodate the situation where the function call is identical, but somehow the side effect files were modified. If sideEffect is logical, then the function will check the cacheRepo; if it is a path, then it will check the path. The function will assess whether the files to be downloaded are found locally prior to download. If it fails the local test, then it will try to recover from a local copy if (makeCopy had been set to TRUE the first time the function was run. Currently, local recovery will only work ifmakeCOpy was set to TRUE the first time Cache was run). Default is FALSE.

Examples

tmpDir <- file.path(tempdir())

# Basic use
ranNumsA <- Cache(rnorm, 10, 16, cacheRepo = tmpDir)

# All same
ranNumsB <- Cache(rnorm, 10, 16, cacheRepo = tmpDir) # recovers cached copy
#>   loading cached result from previous rnorm call, adding to memoised copy
ranNumsC <- rnorm(10, 16) %>% Cache(cacheRepo = tmpDir) # recovers cached copy
#>   loading memoised result from previous 'rnorm' pipe sequence call.
ranNumsD <- Cache(quote(rnorm(n = 10, 16)), cacheRepo = tmpDir) # recovers cached copy
#>   loading memoised result from previous rnorm call.
# For more in depth uses, see vignette
# NOT RUN {
  browseVignettes(package = "reproducible")
# }# Equivalent
a <- Cache(rnorm, 1)
#> No cacheRepo supplied. Using value in getOption('reproducible.cachePath')
#>   loading cached result from previous rnorm call, adding to memoised copy
b %<% rnorm(1)
#> No cacheRepo supplied. Using value in getOption('reproducible.cachePath')
#>   loading memoised result from previous FUN call.