Suggestion: future::with_plan() and/or future::local_plan()

[wlandau]

I am picturing something similar to the with_*() and local_*() functions from withr. Example use case: ropensci/drake#561. When a drake user selects a distributed parallel backend, the imported functions and files are still processed locally. (If we are just hashing an input data file, it does not make sense to send the job to a compute node on a cluster.) In drake, it might be nice to write something like this:

make <- function(...){
  future::with_plan(multiprocess, {
    process_imports(...)
  }
  build_targets(...) # Uses whatever future::plan() the user selects before make().
}

Or alternatively:

make <- function(...){
  process_imports(...)
  build_targets(...) # Uses whatever future::plan() the user selects before make().
}

where

process_imports <- function(...){
  if (jobs > 1L) {
    future::local_plan(multiprocess)
    launch_futures_for_imports(...)
  } else {
    just_loop_through_imports(...)
  }
}

[HenrikBengtsson]

As you've might have noticed (and for "new comers"), I'm really hesitant suggesting that developers could/should set plan locally in their code. I really want to avoid design patterns such as:

foo <- function(..., cores = 1) {
  oplan <- plan("list")
  on.exit(plan(oplan))  # <= super important!
  plan(multiprocess, workers = cores)
  ...
}

that may appear out of habit and will just lock the users into a specific parallel backend although some users may have way more powerful alternatives at hand, e.g. 1,000's of cores on a compute cluster.
Then, it is much better to leave it to the end user to control how to parallelize.

Having said this, I do understand that there are use cases where you need to temporarily use another future backend than what is already set (by the user) with plan(). Then, yes, something like:

withPlan <- function(expr, ..., envir = parent.frame()) {
   expr <- substitute(expr)
   oplan <- plan("list")
   on.exit(plan(oplan))
   plan(...)
   eval(expr, envir = envir)
}

is handy. I prefer to hold back on adding such a function to the future package because of the above concerns. On the other hand, the advantage of providing such a function would be lowered risk for code/packages messing with the user's plan() settings. I need to digest this feature request more before make a decision. In the meanwhile, cut'n'paste the above withPlan() but please consider if it is really needed.

Long-term thoughts: Related to our discussion in Issue #181, it would be better if the developer needs could be handled by only specifying what type of resources they need, e.g. "give me a local parallel backend - I don't care exactly which". Now, such a framework will not be in place anytime soon (it's only being processed on idle speed far back somewhere in my head).

EDIT 2019-06-26: Previous version claimed that oplan <- plan(...) was enough, but that will only return the future strategy on top of the stack, not all of the stack in case there are nested strategies set. Only order to get ahold of the stack, one need to call plan("list").

[wlandau]

That makes sense, and I respect your decision to separate the choice of backend from the hard-coded workflow. Because of ropensci/drake#598 (comment), my original point in this thread might actually become moot.

For the original motivating use case of #181, we might actually be able to stick with a single user-defined future::plan(). Among other things, a couple users and I were hoping that different remote transient workers could have different walltimes, memory limits, etc.

[wlandau]

On further reflection, this feels like something I could implement in drake using the resources argument to future().

[HenrikBengtsson]

Note that the resources argument is only acknowledged by future.batchtools and is not (yet) part of the Future API per because I don't what the exact content/format of resources should be. I'm open to proposal.

[wlandau]

I think that it still suits the use case I was picturing.

[HenrikBengtsson]

UPDATE 2019-06-26: There was a mistake in my example code of comment #263 (comment). I've updated it accordingly. Please see the 'EDIT' note at the very end.

[HenrikBengtsson]

... and, before you guys rush and update your code, I'll see if I can change the default value of plan() to return all of the stack rather than just the strategy on top of the stack. That's certainly a nice behavior. If that's doable, you do not have to change anything.

[HenrikBengtsson]

FYI, in the next release, it'll be slightly easier to undo the future plan, e.g.

my_fcn <- function(...) {
   oplan <- plan("multicore")
   on.exit(plan(oplan))
   ...
}

vs

my_fcn <- function(...) {
  oplan <- plan("list")
  on.exit(plan(oplan))
  plan("multicore")
  ...
}

In other words, when setting a new set of strategies, plan(...) will return the full list of previous strategies (== plan("list")) and not just the next strategy on the stack (== plan("next")). This means that the following will work:

plan(list(A = multicore, B = batchtools_local))

# temporary use sequential processing
oplan <- plan(sequential)
y <- future.apply::future_lapply(1:10, FUN = identical)
# undo strategy stack
plan(oplan)

Previously, the above code would effectively do:

plan(oplan[[1]])

that is, plan(multicore) not plan(list(A = multicore, B = batchtools_local)).