# Introduction to crew

`crew` is a distributed computing framework with a centralized interface and auto-scaling. A `crew` controller is an object in R which accepts tasks, returns results, and launches workers. Workers can be local processes, jobs on traditional clusters such as SLURM, or jobs on cloud services such as AWS Batch, depending on the launcher plugin of the controller.

# Tasks vs workers

A task is a piece of R code, such as an expression or a function call. A worker is a non-interactive R process that runs one or more tasks. When tasks run on workers, the local R session is free and responsive, and work gets done faster. For example, this vignette shows how `crew` and `mirai` work together to speed up Shiny apps.

# How to use `crew`

First, create a controller object to manage tasks and workers.

    library(crew)
    controller <- crew_controller_local(
      name = "example",
      workers = 2,
      seconds_idle = 10
    )

Next, start the controller to create the `mirai` client. Later, when you are done with the controller, call `controller$terminate()` to clean up the workers and dispatcher.

    controller$start()

Use `push()` to submit a new task and `pop()` to return a completed task.

    controller$push(name = "get pid", command = ps::ps_pid())

As a side effect, methods `push()`, `pop()`, and `scale()` also launch workers to run the tasks. If your controller uses transient workers and has a backlog of tasks, you may need to loop over `pop()` or `scale()` multiple times to make sure enough workers are always available.

    controller$pop() # No workers started yet and the task is not done.
    #> NULL

    task <- controller$pop() # Worker started, task complete.
    task
    #> # A tibble: 1 × 12
    #>   name    command result seconds  seed algorithm error trace warnings
    #>   <chr>   <chr>   <list>   <dbl> <int> <chr>     <chr> <chr> <chr>
    #> 1 get pid NA      <int>        0    NA NA        NA    NA    NA
    #> # ℹ 3 more variables: launcher <chr>, worker <int>, instance <chr>

Alternatively, `wait()` is a loop that repeatedly checks tasks and launches workers until all tasks complete.

    controller$wait(mode = "all")

The return value of the task is in the `result` column.

    task$result[[1]] # return value of the task
    #> [1] 69631

Here is the full list of output in the `task` object returned by `pop()`.

- `name`: the task name if given.
- `command`: a character string with the R command if `save_command` was set to `TRUE` in `push()`.
- `result`: a list containing the return value of the R command.
- `seconds`: number of seconds that the task ran.
- `seed`: the single integer originally supplied to `push()`, `NA` if `seed` was supplied as `NULL`.
- `algorithm`: name of the pseudo-random number generator algorithm originally supplied to `push()`, `NA` if `algorithm` was supplied as `NULL`.
- `error`: the first 2048 characters of the error message if the task threw an error, `NA` otherwise.
- `trace`: the first 2048 characters of the text of the traceback if the task threw an error, `NA` otherwise.
- `warnings`: the first 2048 characters. of the text of warning messages that the task may have generated, `NA` otherwise.
- `launcher`: name of the `crew` launcher where the task ran.

If `seed` and `algorithm` are both non-missing in the output, then you can recover the pseudo-random number generator state of the task using `set.seed(seed = seed, kind = algorithm)`. However, it is recommended to supply `NULL` to these arguments in `push()`, in which case you will observe `NA` in the outputs. With `seed` and `algorithm` both `NULL`, the random number generator defaults to the recommended widely spaced worker-specific L’Ecuyer streams supported by `mirai::nextstream()`. See `vignette("parallel", package = "parallel")` for details.

# Functional programming

The `map()` method of the controller supports functional programming similar to `purrr::map()` and `clustermq::Q()`. The arguments of `map()` are mostly the same those of `push()`, but there is a new `iterate` argument to define the inputs of individual tasks. `map()` submits a whole collection of tasks, auto-scales the workers, waits for all the tasks to finish, and returns the results in a `tibble`.

Below, `map()` submits one task to compute `1 + 2 + 5 + 6` and another task to compute `3 + 4 + 5 + 6`. The lists and vectors inside `iterate` vary from task to task, while the elements of `data` and `globals` stay constant across tasks.

    results <- controller$map(
      command = a + b + c + d,
      iterate = list(
        a = c(1, 3),
        b = c(2, 4)
      ),
      data = list(c = 5),
      globals = list(d = 6)
    )

    results
    #> # A tibble: 2 × 12
    #>   name  command result    seconds  seed algorithm error trace warnings
    #>   <chr> <chr>   <list>      <dbl> <int> <chr>     <chr> <chr> <chr>
    #> 1 1     NA      <dbl [1]>       0    NA NA        NA    NA    NA
    #> 2 2     NA      <dbl [1]>       0    NA NA        NA    NA    NA
    #> # ℹ 3 more variables: launcher <chr>, worker <int>, instance <chr>

    as.numeric(results$result)
    #> [1] 14 18

If at least one task in `map()` throws an error, the default behavior is to error out in the main session and not return the results, If that happens, the results are available in the `controller$error`. To return the results instead of setting `controller$error`, regardless of error status, set `error = "warn"` or `"silent"` in `map()`. To conserve memory, consider setting `controller$error <- NULL` when you are done troubleshooting.

# Summaries

The controller summary shows how many tasks each worker ran, how many total seconds it spent running tasks, and how many tasks threw warnings and errors.

    controller$summary()
    #> # A tibble: 2 × 6
    #>   controller worker tasks seconds errors warnings
    #>   <chr>       <int> <int>   <dbl>  <int>    <int>
    #> 1 example         1     2   0.001      0        0
    #> 2 example         2     1   0          0        0

The launcher summary counts the number of times each worker was launched, and it shows the total number of assigned and completed tasks from all past terminated instances of each worker. In addition, it shows whether the current worker instance was actively connected (“online”) or had connected at some point during its life cycle (“discovered”) as of the last call to `controller$launcher$tally()`.

    controller$launcher$summary()
    #> # A tibble: 2 × 6
    #>   worker launches online discovered assigned complete
    #>    <int>    <int> <lgl>  <lgl>         <int>    <int>
    #> 1      1        2 TRUE   TRUE              0        0
    #> 2      2        1 TRUE   TRUE              0        0

Finally, the client summary shows up-to-date worker status from `mirai::daemons()`.

    controller$client$summary()
    #> # A tibble: 2 × 6
    #>   worker online instances assigned complete socket
    #>    <int> <lgl>      <int>    <int>    <int> <chr>
    #> 1      1 FALSE          1        2        2 ws://10.0.0.32:58685/1/15e07250…
    #> 2      2 FALSE          1        1        1 ws://10.0.0.32:58685/2/cb45b3d4…

# Termination

Call `terminate()` on the controller after you finish using it. `terminate()` tries to close the the `mirai` dispatcher and any workers that may still be running. It is important to free up these resources.

    controller$terminate()

The `mirai` dispatcher process should exit on its own, but if not, you can manually terminate the process ID at `controller$client$dispatcher` or call `crew_clean()` to terminate any dispatchers from current or previous R sessions.

    crew_clean()
    #> nothing to clean up

# Tuning and auto-scaling

As explained above, `push()`, `pop()`, and `wait()` launch new workers to run tasks. The number of new workers depends on the number of tasks at the time. In addition, workers can shut themselves down as work completes. In other words, `crew` automatically raises and lowers the number of workers in response to fluctuations in the task workload.

The most useful arguments for down-scaling, in order of importance, are:

1. `seconds_idle`: shut down a worker if it spends too long waiting for a task.
2. `tasks_max`: shut down a worker after it completes a certain number of tasks.
3. `seconds_wall`: soft wall time of a worker.

Please tune these these arguments to achieve the desired balance for auto-scaling. The two extremes of auto-scaling are `clustermq`-like persistent workers and `future`-like transient workers, and each is problematic in its own way.

1. Persistent workers: a persistent worker launches once, typically runs many tasks, and stays running for the entire lifetime of the controller. Persistent workers minimize overhead and quickly complete large numbers of short tasks. However, they risk spending too much time in an idle state if there are no tasks to run. Excessive idling wastes resources, which could impact your colleagues on a shared cluster or drive up costs on Amazon Web Services.
2. Transient workers: a transient worker terminates as soon as it completes a single task. Each subsequent task requires a new transient worker to run it. Transient workers avoid excessive idling, but frequent worker launches cause significant overhead and slows down the computation as a whole.

# Asynchronous management of workers

Some launchers support local processes to launch and terminate workers asynchronously. For example, a cloud-based launcher may need to make HTTP requests to launch and terminate workers on e.g. AWS Batch, and these time-consuming requests should happen in the background. Controllers that support this will have a `processes` argument to specify the number of local R processes to churn through worker launches and terminations. Set `processes = NULL` to disable async, which can be helpful for troubleshooting.