The Workflow Lifecycle

Last updated on 2024-09-24 | Edit this page

Overview

Questions

  • What happens if we re-run a workflow?
  • How does targets know what steps to re-run?
  • How can we inspect the state of the workflow?

Objectives

  • Explain how targets helps increase efficiency
  • Be able to inspect a workflow to see what parts are outdated

Re-running the workflow

One of the features of targets is that it maximizes efficiency by only running the parts of the workflow that need to be run.

This is easiest to understand by trying it yourself. Let’s try running the workflow again:

R 

tar_make()

OUTPUT 

✔ skipped target penguins_csv_file
✔ skipped target penguins_data_raw
✔ skipped target penguins_data
✔ skipped pipeline [0.092 seconds]

Remember how the first time we ran the pipeline, targets printed out a list of each target as it was being built?

This time, it tells us it is skipping those targets; they have already been built, so there’s no need to run that code again.

Remember, the fastest code is the code you don’t have to run!

Re-running the workflow after modification

What happens when we change one part of the workflow then run it again?

Say that we decide the species names should be shorter. Right now they include the common name and the scientific name, but we really only need the first part of the common name to distinguish them.

Edit _targets.R so that the clean_penguin_data() function looks like this:

R 

clean_penguin_data <- function(penguins_data_raw) {
  penguins_data_raw |>
    select(
      species = Species,
      bill_length_mm = `Culmen Length (mm)`,
      bill_depth_mm = `Culmen Depth (mm)`
    ) |>
    remove_missing(na.rm = TRUE) |>
    # Split "species" apart on spaces, and only keep the first word
    separate(species, into = "species", extra = "drop")
}

Then run it again.

R 

tar_make()

OUTPUT 

✔ skipped target penguins_csv_file
✔ skipped target penguins_data_raw
▶ dispatched target penguins_data
● completed target penguins_data [0.059 seconds]
▶ ended pipeline [0.158 seconds]

What happened?

This time, it skipped penguins_csv_file and penguins_data_raw and only ran penguins_data.

Of course, since our example workflow is so short we don’t even notice the amount of time saved. But imagine using this in a series of computationally intensive analysis steps. The ability to automatically skip steps results in a massive increase in efficiency.

Challenge 1: Inspect the output

How can you inspect the contents of penguins_data?

With tar_read(penguins_data) or by running tar_load(penguins_data) followed by penguins_data.

Under the hood

How does targets keep track of which targets are up-to-date vs. outdated?

For each target in the workflow (items in the list at the end of the _targets.R file) and any custom functions used in the workflow, targets calculates a hash value, or unique combination of letters and digits that represents an object in the computer’s memory. You can think of the hash value (or “hash” for short) as a unique fingerprint for a target or function.

The first time your run tar_make(), targets calculates the hashes for each target and function as it runs the code and stores them in the targets cache (the _targets folder). Then, for each subsequent call of tar_make(), it calculates the hashes again and compares them to the stored values. It detects which have changed, and this is how it knows which targets are out of date.

Where the hashes live

If you are curious about what the hashes look like, you can see them in the file _targets/meta/meta, but do not edit this file by hand—that would ruin your workflow!

This information is used in combination with the dependency relationships (in other words, how each target depends on the others) to re-run the workflow in the most efficient way possible: code is only run for targets that need to be re-built, and others are skipped.

Visualizing the workflow

Typically, you will be making edits to various places in your code, adding new targets, and running the workflow periodically. It is good to be able to visualize the state of the workflow.

This can be done with tar_visnetwork()

R 

tar_visnetwork()
Visualization of the targets worklow, showing 'penguins_data' connected by lines to 'penguins_data_raw', 'penguins_csv_file' and 'clean_penguin_data'

You should see the network show up in the plot area of RStudio.

It is an HTML widget, so you can zoom in and out (this isn’t important for the current example since it is so small, but is useful for larger, “real-life” workflows).

Here, we see that all of the targets are dark green, indicating that they are up-to-date and would be skipped if we were to run the workflow again.

Installing visNetwork

You may encounter an error message The package "visNetwork" is required.

In this case, install it first with install.packages("visNetwork").

Challenge 2: What else can the visualization tell us?

Modify the workflow in _targets.R, then run tar_visnetwork() again without running tar_make(). What color indicates that a target is out of date?

Light blue indicates the target is out of date.

Depending on how you modified the code, any or all of the targets may now be light blue.

‘Outdated’ does not always mean ‘will be run’

Just because a target appears as light blue (is “outdated”) in the network visualization, this does not guarantee that it will be re-built during the next run. Rather, it means that at least of one the targets that it depends on has changed.

For example, if the workflow state looked like this:

A -> B* -> C -> D

where the * indicates that B has changed compared to the last time the workflow was run, the network visualization will show B, C, and D all as light blue.

But if re-running the workflow results in the exact same value for C as before, D will not be re-run (will be “skipped”).

Most of the time, a single change will cascade to the rest of the downstream targets and cause them to be re-built, but this is not always the case. targets has no way of knowing ahead of time what the actual output will be, so it cannot provide a network visualization that completely predicts the future!

Other ways to check workflow status

The visualization is very useful, but sometimes you may be working on a server that doesn’t provide graphical output, or you just want a quick textual summary of the workflow. There are some other useful functions that can do that.

tar_outdated() lists only the outdated targets; that is, targets that will be built during the next run, or depend on such a target. If everything is up to date, it will return a zero-length character vector (character(0)).

R 

tar_outdated()

OUTPUT 

character(0)

tar_progress() shows the current status of the workflow as a dataframe. You may find it helpful to further manipulate the dataframe to obtain useful summaries of the workflow, for example using dplyr (such data manipulation is beyond the scope of this lesson but the instructor may demonstrate its use).

R 

tar_progress()

OUTPUT 

# A tibble: 3 × 2
  name              progress
  <chr>             <chr>
1 penguins_csv_file skipped
2 penguins_data_raw skipped
3 penguins_data     completed

Granular control of targets

It is possible to only make a particular target instead of running the entire workflow.

To do this, type the name of the target you wish to build after tar_make() (note that any targets required by the one you specify will also be built). For example, tar_make(penguins_data_raw) would only build penguins_data_raw, not penguins_data.

Furthermore, if you want to manually “reset” a target and make it appear out-of-date, you can do so with tar_invalidate(). This means that target (and any that depend on it) will be re-run next time.

Let’s give this a try. Remember that our pipeline is currently up to date, so tar_make() will skip everything:

R 

tar_make()

OUTPUT 

✔ skipped target penguins_csv_file
✔ skipped target penguins_data_raw
✔ skipped target penguins_data
✔ skipped pipeline [0.133 seconds]

Let’s invalidate penguins_data and run it again:

R 

tar_invalidate(penguins_data)
tar_make()

OUTPUT 

✔ skipped target penguins_csv_file
✔ skipped target penguins_data_raw
▶ dispatched target penguins_data
● completed target penguins_data [0.057 seconds]
▶ ended pipeline [0.154 seconds]

If you want to reset everything and start fresh, you can use tar_invalidate(everything()) (tar_invalidate() accepts tidyselect expressions to specify target names).

Caution should be exercised when using granular methods like this, though, since you may end up with your workflow in an unexpected state. The surest way to maintain an up-to-date workflow is to run tar_make() frequently.

How this all works in practice

In practice, you will likely be switching between running the workflow with tar_make(), loading the targets you built with tar_load(), and editing your custom functions by running code in an interactive R session. It takes some time to get used to it, but soon you will feel that your code isn’t “real” until it is embedded in a targets workflow.

Key Points

  • targets only runs the steps that have been affected by a change to the code
  • tar_visnetwork() shows the current state of the workflow as a network
  • tar_progress() shows the current state of the workflow as a data frame
  • tar_outdated() lists outdated targets
  • tar_invalidate() can be used to invalidate (re-run) specific targets