Chapter 2 Walkthrough

This chapter walks through a minimal example of a targets-powered data analysis project. The source code is here, and other examples are linked from the documentation website.

2.1 Goal of the analysis

The this minimal workflow is to assess the relationship among ozone, wind, and temperature in base R’s airquality dataset. We get the data from a file, preprocess it, visualize it, fit a regression model, and generate an R Markdown report to communicate the results.

2.2 File structure

The file structure of the project looks like this.

├── _targets.R
├── R/
├──── functions.R
├── data/
└──── raw_data.csv

raw_data.csv contains the data we want to analyze.


functions.R contains our custom user-defined functions. (See the best practices chapter for a discussion of function-oriented workflows.)

# functions.R
create_plot <- function(data) {
  ggplot(data) +
    geom_histogram(aes(x = Ozone)) +

Both raw_data.csv and functions.R are custom components. You as the user decide whether to use them, what to name them, and where to put them. However, the “target script” _targets.R file is special. (So special, in fact, that there is a special tar_script() function to help you create one). Every targets project requires a _targets.R script in the project’s root directory to declare and configure the pipeline. Ours looks like this.

# _targets.R
options(tidyverse.quiet = TRUE)
tar_option_set(packages = c("biglm", "rmarkdown", "tidyverse"))
    format = "file"
    read_csv(raw_data_file, col_types = cols())
    raw_data %>%
      mutate(Ozone = replace_na(Ozone, mean(Ozone, na.rm = TRUE)))
  tar_target(hist, create_plot(data)),
  tar_target(fit, biglm(Ozone ~ Wind + Temp, data))

All _targets.R scripts have these requirements.

  1. Load the targets package itself. (_targets.R scripts created with tar_script() automatically insert a library(targets) line at the top by default.)
  2. Load your custom functions and global objects into the R session. In our case, our only such object is the create_plot() function, and we load it into the session by calling source("R/functions.R").
  3. Call tar_option_set() to set the default settings for all you targets, such as the names of required packages and the data storage format. Individual targets can override these settings.
  4. Define individual targets with the tar_target() function. Each target is an intermediate step of the workflow. At minimum, a target must have a name and an R expression. This expression runs when the pipeline builds the target, and the return value is saved as a file in the _targets/objects/ folder. The only targets not stored in _/targets/objects/ are dynamic files such as raw_data_file. Here, format = "file" makes raw_data_file a dynamic file. That means targets watches the data at the file paths returned from the expression (in this case, "data/raw_data.csv").
  5. Gather the targets in a pipeline with the tar_pipeline() function. tar_pipeline() is flexible: it can accept individual tar_target() objects or nested lists of such objects. Every _targets.R script must end with a tar_pipeline() object, which usually means there is an explicit call to tar_pipeline() at the very bottom.

2.3 Inspect the pipeline

Before you run the pipeline for real, you should always inspect the manifest and the graph for errors. tar_manifest() shows you a data frame information about the targets, and it has functionality to specify the targets and columns returned.

tar_manifest(fields = "command")
#> # A tibble: 5 x 2
#>   name         command                                                          
#>   <chr>        <chr>                                                            
#> 1 raw_data     "read_csv(raw_data_file, col_types = cols())"                    
#> 2 fit          "biglm(Ozone ~ Wind + Temp, data)"                               
#> 3 hist         "create_plot(data)"                                              
#> 4 raw_data_fi… "\"data/raw_data.csv\""                                          
#> 5 data         "raw_data %>% mutate(Ozone = replace_na(Ozone, mean(Ozone,  \\n …

There are also graphical displays with tar_glimpse()


and tar_visnetwork().


Both graphing functions above visualize the underlying directed acyclic graph (DAG) and tell you how targets are connected. This DAG is indifferent to the order you write targets in tar_pipeline(). You will still get the same graph even if you rearrange them. This is because targets uses static code analysis to detect the dependencies of each target, and this process does not depend on target order. For details, visit the dependency detection section of the best practices guide.

2.4 Run the pipeline

tar_make() runs the workflow. It creates a fresh clean external R process, reads _targets.R to learn about the pipeline, runs the correct targets in the correct order given by the graph, and saves the necessary data to the _targets/ data store.

#> ● run target raw_data_file
#> ● run target raw_data
#> ● run target data
#> ● run target fit
#> ● run target hist

The next time you run tar_make(), targets skips everything that is already up to date, which saves a lot of time in large projects with long runtimes.

#> ✔ skip target raw_data_file
#> ✔ skip target raw_data
#> ✔ skip target data
#> ✔ skip target fit
#> ✔ skip target hist
#> ✔ Already up to date.

You can use tar_visnetwork() and tar_outdated() to check ahead of time which targets are up to date.

#> character(0)

2.5 Changes

The targets package notices when you make changes to your workflow, and tar_make() only runs the targets that need to build. There are custom rules called “cues” that targets uses to decide whether a target needs to rerun. For the full details on cues, read the “Details” section of the tar_cue() help file. (Enter ?targets::tar_cue into your R console.)

2.5.1 Change code

If you change one of your functions, the targets that depend on it will no longer be up to date, and tar_make() will rebuild them. For example, let’s set the number of bins in our histogram.

# Edit functions.R.
create_plot <- function(data) {
  ggplot(data) +
    geom_histogram(aes(x = Ozone), bins = 10) + # Set number of bins.

targets detects the change. hist is outdated (as would be any targets downstream of hist) and the others are still up to date.

#> [1] "hist"

That means tar_make() reruns hist and nothing else.

#> ✔ skip target raw_data_file
#> ✔ skip target raw_data
#> ✔ skip target data
#> ✔ skip target fit
#> ● run target hist

We would see similar behavior if we changed the R expressions in any tar_target() calls in _targets.R.

2.5.2 Change data

If we change the data file raw_data.csv, targets notices the change. This is because raw_data_file is a dynamic file (i.e. tar_target(format = "file")) that returned "raw_data.csv". Let’s try it out. Below, let’s use only the first 100 rows of the airquality dataset.

write_csv(head(airquality, n = 100), "data/raw_data.csv")

Sure enough, raw_data_file and everything downstream is out of date, so all our targets are outdated.

#> [1] "raw_data"      "fit"           "hist"          "raw_data_file"
#> [5] "data"
#> ● run target raw_data_file
#> ● run target raw_data
#> ● run target data
#> ● run target fit
#> ● run target hist

2.6 Read your data

targets has a convenient functions tar_read() to read your data from the _targets/ data store.


There is also a tar_load() function, which supports tidyselect verbs like starts_with()

#> Loading required package: DBI
#> Large data regression model: biglm(Ozone ~ Wind + Temp, data)
#> Sample size =  100

The purpose of tar_read() and tar_load() is to make exploratory data analysis easy and convenient. Use these functions to verify the correctness of the output from the pipeline and come up with ideas for new targets if needed.

2.7 Read metadata

To read the build progress of your targets while tar_make() is running, you can open a new R session and run tar_progress(). It reads the spreadsheet in _targets/meta/progress and tells you which targets are running, built, errored, or cancelled.
#>            name progress
#> 1 raw_data_file    built
#> 2      raw_data    built
#> 3          data    built
#> 4           fit    built
#> 5          hist    built

Likewise, the tar_meta() function reads _targets/meta/meta and tells you high-level information about the target’s settings, data, and results. The warnings, error, and traceback columns give you diagnostic information about targets with problems.
#>            name     type             data          command           depend
#> 1   create_plot function 658f2a44b31f63bb             <NA>             <NA>
#> 2 raw_data_file     stem 2b16f490030787ce b6df0c34fc22d1b9 ef46db3751d8e999
#> 3      raw_data     stem 81f24e414a848be0 000ed0cc054f0d35 6ef08fe7f06c82ac
#> 4          data     stem 1e1d6c4923db1fa9 df3101ce91c63e21 1e037ea6a50b00ac
#> 5           fit     stem d01f8b0736dde4fe aa0df6c5dbd10537 fb68e10410f0f3c8
#> 6          hist     stem 68c45d373f81c549 68877181ab74e51e 39b2e88285ec367d
#>          seed                      path             time             size bytes
#> 1          NA                        NA             <NA>             <NA>    NA
#> 2  2110307107         data/raw_data.csv 09e1eebeacd2d2f9 9fb65f8dffe0153b  1884
#> 3  -979620141 _targets/objects/raw_data 1e635fd988109aaa 029e3f18865b911c  1152
#> 4  1588979285     _targets/objects/data 6fa8095ebdae2171 a744f0f8f0be9d70  1156
#> 5  1780184594      _targets/objects/fit 3ab68d7827354d48 31f6a3e6158ce3a3  1608
#> 6 -1026346201     _targets/objects/hist 89280bc0088eb234 631abafa35c3f47a 44325
#>   format iteration parent children seconds warnings error
#> 1   <NA>      <NA>     NA       NA      NA       NA    NA
#> 2   file    vector     NA       NA   1.391       NA    NA
#> 3    rds    vector     NA       NA   0.036       NA    NA
#> 4    rds    vector     NA       NA   0.014       NA    NA
#> 5    rds    vector     NA       NA   0.003       NA    NA
#> 6    rds    vector     NA       NA   0.013       NA    NA

The _targets/meta/meta spreadsheet file is critically important. Although targets can still work properly if files are missing from _targets/objects, the pipeline will error out if _targets/meta/meta is corrupted. If tar_meta() works, the project should be fine.

Copyright Eli Lilly and Company