Introduction to jagstargets

The jagstargets package makes it easy to run a single jags model and keep track of the results. R2jags fits the models, and targets manages the workflow and helps avoid unnecessary computation.

Consider the simple regression model below with response variable y and covariate x.

$$ \begin{aligned} y_i &\stackrel{\text{iid}}{\sim} \text{Normal}(x_i \beta, 1) \\ \beta &\sim \text{Normal}(0, 1) \end{aligned} $$

We write this model in the JAGS model file below.

lines <- "model {
  for (i in 1:n) {
    y[i] ~ dnorm(x[i] * beta, 1)
  }
  beta ~ dnorm(0, 1)
}"
writeLines(lines, "x.jags")

A typical workflow proceeds as follows:

1. Prepare a list of input data to JAGS, including vector elements x and y.
2. Fit the JAGS model using the list of input data.
3. Use the fitted model object to compute posterior summaries and convergence diagnostics.
4. Use the fitted model object to extract posterior draws of parameters and store them in a tidy data frame.
5. If there are other models to compare, use the fitted model object to compute the deviance information criterion (DIC).

jagstargets encapsulates this workflow with the tar_jags() function. To use it in a targets pipeline, invoke it from the _targets.R script of the project.

# _targets.R
library(targets)
library(jagstargets)

generate_data <- function(n = 10) {
  true_beta <- stats::rnorm(n = 1, mean = 0, sd = 1)
  x <- seq(from = -1, to = 1, length.out = n)
  y <- stats::rnorm(n, x * true_beta, 1)
  out <- list(n = n, x = x, y = y)
}

# The _targets.R file ends with a list of target objects
# produced by jagstargets::tar_jags(), targets::tar_target(), or similar.
list(
  tar_jags(
    example,
    jags_files = "x.jags",
    parameters.to.save = "beta",
    data = generate_data()
  )
)

tar_jags() only defines the pipeline. It does not actually run JAGS, it declares the targets that will eventually run JAGS. The specific targets are as follows. Run tar_manifest() to show specific details about the targets declared.

tar_manifest()

Each target is responsible for a piece of the workflow.

• example_file_x: Reproducibly track changes to the jags model file.
• example_data: Run the code you supplied to the data argument of tar_jags() and return a dataset compatible with JAGS.
• example_mcmc_x: Run the MCMC and return an object of class rjags from R2jags.
• example_draws_x: Return a friendly tibble of the posterior draws from example.
• example_summaries_x: Return a friendly tibble of the posterior summaries from example. Uses posterior::summarize_draws()
• example_dic_x: Return a friendly tibble with each model’s DIC and penalty.

The suffix _x comes from the base name of the model file, in this case x.jags. If you supply multiple model files to the jags_files argument, all the models share the same dataset, and the suffixes distinguish among the various targets.

The targets depend on one another: for example, example_mcmc_x takes example_data as input. targets can visualize the dependency relationships in a dependency graph, which is helpful for understanding the pipeline and troubleshooting issues.

tar_visnetwork(targets_only = TRUE)

Run the computation with tar_make().

tar_make()

The output lives in a special folder called _targets/ and you can retrieve it with functions tar_load() and tar_read() (from targets).

tar_read(example_summary_x)

At this point, all our results are up to date because their dependencies did not change.

tar_make()

But if we change the underlying code or data, some of the targets will no longer be valid, and they will rerun during the next tar_make(). Below, we change the jags model file, so the MCMC reruns while the data is skipped. This behavior saves time and enhances reproducibility.

write(" ", file = "x.jags", append = TRUE)

tar_outdated()

tar_visnetwork(targets_only = TRUE)

tar_make()

At this point, we can add more targets and custom functions for additional post-processing. See below for a custom summary target (which is equivalent to customizing the summaries argument of tar_jags().)

# _targets.R
library(targets)
library(jagstargets)

generate_data <- function(n = 10) {
  true_beta <- stats::rnorm(n = 1, mean = 0, sd = 1)
  x <- seq(from = -1, to = 1, length.out = n)
  y <- stats::rnorm(n, x * true_beta, 1)
  out <- list(n = n, x = x, y = y)
}

list(
  tar_jags(
    example,
    jags_files = "x.jags",
    parameters.to.save = "beta",
    data = generate_data()
  ),
  tar_target(
    custom_summary,
    posterior::summarize_draws(
      dplyr::select(example_draws_x, -starts_with(".")),
      ~posterior::quantile2(.x, probs = c(0.25, 0.75))
    )
  )
)

In the graph, our new custom_summary target should be connected to the upstream example target, and only custom_summary should be out of date.

tar_visnetwork(targets_only = TRUE)

In the next tar_make(), we skip the expensive MCMC and just run the custom summary.

tar_make()

tar_read(custom_summary)

lines <- "model {
  for (i in 1:n) {
    y[i] ~ dnorm(x[i] * x[i] * beta, 1) # Regress on x^2 instead of x.
  }
  beta ~ dnorm(0, 1)
}"
writeLines(lines, "y.jags")

# _targets.R
library(targets)
library(jagstargets)

generate_data <- function(n = 10) {
  true_beta <- stats::rnorm(n = 1, mean = 0, sd = 1)
  x <- seq(from = -1, to = 1, length.out = n)
  y <- stats::rnorm(n, x * true_beta, 1)
  out <- list(n = n, x = x, y = y)
}

list(
  tar_jags(
    example,
    jags_files = c("x.jags", "y.jags"),
    parameters.to.save = "beta",
    data = generate_data()
  ),
  tar_target(
    custom_summary,
    posterior::summarize_draws(
      dplyr::select(example_draws_x, -starts_with(".")),
      ~posterior::quantile2(.x, probs = c(0.25, 0.75))
    )
  )
)

tar_visnetwork(targets_only = TRUE)