Self-modifying pipelines

Internal pipeline structure

{pipeflow} aims to offer a lean and intuitive interface that enables new users to get started quickly without having to learn a lot of new functions. At the same time, it was designed to provide easy access to the underlying data structures to allow advanced users to modify the pipeline basically in any way they want.

To see this, let’s briefly inspect the internal structure of a pipeline object.

pip <- pip_new("my-pipeline") |>
    pip_add("init", \(xInit = 0) xInit) |>
    pip_add("f1", \(x = ~init) x + 1) |>
    pip_add("f2", \(x = ~f1) x + 2) |>
    pip_add("f3", \(x = ~f2) x + 3)

str(pip)
# List of 3
#  $ name  : chr "my-pipeline"
#  $ view  : NULL
#  $ pipenv:<environment: 0x5606dea62dc0>

There is the name of the pipeline, which is just a character string, as well as a view entry, which is undefined initially.

The most “interesting” part is the pipenv: an environment that holds the pipeline’s actual state. It is shared by the pipeline and all of its views, which is why operations on a view write through to the underlying pipeline.

ls(pip$pipenv)
# [1] "data"

Listing the pipenv1 reveals the dataentry, which contains the pipeline’s step table as a data.table with one row per step:

pip$pipenv$data # or pip_data(pip)
#      step           fun    params    out  state   tags locked   exec                time depends unbound nodeId
#    <char>        <list>    <list> <list> <char> <list> <lgcl> <char>              <POSc>  <list>  <list>  <int>
# 1:   init <function[1]> <list[1]> [NULL]    new         FALSE   auto 2026-09-27 20:21:17           xInit      0
# 2:     f1 <function[1]> <list[1]> [NULL]    new         FALSE   auto 2026-09-27 20:21:17    init              1
# 3:     f2 <function[1]> <list[1]> [NULL]    new         FALSE   auto 2026-09-27 20:21:17      f1              2
# 4:     f3 <function[1]> <list[1]> [NULL]    new         FALSE   auto 2026-09-27 20:21:17      f2              3

Many of the columns should be already familar to you and most of them can be manipulated safely via the [<- and [[<- operators, for example:

pip[["f2", "step"]] <- "my_pretty_f2" # updates downstream 'depends'

pip
# <pipeflow> my-pipeline (4 steps)
# --------------------------------
#            step params      depends state
# 1:         init  xInit                new
# 2:           f1      x         init   new
# 3: my_pretty_f2      x           f1   new
# 4:           f3      x my_pretty_f2   new
# --------------------------------
# <ready> last run: never
pip[step %like% "f", "locked"] <- TRUE

pip
# <pipeflow> my-pipeline (4 steps)
# --------------------------------
#            step params      depends state locked
# 1:         init  xInit                new  FALSE
# 2:           f1      x         init   new   TRUE
# 3: my_pretty_f2      x           f1   new   TRUE
# 4:           f3      x my_pretty_f2   new   TRUE
# --------------------------------
# <ready> last run: never

An exception are the last three columns (depends, unbound, nodeId): these are usually derived indirectly from the step definition and therefore protected against direct assignment.

pip[["init", "nodeId"]] <- 99L
# Error in `[[<-.pipeflow`:
# ! direct assignment to column 'nodeId' is not supported.

Of course, you can even skip the [<- and [[<- operators and work directly on the data.table object, but with an increased risk to invalidate the internal consistency of the overall pipeline structure, for example:

dat <- pip$pipenv$data
dat[2, "step"] <- "new f1 name" # fails to update downstream 'depends'
dat[3, "nodeId"] <- 99L # breaks link to internal DAG node
assign("data", dat, envir = pip$pipenv)

pip
# <pipeflow> my-pipeline (4 steps)
# --------------------------------
#            step params      depends state locked
# 1:         init  xInit                new  FALSE
# 2:  new f1 name      x         init   new   TRUE
# 3: my_pretty_f2      x           f1   new   TRUE
# 4:           f3      x my_pretty_f2   new   TRUE
# --------------------------------
# <ready> last run: never

For this reason, direct manipulation is useful during debugging and you should mostly stick to the provided operators or functions unless you really know what you are doing.

Changing the pipeline structure at runtime

After this little excursion let’s next see how to safely modify our pipeline structure at runtime.

pip <- pip_new("my-pipeline") |>
    pip_add("init", \(xInit = 0) xInit) |>
    pip_add("f1", \(x = ~init) x + 1) |>
    pip_add("f2", \(x = ~f1) x + 2) |>
    pip_add("f3", \(x = ~f2) x + 3)

(pip_run(pip))
# info [2026-09-27 18:21:17.335 UTC]: Starting run of pipeflow 'my-pipeline'
# info [2026-09-27 18:21:17.335 UTC]: Step 1/4 init
# info [2026-09-27 18:21:17.336 UTC]: Step 2/4 f1
# info [2026-09-27 18:21:17.337 UTC]: Step 3/4 f2
# info [2026-09-27 18:21:17.338 UTC]: Step 4/4 f3
# info [2026-09-27 18:21:17.339 UTC]: Finished run of pipeflow 'my-pipeline'
# <pipeflow> my-pipeline (4 steps)
# --------------------------------
#    step params depends state out
# 1: init  xInit          done   0
# 2:   f1      x    init  done   1
# 3:   f2      x      f1  done   3
# 4:   f3      x      f2  done   6
# --------------------------------
# <ready> last run: 2026-09-27 20:21:17

This pipeline just adds 1, 2, and 3 to the initial value, respectively. Let’s modify step f2 that in turn will modify f3 at runtime based on the interim result passed into f2.

Modify steps

pip |> pip_replace(
    "f2",
    \(x = ~f1) {
        if (x > 10) {
            .self$replace("f3", \(x = ~f1) x * 3)
            return(x / 2)
        }
        x + 2
    }
)

Basically, step f2 now checks if the input is greater than 10, and if so, it replaces step f3 with a new step now referencing f1 that multiplies the input passed from f1 by 3 and returns half of the input.

To see this, let’s try it with an input of 15.

pip |>
    pip_set_params(list(xInit = 15)) |>
    pip_run()
# info [2026-09-27 18:21:17.369 UTC]: Starting run of pipeflow 'my-pipeline'
# info [2026-09-27 18:21:17.369 UTC]: Step 1/4 init
# info [2026-09-27 18:21:17.369 UTC]: Step 2/4 f1
# info [2026-09-27 18:21:17.370 UTC]: Step 3/4 f2
# info [2026-09-27 18:21:17.371 UTC]: Step 4/4 f3
# info [2026-09-27 18:21:17.372 UTC]: Finished run of pipeflow 'my-pipeline'

pip
# <pipeflow> my-pipeline (4 steps)
# --------------------------------
#    step params depends state out
# 1: init  xInit          done  15
# 2:   f1      x    init  done  16
# 3:   f2      x      f1  done   8
# 4:   f3      x      f1  done  48
# --------------------------------
# <ready> last run: 2026-09-27 20:21:17

We see that both the output of the pipeline and the dependencies of the last step have changed. Let’s confirm by inspecting the function of the last step.

pip[["f3", "fun"]]
# function (x = ~f1) 
# x * 3
# <environment: 0x5606dbfc7778>

Insert and remove steps

Next, we get even more hacky and instead of just replacing, we will go a bit further to insert and remove steps. The pipeline definition is as follows:

pip <- pip_new("hicky-hacky") |>
    pip_add("init", \(xInit = 0) xInit) |>
    pip_add("f1", \(x = ~init) x + 1) |>
    pip_add(
        "f2",
        \(x = ~f1) {
            if (x > 10) {
                .self |>
                    pip_add("f2a", \(x = ~f1) x + 21, after = "f1") |>
                    pip_add("f2b", \(x = ~f2a) x + 22, after = "f2a") |>
                    pip_replace("f3", \(x = ~f2b) x + 30) |>
                    pip_remove("f2")
            }
            x + 2
        }
    ) |>
    pip_add("f3", \(x = ~f2) x + 3)

If the input is greater than 10, we insert two new steps f2a and f2b after f1, remove f2, and replace f3 with a new step that adds 30 to the input. Let’s first run with the initial value of 0 to see the original output.

pip_run(pip)
# info [2026-09-27 18:21:17.412 UTC]: Starting run of pipeflow 'hicky-hacky'
# info [2026-09-27 18:21:17.412 UTC]: Step 1/4 init
# info [2026-09-27 18:21:17.413 UTC]: Step 2/4 f1
# info [2026-09-27 18:21:17.414 UTC]: Step 3/4 f2
# info [2026-09-27 18:21:17.415 UTC]: Step 4/4 f3
# info [2026-09-27 18:21:17.416 UTC]: Finished run of pipeflow 'hicky-hacky'

pip
# <pipeflow> hicky-hacky (4 steps)
# --------------------------------
#    step params depends state out
# 1: init  xInit          done   0
# 2:   f1      x    init  done   1
# 3:   f2      x      f1  done   3
# 4:   f3      x      f2  done   6
# --------------------------------
# <ready> last run: 2026-09-27 20:21:17

Next, we set the initial value to 11 to trigger the changes.

pip |>
    pip_set_params(list(xInit = 11)) |>
    pip_run()
# info [2026-09-27 18:21:17.431 UTC]: Starting run of pipeflow 'hicky-hacky'
# info [2026-09-27 18:21:17.431 UTC]: Step 1/4 init
# info [2026-09-27 18:21:17.431 UTC]: Step 2/4 f1
# info [2026-09-27 18:21:17.432 UTC]: Step 3/4 f2
# info [2026-09-27 18:21:17.436 UTC]: Step 4/4 f3
# info [2026-09-27 18:21:17.436 UTC]: Finished run of pipeflow 'hicky-hacky'

pip
# <pipeflow> hicky-hacky (5 steps)
# --------------------------------
#    step params depends state    out
# 1: init  xInit          done     11
# 2:   f1      x    init  done     12
# 3:  f2a      x      f1   new [NULL]
# 4:  f2b      x     f2a  done       
# 5:   f3      x     f2b   new [NULL]
# --------------------------------
# <ready> last run: 2026-09-27 20:21:17

While the structure has changed as expected, some steps were not yet run. In fact, since originally step f3came after f2, and in contrast to what the log is showing, instead of step f3, actually the new step f2b was run2 , albeit with x = NULL as input.

So to have the true results, we need to re-init the parameter and need to re-run the pipeline.

pip |>
    pip_set_params(list(xInit = 11)) |>
    pip_run()
# info [2026-09-27 18:21:17.452 UTC]: Starting run of pipeflow 'hicky-hacky'
# info [2026-09-27 18:21:17.452 UTC]: Step 1/5 init
# info [2026-09-27 18:21:17.453 UTC]: Step 2/5 f1
# info [2026-09-27 18:21:17.454 UTC]: Step 3/5 f2a
# info [2026-09-27 18:21:17.454 UTC]: Step 4/5 f2b
# info [2026-09-27 18:21:17.455 UTC]: Step 5/5 f3
# info [2026-09-27 18:21:17.456 UTC]: Finished run of pipeflow 'hicky-hacky'

pip
# <pipeflow> hicky-hacky (5 steps)
# --------------------------------
#    step params depends state out
# 1: init  xInit          done  11
# 2:   f1      x    init  done  12
# 3:  f2a      x      f1  done  33
# 4:  f2b      x     f2a  done  55
# 5:   f3      x     f2b  done  85
# --------------------------------
# <ready> last run: 2026-09-27 20:21:17

Now the output of all steps is as expected. If we want to use {pipeflow} in production, obviously, having to re-run the pipeline and temporarily showing a wrong log is not ideal. That is, ideally, the pipeline run would be aborted once away after all changes were made in f2 and then re-run right away from the beginning. Also, this process potentially should be repeated recursively until the structure does not change anymore.

Invoke restart during run

Luckily, with some minimal changes, this behaviour can be achieved with {pipeflow}. First, for any step where you want to restart the pipeline run, you need to call .self$restart(), so we adapt the f2 function as follows:

pip <- pip_new("hacky-with-restart") |>
    pip_add("init", \(xInit = 0) xInit) |>
    pip_add("f1", \(x = ~init) x + 1) |>
    pip_add(
        "f2",
        \(x = ~f1) {
            if (x > 10) {
                .self |>
                    pip_add("f2a", \(x = ~f1) x + 21, after = "f1") |>
                    pip_add("f2b", \(x = ~f2a) x + 22, after = "f2a") |>
                    pip_replace("f3", \(x = ~f2b) x + 30) |>
                    pip_remove("f2")
                .self$restart() # <-- restart the run
            }
            x + 2
        }
    ) |>
    pip_add("f3", \(x = ~f2) x + 3)

Second, you just run the pipeline as usual.

pip |>
    pip_set_params(list(xInit = 11)) |>
    pip_run()
# info [2026-09-27 18:21:17.484 UTC]: Starting run of pipeflow 'hacky-with-restart'
# info [2026-09-27 18:21:17.484 UTC]: Step 1/4 init
# info [2026-09-27 18:21:17.485 UTC]: Step 2/4 f1
# info [2026-09-27 18:21:17.486 UTC]: Step 3/4 f2
# info [2026-09-27 18:21:17.489 UTC]: Restarting pipeline execution.
# info [2026-09-27 18:21:17.489 UTC]: Restarting run of pipeflow 'hacky-with-restart'
# info [2026-09-27 18:21:17.489 UTC]: Step 1/5 init
# info [2026-09-27 18:21:17.490 UTC]: Step 2/5 f1
# info [2026-09-27 18:21:17.490 UTC]: Step 3/5 f2a
# info [2026-09-27 18:21:17.491 UTC]: Step 4/5 f2b
# info [2026-09-27 18:21:17.492 UTC]: Step 5/5 f3
# info [2026-09-27 18:21:17.493 UTC]: Finished run of pipeflow 'hacky-with-restart'

As you can see, the run was aborted right after step f2 and re-run from the start based on the new structure. As a result, the log now is fully aligned with the performed pipeline run.

Looking at the final pipeline overview, we see that the output matches the expected output of the modified pipeline.

pip
# <pipeflow> hacky-with-restart (5 steps)
# ---------------------------------------
#    step params depends state out
# 1: init  xInit          done  11
# 2:   f1      x    init  done  12
# 3:  f2a      x      f1  done  33
# 4:  f2b      x     f2a  done  55
# 5:   f3      x     f2b  done  85
# ---------------------------------------
# <ready> last run: 2026-09-27 20:21:17

Of course, this was just a toy example to show some possibilities, but I have made use of this feature already in various projects and may present one of them in a more sophisticated example in the future.

Lastly note that since you have full access to the pipeline object, of course, you can get even more hacky, but be aware that some additional operations are done under the hood when steps are added or removed. It is therefore not recommended to “manually” manipulate the internal data.table object in terms of removing or adding rows, or changing important columns such as depends or nodeId as this immediately would invalidate the internal consistency of the dependency graph.

On the other hand, changing entries in columns such as tags, time, state or output is generally not critical. If in doubt, just try and see what works.

  1. By default, ls() hides names starting with a dot. The internal entries, which become visible via ls(pip$pipenv, all.names = TRUE), are not considered in this vignette.↩︎

  2. See the state of f2b in row 4: it is set to done.↩︎