Best way to rerun target after some time elapsed from last run?

[petrbouchal]

Use case: I have a remote resource which I want to update every few days, and I have no way to poll when it's changed (it cannot be collapsed into a URL). For instance, google sheets behind an auth procedure are like this.

What would be the most sensible way to trigger the target, say, every N days?

Currently, do the following:

• the command reading the data from the remote source also writes a file with the current time in it
• this target runs conditional (tar_force()) on the date being older than N days, which is calculated in the _targets.R and stored in a global variable.

This feels convoluted and I suspect I might be missing something obvious. (I am hesitant to check the file-modified date on the target file in _targets/, which feels more straightforward but fragile.)

Intuitively, I would like something like tar_refresh() that does this in one step - but perhaps there is a simple way to achieve this with the existing functions. (I know I could write a target factory to collapse the two steps into one custom target, but I am not sure how best to do the checking of last-modified/last-run time.)

[wlandau]

This came up at #131. As discussed in the thread. A completely satisfying solution would require additions to the core metadata, and I hesitate because this sort of use case is not ubiquitous. I recommend something like tar_change(target_name, regular_command(), get_timestamp()), where get_timestamp() is a custom function that rounds to the nearest week.

[petrbouchal]

Many thanks - understand you don't want to burden every step with time checking as discussed in #131. It's the combination of tar_change() with rounding that does the trick - thanks for that idea. And you just helped me fix my mental model of what tar_change() does...