Question

While working in an R console, I’d like to set up a background task that monitors a particular connection and when an event occurs, another function (an alert) is executed. Alternatively, I can set things up so that an external function simply sends an alert to R, but this seems to be the same problem: it is necessary to set up a listener.

I can do this in a dedicated process of R, but I don’t know if this is feasible from within a console. Also, I’m not interested in interrupting R if it is calculating a function, but alerting or interrupting if the console is merely waiting on input.

Here are three use cases:

1. The simplest possible example is watching a file. Suppose that I have a file called “latestData.csv” and I want to monitor it for changes; when it changes, myAlert() is executed. (One can extend it to do different things, but just popping up with a note that a file has changed is useful.)

2. A different kind of monitor would watch for whether a given machine is running low on RAM and might execute a save.image() and terminate. Again, this could be a simple issue of watching a file produced by an external monitor that saves the output of top or some other command.

3. A different example is like another recent SO question, about : have R halt the EC2 machine it's running on. If an alert from another machine or process tells the program to save & terminate, then being able to listen for that alert would be great.

At the moment, I suspect there are two ways of handling this: via Rserve and possibly via fork. If anyone has examples of how to do this with either package or via another method, that would be great. I think that solving any of these three use cases would solve all of them, modulo a little bit external code.

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Note 1: I realize, per this answer to another SO question that R is single threaded, which is why I suspect fork and Rserve may work. However, I’m not sure about feasibility if one is interfacing with an R terminal. Although R’s REPL is attached to the input from the console, I am trying to either get around this or mimic it, which is where fork or Rserve may be the answer.

Note 2: For those familiar with event handling / eventing methods, that would solve everything, too. I’ve just not found anything about this in R.

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Update 1: I’ve found that the manual for writing R extensions has a section referencing event handling, which mentions the use of R_PolledEvents. This looks promising.

Answer 1

It depends whether you want to interrupt idling or working R. If the first, you can think of bypassing the R default REPL loop by some event listener that will queue the incoming events and evaluate them. The common option is to use tcl/tk or gtk loop; I have made something like this around libev in my triggr package, which makes R digest requests coming from a socket.

The latter case is mostly hopeless, unless you will manually make the computational code to execute if(evenOccured) processIt code periodically.

Multithreading is not a real option, because as you know two interpreters in one process will break themselves by using same global variables, while forked processes will have independent memory contents.