Stateful Agent

This example shows, how VRPC handles stateful interactions with R.

The example here takes up the ideas as shown in the shiny reactivity example, to demonstrate how these concepts map to VRPC.

1

Stateful R code to be called remotely

The fundamental idea behind the shiny example is to show that input parameters can be explicitly wired, such that only specific functions are re-evaluated and not simply everything. Shiny calls this concept "reactivity" and technically it does (at least) two things:

1. Building up dependency chains (a polytree) for functions required to evaluate on a given input change

2. Caching of last-values-changed to avoid unnecessary re-evaluations

As VRPC does not implement the reactivity wiring in R, but in the client application this leads to the following effect:

1. Individual tasks can be implemented using individual functions

2. Caching is naturally done by sharing state between the functions (using variables)

This leads to very simple and clean code:

library("vrpc")

dataset <- NULL

select_dataset <- function(name) {
  dataset <<- switch(name,
    "rock" = rock,
    "pressure" = pressure,
    "cars" = cars
  )
  return(TRUE)
}

get_summary <- function() {
  summary(dataset)
}

get_table <- function(obs) {
  head(dataset, n = obs)
}

start_vrpc_agent(domain = "public.vrpc")

Note, that the caption output from the shiny example is entirely skipped here as there is no computation but only re-rendering involved. In VRPC the caption update is accomplished solely on client-side. Shiny needs a roundtrip as rendering is done server-side.

2

Call R code and manage state

Existing R code is mapped into the Session class on the remote client-side. All functions preceding the start_vrpc_agent line are available as static functions of the Session class and as member functions of Session class instances.

NOTE

Additionally, the generic call function is injected (both statically and as member) which allows calling any other R code.

Within a Session instance all state is maintained, allowing client code like this:

const { VrpcRemote } = require('vrpc')

if (process.argv.length < 3) {
  console.log('Usage: node index.js <agentName>')
  process.exit(1)
}

(async () => {
  const client = new VrpcRemote({
    domain: 'public.vrpc',
    agent: process.argv[2]
  })
  await client.connect()
  const proxy = await client.create({
    className: 'Session',
    instance: 'session1'
  })
  await proxy.select_dataset('rock')
  await proxy.get_table(1) // first line of rock dataset

  await proxy.select_dataset('cars')
  await proxy.get_table(2) // first two lines of cars dataset
  await proxy.get_summary() // summary information of cats dataset
})()