This article is based on this section of the administrator's guide.
Most of the R processes started by RStudio Connect are batch-oriented tasks. R is invoked, does a narrow set of work, and then exits. Shiny applications are different and may see an R process handle many requests for many users over its lifetime.
RStudio Connect launches an R process tied to a Shiny application when the first request arrives for that application. That R process will continue to service requests until it becomes idle and eventually terminated. If there is sufficient traffic against that Shiny application, RStudio Connect may launch additional processes to service those requests.
There are a number of configuration parameters which control the conditions under which processes for Shiny applications are launched and eventually reaped. The default values are appropriate for most applications but occasionally need customization in specialized environments. This section of the administrator's guide explains each of the options. We recommend that adjustment to these runtime properties be done gradually.
Shiny applications have write access to the directory containing the unpackaged R code. This application directory is the working directory when launching Shiny. Data written here will be visible to all processes associated with that Shiny application but are not visible to other R processes. Application directory data remains available until that application is next deployed to RStudio Connect. A deployment creates a new application directory containing only the deployed content.
Note: RStudio Connect may launch multiple processes to service requests for an application. There is no coordination between these processes. Shiny applications that write to local files could experience problems when different processes attempt to write to a single file. We recommend against using the file system for data persistence.