Shiny server Connection closed. Info: {"type":"close","code":4503,"reason":"The application unexpectedly exited","wasClean":true} - r

Question

I've encountered a problem with deploying my shiny app on linux Ubuntu 16.04 LTS.
After I run sudo systemctl start shiny-server, and open up my browser heading to http://192.168..*:3838/StockVis/, the web page greys out in a second.
I found some warnings in the web console as below, and survey some information on the web for like two weeks, but still have no solution. :(
***"Thu Feb 16 2017 12:20:49 GMT+0800 (CST) [INF]: Connection opened. http://192.168.**.***:3838/StockVis/"
Thu Feb 16 2017 12:20:49 GMT+0800 (CST) [DBG]: Open channel 0
The application unexpectedly exited.
Diagnostic information is private. Please ask your system admin for permission if you need to check the R logs.
**Thu Feb 16 2017 12:20:50 GMT+0800 (CST) [INF]: Connection closed. Info: {"type":"close","code":4503,"reason":"The application unexpectedly exited","wasClean":true}
Thu Feb 16 2017 12:20:50 GMT+0800 (CST) [DBG]: SockJS connection closed
Thu Feb 16 2017 12:20:50 GMT+0800 (CST) [DBG]: Channel 0 is closed
Thu Feb 16 2017 12:20:50 GMT+0800 (CST) [DBG]: Removed channel 0, 0 left*****
Please kindly give some suggestions to move on.

Answer 1

This can indicate something in your R code is causing an error. As that R error could be anything, this answer is to help you gather that info. The browser console messages will not tell you what that is. In order to access the error, you need to configure Shiny to not delete the log upon exiting the application.
Assuming you have sudo access:
$ sudo vi /etc/shiny-server/shiny-server.conf
Place the following line in the file after run_as shiny; :
preserve_logs true;
Restart shiny:
sudo systemctl restart shiny-server
Reload your Shiny app.
In the var/log/shiny-sever/ directory there will be a log file with your application name. Viewing that file will give you more information on what is going on.
Warning. After you are done, take out the preserve_logs true; line in the conf file and restart Shiny. If not, you will start generating a bunch of log files you don't want.