Undefined error in httr call. httr output: Failed to connect to localhost port 4445 after 0 ms: Connection refused

Question

I was run shiny app through docker. But it works on RStudio but when I run it through docker it gives an error:

Undefined error in httr call. httr output: Failed to connect to localhost port 4445 after 0 ms: Connection refused

Shiny Code:

# app.R
library(shiny)
library(RSelenium)

ui <- fluidPage(
  actionButton("btn", "Click Me"),
  textOutput("result")
)

server <- function(input, output, session) {
  # Start the remote driver
  remDr <- remoteDriver(
    remoteServerAddr = "localhost",  # Docker container host
    port = 4445L,  # Docker container port
    browserName = "chrome",
  )
  observeEvent(input$btn, {
      output$result <- renderText({
        remDr$open()
  
        # Navigate to a website (e.g., Google)
        remDr$navigate("https://www.google.com")
        remDr$maxWindowSize()
  
        # Perform some actions (e.g., print page title)
        title <- remDr$getTitle()
        
        return(as.character(title))
      })
      
})
  session$onSessionEnded(function() {
    # Close the remote driver
    remDr$close()
  })
}

shinyApp(ui, server)

Dockerfile:

FROM rocker/shiny:4

# Install R packages required 
# Change the packages list to suit your needs
RUN R -e "install.packages(c('shiny', 'RSelenium', 'httr'), dependencies=TRUE)"

# Copy the Shiny app files into the image
COPY app.R /srv/shiny-server/

# Expose port 3838 for Shiny app
EXPOSE 3838

# Run Shiny app on container start
CMD ["R", "-e", "shiny::runApp('/srv/shiny-server/app.R', host = '0.0.0.0', port = 3838)"]

Executed Docker Command:

docker pull selenium/standalone-chrome:4.2.2
docker run -d -p 4445:4444 --shm-size 4g selenium/standalone-chrome:4.2.2
docker build -t shiny-rselenium .
docker run -p 3838:3838 shiny-rselenium

Everything works perfectly on RStudio. But I face that error when I go to localhost:3838. How Can I fix this problem?

Answer 1

In the context of the container, localhost resolves to the container instance itself and not to the host system. If you are using Docker Dektop and want to go though published port, you could try replacing localhost with host.docker.internal, i.e. :

remDr <- remoteDriver(
    remoteServerAddr = "host.docker.internal",  # Docker container host
    port = 4445L,  # Docker container port
    browserName = "chrome",
  )

Though I'd rather use Docker Compose and let it handle networking. First, let's update remoteDriver() call so it can be configured through environment variables and same file could be used in different environments, defaults set to localhost:4445 :

  remDr <- remoteDriver(
    remoteServerAddr = Sys.getenv("SELENIUM_HOST", "localhost"),
    port =  as.integer(Sys.getenv("SELENIUM_PORT", "4445")), 
    browserName = "chrome",
  )

comopse.yaml:

services:
  shiny-rselenium:
    build: .
    ports:
      - "3838:3838"
    environment:
      - SELENIUM_HOST=selenium
      - SELENIUM_PORT=4444
    depends_on:
      - selenium
  selenium:
    image: "selenium/standalone-chrome:4.2.2"
    shm_size: 4g

start:

PS > docker compose up -d
[+] Building 0.0s (0/0)                                                                                                                                                                      docker:default
[+] Running 3/3
 ✔ Network shinydkr_default              Created                                                                                                                                                       0.1s
 ✔ Container shinydkr-selenium-1         Started                                                                                                                                                       0.1s
 ✔ Container shinydkr-shiny-rselenium-1  Started                                                                                                                                                       0.1s

check:

PS > docker compose ps
NAME                         IMAGE                              COMMAND                  SERVICE           CREATED          STATUS          PORTS
shinydkr-selenium-1          selenium/standalone-chrome:4.2.2   "/opt/bin/entry_poin…"   selenium          23 seconds ago   Up 22 seconds   4444/tcp, 5900/tcp
shinydkr-shiny-rselenium-1   shinydkr-shiny-rselenium           "R -e 'shiny::runApp…"   shiny-rselenium   23 seconds ago   Up 22 seconds   0.0.0.0:3838->3838/tcp

stop & remove containers and networks:

PS > docker compose down
[+] Running 3/3
 ✔ Container shinydkr-shiny-rselenium-1  Removed                                                                                                                                                      10.4s
 ✔ Container shinydkr-selenium-1         Removed                                                                                                                                                       4.3s
 ✔ Network shinydkr_default              Removed