How to debug Nodejs app running inside Docker container via Google Cloud

Question

I have found Google provides some guidelines on how to run Nodejs on a custom runtime environment. Everything seems fine and I am managing to start my Nodejs app on local machin

Answer 1

By default, the node debugger will listen only for connections for the same host (127.0.0.1). But in Docker, you need to accept connections from any host (0.0.0.0):

# inside Docker
node --inspect=0.0.0.0:9229 myapp.js

Also you have to expose the debug port (9229). Then the application should be automatically detected and listed as a Remote Target in chrome://inspect/#devices in Chrome (tested in Chrome 67).

Example

Here is a minimal example. It runs a simple JavaScript application in Docker and shows how to attach the Chrome debugger to it:

$ cat example.js
setInterval(() => console.log('Hallo world'), 1000);

$ cat Dockerfile
FROM node
COPY example.js /
CMD node --inspect=0.0.0.0:9229 /example.js

Run with:

$ docker build . -t myapp && docker run -p 9229:9229 --rm -it myapp
Sending build context to Docker daemon  3.072kB
Step 1/3 : FROM node
 ---> aa3e171e4e95
Step 2/3 : COPY example.js /
 ---> Using cache
 ---> 3ef6c0311da2
Step 3/3 : CMD node --inspect=0.0.0.0:9229 /example.js
 ---> Using cache
 ---> e760739c2802
Successfully built e760739c2802
Successfully tagged debug-docker:latest
Debugger listening on ws://0.0.0.0:9229/4177f6cc-85e4-44c6-9ba3-5d8e28e1b124
For help see https://nodejs.org/en/docs/inspector
Hallo world
Hallo world
Hallo world
...

Open Chrome and go to chrome://inspect/#devices. It should soon after the start of the application, detect it and list it.

Troubleshooting

For debugging Docker network issues, docker inspect is useful:

$ docker ps
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                    NAMES
ae83d50e24c8        debug-docker        "/bin/sh -c 'node --…"   2 minutes ago       Up 2 minutes        0.0.0.0:9229->9229/tcp   blissful_sammet
$ docker inspect ae83d50e24c8
...
    "NetworkSettings": {
        "Bridge": "",
        "SandboxID": "682d3ac98b63d4077c5d66a516666b6615327cbea0de8b0a7a2d8caf5995b0ae",
        "HairpinMode": false,
        "LinkLocalIPv6Address": "",
        "LinkLocalIPv6PrefixLen": 0,
        "Ports": {
            "9229/tcp": [
                {
                    "HostIp": "0.0.0.0",
                    "HostPort": "9229"
                }
            ]
        },
   ...

If want to see the requests sent between Docker and Chrome, ngrep can help:

$ sudo ngrep -d any port 9229
interface: any
filter: (ip or ip6) and ( port 9229 )
############################
T ::1:38366 -> ::1:9229 [AP]
  GET /json/version HTTP/1.1..Host: [::1]:9229....                            
#####
T ::1:38368 -> ::1:9229 [AP]
  GET /json HTTP/1.1..Host: [::1]:9229....                                    
##############
T 172.17.0.1:56782 -> 172.17.0.2:9229 [AP]
  GET /json HTTP/1.1..Host: [::1]:9229....                                    
#
T 172.17.0.1:56782 -> 172.17.0.2:9229 [AP]
  GET /json HTTP/1.1..Host: [::1]:9229....                                    
###
T 172.17.0.1:56784 -> 172.17.0.2:9229 [AP]
  GET /json/version HTTP/1.1..Host: [::1]:9229....                            
#
T 172.17.0.1:56784 -> 172.17.0.2:9229 [AP]
  GET /json/version HTTP/1.1..Host: [::1]:9229....                            
###
T 172.17.0.2:9229 -> 172.17.0.1:56782 [AP]
  HTTP/1.0 200 OK..Content-Type: application/json; charset=UTF-8..Cache-Contro
  l: no-cache..Content-Length: 465....                                        
#
T 172.17.0.2:9229 -> 172.17.0.1:56782 [AP]
  HTTP/1.0 200 OK..Content-Type: application/json; charset=UTF-8..Cache-Contro
  l: no-cache..Content-Length: 465....                                        
###
T 172.17.0.2:9229 -> 172.17.0.1:56782 [AP]
  [ {.  "description": "node.js instance",.  "devtoolsFrontendUrl": "chrome-de
  vtools://devtools/bundled/inspector.html?experiments=true&v8only=true&ws=[::
  1]:9229/f29686f9-e92d-45f4-b7a2-f198ebfc7a8e",.  "faviconUrl": "https://node
  js.org/static/favicon.ico",.  "id": "f29686f9-e92d-45f4-b7a2-f198ebfc7a8e",.
    "title": "/example.js",.  "type": "node",.  "url": "file:///example.js",. 
   "webSocketDebuggerUrl": "ws://[::1]:9229/f29686f9-e92d-45f4-b7a2-f198ebfc7a
  8e".} ]..                                                                   
#