How to pretty format the Traefik log in JSON? - json

Question

I have a Traefik service with the following configuration:
version: "3.9"
services:
reverse-proxy:
image: traefik:v2.3.4
networks:
common:
ports:
- target: 80
published: 80
mode: host
- target: 443
published: 443
mode: host
command:
- "--providers.docker.endpoint=unix:///var/run/docker.sock"
- "--providers.docker.swarmMode=true"
- "--providers.docker.exposedbydefault=false"
- "--providers.docker.network=common"
- "--entrypoints.web.address=:80"
# - "--entrypoints.websecure.address=:443"
- "--global.sendAnonymousUsage=true"
# Set a debug level custom log file
- "--log.level=DEBUG"
- "--log.format=json"
- "--log.filePath=/var/log/traefik.log"
- "--accessLog.filePath=/var/log/access.log"
# Enable the Traefik dashboard
- "--api.dashboard=true"
# - "traefik.constraint-label=common" TODO
deploy:
placement:
constraints:
- node.role == manager
labels:
# Expose the Traefik dashboard
- "traefik.enable=true"
- "traefik.http.routers.dashboard.service=api#internal"
- "traefik.http.services.traefik.loadbalancer.server.port=888" # A port number required by Docker Swarm but not being used in fact
- "traefik.http.routers.dashboard.rule=Host(`traefik.learnintouch.com`)"
- "traefik.http.routers.traefik.entrypoints=web"
# - "traefik.http.routers.traefik.entrypoints=websecure"
# Basic HTTP authentication to secure the dashboard access
- "traefik.http.routers.traefik.middlewares=traefik-auth"
- "traefik.http.middlewares.traefik-auth.basicauth.users=stephane:$$apr1$$m72sBfSg$$7.NRvy75AZXAMtH3C2YTz/"
volumes:
# So that Traefik can listen to the Docker events
- "/var/run/docker.sock:/var/run/docker.sock:ro"
- "~/dev/docker/projects/common/volumes/logs/traefik.service.log:/var/log/traefik.log"
- "~/dev/docker/projects/common/volumes/logs/traefik.access.log:/var/log/access.log"
Then I watch the log with the command:
stephane#stephane-pc:~$ tail -f dev/docker/projects/common/volumes/logs/traefik.service.log
{"level":"info","msg":"I have to go...","time":"2021-07-03T10:18:10Z"}
{"level":"info","msg":"Stopping server gracefully","time":"2021-07-03T10:18:10Z"}
{"entryPointName":"web","level":"debug","msg":"Waiting 10s seconds before killing connections.","time":"2021-07-03T10:18:10Z"}
{"entryPointName":"web","level":"error","msg":"accept tcp [::]:80: use of closed network connection","time":"2021-07-03T10:18:10Z"}
I expected the log to be formatted in JSON with indentation.
So I copy-pasted the non indented JSON output in an online JSON formatter but it only indented part of it, making the whole thing useless.

Answer 1

Your problem is that Traefik does not output a single JSON document, but one JSON document per line. You could beautify all documents using xargs and jq:
tail -f dev/docker/projects/common/volumes/logs/traefik.service.log | xargs -n 1 -d "\n" -- bash -c 'echo "$1" | jq' _
In your example, this will result in this output (even with syntax highlighting if your terminal supports that):
{
"level": "info",
"msg": "I have to go...",
"time": "2021-07-03T10:18:10Z"
}
{
"level": "info",
"msg": "Stopping server gracefully",
"time": "2021-07-03T10:18:10Z"
}
{
"entryPointName": "web",
"level": "debug",
"msg": "Waiting 10s seconds before killing connections.",
"time": "2021-07-03T10:18:10Z"
}
{
"entryPointName": "web",
"level": "error",
"msg": "accept tcp [::]:80: use of closed network connection",
"time": "2021-07-03T10:18:10Z"
}