22.05.2018

Docker: Lessons learned - Logging

After some time working with docker here my experiences:

Some days ago i created my own container with a minimal web service.

Here the ncweb.sh:
#!/bin/ash
sed -i  's/Hostname:.*/Hostname: '$HOSTNAME'/g' index.html
while true; do { echo -e 'HTTP/1.1 200 OK\r\n'; cat index.html;}  | nc  -l -p 8080  2>&1 >> logfile; done 
This is the Dockerfile:
FROM alpine

WORKDIR /tmp

RUN mkdir ncweb

ADD .  /tmp

ENTRYPOINT [ "/tmp/ncweb.sh" ]

After building the image
docker build -t ncweb:0.4 .
And starting the container:
docker run -d -p 8080:8080 ncweb:0.4 --name ncweb0.4
I was able to connect to the container and view the log:

To get the right command:
docker ps  |grep  ncweb:0.4 |awk '{print "docker exec -it "$1" ash"}'
and then use the output:
docker exec -it e4f9960fc8e5 ash
alpine:~/ncweb# docker exec -it e4f9960fc8e5 ash
/tmp # ls
Dockerfile  hexdump     index.html  logfile     ncweb       ncweb.sh
/tmp # cat logfile 
GET / HTTP/1.1
Host: 192.168.178.46:8080
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:59.0) Gecko/20100101 Firefox/59.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en,de;q=0.7,en-US;q=0.3
Accept-Encoding: gzip, deflate
Connection: keep-alive
Upgrade-Insecure-Requests: 1
Cache-Control: max-age=0

Thu May 10 10:01:23 UTC 2018 request done
But this is not the right way.
If i change the ncweb.sh to
#!/bin/ash
sed -i  's/Hostname:.*/Hostname: '$HOSTNAME'/g' index.html
while true; do { echo -e 'HTTP/1.1 200 OK\r\n'; cat index.html;}  | nc  -l -p 8080 ;done 
then you can do the following (after building a new container version):

alpine:~/ncweb# docker run -d -p 8080:8080 ncweb:0.5 --name ncweb0.5

9589f77fc289a3713354a365f8f08098279e6d0e893de99a0431d8fbd62c834a

alpine:~/ncweb# docker ps

CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                    NAMES

9589f77fc289        ncweb:0.5           "/tmp/ncweb.sh --n..."   8 seconds ago       Up 7 seconds        0.0.0.0:8080->8080/tcp   gracious_archimedes
To get the logs (which are written to STDOUT):

alpine:~/ncweb# docker logs -f 9589f77fc289

GET / HTTP/1.1

Host: 192.168.178.46:8080

User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:59.0) Gecko/20100101 Firefox/59.0

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8

Accept-Language: en,de;q=0.7,en-US;q=0.3

Accept-Encoding: gzip, deflate

Connection: keep-alive

Upgrade-Insecure-Requests: 1

Cache-Control: max-age=0


Conclusion: It is better to use STDOUT than local logfiles. Or even better: use syslog or other central logging mechanisms.

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