CODEWITHSUNDEEP
dockerlogging
Yao Zhao
Yao Zhao

Reputation: 4613

Can I see logs in rotated files by `docker logs`?

We could set rotation for json-file logs of docker. If I set max-file=3 and max-size=10m, there may be 2 rotated files and 1 connected file for a container. The question is, if I use docker logs to view the logs of this container, can I see the logs in the 2 rotated files?

Upvotes: 5

Views: 1365

Answers (1)

Niels-Ole
Niels-Ole

Reputation: 139

Seems like it, at least for the JSON driver.

You can test it yourself. Let's construct a case where the log files are split into two:

# docker run --log-driver json-file --log-opt max-size=1k --log-opt max-file=2 --name with_logs ubuntu /bin/bash -c 'echo test ; base64 /dev/urandom | head -c 1000'
# docker logs with_logs

Now your output looks something like this:

test
V9L4+Yud3l4+nyuz/J6PecuXNAcXRxiuWHv+6Lvpa34eDb/ktNTgGHxLLpFuJxmol+BCFoatZEX6
PKyqwOzot6PWnrjBwsA+CWpZUUpUMWh/ab1pF+5AYGU+n0nBudMMxehFVAzAP5PSVZdw0TvtZXGm
Trb5eHbMk1O35UF2fw7Mm8VsHEM5RPWUGO60aKODeRlPSkCogDHbGYtGVHtzyPYcL8wC64SEFTnv
(...)

As we can see, the test output is still there, even though it should have spilled into the lograted file. Let's just verify that that is indeed the case. Notice that max-file is set to 1:

# docker run --log-driver json-file --log-opt max-size=1k --log-opt max-file=1 --name without_logs ubuntu /bin/bash -c 'echo test; base64 /dev/urandom | head -c 1000'
# docker logs without_logs
yqdqW+91p2xHO58LTEzqnuwpeQncBshzm6KOutMjQh27m19D5XQ8ffIditEbgNfjc90UmxYHu1t1
arQ+3vYpzWzTSMxmxxOBTOKUKgAC36hVT3JcT+bYF2shLExdKsPA12OJFJj1gmCIF1BWhNVgsE3Z
(...)

As we can see, the echo test is not visible anymore, as the logfile does not exist anymore.

To sum it up: Yes, rotated logs are visible in docker logs

Upvotes: 3

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