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Docker logging

Logging in Docker has always been quite convenient for a developer. Send your logs to stdout/stderr and Docker picks them up, you can view them with docker logs. This command lets you tail the logs, follow them, etc. It can be really nice. In production scenarios this has proven to be a pain point...

As it turns out, in production, people like to actually collect logs from multiple servers, do analytics, and other fun things. Unfortunately docker's logging didn't really handle this well. There are some projects that help you extract container logs out of Docker directly and forward them to another service (e.g. Logspout), however this is less than ideal.

Docker 1.6 changes this. The logging infrastructure in Docker has been driver-ized. A default logging driver can be selected when setting up the daemon, this can get overridden when creating containers. Included drivers are:

  • json-log
  • none
  • syslog

The json-log driver mimics the logging of previous versions of Docker.

The none driver disables logging, especially useful for those really noisy apps.

The syslog driver... well... logs to syslog. Here's an example output from a container using the syslog driver, with the an entry from nginx:

docker/cc198c45b027[16853]: - - [17/Apr/2015:02:00:02 +0000] "GET / HTTP/1.1" 200 2461 "-" "Mozilla/5.0 (Windows NT 5.1; rv:6.0.2) Gecko/20100101 Firefox/6.0.2"

The tag being used here is docker/<container id>[pid]. There's been some discussion on using the container's name here instead of the ID... ultimately in the future this will likely be configurable to use whatever container field you want. The pid in this case is the actual pid as it is seen from the host, not the pid from inside the container.

The docker logs command will only work with the json-file driver, but I'm sure you probably already have your own tool for reading logs that's way better than what docker logs could provide.

It should also be extremely simple to write a custom logging driver, the syslog driver is a grand total of 45 lines of code (syslog.go). Here are the interfaces:

type Message struct {
    ContainerID string
    Line        []byte
    Source      string
    Timestamp   time.Time

type Logger interface {
    Log(*Message) error
    Name() string
    Close() error

So if none of the currently available logging drivers suits you, it should be pretty simple to implement your own!

And as always, pull requests are always welcome!