Writing a Docker Log Driver plugin — Part II

Plugin in action

In Part I we explained the basics of a docker log driver. In this second part we will see this in action with a sample server running in a docker container. you will need to clone the logdriver repository locally to follow along.

Build and Install the plugin

Once you have cloned the repository, cd to plugin directory and run make all . This should build, install and enable a plugin , you will something like below

### enable plugin monmohan/tldriver:v1
docker plugin enable monmohan/tldriver:v1
monmohan/tldriver:v1

Verify the plugin and note the ID 1d9148d984e8 in below example

ID                  NAME                   DESCRIPTION                   ENABLED
1d9148d984e8        monmohan/tldriver:v1   Simple Log Driver as plugin   true

The plugin simply writes any log to its STDOUT . Where is the plugin stdout?

Docker creates it in the /var/docker/plugins/<pluginid> . Doing ls -al in the directory shows three files, plugin stdout, stderr and the unix socket used for communicating with the plugin (this is how docker sends http request to start, stop logging to the plugin http handler)

drwx------ 2 root root 100 Mar 13 20:50 .
drwx------ 4 root root  80 Mar 13 20:50 ..
prwx------ 1 root root   0 Mar 13 20:50 init-stderr
prwx------ 1 root root   0 Mar 13 20:50 init-stdout
srw-rw---- 1 root root   0 Mar 13 20:50 mylogdriver.sock

Since our sample plugin simply writes to its stdout, whatever it reads from the FIFO set by Docker, lets setup a reader in this window on the stdout — cat <init-stdout

Build the Test Server

cd to testserver directory and build the docker image

docker build -t testserverimg .

The Test Server starts an HTTP server at a given port which simply writes the headers and request URI to STDOUT.

Run the Test Server with Plugin

Run the test server in detached mode, mapped to port 9003

docker run --log-driver monmohan/tldriver:v1 -d -p 9003:9003 -e HTTP_PORT='9003' testserverimg

This is what you would see in the terminal window

Start logging request was called for the container : {"File":"/run/docker/logging/10417f2cd63ea34a2928ea4816b6f912e95ffd6683cfcccb399b1489e618cca1","Info":{"Config":{},"ContainerID":"fd21ad91c0c1681ad9157042b2fe6a5515f0460204086eb71071ffa3999beff1","ContainerName":"/eager_blackburn","ContainerEntrypoint":"/usr/bin/testserver","ContainerArgs":[],"ContainerImageID":"sha256:b664862cf96941acc6e2320a6e7c0aaa40e3dc0953849415ee7459ec3855fc42","ContainerImageName":"testserverimg","ContainerCreated":"2019-03-13T13:28:09.281249181Z","ContainerEnv":["HTTP_PORT=9003","PATH=/go/bin:/usr/local/go/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin","GOLANG_VERSION=1.8.7","GOPATH=/go"],"ContainerLabels":{},"LogPath":"","DaemonName":"docker"}}
New Container added for logging : >
 /run/docker/logging/10417f2cd63ea34a2928ea4816b6f912e95ffd6683cfcccb399b1489e618cca1 = fd21ad91c0c1681ad9157042b2fe6a5515f0460204086eb71071ffa3999beff1
fd21ad91c0c1681ad9157042b2fe6a5515f0460204086eb71071ffa3999beff1: [stderr] [1552483689861635661] 2019/03/13 13:28:09 Starting server on host localhost  and port 9003

You can now call this echo server to see more output by the plugin

curl http://localhost:9003/foobar

fd21ad91c0c1681ad9157042b2fe6a5515f0460204086eb71071ffa3999beff1: [stdout] [1552483875130451247] Receieved request:  /foobar
fd21ad91c0c1681ad9157042b2fe6a5515f0460204086eb71071ffa3999beff1: [stdout] [1552483875130606896] Accept=[*/*]
fd21ad91c0c1681ad9157042b2fe6a5515f0460204086eb71071ffa3999beff1: [stdout] [1552483875130615011] User-Agent=[curl/7.58.0]

What happened here is this -

TestServer → Write to its STDOUT → which Docker reads →pipes to FIFO → which Plugin reads → writes to its STDOUT

The last step is totally up to the plugin. It could be posting this message to another server or writing to some file or whatever it wants to do with the logs.

So now we have seen the plugin in action. So feel free to play with the sample plugin code and experiment more !