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docs: document how to use an external logging service

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This is an updated version of projects/logging/README.md

Signed-off-by: David Scott <dave.scott@docker.com>
This commit is contained in:
David Scott
2018-07-01 14:00:01 +01:00
parent f4bbce7a6c
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# Logging
By default LinuxKit will write onboot and service logs directly to files in
`/var/log` and `/var/log/onboot`.
It is tricky to write the logs to a disk or a network service as no disks
or networks are available until the `onboot` containers run. We work around
this by splitting the logging into 2 pieces:
1. `memlogd`: an in-memory circular buffer which receives logs (including
all the early `onboot` logs)
2. a log writing `service` that starts later and can download and process
the logs from `memlogd`
To use this new logging system, you should add the `memlogd` container to
the `init` block in the LinuxKit yml. On boot `memlogd` will be started
from `init.d` and it will listen on a Unix domain socket:
```
/var/run/linuxkit-external-logging.sock
```
The `init`/`service` process will look for this socket and redirect the
`stdout` and `stderr` of both `onboot` and `services` to `memlogd`.
## memlogd: an in-memory circular buffer
The `memlogd` daemon reads the logs from the `onboot` and `services` containers
and stores them together with a timestamp and the name of the originating
container in a circular buffer in memory.
The contents of the circular buffer can be read over the Unix domain socket
```
/var/run/memlogq.sock
```
The circular buffer has a fixed size (overridden by the command-line argument
`-max-lines`) and when it fills up, the oldest messages will be overwritten.
To store the logs somewhere more permanent, for example a disk or a remote
network service, a service should be added to the yaml which connects to
`memlogd` and streams the logs. The example program `logread` in the `memlogd`
package demonstrates how to do this.
Usage examples:
```
/ # logread -f
2017-04-15T15:37:37Z memlogd memlogd started
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: waiting for carrier
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: carrier acquired
2017-04-15T15:37:37Z 002-dhcpcd.stdout DUID 00:01:00:01:20:84:fa:c1:02:50:00:00:00:24
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: IAID 00:00:00:24
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: adding address fe80::84e3:ca52:2590:fe80
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: soliciting an IPv6 router
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: soliciting a DHCP lease
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: offered 192.168.65.37 from 192.168.65.1 `vpnkit'
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: leased 192.168.65.37 for 7199 seconds
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: adding route to 192.168.65.0/24
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: adding default route via 192.168.65.1
2017-04-15T15:37:37Z 002-dhcpcd.stdout exiting due to oneshot
2017-04-15T15:37:37Z 002-dhcpcd.stdout dhcpcd exited
2017-04-15T15:37:37Z rngd.stderr Unable to open file: /dev/tpm0
^C
/ # logwrite echo testing123
testing123
/ # logread | tail -n1
2017-04-15T15:37:45Z echo.stdout testing123
/ # echo -en "GET / HTTP/1.0\n\n" | nc localhost 80 > /dev/null
/ # logread | grep nginx
2017-04-15T15:42:40Z nginx.stdout 127.0.0.1 - - [15/Apr/2017:15:42:40 +0000] "GET / HTTP/1.0" 200 612 "-" "-" "-"
```
Current issues and limitations:
- No docker logger plugin support yet - it could be nice to add support to
memlogd, so the docker container logs would also be gathered in one place
- No syslog compatibility at the moment and `/dev/log` doesn’t exist. This
socket could be created to keep syslog compatibility, e.g. by using
https://github.com/mcuadros/go-syslog. Processes that require syslog should
then be able to log directly to memlogd.
- Kernel messages not read on startup yet (but can be captured with
`logwrite dmesg`)
- Currently no direct external hooks exposed - but options available that
could be added. Should also be possible to pipe output to e.g. `oklog`
from `logread` (https://github.com/oklog/oklog)

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### Logging tools
Experimental logging tools for linuxkit.
This project currently provides three tools for system logs; `logwrite`, `logread` and `memlogd` (+ `startmemlogd` to run `memlogd` with `runc`).
`memlogd` is the daemon that keeps logs in a circular buffer in memory. It is started automatically by `init`/`startmemlogd` in a runc container. It is passed two sockets - one that allows clients to dump/follow the logs and one that can be used to send open file descriptors to `memlogd`. When `memlogd` receives a file descriptor it will read from the file descriptor and timestamp and append the content to the in-memory log until the file is closed.
`logwrite` executes a command and will send stderr and stdout to `memlogd`. It does this by opening a socketpair for stdout and stderr and then sends the file descriptors to memlogd, before executing a specified command. Output is also sent to normal stderr/stdin. For example, `logwrite ls` will show the output both in the console and record it in the logs.
`logread` connects to memlogd and dumps the ring buffer. Parameters `-f` and `-F` can be used to follow the logs and disable the initial log dump (it behaves similar to busybox’ `logread`)
Init is modified to run all `onboot` and `service` containers wrapped in`logwrite` and to run `/usr/bin/startmemlogd`.
New sockets:
`/tmp/memlogd.sock` — sock_dgram which accepts an fd and a null-terminated source description
`/tmp/memlogdq.sock` — sock_stream to ask to dump/follow logs
Usage examples:
```
/ # logread -f
2017-04-15T15:37:37Z memlogd memlogd started
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: waiting for carrier
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: carrier acquired
2017-04-15T15:37:37Z 002-dhcpcd.stdout DUID 00:01:00:01:20:84:fa:c1:02:50:00:00:00:24
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: IAID 00:00:00:24
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: adding address fe80::84e3:ca52:2590:fe80
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: soliciting an IPv6 router
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: soliciting a DHCP lease
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: offered 192.168.65.37 from 192.168.65.1 `vpnkit'
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: leased 192.168.65.37 for 7199 seconds
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: adding route to 192.168.65.0/24
2017-04-15T15:37:37Z 002-dhcpcd.stdout eth0: adding default route via 192.168.65.1
2017-04-15T15:37:37Z 002-dhcpcd.stdout exiting due to oneshot
2017-04-15T15:37:37Z 002-dhcpcd.stdout dhcpcd exited
2017-04-15T15:37:37Z rngd.stderr Unable to open file: /dev/tpm0
^C
/ # logwrite echo testing123
testing123
/ # logread | tail -n1
2017-04-15T15:37:45Z echo.stdout testing123
/ # echo -en "GET / HTTP/1.0\n\n" | nc localhost 80 > /dev/null
/ # logread | grep nginx
2017-04-15T15:42:40Z nginx.stdout 127.0.0.1 - - [15/Apr/2017:15:42:40 +0000] "GET / HTTP/1.0" 200 612 "-" "-" "-"
```
Current issues and limitations:
- The moby tool only supports onboot and service containers. `memlogd` runs as a special container that is managed by init, as it needs fd’s created in advance. To work around this a memlogd container is exported during build. The init-section in the yml is used to extract it to `/containers/init/memlogd` with a pre-created `config.json`.
- No docker logger plugin support yet - it could be nice to add support to memlogd, so the docker container logs would also be gathered in one place
- No syslog compatibility at the moment and `/dev/log` doesn’t exist. This socket could be created to keep syslog compatibility, e.g. by using https://github.com/mcuadros/go-syslog. Processes that require syslog should then be able to log directly to memlogd.
- Kernel messages not read on startup yet (but can be captured with `logwrite dmesg`)
- Currently no direct external hooks exposed - but options available that could be added. Should also be possible to pipe output to e.g. `oklog` from `logread` (https://github.com/oklog/oklog)