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