Use docker logs with remote logging drivers
Overview
You can use the docker logs
command to read container logs regardless of the
configured logging driver or plugin. Docker Engine uses the
local
logging driver to act as cache for reading the latest logs of your containers.
This is called dual logging. By default, the cache has log-file rotation
enabled, and is limited to a maximum of 5 files of 20 MB each (before
compression) per container.
Refer to the configuration options section to customize these defaults, or to the disable dual logging section to disable this feature.
Prerequisites
Docker Engine automatically enables dual logging if the configured logging driver doesn't support reading logs.
The following examples show the result of running a docker logs
command with
and without dual logging availability:
Without dual logging capability
When a container is configured with a remote logging driver such as splunk
, and
dual logging is disabled, an error is displayed when attempting to read container
logs locally:
Step 1: Configure Docker daemon
$ cat /etc/docker/daemon.json { "log-driver": "splunk", "log-opts": { "cache-disabled": "true", ... (options for "splunk" logging driver) } }
Step 2: Start the container
$ docker run -d busybox --name testlog top
Step 3: Read the container logs
$ docker logs 7d6ac83a89a0 Error response from daemon: configured logging driver does not support reading
With dual logging capability
With the dual logging cache enabled, the docker logs
command can be used to
read logs, even if the logging driver doesn't support reading logs. The following
example shows a daemon configuration that uses the splunk
remote logging driver
as a default, with dual logging caching enabled:
Step 1: Configure Docker daemon
$ cat /etc/docker/daemon.json { "log-driver": "splunk", "log-opts": { ... (options for "splunk" logging driver) } }
Step 2: Start the container
$ docker run -d busybox --name testlog top
Step 3: Read the container logs
$ docker logs 7d6ac83a89a0 2019-02-04T19:48:15.423Z [INFO] core: marked as sealed 2019-02-04T19:48:15.423Z [INFO] core: pre-seal teardown starting 2019-02-04T19:48:15.423Z [INFO] core: stopping cluster listeners 2019-02-04T19:48:15.423Z [INFO] core: shutting down forwarding rpc listeners 2019-02-04T19:48:15.423Z [INFO] core: forwarding rpc listeners stopped 2019-02-04T19:48:15.599Z [INFO] core: rpc listeners successfully shut down 2019-02-04T19:48:15.599Z [INFO] core: cluster listeners successfully shut down
メモ
For logging drivers that support reading logs, such as the
local
,json-file
andjournald
drivers, there is no difference in functionality before or after the dual logging capability became available. For these drivers, Logs can be read usingdocker logs
in both scenarios.
Configuration options
The dual logging cache accepts the same configuration options as the
local
logging driver, but with a cache-
prefix. These options
can be specified per container, and defaults for new containers can be set using
the
daemon configuration file.
By default, the cache has log-file rotation enabled, and is limited to a maximum of 5 files of 20MB each (before compression) per container. Use the configuration options described below to customize these defaults.
Option | Default | Description |
---|---|---|
cache-disabled | "false" | Disable local caching. Boolean value passed as a string (true , 1 , 0 , or false ). |
cache-max-size | "20m" | The maximum size of the cache before it is rotated. A positive integer plus a modifier representing the unit of measure (k , m , or g ). |
cache-max-file | "5" | The maximum number of cache files that can be present. If rotating the logs creates excess files, the oldest file is removed. A positive integer. |
cache-compress | "true" | Enable or disable compression of rotated log files. Boolean value passed as a string (true , 1 , 0 , or false ). |
Disable the dual logging cache
Use the cache-disabled
option to disable the dual logging cache. Disabling the
cache can be useful to save storage space in situations where logs are only read
through a remote logging system, and if there is no need to read logs through
docker logs
for debugging purposes.
Caching can be disabled for individual containers or by default for new containers, when using the daemon configuration file.
The following example uses the daemon configuration file to use the
splunk
logging driver as a default, with caching disabled:
$ cat /etc/docker/daemon.json
{
"log-driver": "splunk",
"log-opts": {
"cache-disabled": "true",
... (options for "splunk" logging driver)
}
}
メモ
For logging drivers that support reading logs, such as the
local
,json-file
andjournald
drivers, dual logging isn't used, and disabling the option has no effect.
Limitations
- If a container using a logging driver or plugin that sends logs remotely
has a network issue, no
write
to the local cache occurs. - If a write to
logdriver
fails for any reason (file system full, write permissions removed), the cache write fails and is logged in the daemon log. The log entry to the cache isn't retried. - Some logs might be lost from the cache in the default configuration because a ring buffer is used to prevent blocking the stdio of the container in case of slow file writes. An admin must repair these while the daemon is shut down.