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

console $ cat /etc/docker/daemon.json { "log-driver": "splunk", "log-opts": { "cache-disabled": "true", ... (options for "splunk" logging driver) } }

• Step 2: Start the container

console $ docker run -d busybox --name testlog top

• Step 3: Read the container logs

console $ 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

console $ cat /etc/docker/daemon.json { "log-driver": "splunk", "log-opts": { ... (options for "splunk" logging driver) } }

• Step 2: Start the container

console $ docker run -d busybox --name testlog top

• Step 3: Read the container logs

console $ 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

[!NOTE]

For logging drivers that support reading logs, such as the local, json-file and journald drivers, there is no difference in functionality before or after the dual logging capability became available. For these drivers, Logs can be read using docker 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)
  }
}

[!NOTE]

For logging drivers that support reading logs, such as the local, json-file and journald 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.