Monitoring subsystems


mender-monitor supports monitoring subsystems which perform the actual monitoring and report to the main daemon. Currently, there are three subsystems available out-of-the-box: for services, log files and D-Bus signals, but you can easily extend it and implement your own.

The daemon supports the following directory structure:

tree /etc/mender-monitor/
`-- monitor.d
    |-- available
    |   |--
    |   `--
    |-- enabled
    |   |-- -> /etc/mender-monitor/monitor.d/available/
    |   `-- -> /etc/mender-monitor/monitor.d/available/

In the above example, we enabled the log and service subsystems by providing links from the enabled to the available directory. Each file name in the latter consists of the subsystem name ("service", "log" or "dbus"), an underscore, and a name (being an arbitrary string of letters to distinguish between the files). The main daemon follows links from the enabled directory and sources them to set the environment for the execution of and service.log, using the first part of a file name to decide which file from the monitor.d to run. In other words: we take the first part before an underscore of the file name from the enabled directory, append ".sh", prepend with the path to monitor.d and source resulting path, executing the subsystem check, using the contents of the file in enabled directory as the environment.

The specific check's implementation is common and we store it in the monitor.d/<subsystem_name>.sh files. You can easily extend the mender-monitor service implementing your custom subsystems.


Service monitoring expects two variables from the files in the available directory: SERVICE_NAME and SERVICE_TYPE.

For example, a file could contain:


which means that we monitor the cron systemd service.


The log monitoring works similarly. The file from the above examples could contain:

LOG_PATTERN="session opened for user [a-z0-9]* by"

where SERVICE_NAME is an arbitrary name used in logging and for identification, LOG_PATTERN is the regular expression we are trying to match to each line of the logs and LOG_FILE is the path to the log file.

The log monitoring subsystem saves the number of the last line of logs that it parsed and starts tailing the file skipping the already seen lines.

The log monitoring subsystem uses the grep command to match lines to a given pattern By default, and if supported by grep we use the -P option, which allows you to use the Perl-compatible regular expressions. If you have no support for -P, it falls back to the -E flag, and eventually uses plain grep.

Capturing logs from arbitrary sources

With the log subsystem you are not limited to log files only. You can get the logs from any command. You can achieve it via so-called log streamline extension. It is a simple mechanism that allows you to provide any command that produces data on standard output to which the log monitoring will perform pattern matching and alerting.

The general syntax to enable the streamline logs extension is to use @ character in the LOG_FILE:

mender-monitorctl create log custom_logs "User logged out due to security policy: \w+" "@/opt/bin/"

Where /opt/bin/ is any executable command printing to standard output.

As we saw in the Get started section the journalctl command can be a source of logs, but we do not limit the mechanism to any particular command. The only requirement is that the command prints to standard output so that the log subsystem will parse it.

Docker logs

One example of a command that can output data to standard error is docker logs. In case you want to get alerts based on the patterns that maybe present in the standard error stream, you can configure the check as follows:

mender-monitorctl create log docker_logs "Exception \d+-\w+ found" "@/opt/bin/dockerlogs_wrapper"

where /opt/bin/dockerlogs_wrapper is a wrapper around the docker logs command:

cat <<EOF > /opt/bin/dockerlogs_wrapper
docker logs my_service -f 2>&1
chmod 755 /opt/bin/dockerlogs_wrapper

With the above you can configure alerting based on any source. Please note that the -f flag above is essential, the command that you pass via the @ extension, must not exit.

Log pattern expiration

Once the pattern shows up in the logs, Mender Monitor add-on will send a critical alert. Technically there would be no way out of this situation; the device would stay in the "critical" state forever. Monitor add-on provides a way to send the "OK" alert after a period of time defined by LOG_PATTERN_EXPIRATION in the check. This variable is defined either directly on the check or by the last argument to the mender-monitorctl create log name pattern file expiration command.

See also the configuration of the DEFAULT_LOG_PATTERN_EXPIRATION_SECONDS.


The D-Bus monitoring subsystem expects three variables from the files in the available directory: DBUS_NAME, DBUS_PATTERN and DBUS_WATCH_EXPRESSION.

For example, a file could contain:


Every time a D-Bus signal matching the watch expression is received, an alert will be triggered and sent from the monitoring subsystem. You can adapt the configuration to any D-Bus signal and pattern based on your use case.

We welcome contributions to improve this documentation. To submit a change, use the Edit link at the top of the page or email us at .