Service logging

Logs produced by Sensu services – i.e. sensu-backend and sensu-agent – are often the best place to start when troubleshooting a variety of issues.

Log levels

Each log message is associated with a log level, indicative of the relative severity of the event being logged:

Log level Description
panic Severe errors causing the service to shut down in an unexpected state
fatal Fatal errors causing the service to shut down (status 0)
error Non-fatal service error messages
warn Warning messages indicating potential issues
info Informational messages representing service actions
debug Detailed service operation messages to help troubleshoot issues

These log levels can be configured by specifying the desired log level as the value of log-level in the service configuration file (e.g. agent.yml or backend.yml configuration files), or as an argument to the --log-level command line flag:

sensu-agent start --log-level debug

Changes to log level via configuration file or command line arguments require restarting the service. For guidance on restarting a service, please consult the Operating section of the agent or backend reference, respectively.

Log file locations


Sensu services print structured log messages to standard output. In order to capture these log messages to disk or another logging facility, Sensu services make use of capabilities provided by the underlying operating system’s service management. For example, logs are sent to the journald when systemd is the service manager, whereas log messages are redirected to /var/log/sensu when running under sysv init schemes. If you are running systemd as your service manager and would rather have logs written to /var/log/sensu/, see the guide to forwarding logs from journald to syslog.

In the table below, the common targets for logging and example commands for following those logs are described. The name of the desired service, e.g. backend or agent may be substituted for ${service} variable.

Platform Version Target Command to follow log
RHEL/Centos >= 7 journald
journalctl --follow --unit sensu-${service}
RHEL/Centos <= 6 log file
tail --follow /var/log/sensu/sensu-${service}
Ubuntu >= 15.04 journald
journalctl --follow --unit sensu-${service}
Ubuntu <= 14.10 log file
tail --follow /var/log/sensu/sensu-${service}
Debian >= 8 journald
journalctl --follow --unit sensu-${service}
Debian <= 7 log file
tail --follow /var/log/sensu/sensu-${service}

NOTE: Platform versions described above are for reference only and do not supercede the documented supported platforms.


The Sensu agent stores service logs to the location specified by the log-file configuration flag (default: %ALLUSERSPROFILE%\sensu\log\sensu-agent.log, C:\ProgramData\sensu\log\sensu-agent.log on standard Windows installations). For more information about managing the Sensu agent for Windows, see the agent reference. You can also view agent events using the Windows Event Viewer, under Windows Logs, as events with source SensuAgent.

If you’re running a binary-only distribution of the Sensu agent for Windows, you can follow the service log printed to standard output using the following command.

Get-Content -  Path "C:\scripts\test.txt" -Wait

Sensu backend startup errors

The following errors are expected when starting up a Sensu backend with the default configuration.

{"component":"etcd","level":"warning","msg":"simple token is not cryptographically signed","pkg":"auth","time":"2019-11-04T10:26:31-05:00"}
{"component":"etcd","level":"warning","msg":"set the initial cluster version to 3.3","pkg":"etcdserver/membership","time":"2019-11-04T10:26:31-05:00"}
{"component":"etcd","level":"warning","msg":"serving insecure client requests on, this is strongly discouraged!","pkg":"embed","time":"2019-11-04T10:26:33-05:00"}

The serving insecure client requests error is an expected warning from etcd. TLS configuration is recommended but not required. For more information, see etcd security documentation.

Permission issues

Files and folders within /var/cache/sensu/ and /var/lib/sensu/ need to be owned by the sensu user and group. You will see a logged error similar to the following if there is a permission issue with either the sensu-backend or the sensu-agent:

{"component":"agent","error":"open /var/cache/sensu/sensu-agent/assets.db: permission denied","level":"fatal","msg":"error executing sensu-agent","time":"2019-02-21T22:01:04Z"}
{"component":"backend","level":"fatal","msg":"error starting etcd: mkdir /var/lib/sensu: permission denied","time":"2019-03-05T20:24:01Z"}

You can use a recursive chown to resolve permission issues with the sensu-backend:

sudo chown -R sensu:sensu /var/cache/sensu/sensu-backend

or the sensu-agent:

sudo chown -R sensu:sensu /var/cache/sensu/sensu-agent

Troubleshooting handlers and filters

Whether implementing new workflows or modifying existing ones, its sometimes necessary to troubleshoot various stages of the event pipeline. In many cases generating events using the agent API will save you time and effort over modifying existing check configurations.

Here’s an example using curl with the API of a local sensu-agent process to generate test-event check results:

curl -X POST \
-H 'Content-Type: application/json' \
-d '{
  "check": {
    "metadata": {
      "name": "test-event",
      "namespace": "default"
    "status": 2,
    "output": "this is a test event targeting the email_ops handler",
    "handlers": [ "email_ops" ]
}' \

Additionally, it’s frequently helpful to see the full event object being passed to your workflows. We recommend using a debug handler like this one to write an event to disk as JSON data:

type: Handler
api_version: core/v2
  name: debug
  namespace: default
  type: pipe
  command: cat > /var/log/sensu/debug-event.json
  timeout: 2
  "type": "Handler",
  "api_version": "core/v2",
  "metadata": {
    "name": "debug"
  "spec": {
    "type": "pipe",
    "command": "cat > /var/log/sensu/debug-event.json",
    "timeout": 2

With this handler definition installed in your Sensu backend, you can add the debug to the list of handlers in your test event:

curl -X POST \
-H 'Content-Type: application/json' \
-d '{
  "check": {
    "metadata": {
      "name": "test-event"
    "status": 2,
    "output": "this is a test event targeting the email_ops handler",
    "handlers": [ "email_ops", "debug" ]
}' \

The event data should be written to /var/log/sensu/debug-event.json for inspection. The contents of this file will be overwritten by every event sent to the debug handler.

NOTE: When multiple Sensu backends are configured in a cluster, event processing is distributed across all members. You may need to check the filesystem of each Sensu backend to locate the debug output for your test event.