Logging

One of the main tasks that supervisord performs is logging. supervisord logs an activity log detailing what it’s doing as it runs. It also logs child process stdout and stderr output to other files if configured to do so.

Activity Log

The activity log is the place where supervisord logs messages about its own health, its subprocess’ state changes, any messages that result from events, and debug and informational messages. The path to the activity log is configured via the logfile parameter in the [supervisord] section of the configuration file, defaulting to $CWD/supervisord.log. Sample activity log traffic is shown in the example below. Some lines have been broken to better fit the screen.

Sample Activity Log Output

  1. 2007-09-08 14:43:22,886 DEBG 127.0.0.1:Medusa (V1.11) started at Sat Sep  8 14:43:22 2007
  2.         Hostname: kingfish
  3.         Port:9001
  4. 2007-09-08 14:43:22,961 INFO RPC interface 'supervisor' initialized
  5. 2007-09-08 14:43:22,961 CRIT Running without any HTTP authentication checking
  6. 2007-09-08 14:43:22,962 INFO supervisord started with pid 27347
  7. 2007-09-08 14:43:23,965 INFO spawned: 'listener_00' with pid 27349
  8. 2007-09-08 14:43:23,970 INFO spawned: 'eventgen' with pid 27350
  9. 2007-09-08 14:43:23,990 INFO spawned: 'grower' with pid 27351
  10. 2007-09-08 14:43:24,059 DEBG 'listener_00' stderr output:
  11.  /Users/chrism/projects/supervisor/supervisor2/dev-sandbox/bin/python:
  12.  can't open file '/Users/chrism/projects/supervisor/supervisor2/src/supervisor/scripts/osx_eventgen_listener.py':
  13.  [Errno 2] No such file or directory
  14. 2007-09-08 14:43:24,060 DEBG fd 7 closed, stopped monitoring <PEventListenerDispatcher at 19910168 for
  15.  <Subprocess at 18892960 with name listener_00 in state STARTING> (stdout)>
  16. 2007-09-08 14:43:24,060 INFO exited: listener_00 (exit status 2; not expected)
  17. 2007-09-08 14:43:24,061 DEBG received SIGCHLD indicating a child quit

The activity log “level” is configured in the config file via the loglevel parameter in the [supervisord] ini file section. When loglevel is set, messages of the specified priority, plus those with any higher priority are logged to the activity log. For example, if loglevel is error, messages of error and critical priority will be logged. However, if loglevel is warn, messages of warn, error, and critical will be logged.

Activity Log Levels

The below table describes the logging levels in more detail, ordered in highest priority to lowest. The “Config File Value” is the string provided to the loglevel parameter in the [supervisord] section of configuration file and the “Output Code” is the code that shows up in activity log output lines.

Config File ValueOutput CodeDescription
criticalCRITMessages that indicate a condition that requires immediate user attention, a supervisor state change, or an error in supervisor itself.
errorERROMessages that indicate a potentially ignorable error condition (e.g. unable to clear a log directory).
warnWARNMessages that indicate an anomalous condition which isn’t an error.
infoINFONormal informational output. This is the default log level if none is explicitly configured.
debugDEBGMessages useful for users trying to debug process configuration and communications behavior (process output, listener state changes, event notifications).
traceTRACMessages useful for developers trying to debug supervisor plugins, and information about HTTP and RPC requests and responses.
blatherBLATMessages useful for developers trying to debug supervisor itself.

Activity Log Rotation

The activity log is “rotated” by supervisord based on the combination of the logfile_maxbytes and the logfile_backups parameters in the [supervisord] section of the configuration file. When the activity log reaches logfile_maxbytes bytes, the current log file is moved to a backup file and a new activity log file is created. When this happens, if the number of existing backup files is greater than or equal to logfile_backups, the oldest backup file is removed and the backup files are renamed accordingly. If the file being written to is named supervisord.log, when it exceeds logfile_maxbytes, it is closed and renamed to supervisord.log.1, and if files supervisord.log.1, supervisord.log.2 etc. exist, then they are renamed to supervisord.log.2, supervisord.log.3 etc. respectively. If logfile_maxbytes is 0, the logfile is never rotated (and thus backups are never made). If logfile_backups is 0, no backups will be kept.

Child Process Logs

The stdout of child processes spawned by supervisor, by default, is captured for redisplay to users of supervisorctl and other clients. If no specific logfile-related configuration is performed in a [program:x], [fcgi-program:x], or [eventlistener:x] section in the configuration file, the following is true:

  • supervisord will capture the child process’ stdout and stderr output into temporary files. Each stream is captured to a separate file. This is known as AUTO log mode.
  • AUTO log files are named automatically and placed in the directory configured as childlogdir of the [supervisord] section of the config file.
  • The size of each AUTO log file is bounded by the {streamname}_logfile_maxbytes value of the program section (where {streamname} is “stdout” or “stderr”). When it reaches that number, it is rotated (like the activity log), based on the {streamname}_logfile_backups.

The configuration keys that influence child process logging in [program:x] and [fcgi-program:x] sections are these:

redirect_stderr, stdout_logfile, stdout_logfile_maxbytes, stdout_logfile_backups, stdout_capture_maxbytes, stdout_syslog, stderr_logfile, stderr_logfile_maxbytes, stderr_logfile_backups, stderr_capture_maxbytes, and stderr_syslog.

[eventlistener:x] sections may not specify redirect_stderr, stdout_capture_maxbytes, or stderr_capture_maxbytes, but otherwise they accept the same values.

The configuration keys that influence child process logging in the [supervisord] config file section are these: childlogdir, and nocleanup.

Capture Mode

Capture mode is an advanced feature of Supervisor. You needn’t understand capture mode unless you want to take actions based on data parsed from subprocess output.

If a [program:x] section in the configuration file defines a non-zero stdout_capture_maxbytes or stderr_capture_maxbytes parameter, each process represented by the program section may emit special tokens on its stdout or stderr stream (respectively) which will effectively cause supervisor to emit a PROCESS_COMMUNICATION event (see Events for a description of events).

The process communications protocol relies on two tags, one which commands supervisor to enter “capture mode” for the stream and one which commands it to exit. When a process stream enters “capture mode”, data sent to the stream will be sent to a separate buffer in memory, the “capture buffer”, which is allowed to contain a maximum of capture_maxbytes bytes. During capture mode, when the buffer’s length exceeds capture_maxbytes bytes, the earliest data in the buffer is discarded to make room for new data. When a process stream exits capture mode, a PROCESS_COMMUNICATION event subtype is emitted by supervisor, which may be intercepted by event listeners.

The tag to begin “capture mode” in a process stream is . The tag to exit capture mode is . The data between these tags may be arbitrary, and forms the payload of the PROCESS_COMMUNICATION event. For example, if a program is set up with a stdout_capture_maxbytes of “1MB”, and it emits the following on its stdout stream:

  1. <!--XSUPERVISOR:BEGIN-->Hello!<!--XSUPERVISOR:END-->

In this circumstance, supervisord will emit a PROCESS_COMMUNICATIONS_STDOUT event with data in the payload of “Hello!”.

An example of a script (written in Python) which emits a process communication event is in the scripts directory of the supervisor package, named sample_commevent.py.

The output of processes specified as “event listeners” ([eventlistener:x] sections) is not processed this way. Output from these processes cannot enter capture mode.