Daemontools-encore is a backwards compatible, enhanced version of Daniel J. Bernstein's daemontools package, written by Bruce Guenter. A summary of the features that have been added to daemontools-encore is available here.
- 1 Installation
- 2 Configuration
- 3 Usage
- 3.1 Service directories
- 3.2 The scan directory and supervision tree
- 3.3 The start, stop and notify files, and the daemontools-encore extended service state
- 3.4 Logging
- 3.5 Supervised process execution state changes
- 3.6 Controlling supervised processes
- 3.7 Starting the supervision tree
- 4 Removal
- 5 See also
- 6 External Resources
- 7 References
USE flags for sys-process/daemontools-encore Collection of tools for managing UNIX services
emerge --ask sys-process/daemontools-encore
Daemontools-encore is currently on the testing branch. Users with systems on the stable branch will need to add the package to /etc/portage/package.accept_keywords (if using Portage) to be able to install it. While it is generally not advised to mix packages of stable and testing branches, this package only depends on the libc, so in this case it should be safe
- SUPERVISEDIR - Name of the subdirectory of a service directory used by supervise for its control files. If it is an absolute pathname, it will be suffixed by all daemontools-encore programs that use it with the path to the service directory, with all slashes ('/') replaced with colons (':'). For example, if SUPERVISEDIR is set to /run/supervise, a supervise process running for service directory /service/foo will place its control files in directory /run/supervise/service:foo.
- SOFTLIMIT_ALLBYTES - Alternative to softlimit's
-aoption for specifying the corresponding soft limit.
- SOFTLIMIT_COREBYTES - Alternative to softlimit's
-coption for specifying the corresponding soft limit.
- SOFTLIMIT_DATABYTES - Alternative to softlimit's
-doption for specifying the corresponding soft limit.
- SOFTLIMIT_FILEBYTES - Alternative to softlimit's
-foption for specifying the corresponding soft limit.
- SOFTLIMIT_LOCKEDBYTES - Alternative to softlimit's
-loption for specifying the corresponding soft limit.
- SOFTLIMIT_MEMBYTES - Alternative to softlimit's
-moption for specifying the corresponding soft limits.
- SOFTLIMIT_OPENFILES - Alternative to softlimit's
-ooption for specifying the corresponding soft limit.
- SOFTLIMIT_PROCS - Alternative to softlimit's
-poption for specifying the corresponding soft limit.
- SOFTLIMIT_RSSBYTES - Alternative to softlimit's
-roption for specifying the corresponding soft limit.
- SOFTLIMIT_STACKBYTES - Alternative to softlimit's
-soption for specifying the corresponding soft limit.
- SOFTLIMIT_CPUSECS - Alternative to softlimit's
-toption for specifying the corresponding soft limit.
- /service - Location of the scan directory when using OpenRC, svscanboot or svscan-add-to-inittab.
Bernstein daemontools and daemontools-encore implement process supervision: programs that run as long-lived processes, such as a server program, can be supervised by being run as a child process of a supervisor. The supervisor can detect if the process, also called the service or the daemon in this context, has unexpectedly stopped running, e.g. because it exited with an error status or was killed by a signal, and automatically restart it. The supervisor also provides a reliable interface for controlling both the supervised process and itself, to send signals to the process, and to query status information about it.
All this is based on standard POSIX features: a process reliably knows its child's process ID (PID), because it is the return value of the
fork() call that creates it, knows when its child stops running, because it is notified with a
SIGCHLD signal, and can obtain exit status information when it happens using the
The program implementing the supervisor features in Bernstein daemontools and daemontools-encore is supervise. Supervision for a single process is configured using a service directory (or servicedir). A servicedir is an ordinary directory containing at least one executable file named run. It can also contain an optional, regular file named down. The (absolute or relative to the working directory) pathname of this servicedir is then passed as an argument to supervise. This however is not supposed to be done directly by the user, but to happen indirectly as a consequence of running svscan.
When supervise is invoked, it changes its working directory to the specifed servicedir, and executes the contained run file as a child process, unless there is also a down file, or, for daemontools-encore only, a start file (see the start, stop and notify files, and the daemontools-encore extended service state). Daemontools-encore's supervise also makes the child process the leader of a new session using the POSIX
setsid() call, unless the servicedir contains a regular file named no-setsid. In that case, the child process will run in supervise's session instead. Making the child process a session leader with Bernstein daemontools requires using the pgrphack program inside run (see supervised process execution state changes). If supervise is invoked with a servicedir that contains a down file, the run file won't be executed, but the service can be started later with the svc program (see controlling supervised processes). The contents of the down and no-setsid files are ignored, so they are usually empty.
run can have any file format that the kernel knows how to execute, but is usually a shell script that performs some sort of initialization, and then calls the real program intended to be supervised, using the shell's exec builtin utility. This allows the program to run without creating a new process, so it will have the same PID as the run script, and from there on become the supervised process. supervise waits for 1 second between two child process spawns, so that it does not loop too quickly if the process exits immediately. The daemontools-encore version of supervise also has special behaviour when it receives a signal: if it receives a
SIGTERM signal, it behaves as if an svc -dx command naming the corresponding servicedir had been used (see controlling supervised processes), if it receives a
SIGTSTP signal, it sends a
SIGSTOP signal to the supervised process, as if an svc -p command naming the corresponding servicedir had been used, and if it receives a
SIGCONT signal, it sends a
SIGCONT signal to the supervised process, as if an svc -c command naming the corresponding servicedir had been used.
Programs that fail to adhere to certain design criteria, including those that use
fork() in order to "put the daemon into the background" , might not be able to be supervised. Sometimes programs can meet those criteria if passed certain options (e.g. a 'run in the foreground' option) on invocation.
A minimal (and simplistic) service directory for a hypothetical test-daemon program:
total 4 -rwxr-xr-x 1 user user 27 May 5 12:00 run
#!/bin/sh exec test-daemon
The supervise program keeps control files in a subdirectory of the servicedir, also named supervise. If this subdirectory or any of its files doesn't exist when supervise is invoked, they will be created. If the servicedir contains a supervise symbolic link to directory instead of a subdirectory, supervise will follow it and use the linked-to directory for its control files. Daemontools-encore also allows setting the name of this control directory via the SUPERVISEDIRenvironment variable, see environment variables.
The service directory can be in a read-only filesystem, but files contained in the supervise directory must writable by the effective user of the corresponding supervise process. For Bernstein daemontools, and for daemontools-encore when some of the control files are missing, the directory itself must also be writable. So in this case a symbolic link to an existing directory in a read-write filesystem must be used.
Service directories can also optionally contain a subdirectory or symbolic link to directory named log, which must also have the format of a servicedir, and can be used for setting up a dedicated logger (see logging).
For further details about the supervise program and service directories, please consult the supervise man page.
Service directory with a supervise subdirectory:
total 8 -rwxr-xr-x 1 user user 27 May 5 12:00 run drwx------ 2 user user 4096 May 5 12:00 supervise
supervise subdirectory contents:
ls -l supervise
total 4 prw------- 1 user user 0 May 5 12:00 control -rw------- 1 user user 0 May 5 12:00 lock prw------- 1 user user 0 May 5 12:00 ok -rw-r--r-- 1 user user 18 May 5 12:00 status
The scan directory and supervision tree
Bernstein daemontools and daemontools-encore allow supervising a set of processes running in parallel using the svscan program and a scan directory (or scandir). A scan directory is an ordinary directory containing subdirectories and/or symbolic links to directories, each of them having the structure of a service directory. Invoking svscan with the (absolute or relative to the working directory) path of the scandir as its first argument (and only argument for Bernstein daemontools' svscan) launches one child supervise process for each subdirectory, unless the name of the directory starts with a dot ('.'). If svscan is called with no arguments, it assumes the working directory is the scandir, otherwise it changes its working directory to the specified scandir.
Every 5 seconds, svscan rereads the scan directory (i. e. performs a scan), launching supervise processes for each new servicedir it finds, or old servicedir for which it finds its supervise process has exited. svscan acts as a supervisor for every child supervise, the same way supervise does for its child process. This arrangement of processes is called the supervision tree. The root of the supervision tree is svscan, so it is designed to be robust and to cope with supervise processes exiting unexpectedly, being killed, or being unable to start. The leaves of the supervision tree are the processes that correspond to the service directories in the scandir.
Example scan directory containing three service directories:
total 12 drwxr-xr-x 3 user user 4096 May 5 12:00 test-service1 drwxr-xr-x 3 user user 4096 May 5 12:00 test-service2 drwxr-xr-x 3 user user 4096 May 5 12:00 test-service3
#!/bin/sh exec test-daemon1
#!/bin/sh exec test-daemon2
#!/bin/sh exec test-daemon3
Resulting supervision tree when svscan is run on this scandir as a background process in an interactive shell, assuming it is the working directory (i.e. launched with svscan &):
ps xf -o pid,ppid,pgrp,euser,args
PID PPID PGRP EUSER COMMAND ... 1777 1764 1777 user -bash 2047 1777 2047 user \_ svscan 2048 2047 2047 user \_ supervise test-service3 2053 2048 2047 user | \_ test-daemon3 2049 2047 2047 user \_ supervise test-service1 2052 2049 2047 user | \_ test-daemon1 2050 2047 2047 user \_ supervise test-service2 2051 2050 2047 user \_ test-daemon2 ...
Since processes in a supervision tree are created using the POSIX
fork()call, all of them will inherit svscan's enviroment, which, in the context of this example, is the user's login shell environment. If svscan is launched in some other way (see later), the environment will likely be completely different. This must be taken into account when trying to debug a supervision tree with an interactive shell.
The start, stop and notify files, and the daemontools-encore extended service state
Daemontools-encore service directories can contain executable files named start, stop and notify. The run file is optional for daemontools-encore, but either start or run must exist in the servicedir. If there is a start file and it is executable, it will be executed as a child process instead of run when supervise is invoked, and also when an svc -u or svc -o command is used to manually start the service (see controlling supervised processes). If the start process exits with a an exit code of 0, supervise will then execute the run file just like Bernstein daemontools' supervise. start can be used to perform some kind of first time-only initialization for the program intended to be supervised.
If the supervised process exits with an exit code of 100, or if the start process is killed by a signal or exits with a nonzero exit code, supervise enters the failed extended state (see later) and does not restart the service. If the service is manually stopped with an svc -d command or, after an svc -o command is used, the supervised or start process stop running and the conditions for entering the failed state are not met, supervise enters the stopped extended state. In stopped state, if there is a stop file in the servicedir and it is executable, supervise will execute it as a child process. stop can be used to perform some kind of last time-only cleanup for the program intended to be supervised, or for the start process. If the stop process exits with an exit code of 100, supervise will also enter the failed state, otherwise it will remain in stopped state.
If there is a notify file, supervise will execute it as a child process each time a start, run or stop process is spawned or stops running for whatever reason, so it can be used to perfom some action after one or more of those events. supervise invokes notify with four arguments: the first one is either start, run or stop, indicating which process caused notify to be invoked, the second one is either start, exit or killed, indicating that the corresponding process was spawned, exited or was killed by a signal, respectively, the third one is the process ID of the corresponding process, and the fourth one is the exit code or signal number, as determined by POSIX
waitpid(), or 0 if the second argument is start. So, for example, notify could be used to perfom cleanup actions each time the supervised process stops, possibly depending on its exit status information, by checking that its first argument is run and its second argument is other than start, behaving like a runit or s6 finish file.
The effect of the no-setsid file applies to all start, stop, run and notify processes.
Because Bernstein daemontools' supervise only executes a run file, the corresponding service can only be in two states: up if supervise has a child process, or down if it doesn't. Daemontools-encore's supervise, on the other hand, can have at any given time either a start, run, stop or notify child process, so for compatibility with Bernstein daemontools, when the service state is queried with the svstat program (see controlling supervised processes), an up or down state will still be displayed, but also an additional extended state: starting, started, running, stopping, stopped or failed.
- Extended state starting will be displayed after supervise launches a start process.
- Extended state started will be displayed after the start process exits successfully if there is no run file. In started state, supervise does nothing but wait for commands sent with the svc program.
- Extended state running will be displayed after supervise launches a run process.
- Extended state stopping will be displayed after supervise sends its child process a
SIGTERMsignal followed by a
SIGCONTsignal to make it stop, until the process dies.
- Otherwise, either extended state stopped or extended state failed will be displayed. In failed state, or in stopped state after the stop process has exited, supervise does nothing but wait for commands sent with the svc program.
Whether a notify process is currently running or not does not modify the service state, it will be whatever it was immediatly before notify was executed.
Services directories using down, start, stop and notify files:
total 8 drwxr-xr-x 3 user user 4096 May 6 12:00 test-service1 drwxr-xr-x 3 user user 4096 May 6 12:00 test-service2
ls -l *
test-service1: total 4 -rw-r--r-- 1 user user 0 May 6 12:00 down -rwxr-xr-x 1 user user 28 May 6 12:00 run test-service2: total 16 -rwxr-xr-x 1 user user 163 May 6 12:00 notify -rwxr-xr-x 1 user user 24 May 6 12:00 run -rwxr-xr-x 1 user user 24 May 6 12:00 start -rwxr-xr-x 1 user user 24 May 6 12:00 stop
#!/bin/sh exec test-daemon1
#!/bin/sh exec sleep 20
#!/bin/sh exec sleep 10
#!/bin/sh exec sleep 10
#!/bin/sh case $2 in start) echo "'$1' started (PID $3)";; exit) echo "'$1' (PID $3) exited with $4";; killed) echo "'$1' (PID $3) was killed by signal $4";; esac
Resulting supervision tree:
ps xf -o pid,ppid,pgrp,euser,args
PID PPID PGRP EUSER COMMAND ... 1777 1764 1777 user -bash 2114 1777 2114 user \_ svscan 2115 2114 2114 user \_ supervise test-service1 2116 2114 2114 user \_ supervise test-service2 2146 2116 2146 user \_ sleep 20 ...
Note that since test-service1 has a down file, the corresponding supervise process has no children.
Messages printed to svscan's standard output (stdout) by the notify script:
svscan scan &
'start' started (PID 2253) 'start' (PID 2253) exited with 0 'run' started (PID 2258) 'run' (PID 2258) exited with 0 'run' started (PID 2287)
SIGTERM signal to the run process (PID 2287):
'run' (PID 2287) was killed by signal 15 'run' started (PID 2295)
Manually stopping service test-service2:
svc -d scan/test-service2
'run' (PID 2295) was killed by signal 15 'stop' started (PID 2300) 'stop' (PID 2300) exited with 0
The multilog program
If a servicedir S in the scan directory contains a subdirectory or symbolic link to directory named log, svscan will launch two supervise processes in parallel, one executing S/run as a child process, and the other executing S/log/run with its standard input (stdin) connected to S/run's standard output (stdout) by a pipe. If any of the two processes or their supervise parents stops running and is restarted, the same pipe is reused so that no data is lost. This allows per-service logging by having S/log/run execute a logger program. Bernstein daemontools and daemontools-encore packages provide such a logger: the multilog program. This type of logging works for programs that send messages to their standard error (stderr).
Daemontools-encore's supervise allows log to be an executable file instead of subdirectory or symlink to directory. In this case, the same supervise process will supervise both the 'main' service and the logger, and maintain the pipe between then. This is similar to having a subdirectory with a run file equivalent to the log file, and no start or stop files. The notify file of the 'main' service will be invoked with log as its first argument when the log process starts, exits or gets killed by a signal. The down and no-setsid files of the 'main' service also apply to the log process in this case.
multilog is invoked as multilog arg1 arg2 arg3 ..., and expects to read a sequence of newline-terminated lines of text from its standard input. The arguments arg1, arg2, arg3, ... make up a logging script that tells multilog what to do with them. Each argument specifies an action, actions are carried out sequentially in argument order.
The simplest form of multilog invocation is multilog dir, where dir starts with a dot ('.') or a slash ('/'): it is interpreted as a pathname specifying an automatically rotated logging directory (or logdir). The logdir contains control files used by multilog, a current log file named current, and may also contain a set of old log files. The current log file contains a selection of the lines read by multilog, possibly modified by other actions specified in the logging script, and old log files are produced by rotations: when current approaches a certain maximum size, its contents are copied to a another file, after some optional processing, and current is emptied. Old log file have names beginning with '@', continuing with a timestamp in external TAI64N format showing when the file was finished, and ending with either .s or .u. The .s files are files that have been completely processed and safely written, i.e. files produced by a complete rotation. The .u files, if any, are files created by an incomplete rotation, they are not completely processed and may be truncated. Rotations respect line boundaries, i.e. old log files will always contain whole lines. Also, to avoid indefinite accumulation of old log files, if, after completing a rotation, their number exceeds a certain value, multilog deletes the oldest log file (the file with the smallest TAI64N timestamp). The default maximum size of a current file is 99999 bytes, and the default maximum number of old log files is 10. A rotation can be forced by sending multilog a
SIGALRM signal. If it is running as a supervised process, the svc program can be used to do that (see controlling supervised processes).
Some other actions recognized by multilog in a logging script are:
- + (plus sign) and - (minus sign) followed by a pattern: By default, multilog logs all the lines read from its standard input without modifications; these actions allow selection and deselection of lines, respectively, based on the specified pattern. Because all lines are initially selected, + actions are only effective after a - action that deselects some lines first. The pattern uses the shell's notation for matching strings, and must match whole lines (i.e. partial matching doesn't count), with the following restrictions:
- Berstein daemontools' multilog only treats asterisks ('*') as special characters in the pattern: they match any string, including the null string, that does not include the next character in pattern instead of the POSIX behaviour of matching the greatest possible number of characters that still allows the remainder of the pattern to match the line, and cannot be escaped with a backslash ('\').
- Daemontools-encore's multilog can switch between Bernstein daemontools behaviour and using the full range of patterns allowed by POSIX
fnmatch(), including backslash escaping. The latter is more flexible at the expense of being less efficient. At the start of logging script action processing, Bernstein daemontools match mode is selected.
- An F action selects
fnmatch()match mode for all subsequent + and - actions until the next mode-changing action in the logging script.
- An S action selects Bernstein daemontools match mode for all subsequent + and - actions until the next mode-changing action in the logging script.
- An F action selects
- t: prepends each logged line with '@', followed by a timestamp in external TAI64N format (printed as 24 lowercase hexadecimal characters), and a space. It must be specified as the first action, and any subsequent + and - actions match against lines with the prepended timesptamp. Daemontools-encore also supports a T action, which must also be the first one, that prepends an accustamp-style timestamp and a space instead (accustamp was a program shipped with Bernstein daemontools that was eventually dropped).
- s followed by an integer: sets the maximum size of the current file to the specified value (in bytes), for all subsequent actions in the logging script specifying a logdir, until the next s action.
- n followed by an integer: sets the maximum number of old log files to the specified value, for all subsequent actions in the logging script specifying a logdir, until the next n action.
- ! (exclamation mark) followed by a string args: allows multilog to perform some processing of the contents of the current log file during a rotation. multilog performs the equivalent of calling sh -c 'args', and feeds the resulting process the contents of current on its standard input, so args should be the name of a program (that can be found via PATH search if no absolute pathname is used) and its arguments, if any. The .s or .u file produced as a result will contain that program's output.
For details about all actions supported multilog, and about the protocol used for communication between multilog and the processor specified in an ! action, please consult the multilog man page.
info: Message 1 warning: Message 2 error: Message 3 error: Message 4 warning: Message 5 info: Message 6
cat input | multilog '-*' '+warning: *' ./log1 '+error: *' ./log2
drwx------ 2 user user 4096 May 6 12:00 log1 drwx------ 2 user user 4096 May 6 12:00 log2
ls -l log*
log1: total 4 -rwxr--r-- 1 user user 38 May 6 12:00 current -rw------- 1 user user 0 May 6 12:00 lock -rw-r--r-- 1 user user 0 May 6 12:00 state log2: total 4 -rwxr--r-- 1 user user 72 May 6 12:00 current -rw------- 1 user user 0 May 6 12:00 lock -rw-r--r-- 1 user user 0 May 6 12:00 state
When there's no multilog process running on a logdir, the current file has the executable by user flag set.
warning: Message 2 warning: Message 5
warning: Message 2 error: Message 3 error: Message 4 warning: Message 5
Note that the second current log file also contains the warning messages. This shows that actions that aren't + or - do not affect the currently selected lines, so warning messages selected by +warning: * stay selected after processing the ./log1 action. To only have the error messages in the log2/current log file, a -* action should have immediatly preceded the +error: * action.
Service directory test-service containing a logger:
total 8 drwxr-xr-x 2 user user 4096 May 6 12:00 log -rwxr-xr-x 1 user user 110 May 6 12:00 run
ls -l log
total 4 -rwxr-xr-x 1 user user 40 May 6 12:00 run
#!/usr/bin/python from time import sleep from sys import stderr for i in range(1,30): print("Message", i, file=stderr, flush=True) sleep(2)
#!/bin/sh exec /home/user/test-daemon 2>&1
#!/bin/sh exec multilog t ../../../logs
Note that since supervise makes the service directory its working directory, the relative pathname in multilog's invocation results in the logging directory being placed wherever the scan directory is.
Resulting supervision tree:
ps xf -o pid,ppid,pgrp,euser,args
PID PPID PGRP EUSER COMMAND ... 1777 1764 1777 user -bash 2702 1777 2702 user \_ svscan 2703 2702 2702 user \_ supervise test-service 2706 2703 2702 user | \_ /usr/lib/python-exec/python3.4/python /home/user/test-daemon 2704 2702 2702 user \_ supervise log 2705 2704 2702 user \_ multilog t ../../../logs ...
Contents of the logs logging directory:
ls -l ../logs
total 20 -rwxr--r-- 1 user user 361 May 6 17:20 @40000000590e2ffe0bf16944.s -rwxr--r-- 1 user user 2202 May 6 17:22 @40000000590e30941e642c9c.s -rw-r--r-- 1 user user 1101 May 6 17:23 current -rw------- 1 user user 0 May 6 17:19 lock -rw-r--r-- 1 user user 0 May 6 17:19 state
This logdir contains two old log files and, since multilog is running, the current file has its executable by user flag cleared.
@40000000590e30983973bda4 Message 1 @40000000590e309a39b02a64 Message 2 @40000000590e309c3a353ccc Message 3 @40000000590e309e3a93b84c Message 4 @40000000590e30a03aec8a94 Message 5 @40000000590e30a23b3bbc04 Message 6 @40000000590e30a5003a28ac Message 7 @40000000590e30a70097255c Message 8 @40000000590e30a900bc221c Message 9 @40000000590e30ab0100ff04 Message 10
The TAI64N timestamps can be shown in human-readable form with tai64nlocal, for further details please consult the respective man page.
cat ../logs/current | tai64nlocal
2017-05-06 17:22:38.963886500 Message 1 2017-05-06 17:22:40.967846500 Message 2 2017-05-06 17:22:42.976567500 Message 3 2017-05-06 17:22:44.982759500 Message 4 2017-05-06 17:22:46.988580500 Message 5 2017-05-06 17:22:48.993770500 Message 6 2017-05-06 17:22:51.003811500 Message 7 2017-05-06 17:22:53.009905500 Message 8 2017-05-06 17:22:55.012329500 Message 9 2017-05-06 17:22:57.016842500 Message 10
The readproctitle program
The readproctitle program expects to read a sequence of newline-terminated lines of text from its standard input, and saves them to an automatically rotated log it keeps in memory. The log can be seen in the output of the ps utility. The number of characters displayed is specified by readproctitle's arguments.
readproctitle is invoked as readproctitle arg1 arg2 ... argn dots. The arguments arg1, arg2, ..., argn can be anything, they are shown as-is in the output of ps, and can used for displaying some kind of heading that introduces the log. The last argument, dots, must be at least five dots ('.'), and its length specifies the number of characters the log kept in memory will have. In the output of ps, readproctitle's last shown argument will be the contents of the log up to that time instead of dots. Characters shift to the left as readproctitle reads new lines of text, and older characters are discarded to keep the log of constant size. Therefore, the ps utility will show each time the most recent data. For further details, please consult the readproctitle man page.
#!/bin/sh echo Print this message! sleep 10 echo Second message sleep 10
./doit | readproctitle Log header: ........................ &
ps x -o pid,args
PID COMMAND ... 2860 readproctitle Log header: ....Print this message! ...
After enough seconds have elapsed:
ps x -o pid,args
PID COMMAND ... 2860 readproctitle Log header: ...sage! Second message ...
The best known use of readproctitle is in the svscanboot script included in Bernstein daemontools and daemontools-encore.
The logging chain
A supervision tree where all leaf processes have a logger can be arranged into what the author of s6 calls the logging chain , which he considers to be technically superior to the traditional syslog-based centralized approach .
Since processes in a supervision tree are created using the POSIX
fork() call, each of them will inherit svscan's standard input, output and error. A logging chain arrangement using Bernstein daemontools and daemontools-encore is as follows:
- Leaf processes should normally have a logger, so their standard output and error connect to their logger's standard input. Therefore, all their messages are collected and stored in dedicated, per-service logs by their logger. Some programs might need to be invoked with special options to make them send messages to their standard error, and redirection of standard error to standard output (i.e. 2>&1 in a shell script) must be performed in the servicedir's run file.
- Leaf processes with a controlling terminal are an exception: their standard input, output and error connect to the terminal.
- supervise, the loggers, and leaf processes that exceptionally don't have logger for some reason, inherit their standard input, output and error from svscan, so their messages are sent wherever the ones from svscan are.
- Leaf processes that still unavoidably report their messages using
syslog()have them collected and logged by a (possibly supervised) syslog server.
Specifying a logger for svscan
Daemontools-encore's svscan allows the pathname of a service directory to be passed as a second argument after the scan directory's pathname. If this argument is present, svscan will launch one supervise process for the specified directory, and connect its standard output and error to the corresponding supervised process' standard input by a pipe. This makes it possible to set up a logger for svscan in the same way a logger can be set up for the leaf processes of the supervision tree.
If the service directory of svscan's logger is specified as a relative pathname, it is interpreted as relative to the scan directory
Supervised process execution state changes
Bernstein's daemontools and daemontools-encore provide a set of tools for modifying a supervised process' execution state. These tools employ a technique called chain loading by some people, and Bernstein chaining by others. A program prog1 designed to use chain loading is invoked as prog1 arg11 ... arg1n prog2 arg21 ... arg2n, where prog2 is the name of another program. When prog1 is invoked, it performs some action based on arguments arg11, ..., arg1n, and then executes prog2 without creating a new process, using one of the POSIX
exec...() calls. Arguments arg21, ..., arg2n are not interpreted by prog1 and are passed along to prog2.
If prog2 is also designed to use chain loading, some of those arguments might in turn be the name of a third program prog3 and its corresponding arguments, which will be executed after prog2 completes its task. As a consequence, it is possible to build a chain of programs that will run one after the other as a single process, therefore preserving the process ID (PID), which makes the technique suitable for the run script of a service directory. The final one in such a chain of programs would be the real program intended to be supervised.
The tools provided by Bernstein's daemontools and daemontools-encore to change a process' execution state are:
- envdir: runs another program with environment modified according to files in a specified directory.
- envuidgid: runs another program with environment variables indicating a specified account's user ID and group ID
- pgrphack: runs a program in a new session (using POSIX
- setlock: runs another program with a file locked (using Linux
- setuidgid: runs another program under a specified account's user ID and group ID. Requires root privileges.
- softlimit: runs another program with new soft resource limits (using POSIX
setrlimit()). These limits are set via options specified as softlimit's arguments. Daemontools-encore also allows setting those limits via SOFTLIMIT_* environment variables, applied at softlimit invocation; see environment variables.
For further details on these tools please consult their respective man page.
The directory supplied to the envdir program is called an environment directory (or envdir). Each file in the envdir controls a single environment variable. If the file is empty, and a variable with the same name as the file exists in the process' environment, it is removed from it. If the file is nonempty, the contents of its first line become the value of a new environment variable with the same name as the file. If the variable existed before envdir's invocation, it is first removed from the environment, and then readded with the new value. Enviroment variables not referred to by a file in the envdir remain unchanged.
#!/bin/sh exec \ softlimit -o 5 \ setuidgid daemon \ test-daemon 2>&1
This script executes program test-daemon with effective user daemon and the maximum number of open file descriptors set to 5. This is the same as if test-daemon performed a
setrlimit(RLIMIT_NOFILE, &rl) call itself with
rl.rlim_cur set to 5, provided that value does not exceed the corresponding hard limit. As in previous examples, the redirection of stderr to stdout allows setting up a dedicated logger for test-daemon.
Example service directory with an enviroment directory env:
total 8 drwxr-xr-x 2 root root 4096 May 7 12:00 env -rwxr-xr-x 1 root root 125 May 7 12:00 run
ls -l env
total 4 -rw-r--r-- 1 root root 2 May 7 12:00 SOFTLIMIT_OPENFILES
#!/bin/sh exec \ envdir env \ envuidgid daemon \ softlimit \ test-daemon 2>&1
This script adds variables UID, GID and SOFTLIMIT_OPENFILES to the environment, the first two set to the user ID and group ID of account daemon via envuidgid, and the last one via the enviroment directory env, which is used by the softlimit invocation to set the maximum number of open file descriptors, provided it is the daemontools-encore version of that program. It is assumed that test-daemon can use environment variables UID and GID to drop privileges.
Controlling supervised processes
Bernstein daemontools and daemontools-encore provide two programs for controlling a supervised process and to query status information about it: svc and svstat, respectively.
The svc program accepts a set of service directory pathnames and options that specify what to do. Some of these options are:
- svc -d (down): If the supervised process is running, send it a
SIGTERMsignal followed by a
SIGCONTsignal. The run file won't be reexecuted after that, and, for daemontools-encore, the stop file will be executed, if present. This is the standard way to manually stop a supervised process. If the process is not running (e.g. because it has a down file or after a previous svc -d command) this is a no operation.
- svc -u (up): If the supervised process is not running, start it by executing its run file, or, for daemontools-encore, its start file, if present. If the process is running, this is mostly a no operation, except that it cancels the effect of a previous svc -o command. This is the standard way to manually start a supervised process that has a down file, or that has been previously stopped with an svc -d command.
- svc -o (once): If the supervised process is not running, start it by executing its run file, or, for daemontools-encore, its start file, if present, but don't restart it if stops. This still allows the process to be monitored and to be reliably sent signals, but it won't actually be supervised. If the process is running, it just asks supervise to not restart it if it stops. The effect of this option can be cancelled by a subsequent svc -u command, and restored by another svc -o command.
- svc -x (exit): Asks supervise to exit if the supervised process stops. This is only effective if its svscan parent stopped running without supervise being killed, otherwise a new supervise process will be launched on the next periodic scan.
Other svc options allow reliably sending signals to a supervised process. For further details, please consult the svc man page. In particular, svc -a can be used to send a
SIGALRM signal to a supervised multilog process to force it to perform a rotation.
The svstat program accepts a set of service directory pathnames. Its output is intended to be displayed on a user interface or logged, not to be fed as input to a program for parsing. It displays whether the supervised process is running ('up') or not ('down'), its process ID (PID) if it is up, how long it has been in the current state, whether its current up or down status matches the presence or absence of a down file in the servicedir ('normally up' or 'normally down'), and whether it is transitioning to the desired state or already there ('want up' or 'want down'). The daemontools-encore version also displays the service's extended state (see the start, stop and notify files, and the daemontools-encore extended service state), and if there is a log executable file, it will print a second line of output with information about the logger process. If the
-l option is passed as an argument, the information about the logger process will be suppressed from the output, and if the
-L option is passed as an argument, only information about the logger process will be displayed, or the message 'no log service' if the servicedir has no log executable file. For further details, please consult the svstat man page.
Bernstein daemontools and daemontools-encore also provide the svok program, that accepts a service directory pathname, and exits with an exit code of 0 if there is a supervise process successfully running for that directory, and an exit code of 100 if there isn't. It produces no output if no error was encountered, otherwise it displays an error message and exits with an exit code of 111. Finally, daemontools-encore provides the svup program, that accepts a service directory pathname, and exits with an exit code of 0 if there is a supervise process successfully running for that directory and the service's extended state is either started or running, and an exit code of 100 if the service is in any other state, or if there is no supervise process. It produces no output if no error was encountered, otherwise it displays an error message and exits with an exit code of 111. If there is a log executable file, svup exits with an exit code of 0 only if both the 'main' service and the logger are in started or running state. If the
-l option is passed as an argument, only the 'main' service will be checked, and if the
-L option is passed as an argument, only the logger will be checked. svup -L exits with an exit code of 100 if the servicedir has no log executable file. For further details about svok and svup, please consult their respective man page.
For the operation performed by svc to be allowed, its effective user must have write access to the supervise/control FIFO in the service directory, otherwise, it will fail with an 'access denied' error. For the operation performed by svstat and svup to be allowed, its effective user must have write access to the supervise/ok FIFO and read access to the supervise/status file in the service directory, otherwise, it will fail with an 'access denied' error. Using svc or svstat on a service directory for which no supervise process is running will result in a 'supervise not running' error.
Service directories with a log subdirectory and a log executable file:
total 8 drwxr-xr-x 3 user user 4096 Jan 28 12:00 test-service1 drwxr-xr-x 2 user user 4096 Jan 28 12:00 test-service2
ls -l * test-service1/log
test-service1: total 8 drwxr-xr-x 2 user user 4096 Jan 28 12:00 log -rwxr-xr-x 1 user user 32 Jan 28 12:00 run test-service1/log: total 4 -rwxr-xr-x 1 user user 41 Jan 28 12:00 run test-service2: total 8 -rwxr-xr-x 1 user user 38 Jan 28 12:00 log -rwxr-xr-x 1 user user 32 Jan 28 12:00 run
#!/bin/sh exec test-daemon 2>&1
#!/bin/sh exec multilog ../../../logdir1
#!/bin/sh exec test-daemon 2>&1
#!/bin/sh exec multilog ../../logdir2
Resulting supervision tree:
ps xf -o pid,ppid,pgrp,euser,args
PID PPID PGRP EUSER COMMAND ... 2284 2122 2284 user -bash 2353 2284 2353 user \_ svscan 2354 2353 2353 user \_ supervise test-service1 2357 2354 2357 user | \_ test-daemon 2355 2353 2353 user \_ supervise log 2359 2355 2359 user | \_ multilog ../../../logdir1 2356 2353 2353 user \_ supervise test-service2 2358 2356 2358 user \_ multilog ../../logdir2 2360 2356 2360 user \_ test-daemon ...
Note that there are separate supervise processes (with PIDs 2354 and 2355) for test-service1 and its logger, but a single supervise process (with PID 2356) supervising both test-service2 and its logger.
test-service1: up (pid 2357) 157 seconds, running test-service2: up (pid 2360) 156 seconds, running test-service2 log: up (pid 2358) 157 seconds, running
svstat -l *
test-service1: up (pid 2357) 170 seconds, running test-service2: up (pid 2360) 169 seconds, running
svstat -L *
test-service1: no log service test-service2 log: up (pid 2358) 182 seconds, running
To get the status of the test-service1 logger, the pathname of the corresponding log subdirectory must be specified to svstat:
test-service1/log: up (pid 2359) 241 seconds, running
Starting the supervision tree
In a supervision tree arrangement each process supervises its children, therefore the robustness of the supervision tree ultimately relies on the svscan process. So the problem of how to supervise svscan itself arises, as well as what to do with svscan's messages in a logging chain.
Gentoo's packaging of Bernstein daemontools and daemontools-encore provides an OpenRC service script for running svscan, also named svscan. If this service script is used, the scan directory will be /service. Thus, svscan can be started when the machine boots by adding it to an OpenRC runlevel using rc-update:
rc-update add svscan default
Or it can also be started manually:
rc-service svscan start
The service script launches svscan using OpenRC's start-stop-daemon program, so it will run unsupervised, and with its standard output and error redirected to /dev/null by default, so supervision tree messages other than those from leaf processes that have a dedicated logger will be lost.
The svscanboot script
Bernstein daemontools and daemontools-encore provide a script called scvscanboot, that can be spawned and supervised by sysvinit (sys-apps/sysvinit) by adding a 'respawn' line for it in /etc/inittab. scvscanboot is a shell script that launches an svscan process and a readproctitle process, with the former's standard output connected to the latter's standard input by a pipe, and every other standard descriptor redirected to /dev/null. The enviroment will be empty, except for the PATH variable, set to a known value in the script. The scan directory will be /service.
Used in this way, the supervision tree becomes rooted in process 1, which cannot die without crashing the machine. Also, svscan's messages will go to readproctitle, so they can be seen using the ps utility.
Gentoo users wanting to use svscanboot will need to manually edit /etc/inittab, and then call telinit
This will run svscanboot when entering runlevels 1 to 5. Because svscanboot calls svc using absolute path /command/svc, a symlink to the correct path for Gentoo must be created:
mkdir -p /command
ln -s /usr/bin/svc /command/svc
The supervise-scripts package
Bruce Guenter's supervise-scripts package (sys-process/supervise-scripts) provides an svscan-add-to-inittab script that also allows sysvinit to launch and supervise svscan. The script will make a backup of /etc/inittab (with a name of the form /etc/inittab-YYYY-MM-DD) and then modify it so that another script, svscan-start is run when entering runlevels 1 to 5.
The svscan-start script is a wrapper around svscan. After some initialization it will replace itself with svscan using the shell's exec builtin utility, with its standard input, output and error redirected to /dev/console. Therefore, svscan's messages will be shown on the console. The enviroment will be empty, except for the PATH variable, set to a known value in the script. The scan directory will be /service.
The package also includes a svscan-stopall script, that will be called when entering sysvinit's single user mode or runlevels 0, 1 and 6, to perform cleanup. It will stop all lingering supervise processes and their children using an svc -dx command.
emerge --ask --depclean sys-process/daemontools-encore
All scan directories, service directories, the /command/svc symlink, etc. must be manually deleted if no longer wanted after removing the package. Also, all modifications to /etc/inittab must be manually reverted, even when using the supervise-scripts package, because svscan-add-to-inittab does not have an 'undo' option. Lines for svscanboot, svscan-start and svscan-stopall must be deleted, and a telinit q command must be used afterwards. If no further changes to /etc/inittab have been made after invoking svscan-add-to-inittab, the backup copy created by that script before the changes are applied can be used to revert them by just replacing /etc/inittab with that file.
- daemontools — a collection of tools for managing UNIX services
- runit — a daemontools-inspired process supervision suite that also provides a program suitable for running as process 1
- s6 — a package that provides a daemontools-inspired process supervision suite, a notification framework, a UNIX domain super-server, and tools for file descriptor holding and suidless privilege gain
- Process-Supervision — the ability to manage (long lived) processes or rather daemons and be able to get (automated) process restart if needed
- D. J. Bernstein, daemontools FAQ, which includes one about the benefits of process supervision. Retrieved on April 23rd, 2017.
- Gerrit Pape, runit benefits, which includes a short description of process supervision in general. Retrieved on April 23rd, 2017.
- Laurent Bercot, s6 overview, which contains an introduction to process supervision. Retrieved on April 23rd, 2017.
- Jonathan de Boyne Pollard, Mistakes to avoid when designing Unix dæmon programs. Retrieved on May 5th, 2017.
- Laurent Bercot, the logging chain, Retrieved on May 1st, 2017.
- Laurent Bercot, on the syslog design, Retrieved on May 1st, 2017.