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Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
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Sentry - Simple process monitoring

This project is in early stage.

Sentry is a process monitoring tool written and configured in Haskell. Its aim is to keep running programs. For each configured program (called an entry in its configuration) , multiple processes can be started.

Currently Sentry runs on Linux only.


The development version can be installed by cloning the Git repository and using cabal:

> git clone git://github.com/noteed/sentry.git
> cd sentry && cabal install


Sentry is written in Haskell and is also configured in Haskell. The principle is to write a short script in ~/.sentry/conf/. For instance a sample.hs configuration would look like the following:

import Sentry.Server

main :: IO ()
main = sentry
  [ entry "dummy" "sleep" ["4"] 1000 1

"dummy" is the entry name, also called the process type. "sleep" and ["4"] is the command that need to be run (and kept running), together with its arguments. The 1000 value specifies how long in milliseconds Sentry has to wait before restarting the command when it dies. Finally 1 is the number of processes you want to run that particular process type. You can then compile sample.hs and obtain a sample binary like this:

> ghc --make -threaded sample.hs

Start your Sentry with:

> ~/.sentry/conf/sample start

Sentry will save its PID to sample.pid and start running the "dummy" entry:

Sentry started (PID: 1511s aved in /home/thu/.sentry/conf/sample.pid).
20:17:51 dummy.1514      Started at 1334081871.
20:17:55 dummy.1514      Exited at 1334081875 with ExitSuccess.
20:17:56 dummy.1516      Started at 1334081876.
20:18:00 dummy.1516      Exited at 1334081880 with ExitSuccess.

Whenever sleep exits, Sentry will run it again, and again.

Recompiling a configuration (i.e. sample.hs in the example) can be done while Sentry is running. A SIGHUP will instruct Sentry to re-exec itself, using the new configuration. The command (while Sentry is already running)

> ~/.sentry/conf/sample reload

will just do that:

> .sentry/conf/sample start
Sentry started (PID: 1530saved in /home/thu/.sentry/conf/sample.pid).
20:32:01 dummy.1536      Started at 1334082721.
20:32:05 dummy.1536      Exited at 1334082725 with ExitSuccess.
/home/thu/.sentry/conf/sample.hs successfully compiled.
Sentry reexec'd. Initially started at 1334082711 (Previously reexec'd at 1334082711).
20:32:08 dummy.1550      Started at 1334082728.
20:32:12 dummy.1550      Exited at 1334082732 with ExitSuccess.

HTTP interface

Sentry has an embedded web server to exposes its state over HTTP. It is also used to accept reconfiguration commands (e.g. scale a given process type).


This README.md file should be a good starting point. Additional information can be found in the Haddock pages. Those pages are currently available from http://hypered.be/haddock/sentry.


Sentry is configured with a list of process specifications. For each specification, multiple processes can be spawn and monitored.

Sentry spawns a process by forking itself and then exec'ing the process' command. Waiting for the process to complete is done in a lightweight thread. Forking and waiting a child process is a more accurate way to know when the process dies than polling. (Another good way to do that is to use netlink with the process events connector.)

A main thread reads commands from a concurrent queue (a Chan in Haskell). Commands are pushed by signal handlers and monitoring threads to instruct the main thread to act on its configuration (and thus change its set of monitored processes).


  • Dynamically resize the number of processes for a specific entry.
  • Let Sentry start a configuration instead of manually run it. I.e. sentryd start -c sample instead of ~/.sentry/conf/sample. Default configuration could be sentry.hs.
  • Separate data types for save/restore (i.e. with SafeCopy instances) and data types actually used at runtime.
  • Build on 7.0 and 7.4 (was tested only on 7.0.3 so far).