Angel

angel is a daemon that runs and monitors other processes. It is similar to djb's daemontools or the Ruby project god.

It's goals are to keep a set of services running, and to facilitate the easy configuration and restart of those services.

Motivation

The author is a long-time user of daemontools due to its reliability and simplicity; however, daemontools is quirky and follows many unusual conventions.

angel is an attempt to recreate daemontools's capabilities (though not the various bundled utility programs which are still quite useful) in a more intuitive and modern unix style.

Functionality

angel is driven by a configuration file that contains a list of program specifications to run. angel assumes every program listed in the specification file should be running at all times.

angel starts each program, and optionally sets the program's stdout and stderr to some file(s) which have been opened in append mode; at this point, the program is said to be "supervised".

If the program dies for any reason, angel waits a specified number of seconds (default, 5), then restarts the program.

The angel process itself will respond to a HUP signal by re-processing its configuration file, and synchronizing the run states with the new configuration. Specifically:

• If a new program has been added to the file, it is started and supervised
• If a program's specification has changed (command line path, stdin/stdout path, delay time, etc) that supervised child process will be sent a TERM signal, and as a consequence of normal supervision, will be restarted with the updated spec
• If a program has been removed from the configuration file, the corresponding child process will be sent a TERM signal; when it dies, supervision of the process will end, and therefore, it will not be restarted

Safety and Reliability

Because of angel's role in policing the behavior of other daemons, it has been written to be very reliable:

• It is written in Haskell, which boasts a combination of strong, static typing and purity-by-default that lends itself to very low bug counts
• It uses multiple, simple, independent lightweight threads with specific roles, ownership, and interfaces
• It uses STM for mutex-free state synchronization between these threads
• It falls back to polling behavior to ensure eventual synchronization between configuration state and run state, just in case odd timing issues should make event-triggered changes fail
• It simply logs errors and keeps running the last good configuration if it runs into problems on configuration reloads
• It has logged hundreds of thousands of uptime-hours since 2010-07 supervising all the daemons that power http://bu.mp without a single memory leak or crash

Building

1. Install the haskell-platform 2011 (or somehow, ghc 7.0 + cabal-install)
2. Run cabal install in the project root (this directory)
3. Either add the ~/.cabal/bin file to your $PATH or copy the angel executable to /usr/local/bin

Notes:

• I have not tried building angel against ghc 6.10 or earlier; 6.12 and 7.2 are known to work

Configuration and Usage Example

The angel executable takes exactly one argument: a path to an angel configuration file.

angel's configuration system is based on Bryan O'Sullivan's configurator package. A full description of the format can be found here:

http://hackage.haskell.org/packages/archive/configurator/0.1.0.0/doc/html/Data-Configurator.html

A basic configuration file might look like this:

watch-date {
    exec = "watch date"
}

ls {
    exec = "ls"
    stdout = "/tmp/ls_log"
    stderr = "/tmp/ls_log"
    delay 7
}

Each program that should be supervised starts a program-id block:

watch-date {

Then, a series of corresponding configuration commands follow:

• exec is the exact command line to run (required)
• stdout is a path to a file where the program's standard output should be appended (optional, defaults to /dev/null)
• stderr is a path to a file where the program's standard error should be appended (optional, defaults to /dev/null)
• delay is the number of seconds (integer) angel should wait after the program dies before attempting to start it again (optional, defaults to 5)
• directory is the current working directory of the newly executed program (optional, defaults to angel's cwd)

Assuming the above configuration was in a file called "example.conf", here's what a shell session might look like:

jamie@choo:~/random/angel$ angel example.conf 
[2010/08/24 15:21:22] {main} Angel started
[2010/08/24 15:21:22] {main} Using config file: example.conf
[2010/08/24 15:21:22] {process-monitor} Must kill=0, must start=2
[2010/08/24 15:21:22] {- program: watch-date -} START
[2010/08/24 15:21:22] {- program: watch-date -} RUNNING
[2010/08/24 15:21:22] {- program: ls -} START
[2010/08/24 15:21:22] {- program: ls -} RUNNING
[2010/08/24 15:21:22] {- program: ls -} ENDED
[2010/08/24 15:21:22] {- program: ls -} WAITING
[2010/08/24 15:21:29] {- program: ls -} RESTART
[2010/08/24 15:21:29] {- program: ls -} START
[2010/08/24 15:21:29] {- program: ls -} RUNNING
[2010/08/24 15:21:29] {- program: ls -} ENDED
[2010/08/24 15:21:29] {- program: ls -} WAITING

.. etc

You can see that when the configuration is parsed, the process-monitor notices that two programs need to be started. A supervisor is started in a lightweight thread for each, and starts logging with the context program: <program-id>.

watch-date starts up and runs. Since watch is a long-running process it just keeps running in the background.

ls, meanwhile, runs and immediately ends, of course; then, the WAITING state is entered until delay seconds pass. Finally, the RESTART event is triggered and it is started again, ad naseum.

Now, let's see what happens if we modify the config file to look like this:

#watch-date {
#    exec = "watch date"
#}

ls {
    exec = "ls"
    stdout = "/tmp/ls_log"
    stderr = "/tmp/ls_log"
    delay 7
}

.. and then send HUP to angel.

[2010/08/24 15:33:59] {config-monitor} HUP caught, reloading config
[2010/08/24 15:33:59] {process-monitor} Must kill=1, must start=0
[2010/08/24 15:33:59] {- program: watch-date -} ENDED
[2010/08/24 15:33:59] {- program: watch-date -} QUIT
[2010/08/24 15:34:03] {- program: ls -} RESTART
[2010/08/24 15:34:03] {- program: ls -} START
[2010/08/24 15:34:03] {- program: ls -} RUNNING
[2010/08/24 15:34:03] {- program: ls -} ENDED
[2010/08/24 15:34:03] {- program: ls -} WAITING

As you can see, the config monitor reloaded on HUP, and then the process monitor marked the watch-date process for killing. TERM was sent to the child process, and then the supervisor loop QUIT because the watch-date program no longer had a config entry.

Advanced Configuration

The configurator package supports import statements, as well as environment variable expansion. Using collections of configuration files and host-based or service-based environment variables, efficient, templated angel configurations can be had.

FAQ

Can I have multiple programs logging to the same file?

Yes, angel dup()s file descriptors and makes effort to safely allow concurrent writes by child programs; you should DEFINITELY make sure your child program is doing stdout/stderr writes in line-buffered mode so this doesn't result in a complete interleaved mess in the log file.

Will angel restart programs for me?

No; the design is just to send your programs TERM, then angel will restart them. angel tries to work in harmony with traditional Unix process management conventions.

How can I take a service down without wiping out its configuration?

Currently, the only way to achieve this is by employing the "commenting out" method illustrated above.

Author

Jamie Turner [email protected]

Thanks to Bump Technologies, Inc. (http://bu.mp) for sponsoring some of the work on angel.

And, of course, thanks to all Angel's contributors:

https://github.com/jamwt/Angel/contributors