183 lines
7.0 KiB
Plaintext
183 lines
7.0 KiB
Plaintext
Daemontools and runit
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Tired of PID files, needing root access, and writing init scripts just
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to have your UNIX apps start when your server boots? Want a simpler,
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better alternative that will also restart them if they crash? If so,
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this is an introduction to process supervision with runit/daemontools.
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Background
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Classic init scripts, e.g. /etc/init.d/apache, are widely used for
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starting processes at system boot time, when they are executed by init.
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Sadly, init scripts are cumbersome and error-prone to write, they must
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typically be edited and run as root, and the processes they launch do
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not get restarted automatically if they crash.
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In an alternative scheme called "process supervision", each important
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process is looked after by a tiny supervising process, which deals with
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starting and stopping the important process on request, and re-starting
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it when it exits unexpectedly. Those supervising processes can in turn
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be supervised by other supervising processes.
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Dan Bernstein wrote the process supervision toolkit, "daemontools",
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which is a set of small, reliable programs that cooperate in the
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UNIX tradition to manage process supervision trees.
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Runit is a more conveniently licensed and more actively maintained
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reimplementation of daemontools, written by Gerrit Pape.
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Here I’ll use runit, however, the ideas are the same for other
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daemontools-like projects (there are several).
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Service directories and scripts
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In runit parlance a "service" is simply a directory containing a script
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named "run".
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There are just two key programs in runit. Firstly, runsv supervises the
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process for an individual service. Service directories themselves sit
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inside a containing directory, and the runsvdir program supervises that
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directory, running one child runsv process for the service in each
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subdirectory. A typical choice is to start an instance of runsvdir
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which supervises services in subdirectories of /var/service/.
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If /var/service/log/ exists, runsv will supervise two services,
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and will connect stdout of main service to the stdin of log service.
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This is primarily used for logging.
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You can debug an individual service by running its SERVICE_DIR/run script.
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In this case, its stdout and stderr go to your terminal.
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You can also run "runsv SERVICE_DIR", which runs both the service
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and its logger service (SERVICE_DIR/log/run) if logger service exists.
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If logger service exists, the output will go to it instead of the terminal.
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"runsvdir /var/service" merely runs "runsv SERVICE_DIR" for every subdirectory
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in /var/service.
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Examples
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This directory contains some examples of services:
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var_service/getty_<tty>
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Runs a getty on <tty>. (run script looks at $PWD and extracts suffix
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after "_" as tty name). Create copies (or symlinks) of this directory
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with different names to run many gettys on many ttys.
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var_service/gpm
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Runs gpm, the cut and paste utility and mouse server for text consoles.
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var_service/inetd
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Runs inetd. This is an example of a service with log. Log service
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writes timestamped, rotated log data to /var/log/service/inetd/*
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using "svlogd -tt". p_log and w_log scripts demonstrage how you can
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"page log" and "watch log".
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Other services which have logs handle them in the same way.
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var_service/nmeter
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Runs nmeter '%t %c ....' with output to /dev/tty9. This gives you
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a 1-second sampling of server load and health on a dedicated text console.
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Networking examples
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In many cases, network configuration makes it necessary to run several daemons:
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dhcp, zeroconf, ppp, openvpn and such. They need to be controlled,
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and in many cases you also want to babysit them.
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They present a case where different services need to control (start, stop,
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restart) each other.
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var_service/dhcp_if
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controls a udhcpc instance which provides dhpc-assigned IP
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address on interface named "if". Copy/rename this directory as needed to run
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udhcpc on other interfaces (var_service/dhcp_if/run script uses _foo suffix
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of the parent directory as interface name).
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When IP address is obtained or lost, var_service/dhcp_if/dhcp_handler is run.
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It saves new config data to /var/run/service/fw/dhcp_if.ipconf and (re)starts
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/var/service/fw service. This example can be used as a template for other
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dynamic network link services (ppp/vpn/zcip).
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This is an example of service with has a "finish" script. If downed ("sv d"),
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"finish" is executed. For this service, it removes DHCP address from
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the interface. This is useful when ifplugd detects that the the link is dead
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(cable is no longer attached anywhere) and downs us - keeping DHCP configured
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addresses on the interface would make kernel still try to use it.
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var_service/zcip_if
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Zeroconf IP service: assigns a 169.254.x.y/16 address to interface "if".
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This allows to talk to other divices on a network without DHCP server
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(if they also assign 169.254 addresses to themselves).
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var_service/ifplugd_if
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Watches link status of interface "if". Downs and ups /var/service/dhcp_if
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service accordingly. In effect, it allows you to unplug/plug-to-different-network
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and have your IP properly re-negotiated at once.
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var_service/dhcp_if_pinger
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Uses var_service/dhcp_if's data to determine router IP. Pings it.
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If ping fails, restarts /var/service/dhcp_if service.
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Basically, an example of watchdog service for networks which are not reliable
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and need babysitting.
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var_service/supplicant_if
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Wireless supplicant (wifi association and encryption daemon) service for
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inteface "if".
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var_service/fw
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"Firewall" script, although it is tasked with much more than setting up firewall.
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It is responsible for all aspects of network configuration.
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This is an example of *one-shot* service.
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It reconfigures network based on current known state of ALL interfaces.
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Uses conf/*.ipconf (static config) and /var/run/service/fw/*.ipconf
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(dynamic config from dhcp/ppp/vpn/etc) to determine what to do.
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One-shot-ness of this service means that it shuts itself off after single run.
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IOW: it is not a constantly running daemon sort of thing.
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It starts, it configures the network, it shuts down, all done
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(unlike infamous NetworkManagers which sit in RAM forever).
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However, any dhcp/ppp/vpn or similar service can restart it anytime
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when it senses the change in network configuration.
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This even works while fw service runs: if dhcp signals fw to (re)start
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while fw runs, fw will not stop after its execution, but will re-execute once,
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picking up dhcp's new configuration.
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This is achieved very simply by having
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# Make ourself one-shot
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sv o .
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at the very beginning of fw/run script, not at the end.
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Therefore, any "sv u /var/run/service/fw" command by any other
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script "undoes" o(ne-shot) command if fw still runs, thus
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runsv will rerun it; or start it in a normal way if fw is not running.
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This mechanism is the reason why fw is a service, not just a script.
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System administrators are expected to edit fw/run script, since
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network configuration needs are likely to be very complex and different
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for non-trivial installations.
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var_service/ftpd
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var_service/httpd
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var_service/tftpd
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var_service/ntpd
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Examples of typical network daemons.
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