[systemd-devel] multiple starts for a socket-based service

[Ross Boylan]

Hi, systemd-ers.  I'm trying to do something that seems at cross
purposes with systemd's assumptions, and I'm hoping for some guidance.

Goal: remote client sends a 1 line command to a server, which executes
a script that does not create a long-running service.
These events will be rare.  I believe the basic model for systemd
sockets is that service is launched on first contact, and is then
expected to hang around to handle later requests.  Because such
requests are rare, I'd rather that the service exit and the process be
repeated for later connections.

Is there a way to achieve this with systemd?

It looks as if Accept=yes in the [Socket] section might work, but I'm
not sure about the details, and have several concerns:
1. systemd may attempt to restart my service script if it exits (as it
seems to have in the logs below).
2. man systemd.socket recommends using Accept=no "for performance
reasons".  But that seems to imply the service that is activated must
hang around to handle future requests.
3. Accept=yes seems to imply that several instances of the service
could share the same socket, which seems dicey.  Is systemd
automatically doing the handshake for TCP sockets, in which the server
generates a new port, communicates it to the client, and it is this
second port that gets handed to the service?  Or is the expectation
that the client service will do that early on and close the original
port?
Although calls to the service should be rare, it's easy to imagine
that during development I inadvertently generate rapidly repeated
calls so that several live processes could end up accessing the same
socket.

Finally, I'd like to ignore rapid bursts of requests.  Most systemd
limits seem to put the unit in a permanently failed state if that
happens, but I would prefer if that didn't happen, ie. ignore but
continue, rather than ignore and fail.

My first attempt follows.  It appears to have generated 5 quick
invocations of the script and then permanent failure of the service
with service-start-limit-hit.  So even when the burst window ended,
the service remained down.  I think what happened was that when my
script finished the service unit took that as a failure (? the logs do
show the service succeeding, though RemainAfterExit=no)  and tried to
restart it.  It did this 5 times and then hit a default limit.  Since
the service and the socket were then considered failed, no more
traffic on the socket triggered any action.

Here are the last few log entries:
Aug 01 01:42:55 barley systemd[1]: Finished Update kernel netboot info
for family system.
Aug 01 01:42:55 barley systemd[1]: Starting Update kernel netboot info
for family system...
Aug 01 01:42:55 barley systemd[1]: family.service: Succeeded.
Aug 01 01:42:55 barley systemd[1]: Finished Update kernel netboot info
for family system.
Aug 01 01:42:55 barley systemd[1]: family.service: Start request
repeated too quickly.
Aug 01 01:42:55 barley systemd[1]: family.service: Failed with result
'start-limit-hit'.
Aug 01 01:42:55 barley systemd[1]: Failed to start Update kernel
netboot info for family system.
Aug 01 01:42:55 barley systemd[1]: family.socket: Failed with result
'service-start-limit-hit'.

And the config files
# /etc/systemd/system/family.service
[Unit]
Description=Update kernel netboot info for family system

[Service]
Type=oneshot
# next is the default. will be considered failed, and since we want it
# to run multiple times that is good. [Well, maybe not]
RemainAfterExit=no
ExecStart=sh -c "date >> /root/washere"  #for testing

[Install]
WantedBy=network-online.target

# /etc/systemd/system/family.socket
[Unit]
Description=Socket to tickle to update family netboot config

[Install]
WantedBy=network-online.target

[Socket]
ListenStream=192.168.1.10:14987
BindToDevice=br0
# 2s is default
TriggerLimitIntervalSec=5s

Thanks.
Ross