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I'm trying to understand the difference between service restart [someservice] and service reload [someservice]. I understand that "restart" restarts the service whereas "reload" reloads the configuration, but I don't understand the practical implications of this well enough to determine which I should use in a given context.

An example: most guides I've read for setting up PostgreSQL say that, once I've edited postgresql.conf and pg_hba.conf to allow remote connections, I should issue sudo service postgresql restart. However, if I were guessing which to use based on the description above I would choose "reload".

In case it matters, I'm on Ubuntu 11.10 - though I'm hoping for as generally applicable an explanation as possible.

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    A couple years ago, I was running a server with several thousand users. restarting Apache wasn’t fast for whatever reason, it would take about 3 minutes to restart apache(reasons). Anyhow, if the server was down for 3 minutes my boss would get about 800 phone calls. With service reload, 0 phone calls. Thats the difference. – j0h Jan 7 '16 at 14:13
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What you said is correct, reload tells the service to reload its configuration files. That means it should be sufficient to reload the configuration; however there may be certain services that "don't follow the rule" or that won't reload config files. Due to this you're probably safer with restart. I personally do not use postgresql, so I don't know.

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    Meanwhile, there is reload-or-restart – Suuuehgi Apr 30 '18 at 13:25
  • For PostgreSQL, you can apply changes to the /etc/postgresql/version/main/postgresql.conf with a reload. Applying changes to pg_hba.conf requires a restart. – Aidan Melen May 22 '19 at 17:50
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  • restart = stop + start
  • reload = remain running + re-read configuration files.
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Not all services support reload. For those that do, it is usually preferable to restarting (i.e. reloading causes less or no downtime).

The Debian Policy Manual specifies that every /etc/init.d/ script should support a force-reload action, which means reload if the service supports it, and restart if the service doesn't support reloading.

I'm not sure how that translates into the modern Ubuntu upstart world.

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    The Debian Policy Manual was really useful link, thanks. – notapatch Mar 22 '14 at 15:52
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    most support reload, if not through the system service, thought /etc/init.d/SomeProgram reload – j0h Jan 7 '16 at 14:15
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To expand عبد النور التومي answer with my experience with systemd.

In systemd whenever a process is started it is run within systemd context, most clear example of this is with the environment variables defined in its unit file.

So when you send a systemctl reload [someservice] signal it sends a signal to the service to reload itself gracefully if it is supported. If not, the process will simply ignore the signal. This is configurable though.

What do I mean with gracefully? to start new workers with the new configuration or code and stop the old workers as they finish serving current requests if any.

And if you make systemctl restart [someservice] it will tell systemctl to tell the service to stop, destroy the current systemd context, create a new one and run the service again. This makes sense for example to reload environment variables in a systemd context or if a reload is not supported.

Hope this clarifies a little and if I'm wrong in something please let me know.

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Signal detail may be important for reload.
Most of times it is all about sending a proper signal (SIGHUP) to service process. If the underlying application has some kind of 'live configuration re-apply' strategy, then most probably there should be a line starting with "ExecReload=" in the service unit file. Which means reload is optional and you need to check if the application implements reload: (docker i.e.)

cat $(echo $(systemctl show -p FragmentPath docker) | cut -d "=" -f 2)

Restart is just stop-start. Systemd only needs to know proper "ExecStart" command to start.

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currently if a service needs kicking (e.g. a config file changed) you can notify the service, but this leads to a restart. It would be nice if a reload was done if the service was already running (although I suppose there are bound to be some services which require restarts for some files, reloads for others).

The most complex example I can think of is something like Apache. Normally you can just ask it to reload, however sometimes you need to request a restart instead (if you add/remove modules for example).

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postgres is a good example for big differences between reload and restart, because the later has to disconnect all database-clients.

when the connections should not rollback, you can stop the service without a "--force" at first by using pg_ctlcluster.

in /etc/postgres/{version}/{dbname}/postgresql.conf and at http://www.postgresql.org/docs/manuals/ every parameter has a remark like "This parameter can only be set at server start."

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