[systemd-devel] .service and .socket

[Jérémy Rosen]

On 02/01/2019 07:31, Michael Chapman wrote:
> On Tue, 1 Jan 2019, Olaf van der Spek wrote:
>> Hi,
>>
>> AFAIK socket units require a separate file, which seems more complex
>> then it has to be.
>>
>> 1. Could sockets be specified directly in the .service file?
> If anything, I should think it would work the other way around: a .socket
> without any activatable .service is relatively useless.
>
> But I'm not proposing that this actually be implemented.
A socket can trigger a service with a different name using Service=

moreover a service can be triggered by multiple sockets. It's a bit
complicated to use (check Socket= in [Service]) but it can be handy
if you want your web server to be started both by 80 and 8080
>
>> 2. If not, could the .service file gain a default / implicit
>> dependency on the .socket file?
> There are a some reasons for not having a .service dependent upon its
> .socket. Many services can be started directly and will work correctly
> even when not passed any sockets from systemd.
>
>> 3. AFAIK Install.WantedBy doesn't have a default. Could it get a proper default?
WantedBy is valid for all units, not just services and can have lots of 
proper values, not just
default.target.

Lots of on demand daemons do not have a WantedBy at all. they have 
Also=<a trigger unit>

> That doesn't make much sense.
>
> Take your example:
>
>> xhp.service:
>> [Unit]
>> Requires=xhp.socket
>>
>> [Install]
>> WantedBy=default.target
>>
>> xhp.socket:
>> [Socket]
>> ListenStream=/run/xhp.socket
> This would start xhp.service at daemon startup (i.e. boot, for the system
> daemon) whether or not the service is actually required. One of the
> reasons for using socket activation is to _not_ start services when
> they're not required.
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