COME FROM [LWN.net]

COME FROM

Posted Feb 11, 2014 12:46 UTC (Tue) by cesarb (subscriber, #6266)
Parent article: Systemd programming part 2: activation and language issues

> There is a key difference here, though, and it isn't really about events or dependencies, but about causality. In upstart, a job can declare "start on" to identify which event it should start on. So each job declares the events which cause it to run. Systemd, despite its rich dependency language, has no equivalent to "start on", an omission that appears to be deliberate. Instead, each event — the starting or stopping of a unit — declares which jobs (or units) need to be running. The dependency language is exactly reversed. With upstart, each job knows what causes it to start. With systemd, each job knows what it causes to start.

Let's see if I understood this correctly. Upstart's behavior is like COME FROM, while systemd's is more like GO TO.

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COME FROM

Posted Feb 11, 2014 13:34 UTC (Tue) by hummassa (guest, #307) [Link] (1 responses)

Exactly that. :)

Not that COME FROM is totally useless (no GUI-based programming without it).

COME FROM

Posted Feb 14, 2014 21:15 UTC (Fri) by giraffedata (guest, #1954) [Link]

The dependency declaration looks backwards only if you look at the meaning of "multiuser" backwards.

The Upstart philosophy is the "multiuser" unit means the user has authorized or requested multiuser service. NFS is appropriate only in multiuser service, so NFS depends upon multiuser. The Systemd philosophy is that the multiuser unit refers to the system providing the multiuser service. You can't claim to providing multiuser service if you aren't providing NFS, so multiuser depends upon NFS.

The Systemd definition of multiuser seems more useful and easy to understand to me.

Here's an analogy: a house is inhabitable only if the heat is on, but it's wasteful to heat an uninhabited house. So you could say inhabitability depends upon the heat being on and therefore the landlord should turn on the heat before signing a lease. Or you could say the appropriateness of the heat being on depends upon the house being inhabited, so the furnace should turn on only when it detects that a lease has been signed.