“before” and “want” for the same systemd service?2019 Community Moderator ElectionWhich service provides time-sync.target in systemd?Systemd optional service dependency that checks for errorHow to start and stop a listener service together with a custom serviceHow to start a service after a specified time in SystemD?Force systemd mounting some partititions before anythingSystemd: Start lots of services one after the other without specific depenciessystemd multiple unit files for a single serviceIs it possible to make a systemd unit wait until all its Conflicts= are stopped before trying to start?If the “transaction” for default.target fails, will systemd try to boot to emergency.target?Why is one of my mount units missing `Before=local-fs.target`?Systemd Unit File - WantedBy and After
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“before” and “want” for the same systemd service?
2019 Community Moderator ElectionWhich service provides time-sync.target in systemd?Systemd optional service dependency that checks for errorHow to start and stop a listener service together with a custom serviceHow to start a service after a specified time in SystemD?Force systemd mounting some partititions before anythingSystemd: Start lots of services one after the other without specific depenciessystemd multiple unit files for a single serviceIs it possible to make a systemd unit wait until all its Conflicts= are stopped before trying to start?If the “transaction” for default.target fails, will systemd try to boot to emergency.target?Why is one of my mount units missing `Before=local-fs.target`?Systemd Unit File - WantedBy and After
In this example of a systemd unit file:
# systemd-timesyncd.service
...
Before=time-sync.target sysinit.target shutdown.target
Conflicts=shutdown.target
Wants=time-sync.target
systemd-timesyncd.service
should start before time-sync.target
.
This defines an ordering dependency.
But at the same systemd-timesyncd.service
wants time-sync.target
. So time-sync.target
is it's requirement dependency
What is the use case for this relation and why aren't they in some conflict with one another?
systemd dependencies systemd-unit
add a comment |
In this example of a systemd unit file:
# systemd-timesyncd.service
...
Before=time-sync.target sysinit.target shutdown.target
Conflicts=shutdown.target
Wants=time-sync.target
systemd-timesyncd.service
should start before time-sync.target
.
This defines an ordering dependency.
But at the same systemd-timesyncd.service
wants time-sync.target
. So time-sync.target
is it's requirement dependency
What is the use case for this relation and why aren't they in some conflict with one another?
systemd dependencies systemd-unit
add a comment |
In this example of a systemd unit file:
# systemd-timesyncd.service
...
Before=time-sync.target sysinit.target shutdown.target
Conflicts=shutdown.target
Wants=time-sync.target
systemd-timesyncd.service
should start before time-sync.target
.
This defines an ordering dependency.
But at the same systemd-timesyncd.service
wants time-sync.target
. So time-sync.target
is it's requirement dependency
What is the use case for this relation and why aren't they in some conflict with one another?
systemd dependencies systemd-unit
In this example of a systemd unit file:
# systemd-timesyncd.service
...
Before=time-sync.target sysinit.target shutdown.target
Conflicts=shutdown.target
Wants=time-sync.target
systemd-timesyncd.service
should start before time-sync.target
.
This defines an ordering dependency.
But at the same systemd-timesyncd.service
wants time-sync.target
. So time-sync.target
is it's requirement dependency
What is the use case for this relation and why aren't they in some conflict with one another?
systemd dependencies systemd-unit
systemd dependencies systemd-unit
asked yesterday
TheMeaningfulEngineerTheMeaningfulEngineer
1,79173776
1,79173776
add a comment |
add a comment |
3 Answers
3
active
oldest
votes
The use case of this double relation is similar to a “provides” relation. systemd-timesyncd
provides a time synchronisation service, so it satisfies any dependency a unit has on time-sync.target
. It must start before time-sync.target
because it’s necessary for any service which relies on time synchronisation, and it wants time-sync.target
because any unit relying on time synchonisation should be started along with the systemd-timesyncd
service.
I think the misunderstanding comes from your interpretation of “wants”. The “wants” relation in systemd isn’t a dependency: systemd-timesyncd
doesn’t need time-sync
to function. It’s a “start along with” relation: it says that the configuring unit (systemd-timesyncd.service
) wants the listed units (time-sync.target
) to start along with it.
See also Which service provides time-sync.target in systemd?
add a comment |
The purpose of this mechanism is to ensure that ordering relationships can be made but do not take effect unless necessary.
time-sync.target
is an ordering milestone. All of the services that provide "time synchronization" specify that they are Before
the time-sync.target
, so that the target only becomes ready once "time synchronization" is in effect. All of the services that need "time synchronization" to be in effect when they run specify that they are After
the time-sync.target
.
If the latter also had a Wants
relationship to that target, then they would always end up being ordered by it, as it would always be included in the set of things that are put into order.
This is seen as being suboptimal in the case where there is in fact no concrete "time synchronization" service; and the thinking of the systemd people is that such ordering should not be in effect in such a case. Rather, services should be ordered as if time-sync.target
were not there, allowing some of them to be started much earlier if that is their "natural" position without the milestone.
The solution is for time-sync.target
to actually not be there. It isn't wanted by the services that expect to start after time synchronization is available. So it does not exist in the set of ordered things if only those services are started. It is only brought into the set if an actual "time synchronization" service is started, with that (rather than the client services) having the Wants
relationship that brings it in.
Targets do not necessarily have to be collections of services. They can also be ordering milestones.
There are a fair number of such pure milestones, in systemd and elsewhere. The name-services
target in the nosh toolset's service bundle collection is a similar pure ordering milestone.
Further reading
- Jonathan de Boyne Pollard (2018).
system-control
. nosh Guide. Softwares.
add a comment |
time-sync.target
is kind of a flag in system, so that services depending on a correct time do not have to depend on systemd-timesyncd, ntpd, whatever.
The Before
entry tells systemd to start systemd-timesyncd, then time-sync.target (this is just for ordering). The Wants
tells it to actually set the flag.
New contributor
add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
The use case of this double relation is similar to a “provides” relation. systemd-timesyncd
provides a time synchronisation service, so it satisfies any dependency a unit has on time-sync.target
. It must start before time-sync.target
because it’s necessary for any service which relies on time synchronisation, and it wants time-sync.target
because any unit relying on time synchonisation should be started along with the systemd-timesyncd
service.
I think the misunderstanding comes from your interpretation of “wants”. The “wants” relation in systemd isn’t a dependency: systemd-timesyncd
doesn’t need time-sync
to function. It’s a “start along with” relation: it says that the configuring unit (systemd-timesyncd.service
) wants the listed units (time-sync.target
) to start along with it.
See also Which service provides time-sync.target in systemd?
add a comment |
The use case of this double relation is similar to a “provides” relation. systemd-timesyncd
provides a time synchronisation service, so it satisfies any dependency a unit has on time-sync.target
. It must start before time-sync.target
because it’s necessary for any service which relies on time synchronisation, and it wants time-sync.target
because any unit relying on time synchonisation should be started along with the systemd-timesyncd
service.
I think the misunderstanding comes from your interpretation of “wants”. The “wants” relation in systemd isn’t a dependency: systemd-timesyncd
doesn’t need time-sync
to function. It’s a “start along with” relation: it says that the configuring unit (systemd-timesyncd.service
) wants the listed units (time-sync.target
) to start along with it.
See also Which service provides time-sync.target in systemd?
add a comment |
The use case of this double relation is similar to a “provides” relation. systemd-timesyncd
provides a time synchronisation service, so it satisfies any dependency a unit has on time-sync.target
. It must start before time-sync.target
because it’s necessary for any service which relies on time synchronisation, and it wants time-sync.target
because any unit relying on time synchonisation should be started along with the systemd-timesyncd
service.
I think the misunderstanding comes from your interpretation of “wants”. The “wants” relation in systemd isn’t a dependency: systemd-timesyncd
doesn’t need time-sync
to function. It’s a “start along with” relation: it says that the configuring unit (systemd-timesyncd.service
) wants the listed units (time-sync.target
) to start along with it.
See also Which service provides time-sync.target in systemd?
The use case of this double relation is similar to a “provides” relation. systemd-timesyncd
provides a time synchronisation service, so it satisfies any dependency a unit has on time-sync.target
. It must start before time-sync.target
because it’s necessary for any service which relies on time synchronisation, and it wants time-sync.target
because any unit relying on time synchonisation should be started along with the systemd-timesyncd
service.
I think the misunderstanding comes from your interpretation of “wants”. The “wants” relation in systemd isn’t a dependency: systemd-timesyncd
doesn’t need time-sync
to function. It’s a “start along with” relation: it says that the configuring unit (systemd-timesyncd.service
) wants the listed units (time-sync.target
) to start along with it.
See also Which service provides time-sync.target in systemd?
answered yesterday
Stephen KittStephen Kitt
177k24402480
177k24402480
add a comment |
add a comment |
The purpose of this mechanism is to ensure that ordering relationships can be made but do not take effect unless necessary.
time-sync.target
is an ordering milestone. All of the services that provide "time synchronization" specify that they are Before
the time-sync.target
, so that the target only becomes ready once "time synchronization" is in effect. All of the services that need "time synchronization" to be in effect when they run specify that they are After
the time-sync.target
.
If the latter also had a Wants
relationship to that target, then they would always end up being ordered by it, as it would always be included in the set of things that are put into order.
This is seen as being suboptimal in the case where there is in fact no concrete "time synchronization" service; and the thinking of the systemd people is that such ordering should not be in effect in such a case. Rather, services should be ordered as if time-sync.target
were not there, allowing some of them to be started much earlier if that is their "natural" position without the milestone.
The solution is for time-sync.target
to actually not be there. It isn't wanted by the services that expect to start after time synchronization is available. So it does not exist in the set of ordered things if only those services are started. It is only brought into the set if an actual "time synchronization" service is started, with that (rather than the client services) having the Wants
relationship that brings it in.
Targets do not necessarily have to be collections of services. They can also be ordering milestones.
There are a fair number of such pure milestones, in systemd and elsewhere. The name-services
target in the nosh toolset's service bundle collection is a similar pure ordering milestone.
Further reading
- Jonathan de Boyne Pollard (2018).
system-control
. nosh Guide. Softwares.
add a comment |
The purpose of this mechanism is to ensure that ordering relationships can be made but do not take effect unless necessary.
time-sync.target
is an ordering milestone. All of the services that provide "time synchronization" specify that they are Before
the time-sync.target
, so that the target only becomes ready once "time synchronization" is in effect. All of the services that need "time synchronization" to be in effect when they run specify that they are After
the time-sync.target
.
If the latter also had a Wants
relationship to that target, then they would always end up being ordered by it, as it would always be included in the set of things that are put into order.
This is seen as being suboptimal in the case where there is in fact no concrete "time synchronization" service; and the thinking of the systemd people is that such ordering should not be in effect in such a case. Rather, services should be ordered as if time-sync.target
were not there, allowing some of them to be started much earlier if that is their "natural" position without the milestone.
The solution is for time-sync.target
to actually not be there. It isn't wanted by the services that expect to start after time synchronization is available. So it does not exist in the set of ordered things if only those services are started. It is only brought into the set if an actual "time synchronization" service is started, with that (rather than the client services) having the Wants
relationship that brings it in.
Targets do not necessarily have to be collections of services. They can also be ordering milestones.
There are a fair number of such pure milestones, in systemd and elsewhere. The name-services
target in the nosh toolset's service bundle collection is a similar pure ordering milestone.
Further reading
- Jonathan de Boyne Pollard (2018).
system-control
. nosh Guide. Softwares.
add a comment |
The purpose of this mechanism is to ensure that ordering relationships can be made but do not take effect unless necessary.
time-sync.target
is an ordering milestone. All of the services that provide "time synchronization" specify that they are Before
the time-sync.target
, so that the target only becomes ready once "time synchronization" is in effect. All of the services that need "time synchronization" to be in effect when they run specify that they are After
the time-sync.target
.
If the latter also had a Wants
relationship to that target, then they would always end up being ordered by it, as it would always be included in the set of things that are put into order.
This is seen as being suboptimal in the case where there is in fact no concrete "time synchronization" service; and the thinking of the systemd people is that such ordering should not be in effect in such a case. Rather, services should be ordered as if time-sync.target
were not there, allowing some of them to be started much earlier if that is their "natural" position without the milestone.
The solution is for time-sync.target
to actually not be there. It isn't wanted by the services that expect to start after time synchronization is available. So it does not exist in the set of ordered things if only those services are started. It is only brought into the set if an actual "time synchronization" service is started, with that (rather than the client services) having the Wants
relationship that brings it in.
Targets do not necessarily have to be collections of services. They can also be ordering milestones.
There are a fair number of such pure milestones, in systemd and elsewhere. The name-services
target in the nosh toolset's service bundle collection is a similar pure ordering milestone.
Further reading
- Jonathan de Boyne Pollard (2018).
system-control
. nosh Guide. Softwares.
The purpose of this mechanism is to ensure that ordering relationships can be made but do not take effect unless necessary.
time-sync.target
is an ordering milestone. All of the services that provide "time synchronization" specify that they are Before
the time-sync.target
, so that the target only becomes ready once "time synchronization" is in effect. All of the services that need "time synchronization" to be in effect when they run specify that they are After
the time-sync.target
.
If the latter also had a Wants
relationship to that target, then they would always end up being ordered by it, as it would always be included in the set of things that are put into order.
This is seen as being suboptimal in the case where there is in fact no concrete "time synchronization" service; and the thinking of the systemd people is that such ordering should not be in effect in such a case. Rather, services should be ordered as if time-sync.target
were not there, allowing some of them to be started much earlier if that is their "natural" position without the milestone.
The solution is for time-sync.target
to actually not be there. It isn't wanted by the services that expect to start after time synchronization is available. So it does not exist in the set of ordered things if only those services are started. It is only brought into the set if an actual "time synchronization" service is started, with that (rather than the client services) having the Wants
relationship that brings it in.
Targets do not necessarily have to be collections of services. They can also be ordering milestones.
There are a fair number of such pure milestones, in systemd and elsewhere. The name-services
target in the nosh toolset's service bundle collection is a similar pure ordering milestone.
Further reading
- Jonathan de Boyne Pollard (2018).
system-control
. nosh Guide. Softwares.
answered yesterday
JdeBPJdeBP
37.4k477179
37.4k477179
add a comment |
add a comment |
time-sync.target
is kind of a flag in system, so that services depending on a correct time do not have to depend on systemd-timesyncd, ntpd, whatever.
The Before
entry tells systemd to start systemd-timesyncd, then time-sync.target (this is just for ordering). The Wants
tells it to actually set the flag.
New contributor
add a comment |
time-sync.target
is kind of a flag in system, so that services depending on a correct time do not have to depend on systemd-timesyncd, ntpd, whatever.
The Before
entry tells systemd to start systemd-timesyncd, then time-sync.target (this is just for ordering). The Wants
tells it to actually set the flag.
New contributor
add a comment |
time-sync.target
is kind of a flag in system, so that services depending on a correct time do not have to depend on systemd-timesyncd, ntpd, whatever.
The Before
entry tells systemd to start systemd-timesyncd, then time-sync.target (this is just for ordering). The Wants
tells it to actually set the flag.
New contributor
time-sync.target
is kind of a flag in system, so that services depending on a correct time do not have to depend on systemd-timesyncd, ntpd, whatever.
The Before
entry tells systemd to start systemd-timesyncd, then time-sync.target (this is just for ordering). The Wants
tells it to actually set the flag.
New contributor
New contributor
answered yesterday
Uwe OhseUwe Ohse
1013
1013
New contributor
New contributor
add a comment |
add a comment |
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