A Service Discovery (SD) mechanism has to discover targets and provide them to Prometheus. We expect similar targets to be grouped together, in the form of a [`TargetGroup`](https://godoc.org/github.com/prometheus/prometheus/config#TargetGroup). The SD mechanism sends the targets down to prometheus as list of `TargetGroups`.
An SD mechanism has to implement the `TargetProvider` Interface:
```go
type TargetProvider interface {
Run(ctx context.Context, up chan<-[]*config.TargetGroup)
}
```
Prometheus will call the `Run()` method on a provider to initialise the discovery mechanism. The mechanism will then send *all* the `TargetGroup`s into the channel. Now the mechanism will watch for changes and then send only changed and new `TargetGroup`s down the channel.
For example if we had a discovery mechanism and it retrieves the following groups:
```
[]config.TargetGroup{
{
Targets: []model.LabelSet{
{
"__instance__": "10.11.150.1:7870",
"hostname": "demo-target-1",
"test": "simple-test",
},
{
"__instance__": "10.11.150.4:7870",
"hostname": "demo-target-2",
"test": "simple-test",
},
},
Labels: map[LabelName][LabelValue] {
"job": "mysql",
},
"Source": "file1",
},
{
Targets: []model.LabelSet{
{
"__instance__": "10.11.122.11:6001",
"hostname": "demo-postgres-1",
"test": "simple-test",
},
{
"__instance__": "10.11.122.15:6001",
"hostname": "demo-postgres-2",
"test": "simple-test",
},
},
Labels: map[LabelName][LabelValue] {
"job": "postgres",
},
"Source": "file2",
},
}
```
Here there are two `TargetGroups` one group with source `file1` and another with `file2`. The grouping is implementation specific and could even be one target per group. But, one has to make sure every target group sent by an SD instance should have a `Source` which is unique across all the `TargetGroup`s of that SD instance.
In this case, both the `TargetGroup`s are sent down the channel the first time `Run()` is called. Now, for an update, we need to send the whole _changed_`TargetGroup` down the channel. i.e, if the target with `hostname: demo-postgres-2` goes away, we send:
```
&config.TargetGroup{
Targets: []model.LabelSet{
{
"__instance__": "10.11.122.11:6001",
"hostname": "demo-postgres-1",
"test": "simple-test",
},
},
Labels: map[LabelName][LabelValue] {
"job": "postgres",
},
"Source": "file2",
}
```
down the channel.
If all the targets in a group go away, we need to send the target groups with empty `Targets` down the channel. i.e, if all targets with `job: postgres` go away, we send:
