icinga2/doc/3.1-hosts-and-services.md

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## Hosts and Services
Icinga 2 can be used to monitor the availability of hosts and services. Services
can be virtually anything which can be checked in some way:
* Network services (HTTP, SMTP, SNMP, SSH, etc.)
* Printers
* Switches / Routers
* Temperature Sensors
* Other local or network-accessible services
Host objects provide a mechanism to group together services that are running
on the same physical device.
Here is an example of a host object which defines two child services:
object Host "my-server1" {
services["ping4"] = {
check_command = "ping4"
},
services["http"] = {
check_command = "http_ip"
},
check = "ping4",
macros["address"] = "10.0.0.1"
}
The example host *my-server1* creates two services which belong to this host:
*ping4* and *http*.
It also specifies that the host should inherit its availability state from the
*ping4* service.
> **Note**
>
> In Icinga 1.x hosts had their own check command, check interval and
> notification settings. Instead, in Icinga 2 hosts inherit their state
> from one of its child services. No checks are performed for the host
> itself.
The *address* macro is used by check commands to determine which network
address is associated with the host object.
### Host States
Hosts inherit their state from the host check service that is specified using
the *check* attribute.
Hosts can be in any of the following states:
Name | Description
------------|--------------
UP | The host is available.
DOWN | The host is unavailable.
UNREACHABLE | At least one of the host's dependencies (e.g. its upstream router) is unavailable causing the host to be unreachable.
### Service States
Services can be in any of the following states:
Name | Description
------------|--------------
OK | The service is working properly.
WARNING | The service is experiencing some problems but is still considered to be in working condition.
CRITICAL | The service is in a critical state.
UNKNOWN | The check could not determine the service's state.
### Hard and Soft States
When detecting a problem with a service Icinga re-checks the service a number of
times (based on the *max_check_attempts* and *retry_interval* settings) before sending
notifications. This ensures that no unnecessary notifications are sent for
transient failures. During this time the service is in a *SOFT* state.
After all re-checks have been executed and the service is still in a non-OK
state the service switches to a *HARD* state and notifications are sent.
Name | Description
------------|--------------
HARD | The host/service's state hasn't recently changed.
SOFT | The host/service has recently changed state and is being re-checked.