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The ObjectMapper

The xyz.openbmc_project.ObjectMapper service, commonly referred to as just the mapper, is an OpenBMC application that attempts to ease the pain of using D-Bus by providing APIs that help in discovering and associating other D-Bus objects.

The mapper has two major pieces of functionality:

  • Methods - Provides D-Bus discovery related functionality.
  • Associations - Associates two different objects with each other.

Methods

The official YAML interface definition can be found here.

GetObject

Use this method to find the services, with their interfaces, that implement a certain object path. The output is a map of service names to their implemented interfaces. An optional list of interfaces may also be passed in to constrain the output to services that implement those specific interfaces.

Inputs:

  • path: object path
  • param: interfaces - an optional list of interfaces to constrain the search to

Output:

  • Map of service names to their interfaces
dbus-send --system --print-reply \
--dest=xyz.openbmc_project.ObjectMapper \
/xyz/openbmc_project/object_mapper \
xyz.openbmc_project.ObjectMapper.GetObject \
string:"/xyz/openbmc_project/sensors/voltage/ps1_input_voltage" array:string:

   array [
      dict entry(
         string "xyz.openbmc_project.Hwmon-1025936882.Hwmon1"
         array [
            string "xyz.openbmc_project.Sensor.Threshold.Critical"
            string "xyz.openbmc_project.Sensor.Threshold.Warning"
            string "xyz.openbmc_project.Sensor.Value"
         ]
      )
   ]

Example Use Case

Find the service name that has the desired object path so it can be passed into a get property call.

GetSubTree

Use this method to find the objects, services, and interfaces in the specified subtree that implement a certain interface. If no interfaces are passed in, then all objects/services/interfaces in the subtree are returned. If interfaces are passed in, then only those interfaces are returned in the output.

Inputs:

  • param: subtree - the root of the tree. Using "/" will search the whole tree
  • param: depth - the maximum depth of the tree past the root to search. Use 0 to search all
  • param: interfaces - an optional list of interfaces to constrain the search to

Output:

  • Map of object paths to a map of service names to their interfaces
dbus-send --system --print-reply \
--dest=xyz.openbmc_project.ObjectMapper \
/xyz/openbmc_project/object_mapper \
xyz.openbmc_project.ObjectMapper.GetSubTree \
string:"/" int32:0 array:string:"xyz.openbmc_project.Sensor.Threshold.Warning"

   array [
      dict entry(
         string "/xyz/openbmc_project/sensors/current/ps0_output_current"
         array [
            dict entry(
               string "xyz.openbmc_project.Hwmon-1040041051.Hwmon1"
               array [
                  string "xyz.openbmc_project.Sensor.Threshold.Critical"
                  string "xyz.openbmc_project.Sensor.Threshold.Warning"
                  string "xyz.openbmc_project.Sensor.Value"
               ]
            )
         ]
      )
      dict entry(
         string "/xyz/openbmc_project/sensors/current/ps1_output_current"
         array [
            dict entry(
               string "xyz.openbmc_project.Hwmon-1025936882.Hwmon1"
               array [
                  string "xyz.openbmc_project.Sensor.Threshold.Critical"
                  string "xyz.openbmc_project.Sensor.Threshold.Warning"
                  string "xyz.openbmc_project.Sensor.Value"
               ]
            )
         ]
      )
...

Example Use Case

Find all object paths and services that implement a specific interface.

GetSubTreePaths

This is the same as GetSubTree, but only returns object paths

Inputs:

  • param: subtree - the root of the tree. Using "/" will search the whole tree
  • param: depth - the maximum depth of the tree past the root to search. Use 0 to search all
  • param: interfaces - an optional list of interfaces to constrain the search to

Output:

  • array of object paths in that subtree
dbus-send --system --print-reply \
--dest=xyz.openbmc_project.ObjectMapper \
/xyz/openbmc_project/object_mapper \
xyz.openbmc_project.ObjectMapper.GetSubTreePaths \
string:"/" int32:0 array:string:"xyz.openbmc_project.Sensor.Threshold.Warning"

   array [
      string "/xyz/openbmc_project/sensors/current/ps0_output_current"
      string "/xyz/openbmc_project/sensors/current/ps1_output_current"
      string "/xyz/openbmc_project/sensors/power/ps0_input_power"
...
   ]

Example Use Case

Find all object paths that implement a specific interface.

GetAncestors

Use this method to find all ancestors of an object that implement a specific interface. If no interfaces are passed in, then all ancestor paths/services/interfaces are returned.

Inputs:

  • param: path - the object path to find the ancestors of
  • param: interfaces - an optional list of interfaces to constrain the search to

Output:

  • A map of object paths to a map of services names to their interfaces

dbus-send --system --print-reply \
--dest=xyz.openbmc_project.ObjectMapper \
/xyz/openbmc_project/object_mapper \
xyz.openbmc_project.ObjectMapper.GetAncestors \
string:"/xyz/openbmc_project/inventory/system" array:string:

   array [
      dict entry(
         string "/xyz/openbmc_project"
         array [
            dict entry(
               string "xyz.openbmc_project.ObjectMapper"
               array [
                  string "org.freedesktop.DBus.ObjectManager"
               ]
            )
         ]
      )
      dict entry(
         string "/xyz/openbmc_project/inventory"
         array [
            dict entry(
               string "xyz.openbmc_project.Inventory.Manager"
               array [
                  string "xyz.openbmc_project.Inventory.Manager"
                  string "org.freedesktop.DBus.ObjectManager"
               ]
            )
         ]
      )
      dict entry(
         string "/"
         array [
            dict entry(
               string "xyz.openbmc_project.Settings"
               array [
                  string "org.freedesktop.DBus.ObjectManager"
               ]
            )
         ]
      )
   ]

Example Use Case

Find a parent object that implements a specific interface.

Associations

Associations are special D-Bus objects created by the mapper to associate two objects with each other. For this to occur, some application must implement the xyz.openbmc_project.Association.Definitions interface, and then when an association is desired, the Associations property on that interface needs to be written.

This Associations property is an array of tuples of the form:

[forward, reverse, object path]
  • forward: this is the name of the forward association object
  • reverse: this is the name of the reverse association object
  • object path: this is the other object to associate with

When an object with, for example, an object path of pathA uses the following values:

["foo", "bar", "pathB"]

The mapper will create 2 new objects:

  1. pathA/foo
  2. pathB/bar

On each of these objects, the interface xyz.openbmc_project.Association will be implemented, which has a single endpoints property. This property is an array that holds the object paths to the other end of the association.

So, pathA/foo->endpoints will contain pathB, and pathB/bar->endpoints will contain pathA.

If another object, say pathC, also has an association to pathB, then a second entry, pathC, will be added into pathB's endpoints property.

These new objects will match the lifetime of the associated objects. For example, if pathA is deleted, then pathA/foo will also be deleted, and pathA will be removed from the endpoints property of pathB/bar. If that was the last entry in that property, then pathB/bar will also be deleted. In addition, if the endpoint path is removed from D-Bus, in this case pathB, then the mapper will remove the 2 association paths until pathB shows back up again.

Note: The original name of the association definition interface was org.openbmc.Associations. While the mapper still supports this interface as well for the time being, new code should use the xyz version.

Example Use Case

Associate an error log with the inventory item that caused it.

# Error log
"/xyz/openbmc_project/logging/entry/3": {
...
 "associations": [
   [
     "callout",
     "fault",
     "/xyz/openbmc_project/inventory/system/chassis/motherboard/powersupply0"
   ]
 ]
}

# Newly created forward association object
"/xyz/openbmc_project/logging/entry/3/callout": {
    "endpoints": [
    "/xyz/openbmc_project/inventory/system/chassis/motherboard/powersupply0"
    ]
}

# Newly created reverse association object
"/xyz/openbmc_project/inventory/system/chassis/motherboard/powersupply0/fault": {
    "endpoints": [
    "/xyz/openbmc_project/logging/entry/3"
    ]
}