ifndef::external_title[]
[[daisychain]]
NUT daisychain support notes
============================
endif::external_title[]

NUT supports daisychained devices for any kind of device that proposes
it. This chapter introduces:

* for developers: how to implement such mechanism,
* for users: how to manage and use daisychained devices in NUT in general,
  and how to take advantage of the provided features.

Introduction
------------

It's not unusual to see some daisy-chained PDUs or UPSs, connected together
in master-slave mode, to only consume 1 IP address for their communication
interface (generally, network card exposing SNMP data) and expose only one
point of communication to manage several devices, through the daisy-chain
master.

This breaks the historical consideration of NUT that one driver provides
data for one unique device. However, there is an actual need, and a smart
approach was considered to fulfill this, while not breaking the standard
scope (for base compatibility).


Implementation notes
--------------------

General specification
~~~~~~~~~~~~~~~~~~~~~

The daisychain support uses the device collection to extend the historical
NUT scope (1 driver -- 1 device), and provides data from the additional
devices accessible through a single management interface.

A new variable was introduced to provide the number of devices exposed: the
`device.count`, which:

* defaults to 1
* if higher than 1, enables daisychain support and access to data of each
individual device through `device.X.{...}`

To ensure backward compatibility in NUT, the data of the various devices
are exposed the following way:

* `device.0` is a special case, for the whole set of devices (the whole
daisychain). It is equivalent to `device` (without `.X` index) and root
collections. The idea is to be able to get visibility and control over the
whole daisychain from a single point.
* daisy-chained devices are available from `device.1` (master) to `device.N`
(slaves).

That way, client applications that are unaware of the daisychain support,
will only see the whole daisychain, as it would normally seem, and not
nothing at all.

Moreover, this solution is generic, and not specific to the ePDU use case
currently considered. It thus support both the current NUT scope, along with
other use cases (parallel / serial UPS setups), and potential evolution and
technology change (hybrid chain with UPS and PDU for example).


Devices status handling
^^^^^^^^^^^^^^^^^^^^^^^

FIXME: To be clarified...


Devices alarms handling
^^^^^^^^^^^^^^^^^^^^^^^

Devices (master and slaves) alarms are published in `device.X.ups.alarm`,
which may evolve into `device.X.alarm`. If any of the devices has an alarm,
the main `ups.status` will publish an `ALARM` flag. This flag is be cleared
once all devices have no alarms anymore.

NOTE: `ups.alarm` behavior is not yet defined (all devices alarms vs. list of
device(s) that have alarms vs. nothing?)

Example
^^^^^^^

Here is an example excerpt of three PDUs, connected in daisychain mode, with
one master and two slaves:

	device.count: 3
	device.mfr: EATON
	device.model: EATON daisychain PDU
	device.1.mfr: EATON
	device.1.model: EPDU MI 38U-A IN: L6-30P 24A 1P OUT: 36XC13:6XC19
	device.2.mfr: EATON
	device.2.model: EPDU MI 38U-A IN: L6-30P 24A 1P OUT: 36XC13:6XC19
	device.3.mfr: EATON
	device.3.model: EPDU MI 38U-A IN: L6-30P 24A 1P OUT: 36XC13:6XC19
	...
	device.3.ups.alarm: high current critical!
	device.3.ups.status: ALARM
	...
	input.voltage: ??? (proposal: range or list or average?)
	device.1.input.voltage: 237.75
	device.2.input.voltage: 237.75
	device.3.input.voltage: 237.75
	...
	outlet.1.status: ?? (proposal: "on, off, off)
	device.1.outlet.1.status: on
	device.2.outlet.1.status: off
	device.3.outlet.1.status: off
	...
	ups.status: ALARM


Information for developers
~~~~~~~~~~~~~~~~~~~~~~~~~~

NOTE: These details are dedicated to the `snmp-ups` driver!

In order to enable daisychain support for a range of devices, developers
have to do two things:

* Add a `device.count` entry in a mapping file (see `*-mib.c`)
* Modify mapping entries to include a format string for the daisychain index

Optionally, if there is support for outlets and / or outlet-groups, there
is already a template formatting string. So you have to tag such templates
with multiple definitions, to point if the daisychain index is the first
or second formatting string.

Base support
^^^^^^^^^^^^

In order to enable daisychain support on a mapping structure, the following
steps have to be done:

* Add a "device.count" entry in the mapping file: snmp-ups will determine
if the daisychain support has to be enabled (if more than 1 device).
To achieve this, use one of the following type of declarations:
+
a) point at an OID which provides the number of devices:

	{ "device.count", 0, 1, ".1.3.6.1.4.1.13742.6.3.1.0", "1",
		SU_FLAG_STATIC, NULL },
+
b) point at a template OID to guesstimate the number of devices, by walking
through this template, until it fails:
+
	{ "device.count", 0, 1, ".1.3.6.1.4.1.534.6.6.7.1.2.1.2.%i", "1",
		SU_FLAG_STATIC, NULL, NULL },

* Modify all entries so that OIDs include the formatting string for the
daisychain index. For example, if you have the following entry:
+
	{ "device.model", ST_FLAG_STRING, SU_INFOSIZE,
		".1.3.6.1.4.1.534.6.6.7.1.2.1.2.0", ... },
+
And if the last "0" of the 4th field represents the index of the device
in the daisychain, then you would have to adapt it the following way:
+
	{ "device.model", ST_FLAG_STRING, SU_INFOSIZE,
		".1.3.6.1.4.1.534.6.6.7.1.2.1.2.%i", ... },


Templates with multiple definitions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If there already exist templates in the mapping structure, such as for
single outlets and outlet-groups, you also need to specify the position
of the daisychain device index in the OID strings for all entries in the
mapping table, to indicate where the daisychain insertion point is exactly.

For example, using the following entry:

	{ "outlet.%i.current", 0, 0.001, ".1.3.6.1.4.1.534.6.6.7.6.4.1.3.0.%i",
		NULL, SU_OUTLET, NULL, NULL },

You would have to translate it to:

	{ "outlet.%i.current", 0, 0.001, ".1.3.6.1.4.1.534.6.6.7.6.4.1.3.%i.%i",
		NULL, SU_OUTLET | SU_TYPE_DAISY_1, NULL, NULL },

`SU_TYPE_DAISY_1` flag indicates that the daisychain index is the first
`%i` specifier in the OID template string. If it is the second one, use
`SU_TYPE_DAISY_2`.


Devices alarms handling
^^^^^^^^^^^^^^^^^^^^^^^

Two functions are available to handle alarms on daisychain devices in your
driver:

* `device_alarm_init()`: clear the current alarm buffer
* `device_alarm_commit(const int device_number)`: commit the current alarm
  buffer to `device.<device_number>.ups.alarm`, and increase the count of
  alarms. If the current alarms buffer is empty, the count of alarm is
  decreased, and the variable `device.<device_number>.ups.alarm` is removed
  from publication. Once the alarm count reaches "0", the main (`device.0`)
  `ups.status` will also remove the "ALARM" flag.

[NOTE]
======
When implementing a new driver, the following functions have to be
called:

* `alarm_init()` at the beginning of the main update loop, for the whole
daisychain. This will set the alarm count to "0", and reinitialize all
alarms,
* `device_alarm_init()` at the beginning of the per-device update loop.
This will only clear the alarms for the current device,
* `device_alarm_commit()` at the end of the per-device update loop.
This will flush the current alarms for the current device,
* also `device_alarm_init()` at the end of the per-device update loop.
This will clear the current alarms, and ensure that this buffer will not
be considered by other subsequent devices,
* `alarm_commit()` at the end of the main update loop, for the whole
daisychain. This will take care of publishing or not the "ALARM" flag
in the main ups.status (`device.0`, root collection).
======
