Driver/server socket protocol
=============================

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// You can find a rendered variant of this document on the web at
// https://networkupstools.org/docs/developer-guide.chunked/sock-protocol.html
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Here's a brief explanation of the text-based protocol which is used
between the drivers and server.

The drivers may send things on the socket at any time.  They will send
out changes to their local storage immediately, without any sort of
prompting from the server.  As a result, the server must always check on
any driver sockets for activity.

In terms of communications, each driver is a server on the Unix socket
(or Windows named pipe) which it creates, and the data server `upsd` is
a client which knows where to find such sockets, how they are named,
and connects to all of them to send commands and receive data updates.

During development, it is possible to use tools like `socat` to connect
to the socket (you may want to enable `NOBROADCAST` mode soon), e.g.

	socat - UNIX-CONNECT:/var/state/ups/dummy-ups-UPS1

For more insight, NUT provides an optional tool of its own (not built
by default): the `sockdebug` which is built when `configure --with-dev`
is in effect, or can be requested from the root directory of the build
workspace:

	make sockdebug && \
	./server/sockdebug dummy-ups-UPS1

Formatting
----------

All parsing on either side of the socket is done by parseconf, so the
same rules about escaping characters and "quoting multi-word elements"
apply here.  Values which may contain odd characters are typically
sent through pconf_encode to apply \ characters where necessary.

The "" construct is used throughout to force a multi-word value to stay
together on its way to the other end.

Commands used by the drivers
----------------------------

These commands (or semantically responses to server commands in some cases)
can be sent by drivers to the data server over the socket protocol.

SETINFO
~~~~~~~

	SETINFO <varname> "<value>"

	SETINFO ups.status "OB LB"

There is no "ADDINFO" -- if a given variable does not exist, it is
created upon receiving the first SETINFO command.

DELINFO
~~~~~~~

	DELINFO <varname>

	DELINFO ups.temperature

ADDENUM
~~~~~~~

	ADDENUM <varname> "<value>"

	ADDENUM input.transfer.low "95"

DELENUM
~~~~~~~

	DELENUM <varname> "<value>"

	DELENUM input.transfer.low "98"

ADDRANGE
~~~~~~~~

	ADDRANGE <varname> <minvalue> <maxvalue>

	ADDRANGE input.transfer.low 95 100

DELRANGE
~~~~~~~~

	DELRANGE <varname> <minvalue> <maxvalue>

	DELRANGE input.transfer.low 95 100

SETAUX
~~~~~~

	SETAUX <varname> <numeric value>

	SETAUX ups.id 8

This overrides any previous value.  The auxiliary value is presently
used as a length byte for read-write variables that are strings.

SETFLAGS
~~~~~~~~

	SETFLAGS <varname> <flag>...

	SETFLAGS ups.id RW STRING

Note that this command takes a variable number of arguments, as multiple
flags are supported.  Also note that they are not crammed together in
"" quotes, since "RW STRING" would mean something completely different.

This also replaces any previous flags for a given variable.

Currently supported flags include `RW`, `STRING` and `NUMBER`
(detailed in the NUT Network Protocol documentation); unrecognized values
are quietly ignored.

ADDCMD
~~~~~~

	ADDCMD <cmdname>

	ADDCMD load.off

DELCMD
~~~~~~

	DELCMD <cmdname>

	DELCMD load.on

PID
~~~

	PID <id>

	PID 12345

	PID "StrangeOS process identifier"

Response to `GETPID` query, where we serve platform-specific process
identifier. On POSIX and many other platforms this would be a numeric
value, but most generally it should be treated as an opaque string.

DUMPDONE
~~~~~~~~

	DUMPDONE

This is only used to tell the server that every possible item has been
transmitted in response to its DUMPALL request.  Once this has been
received by the server, it can be sure that it knows everything that the
driver does.

PONG
~~~~

	PONG

This is sent in response to a PING from the server.  It is only used as
a sanity check to make sure that the driver has not gotten stuck
somewhere.

OK
~~

	OK Goodbye

This is sent in response to a LOGOUT from the server (or more likely from
a sibling driver or `upsdrvctl` program).

DATAOK
~~~~~~

	DATAOK

This means that the driver is able to communicate with the UPS, and the
data should be treated as usable.  It is always sent at the end of the
dump if the data is not stale.  It may also be sent at other times.

DATASTALE
~~~~~~~~~

	DATASTALE

This is sent by the driver to inform any listeners that the data is no
longer usable.  This usually means that the driver is unable to get any
sort of meaningful response from the UPS.  You must not rely on any
status information once this has been sent.

This will be sent in the beginning of a dump if the data is stale, and
may be repeated.  It is cleared by DATAOK.

TRACKING
~~~~~~~~

	TRACKING <id> <value>

This is sent in response to an INSTCMD or SET VAR that includes a TRACKING,
upon completion of request execution by the driver. <value> is the integer
return value from the driver handlers instcmd and setvar (see
drivers/upshandler.h). The server is in charge of translating these codes into
strings, as per docs/net-protocol.txt GET TRACKING.


Commands sent by the server
---------------------------

The data server `upsd` (or technically any client that connects to a Unix
socket or Windows named pipe provided by each NUT driver) can send the
following commands to the driver:

PING
~~~~

	PING

This is sent to check on the health of a driver.  The server should only
send this when it hasn't heard anything valid from a driver recently.
Some drivers have very little to say in terms of updates, and this may
be the only communications they have with the server on a normal basis.

If a driver does not respond with the PONG within a few seconds at the
most, it should be treated as dead/unavailable.  Data stored in the
server must not be passed on to the clients when this happens.

NOTE: For the `upsd` data server, the MAXAGE setting in upsd.conf controls
how long since the last message from the driver it is considered stale.
At 1/3 of this time the server sends a `PING` command to the driver, so
there is some time for a `PONG` to arrive and reset the timer (any other
message would serve that goal as well).

INSTCMD
~~~~~~~

	INSTCMD <cmdname> [<cmdparam>] [TRACKING <id>]

	INSTCMD panel.test.start
	INSTCMD load.off 10
	INSTCMD load.on 10 TRACKING 1bd31808-cb49-4aec-9d75-d056e6f018d2

NOTE:

* <cmdparam> is an additional and optional parameter for the command,
* "TRACKING <id>" can be provided to track commands execution status, if
TRACKING was set to ON on upsd. In this case, driver will later return
the execution status, using TRACKING.

SET
~~~

	SET <varname> "<value>" [TRACKING <id>]

	SET ups.id "Data room"
	SET ups.id "Data room" TRACKING 2dedb58a-3b91-4fab-831f-c8af4b90760a

NOTE:

* "TRACKING <id>" can be provided to track commands execution status, if
TRACKING was set to ON on upsd. In this case, driver will later return
the execution status, using TRACKING.

GETPID
~~~~~~

The server (or sibling driver instances, or `upsdrvctl` tool) can use
this to request the platform-specific process identifier of the driver
process. On POSIX and many other platforms this would be a numeric value,
but most generally it should be treated as an opaque string.

DUMPALL
~~~~~~~

	DUMPALL

The server uses this to request a complete copy of everything the driver
knows.  This is returned in the form of the same commands (SETINFO,
etc.) that would be used if they were being updated normally.  As a
result, the same parsing happens either way.

The server can tell when it has a full copy of the data by waiting for
DUMPDONE.  That special response from the driver is sent once the entire
set has been transmitted.

DUMPVALUE
~~~~~~~~~

	DUMPVALUE <varname>

	DUMPVALUE driver.version

Only request the value of specified variable name (and its additional
metadata in other lines), same as when `DUMPALL` iterates all such names.

The NUT data server or other socket-protocol client should parse the
response line by line, looking for the SETINFO line to get the value;
if a DUMPDONE is seen first, the value was not available in the driver.

DUMPSTATUS
~~~~~~~~~~

	DUMPSTATUS

Effectively an alias to `DUMPVALUE ups.status`.

NOBROADCAST
~~~~~~~~~~~

This connection does not want to receive broadcast messages (implemented
by `send_to_all()` method in `dstate.c`). Default is to receive everything.

BROADCAST (NUM)
~~~~~~~~~~~~~~~

This connection specified whether it wants to receive broadcast messages
(implemented by `send_to_all()` method in `dstate.c`), and by default
enables that -- unless disabled by providing an optional zero or negative
numeric argument. Note that initial default is to receive everything, so
this command may be useful for connections that disabled broadcasts at
some point.

LOGOUT
~~~~~~

Primarily used by communications between driver processes and/or `upsdrvctl`,
this command allows clients to gracefully close connection to the NUT driver
which acts as the server on the socket/pipe, avoiding noisy logs about sudden
disconnection.

	LOGOUT
	OK Goodbye

Design notes
------------

Requests
~~~~~~~~

There is no way to request just one variable.  This was done on purpose
to limit the complexity of the drivers.  Their job is to send out
updates and handle a few simple requests.  DUMPALL is provided to give
the server a known foundation.

To track a limited set of variables, a server just needs to do DUMPALL,
then only have handlers that remember values for the variables that
matter.  Anything else should be ignored.

Access/Security
~~~~~~~~~~~~~~~

There are no access controls in the drivers.  Anything that can connect
to their sockets can make requests, including SET and INSTCMD if
supported by the driver and hardware.  These sockets must be kept
secure.  If your operating system does not honor permissions or modes on
sockets, then you must store them in a directory with suitable
permissions to limit access.

Command limitations
~~~~~~~~~~~~~~~~~~~

As parseconf is used to handle decoding and chunking of the data, there
are some limits on what may be used.  These default to 32 arguments of
512 characters each, which should be more than enough for everything
which is currently needed by the software.

These limits are strictly for sanity purposes, and may be raised if
necessary.  parseconf itself can handle vast numbers of arguments and
characters, with some speed penalty as things get really big.

Re-establishing communications
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If the server loses its connection to the driver and later reconnects,
it must flush any local storage and start again with DUMPALL.  The
driver may have changed the internal state considerably during that
time, and any other approach could leave old elements behind.
