path: root/www/protocol-transition.txt

= Moving to GPSD-NG: a Guide for Client Developers =
Eric S. Raymond <esr@thyrsus.com>
v1.0, August 2009

== Why a new protocol? ==

GPSD is moving to a new request/response protocol.  This move has been
forced upon us because the old one ran out of namespace.  It was
designed with case-insensitive single-character command/response
codes. 25 of 26 ASCII alphabetics were already in use by 2006, and
there have been functional challenges accumulating over the last three
years that will require several more request/response codes - things
like reporting AIS data, reporting raw pseudoranges, and reporting
RTCM3.  

Yes, we could as a desperate expedient have pressed non-alphabetic
printables into service - but the result would have looked like
line noise and only delayed the day of reckoning.  Instead, we're
writing a new protocol that is upward-compatible with the old one 
in that you can mix old and new syntax.

The old protocol will continue to be available for some time, but not
indefinitely.  Eventually we're going to want to shed the code
complexity and overhead of maintaining both protocols at once. This
will happen sooner than it otherwise might have because gpsd is in
part targeted for SBCs and other constrained environments where 
shaving a few K off the runtime image can actually matter.  When
it comes to keeping the codebase lean and mean, we try harder.

We'll try to make the transition easy, but we cannot guarantee no
problems. The sooner you start adapting your code, the less pain you
are likely to experience.  The rest of this document will explain both
theory and practice, and give you specific questions on how to fix
client code.

== Virtue is rewarded ==

Since 2004, the way you were *supposed* to be using gpsd was through
one of the client libraries (in C or Python). If you have been doing
it right, you have been telling gpsd to stream data at you with the
'w' command, via application code probably looked something like this:

-------------------------------------------------------------------

   gpsdata = gps_open(source.server, source.port);

   (void)gps_query(gpsdata, "w+x\n");

   // This is in your application's main event loop somewhere.
   // It polls gpsd for new data.
   ... gps_poll(gpsdata) ...
   
   gps_close()

-------------------------------------------------------------------

If you have been virtuous, you need only to make one small change to
your code.  Replace the gps_query() call with:

-------------------------------------------------------------------
gps_stream(gpsdata, WATCH_ENABLE) 
-------------------------------------------------------------------

This will tell whatever version of the client library your application
dynamically links to emit shat it should under the hood, either old
or new protocol. Unless a target system carries a version of the
libgps shared library different from the gpsd version, everything
should work and continue to work through future updates.

There. You're probably done, unless you relied on some parts of 
struct gpsdata_t that application developers usually don't or issued
unusual configuration commands. Here are the exceptions:

* You issued other gps_query() commands, such as "J=1".  If so,
  you'll need to learn how to say them in the new API (the J command
  itself is dead, and you can just remove it entirely). That
  is not difficult, and this document will cover what you
  need to know.

* Your application code referenced struct gpsdata_t members that no
  longer exist. This will be obvious because your compilation will
  fail.  Later in this document you will learn how to fix these
  problems.

* You set a per-packet raw hook.  This feature was documented
  incorrectly.  It no longer takes a level argument.

You can probably ignore the rest of this document, unless
either (a) you want to learn about gpsd's new capabilities so you
can use them in creative ways, or (b) you want to caper with unholy glee 
as you contemplate the trials awaiting the non-virtuous.

If you are non-virtuous - that is, you rolled your own client-side
interface - you've had years of warning that this choice would 
fail to insulate you from protocol details and cost you pain in the
future -- and that future is now.  

In the remainder of this document we will try to help you minimize the
pain.  The main strategy for doing so is to *use libgps* (or its
functional equivalents in languages other than C).  Scrap your
hand-rolled interface code!  When you use libgps, compatibility issues
become *our* problem to solve rather than *yours*.

== When the bough breaks ==

Even virtuous clients have to worry about version skew. Supposing you
have used libgps and not done anything exotic, you will still have
problems if the client library you linked and the instance of gpsd it
speaks to are using different protocols.

We're working to minimize problems by scheduling the changes like
this:

1. In 2.40 (shipping sometime in the fourth quarter of 2009), we will
roll out a daemon that speaks both old and new protocols and a client
library that speaks old protocol by default - that is, under the hood
gps_stream() still says "w+" rather than "?WATCH".

2. When we have reason to believe all major Linux distros have 
deployed 2.40, we will change the client library to ship "?WATCH"
by default.  This is likely to happen in late 2010.

3. At some point in the far future, we will disable old protocol
in the default builds of both client and daemon.  This is
probably at least three years or more out.

The possible failure modes are pretty obvious.  Transitions are
difficult.  We're essentially relying on the distribution integrators
to ship libgps and gpsd updates at the same time, with sane
package dependencies. If that goes smoothly, applications may
not even notice the changes.  We can hope...

== On not doing things by halves ==

At the same time that pressure has been building to redesign the
protocol, we've been gaining experience in gpsd's application domain
that has made us rethink some of the assumptions behind the old one.

Since we knew we were going to have a forced compatibility break at the
wire-protocol level anyway, we decided not to do things by halves.  One
big break - in the application model, struct gpsdata_t, and the
wire protocol behind it - is better than three or four spread out
over a period of time.  

As a result, the new protocol is not an exact superset of the old one.
It reflects a different way of carving up the behavior space in gpsd's
application domain.  And the shape of struct gpsdata_t, the
client-side interface structure, has changed in corresponding ways.

Accordingly, there are three things a client developer will need to
understand about the new protocol.  The first is theory: how its model
of the gpsd application domain is different. The second is practice:
how to issue new-style commands and interpret responses. The third, if
you have relied on the structure in a way that now breaks your
compile, is how that structure has changed.

== How the theory has changed ==

=== Channels are gone ===

In old protocol, when you requested data from the daemon, it would
search for a device supplying the kind of data you had told it you
wanted (GPS, by default) and connect your listening channel to *that
single device*. The association between channel and device was set
when channel was first bound to device and implicit; reports weren't
labeled with the device name. You could request a specific device if
you wanted to.

In the new protocol, channels are gone.  You tell gpsd to stream
reports at you; thereafter, every time an attached GPS or other device
generates a report, you'll get it.  There may be multiple devices
reporting; each report will come labeled with the name of the
originating device, and that name will be left in your client
structure along with the rest of the new data.

In both protocols, when you poll gpsd and get data the client library
takes care of interpreting what comes up the wire from the daemon, and
merges the resulting data into your client structure (struct
gpsdata_t).  

The difference is that before, the API locked you to one device during
the life of the session.  Now it potentially has to deal with a *set* 
of devices, treated symmetrically.

There are multiple reasons this change is a good idea.  One is that it
makes coping with devices being hotplugged in or out completely
trivial from the client's point of view - it can just choose to ignore
the fact that the device IDs in the reports have changed.  Also, the
channel-management hair in the interface goes away. Also, it means
that clients can treat identically the cases where (a) you have one
device reporting different kinds of data (e.g. a marine navigation
system reporting both GPS and AIS) and (b) you have several devices
reporting different kinds of data.

=== From lockstep to streaming ===

A subtler change has to do with the difference between a lockstep 
or conversational interface and a streaming, stateless one.

In the earliest versions of GPSD, clients requested various pieces of
data by command. After each request, they would need to wait until a
response came back.  Then, watcher mode was added.  By saying "w+",
you could ask gpsd to stream GPS reports at you whenever it got them.

In the new protocol, streaming is all there is.  Every report coming
up from the daemon is tagged with its device and type.  Instead of
issuing commands and then ewaiting for specific responses, clients
should expect any kind of report at any time and merge it into
client-local storage (libgps does this for you).

This change is necessary to cope with devices that may send (for
example) mixed GPS and AIS data. In the future, the stream from
gpsd could include other kinds of data, such as the take from
a digital compass, water-temperature sensors, or even aircraft
transponders.

== How the command set has changed == 

If your code issues old-protocol commands A, D, E, M, P, T, U, or V,
it is a wretched hive of scum and villainy that probably hasn't
changed since before the introduction of "W".  You are using the
oldest single-shot commands and will have to rewrite your interface
significantly, as the new protocol does not support equivalents.  Use
libgps.

If your code issues B, C, or N commands, they need to change to
?DEVICE commands.  See the protocol reference for details.

The old 'F' and 'G' commands do not have equivalents.  It would be
possible to implement these, but we probably won't do it unless there is
actual demand.  Consider teaching your client to ignore fix updates
when they don't have a specified "device" or "class" tag, respectively.

The old 'I' command has no equivalent. You probably issued it as part
of an initialization string, hoping that a subtype string would later
show up in gps_id so you could post it somewhere.  In the new
protocol, when a device sends back subtype information the daemon
ships the client an object of class DEVICE with a device tag and
subtype fields.  Watch for that and process appropriately.

The old 'J' command is dead. gpsd now detects the end of the reporting
cycle reliably and ships on that, buffering data during the individual
reporting cycle.

The old 'K' command is replaced by ?DEVICES.

The old 'L' command is replaced by ?VERSION.  Note that the daemon now
ships a version response to each client on connect, so it will
probably never be necessary for you to issue a ?VERSION request.

The old 'M' command has no equivalent.  Mode is reported in the TPV response.

The old 'O' and 'Y' commands are gone.  Use ?WATCH and sample the
stream instead.

The old 'Q' command has no equivalent.  DOPs are reported in the SKY response.

The 'S' command has no equivalent, because it is not well defined what
value should be presented for binary protocols.

The old 'R' command has been replaced by an optional attribute in
?WATCH.  Include the WATCH_RAW mask in the argument of
gps_stream(), or set a raw hook before alling gps_stream().

The old 'W' command has been replaced by ?WATCH. Call gps_stream()
with whatever options you want to set.

The old 'X' command is gone.  Instead, you will see an object of
class DEVICE from the daemon whenever a device is opened or closed.

The old 'Z' and '$' commands, used by the developers for profiling, 
have equivalents, which are presently considered unstable and thus
are undocumented.

== How the gps_data_t structure has changed == 

The client libary's reporting structure, struct gpsdata_t, has a new
substructure (struct devconfig_t) named "dev" that groups together
information about the device that shipped the last update to the
client.  The members of this structure replace several top-level
struct gpsdata members in older versions.

Most notably, the gps_device member has been replaced by dev.path.
It is valid after every response with a device tag (DEVICE, TPV, SKY,
AIS, RTCM2, RTCM3).

The top-level gps_id member is replaced by dev.subtype.  This data
should be considered valid only when DEVICEID_SET is on in the
top-level set member.

The dev members baudrate, parity, stopbits, cycle, mincycle, and
driver_mode replace older top-level members.  They should be
considered valid only when DEVICE_SET is on in the top-level set
member.

The top-level members ndevices and devicelist (used only on the client
side) have been replaced by an array of struct devconfig_t structures.
Data in this structure should be considered valid only when
DEVICELIST_SET is on in the top-level set member.  Storage for
pathnames is no longer dynamically allocated, but static; to save
space, it lives in a union with several other substructures.

The signature of the raw_hook member has changed.  It no longer takes
the final 'leve' argument, which libgps had always set to 1.

== Python client library changes ==

There is a gps_stream() entry point like the one in the C library.
gps_query() is deprecated.