A delegation language to request weather products
and a scheme of its interpretation

A poster presentation at the 1998 International Conference on Functional Programming (ICFP'98)
Mt. Washington Conference Center, Baltimore, MD, 27-29 September 1998

What is Metcast

a request-reply and subscription -- channel -- system for distributing weather information
a projection of MIME and HTTP into the meteorological domain
Metcast packs weather products into MIME messages; it relies on http/smtp to distribute, disseminate, publish and broadcast these messages

Metcast serves:

weather observation reports, forecasts and advisories (METAR, SPECI, SIGMET, TAF)
tropical cyclone and other warnings
satellite imagery
current and forecast pressure, temperature, wind speed and direction,... grids

Metcast's Request Language

This is a language a Metcast client uses to communicate its wishes to a server in a POSTed request. This language:

implies a single specific area of interest
which may as well be the entire globe
interprets each phrase in a context
the area of interest: the top-most context
there may be children (nested) contexts
accounts for different aspects of products: geographical registration, layer, type, time
accounts for different types of products:
grids; 3D data; point data; area, polygon data; satellite imagery
each type of product => its own request spec
spares a user from specifying all parameters
=>
infers the values of missing parameters
from profiles and defaults,
which apply to a particular product category, to particular users, to all products requested in a current transaction, etc.

Examples

Three request language phrases:

     (some-area
     	(bounding-box  90.0 178W -90.0 179E)
     	(source FNMOC)
     	(model NOGAPS)(resolution 1.0)
     	(products
     	  ((geopotential-height)
     		(layer isobar 500)(tau 6))
     	  ((geopotential-height)
     		(layer msl)(tau 6))
     	  ((temperature)
     		(layer isobar 1000)(tau 6)
	     	(model NORAPS))
	      (METAR) (SIGMET)))

     (Area51
       (use special-area-51))

     (Area51
       (use special-area-51) (user 007)
       (products (TAF)))

Implementation

"Compilation"
- un-serializing objects by applying the head to the body
- a stack-compilation of the request
"Interpretation"
- converting product requests to product promises
- deducing the complete set of query parameters and querying the database
Execution
- calling the promises and composing a multi-part MIME reply
  (if there are several promises)

Walk through a sample transaction

Initial Request

     (some-area
       (bounding-box  90.0 178W -90.0 179E)
       (model NOGAPS)(resolution 1.0)
       (products
         (geopotential-height
            (layer isobar 500)(tau 12))
         (temperature
            (source NOAA)
            (model NOGAPS) (layer msl))))

Note a (object-name argument...) format of each sentence in the request. When a server internalizes -- compiles -- a sentence, it looks up a constructor named OBJ-LOADER:object-name in the current environment, and applies this constructor to the argument.... In other words, the server un-serializes each object mentioned in the request.

Initial Stack

This is the initial environment of the server, which specifies global defaults for every request

     (OBJ-LOADER:products . #<procedure #x1006ECB3>)
     (OBJ-LOADER:bounding-box . #<procedure #x1006ECA8>)
     (OBJ-LOADER:model . #<procedure #x1006EC00>)
     (OBJ-LOADER:source . #<procedure #x1006EBF5>)
     (executor . #<procedure serve-grids>)
     (center-id . 58)
     (process-id . 58)
     (resolution . 1.)
     (grid-id . 240)
     (pressure (product-GRIB-code . 1))
     (geopotential-height (product-GRIB-code . 7))
     (temperature (product-GRIB-code . 11))
     (GlobalWarnings
       (executor . #<procedure serve-global-warnings>))
     (SIGMET
       (executor . #<procedure perform-SIGMET-request>)
              (mime-type . "text/x-omf-advisories"))
     (|Env Name Key!| . "*** initial env ***"))

After the Stack-Compilation

This is the current environment after the sample request has been precompiled and its objects loaded

     (products
       (geopotential-height (layer isobar 500) (tau 12))
       (temperature (source NOAA) (model NOGAPS)
         (layer msl)))

		;  the top-level request context
      (resolution . 1.)	; request defaults

      (process-id . 58)	; model defaults

      (resolution . 2.5)
      (grid-id . 223)

		; Note an internal format of the bounding-box
      (bounding-box . #(90. -178. -90. 179.))
      (area-id . some-area)

		;  inherited from the standard prelude
      (OBJ-LOADER:products . #<procedure #x1006ECB3>)
      (center-id . 58)
      (process-id . 58)
      (resolution . 1.)
      (grid-id . 240)
	; etc., see above

Processing the first request

After the submitted request has been compiled, the server turns its attention to product request clauses, and recursively stack-compiles them. This is how the stack looks after the first product request has been fully loaded:

     ((tau . 12)			; product specs
      (layer isobar 500)
      (product-GRIB-code . 7)		; ... means geopotential-height
       (g3)		; mark!

      (products
        (geopotential-height (layer isobar 500) (tau 12))
        (temperature (source NOAA) (model NOGAPS)
     	 (layer msl)))

		; inherited from the top-level request context
      (resolution . 1.)		; request defaults
      (process-id . 58)		; model defaults
      (resolution . 2.5)
      (grid-id . 223)

      (bounding-box . #(90. -178. -90. 179.))
      (area-id . some-area)
        ...
      (center-id . 58)		; global defaults
      (process-id . 58)
      (resolution . 1.)
      (grid-id . 240)

     (geopotential-height (product-GRIB-code . 7))
     (temperature (product-GRIB-code . 11))
     (|Env Name Key!| . "*** initial env ***"))

Using this environment, the following database query is issued:

     AREA_ID=some-area
     MODIFIED_SINCE=0
     BOUNDING_BOX=90. -178. -90. 179.
     OUTPUT_FILE=/var/tmp/baaa12089
     PARAMETER=7
     CENTER=58 0 58
     LAYER=100 500
     GRID_ID=223
     TAU=12
     RESOLUTION=1. 1.

Some database keys (set in bold) are specified several times in the environment, within defaults of different depth. The most specific -- the least default -- value is chosen for the query.

Interpreting the `temperature` request

The second product request is processed similarly. The server compiles it and loads in the environment:

     ((layer . msl)			; product specs
      (process-id . 58)
      (resolution . 2.5)
      (grid-id . 223)			; model defaults

      (center-id . 59)
      (subcenter-id . 0)
      (product-GRIB-code . 11)	; ... means temperature

      (g4)	; mark!
      ...
      (resolution . 1.)			; request defaults

      (process-id . 58)			; model defaults

      (resolution . 2.5)
      (grid-id . 223)
      (bounding-box . #(90. -178. -90. 179.))
      (area-id . some-area)
      ...
      (center-id . 58)			; global defaults

      (process-id . 58)
      (resolution . 1.)
      (grid-id . 240)
       ...
      (|Env Name Key!| . "*** initial env ***"))

This environment is then used to look up all the necessary keys for the query:

     AREA_ID=some-area
     MODIFIED_SINCE=0
     BOUNDING_BOX=90. -178. -90. 179.
     OUTPUT_FILE=/var/tmp/daaa12089
     PARAMETER=11
     CENTER=59 0 58
     LAYER=102
     GRID_ID=223
     TAU=0
     RESOLUTION=2.5 2.5

Note that the environment contains not only simple values but procedures as well, which may add new or shadow old bindings in the environment.

REFERENCES

A one-page abstract of this presentation [.ps.gz, 5K]

Metcast home page
http://www.metnet.navy.mil/Metcast/

Request Language specification
http://www.metnet.navy.mil/Metcast/Request-Lang.html

Vocabulary package, (poly/homo)morphic dictionaries with a dynamic "inheritance" path

Hierarchical repository in Scheme
that implements the request language "stack"

Metcast Server URL
http://www.metnet.navy.mil/cgi-bin/oleg/server

Last updated October 9, 1998

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A delegation language to request weather products and a scheme of its interpretation