Difference between revisions of "DAP4: Possible Notation for Server Commands"

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# Server-side processing.
 
# Server-side processing.
  
I want to propose a notation for everything in the URL after the "?". I think this notation has ability to represent a wide variety of features without, I hope, being too generic.
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I want to propose a notation for everything in the URL after the "?". I think this notation has the ability to represent a wide variety of features without, I hope, being too generic.
  
The notation is basically nested functions combined with single assignment variables. A semantically non-sensical, but grammatical example would look something like this.
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The notation is basically nested functions combined with single assignment variables. A semantically nonsensical, but grammatical example would look something like this.
  ?_x=f(17,g(h(12))),f2(_x,[0:3:10])
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  ?svc("cmd");_x=f("string17",g(h(12))),f2(_x,[0:3:10])
Everthing to past the "?" is in the form of a comma separated list of nested function invocations. Anything that begins with an underscore is considered a local, temporary, variable, anything that does not look like a function call (i.e. name followed immediately by left paren) is assume to be string constant. Each function has an arbitrary number of argument expressions separated by commas.
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Everything  past the "?" is in the form of a semi-colon separated list of expression lists. An expression list is a comma separated list of nested function invocations, possibly assigned to a variable. Anything that begins with an underscore is considered a local, temporary, variable, anything that does not look like a function call (i.e. name followed immediately by left paren) is assume to what I will call a non-quoted string constant; standard quoted string constants are also allowed. Each function has an arbitrary number of argument expressions separated by commas.
  
There would be several semantic rules.
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BTW, the term "single assignment" means that a variable may only be assigned to once, but may be referenced as many times as desired after that.
# A variable may only be assigned to once (single assignment), but may be referenced as many times as desired after that.
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# All functions have a defined "return type", which looks like a legal DDX minus certain things like groups, enumeration declarations, and dimension declarations; in addition, a function may be defined to have a "void" return type, which means it is executed for its side-effects on the server.
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== Syntactic and Lexical Structure ==
# Any expression that is not assigned to a variable and does not have a void return type will have its return value returned to the caller as part of a DATADDX.
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I have defined a preliminary [http://dl.dropbox.com/u/53929684/query.y syntax document] and [http://dl.dropbox.com/u/53929684/query.lex lexical document] for the ideas presented here.
  
 
== Notes ==
 
== Notes ==
My hypothesis is that this notation should also be able to handle most kinds of server side processing by defining and composing functions.
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# The purpose of providing multiple expression lists separated by semi-colons is to support the a visible notation for specififying server commands separate from constraint evaluation.
 
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# My hypothesis is that this notation should also be able to handle most kinds of server side processing by defining and composing functions.
The standard projection+selection constraints of DAP2 can be represented using a special query() function whose argument is the standard DAP2 constraint [or alternatively, one could define a collection of nested functions to do the same thing], or alternatively, we could split the query part into two pieces separated by a semicolon. The first piece would be a constraint expression and the second piece (after the semicolon) would be in the nest function call form defined above.
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# The standard projection+selection constraints of DAP2 can be represented using a special constraint() function whose argument is the standard DAP2 constraint. Alternatively, one could define a collection of nested functions to do the same thing.
 
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# I hypothesize that Ferret notations could be represented in my proposed function notation without having to clutter up the URL format. E.g.
An important aspect has to do with the construction of what may be referred to as a DATADDX. It defines the structure of a DDX that is the composition of the return types of the invoked functions that will return a (possibly structured) value. I need to work this out. BUT, in any case, the resulting DATADDX may have only have a loose relation to any DDX representing the raw dataset. This is because server-side computations will not have been represented in the original DDX, but only in the DATADDX.
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#: <nowiki>http://.../thredds/dodsC/hfrnet/agg/6km_expr_{}{let deq1ubar=u[d=1,l=1:24@ave]}</nowiki>
 
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# An important issue involves the construction of a DDX from a constraint. I have begun this discussion [[DAP4: Constructing a DDX from a Query | here ]]
I also hypothesize that Ferret notations
 
http://.../thredds/dodsC/hfrnet/agg/6km_expr_{}{let deq1ubar=u[d=1,l=1:24@ave]}
 
could be represented in my proposed function notation without having to clutter up the URL format.
 
  
 
''-Dennis Heimbigner''
 
''-Dennis Heimbigner''

Revision as of 19:13, 12 April 2012

Looking to the future, it is clear that eventually our query language, or more generically our server commands must encompass three classes of computations.

  1. Queries in the DAP2 sense,
  2. Commands to control the processing of requests on the server (i.e. thing like caching),
  3. Server-side processing.

I want to propose a notation for everything in the URL after the "?". I think this notation has the ability to represent a wide variety of features without, I hope, being too generic.

The notation is basically nested functions combined with single assignment variables. A semantically nonsensical, but grammatical example would look something like this.

?svc("cmd");_x=f("string17",g(h(12))),f2(_x,[0:3:10])

Everything past the "?" is in the form of a semi-colon separated list of expression lists. An expression list is a comma separated list of nested function invocations, possibly assigned to a variable. Anything that begins with an underscore is considered a local, temporary, variable, anything that does not look like a function call (i.e. name followed immediately by left paren) is assume to what I will call a non-quoted string constant; standard quoted string constants are also allowed. Each function has an arbitrary number of argument expressions separated by commas.

BTW, the term "single assignment" means that a variable may only be assigned to once, but may be referenced as many times as desired after that.

1 Syntactic and Lexical Structure

I have defined a preliminary syntax document and lexical document for the ideas presented here.

2 Notes

  1. The purpose of providing multiple expression lists separated by semi-colons is to support the a visible notation for specififying server commands separate from constraint evaluation.
  2. My hypothesis is that this notation should also be able to handle most kinds of server side processing by defining and composing functions.
  3. The standard projection+selection constraints of DAP2 can be represented using a special constraint() function whose argument is the standard DAP2 constraint. Alternatively, one could define a collection of nested functions to do the same thing.
  4. I hypothesize that Ferret notations could be represented in my proposed function notation without having to clutter up the URL format. E.g.
    http://.../thredds/dodsC/hfrnet/agg/6km_expr_{}{let deq1ubar=u[d=1,l=1:24@ave]}
  5. An important issue involves the construction of a DDX from a constraint. I have begun this discussion here

-Dennis Heimbigner