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MS RFC 64 - MapServer Expression Parser Overhaul

Date:2010/11/2
Author:Steve Lime
Contact:sdlime at comcast.net
Last Edited:2010-11-2
Status:Adopted

Aperçu

This is a draft RFC addressing 1) how the Bison/Yacc parser for logical expressions is implemented and 2) where in the MapServer code the parser can be used. This RFC could have broader impacts on query processing depending on additional changes at the driver level, specifically the RDBMS ones. Those changes don’t have to occur for this to be a useful addition.

A principle motivation for this work is to support OGC filter expressions in a single pass in a driver-independent manner.

All of the work detailed here is being prototyped in a sandbox, visit:

http://svn.osgeo.org/mapserver/sandbox/sdlime/common-expressions/mapserver <http://svn.osgeo.org/mapserver/sandbox/sdlime/common-expressions/mapserver>

Existing Expression Parsing

The existing logical expression handling in MapServer works like so:

  1. duplicate expression string[[BR]]
  2. substitute shape attributes into string (e.g. ‘[name]’ => ‘Anoka’)[[BR]]
  3. parse with yyparse()

The parser internally calls yylex() for its tokens. Tokens are the smallest pieces of an expression.

Advantages

  • it’s simple and it works

Disadvantages

  • limited by substitution to strings, no complex types can be handled
  • have to perform the substitution and tokenize the resulting string for every feature

Proposed Technical Changes

This RFC proposes a number of technical changes. The core change, however, involved updating the way logical expressions work. Additional features capitalize on this core change to bring additional capabilities to MapServer.

Core Parser Update

I propose moving to a setup where a logical expression is tokenized once (via our Flex-generated lexer) and then Bison/Yacc parser works through tokens (via a custom version of yylex() defined in mapparser.y) as necessary for each feature. This eliminates the substitution and tokenize steps currently necessary and opens up possibilities for supporting more complex objects in expressions. Basically we’d hang a list of tokens off an expressionObj, populate it in msLayerWhichItems() and leverage the tokens as needed in the parser. The following new structs and enums are added to mapserver.h:

enum MS_TOKEN_LOGICAL_ENUM { MS_TOKEN_LOGICAL_AND=100, MS_TOKEN_LOGICAL_OR, MS_TOKEN_LOGICAL_NOT };
enum MS_TOKEN_LITERAL_ENUM { MS_TOKEN_LITERAL_NUMBER=110, MS_TOKEN_LITERAL_STRING, MS_TOKEN_LITERAL_TIME, MS_TOKEN_LITERAL_SHAPE };
enum MS_TOKEN_COMPARISON_ENUM {
  MS_TOKEN_COMPARISON_EQ=120, MS_TOKEN_COMPARISON_NE, MS_TOKEN_COMPARISON_GT, MS_TOKEN_COMPARISON_LT, MS_TOKEN_COMPARISON_LE, MS_TOKEN_COMPARISON_GE, MS_TOKEN_COMPARISON_IEQ,
  MS_TOKEN_COMPARISON_RE, MS_TOKEN_COMPARISON_IRE,
  MS_TOKEN_COMPARISON_IN, MS_TOKEN_COMPARISON_LIKE,
  MS_TOKEN_COMPARISON_INTERSECTS, MS_TOKEN_COMPARISON_DISJOINT, MS_TOKEN_COMPARISON_TOUCHES, MS_TOKEN_COMPARISON_OVERLAPS, MS_TOKEN_COMPARISON_CROSSES, MS_TOKEN_COMPARISON_WITHIN, MS_TOKEN_COMPARISON_CONTAINS,
  MS_TOKEN_COMPARISON_BEYOND, MS_TOKEN_COMPARISON_DWITHIN
};
enum MS_TOKEN_FUNCTION_ENUM { MS_TOKEN_FUNCTION_LENGTH=140, MS_TOKEN_FUNCTION_TOSTRING, MS_TOKEN_FUNCTION_COMMIFY, MS_TOKEN_FUNCTION_AREA, MS_TOKEN_FUNCTION_ROUND, MS_TOKEN_FUNCTION_FROMTEXT };
enum MS_TOKEN_BINDING_ENUM { MS_TOKEN_BINDING_DOUBLE=150, MS_TOKEN_BINDING_INTEGER, MS_TOKEN_BINDING_STRING, MS_TOKEN_BINDING_TIME, MS_TOKEN_BINDING_SHAPE };

typedef union {
  double dblval;
  int intval;
  char *strval;
  struct tm tmval;
  shapeObj *shpval;
  attributeBindingObj bindval;
} tokenValueObj;

typedef struct {
  int token;
  tokenValueObj tokenval;
} tokenObj;

Some of these definitions hint at other features that will be detailed later. When we convert an expression string into a series of tokens we also store away the value associated with that token (if necessary). In many cases the token value is a literal (string or number), in other cases its a reference to a feature attribute. In the latter case we use the attributeBindingObj already in use by MapServer to encapsulate the information necessary to quickly access the correct data (typically an item index value).

We always have had to make expression data available to the parser (and lexer) via temporary global variables. That would continue to be the case here, although different data are shared in this context. One thing to note is that once an expression is tokenized we no longer have to rely on the flex-generated lexer so, in theory, it should be easier to implement a thread-safe parser should we choose to do so.

Extending the Yacc grammar to support spatial operators

The mapserver.h definitions above allow for using shapeObj’s within the Yacc grammar (we also define a shapeObj as a new base token type within mapparser.y). There are two types of shape-related tokens: 1) a shape binding, that is, a reference to the geometry being evaluated and 2) shape literals, shapes described as WKT within the expression string. For example:

In the expression:
  EXPRESSION (fromText('POINT(500000 5000000)') Intersects [shape])

  1) fromText('POINT(500000 5000000)') defines a shape literal (the WKT to shapeObj conversion is done only once)
  2) [shape] is a shape binding

We can use these tokens in the grammar to implement all of the MapServer supported (via GEOS) logical operators. Note that in the above example fromText() appears as a function operating on a string. This is handled as a special case when tokenizing the string since we only want to do this once. So we create a shape literal based on the enclosed WKT string at this time.

Extending the grammar to support spatial functions

By supporting the use of more complex objects we can support functions on those objects. We could write:

EXPRESSION (area([shape]) < 100000)

or rely on even more of the GEOS operators. (Note: only the area function is present in the sandbox.) To do this we need to somehow store a shapeObj’s scope so that working copies can be free’d as appropriate. I would propose adding a ‘’int scope’’ to the shapeObj structure definition. Shapes created in the course of expression evaluation would be tagged as having limited scope while literals or bindings would be left alone and presumably destroyed later. This saves having to make copies of shapes which can be expensive.

Context Sensitive Parsing

We could have done this all along but this would be an opportune time to implement context sensitive parser use. Presently we expect the parser to produce a true or false result, but certainly aren’t limited to that. The idea is to use the parser to compute values in other situations. Two places are working in the sandbox and are detailed below.

Class TEXT Parameter

Currently you can write:

CLASS
  ...
  TEXT ([area] acres)
END

It looks as if the TEXT value is an expression (and it is stored as such) but it’s not evaluated as one. It would be very useful to treat this as a true expression. This would open up a world of formatting options for drivers that don’t support it otherwise (e.g. shapefiles). Ticket 2950 <http://trac.osgeo.org/mapserver/ticket/2950> is an example where this would come in handy. Within the sandbox I’ve added toString, round and commify functions so that you can write:

TEXT (commify(toString([area]*0.000247105381,"%.2f")) + ' ac')

Which converts area from sq. meters to acres, truncates the result to two decimal places adds commas (213234.123455 => 213,234.12) for crowd pleasing display. To add this support, in addition to grammar changes, we add these declarations to mapserver.h:

enum MS_PARSE_RESULT_TYPE_ENUM { MS_PARSE_RESULT_BOOLEAN, MS_PARSE_RESULT_STRING, MS_PARSE_RESULT_SHAPE };

typedef union {
  int intval;
  char *strval;
  shapeObj *shpval;
} parseResultObj;

Then in the grammar we set a parse result type and set the result accordingly. One side effect is that we have to define a standard way to convert numbers to strings when in the string context and simply using “%g” as a format string for snprintf does wonders to output.

Style GEOMTRANSFORM

Within the sandbox I’ve implemented GEOMTRANSFORMs as expressions as opposed to the original implementation. The parser has also been extended to support the GEOS buffer operator. So you can write:

STYLE
  GEOMTRANSFORM (buffer([shape], -3))
  ...
END

This does executes a buffer on the geometry before rendering (see test.buffer.png attachment). Because the GEOMTRANSFORM processing occurs this transformation happens after the feature is converted from map to image coordinates, but the effect is still valuable and the buffer value is given in pixels. In the future we might consider implementing a GEOMTRANSFORM at the layer level so that the transformation is available to all classes and/or styles (and consequently in query modes too).

Expression Use Elsewhere

Currently the logical expression syntax is also used with REQUIRES/LABELREQUIRES and with rasters. In the REQUIRES/LABELREQUIRES case the code would remain mostly “as is” we’d still do the substitutions bases on layer status, then explicitly tokenize and parse. Since this done at most once per layer there’s really no need to do anything more.

Rasters present more of a challenge. We’d need to handle them as a special case when tokenizing an expression by defining pixel bindings and then pass a pixel to the parser when evaluating the expression. This should be much, much faster than the current method where each pixel value is converted to a string representation (and then back to a number), especially given the number of pixels often evaluated. Some interesting drawing effects are also possible if you could expose a pixel location to the GEOS operators. For example, one could create a mask showing only pixels within a particular geometry.

Query Impact

This is where things get interesting. I’m proposing adding a new query, msQueryByFilter() that would work off an expression string (this is working in the sandbox). The expression string would still have to be accompanied by an extent parameter. Drivers like shapefiles still need to first apply a bounding box before applying a secondary filter. Other drivers could choose to combine the extent and expression string (more likely the tokens) if they so choose. msQueryByFilter() works much like msQueryByAttributes(). The layer API has been extended to include a prototype, msLayerSupportsCommonFilters(), that allows the driver to say if it could process this common expression format natively somehow (e.g. via a FILTER and msLayerWhichShapes()/ msLayerNextShape()) or if the expression would need to be applied after msLayerNextShape() is called repeatedly. The shapefile and tiled shapefile drivers would work natively, as would any driver that uses msEvalExpression(). My hope is that the RDBMS drivers could somehow translate (via the tokens) an expression into their native SQL but if not, we would still be able to use those sources.

Backwards Compatibility Issues

Surprisingly few. The parser changes would all be transparent to the user. Truly handling TEXT expressions as expressions is a regression, albeit a positive one IMHO. I would also propose a few expression level changes especially around case-insensitive string and regex comparisons within logical expression. I think it makes more sense and is more user friendly to simply define case-insensitive operators (e.g. EQ and IEQ for straight string equality, and ~ and ~* for regex (modeled after Postgres).

The additional operators, functions and parsing contexts are new functionality.

A great deal of code would be made obsolete if this were pursued. Much of the OWS filter evaluation would be handled by the msQueryByFilter() function and numerous associated enums, defines, etc... could go away.

Grammar summary (bold means new):

  • Logical operators: AND, OR, NOT
  • Comparison operators: =, =*, !=, >, <, >=, <=, ~, ~*, in
  • Spatial comparison operators: intersects, disjoint, touches, overlaps, crosses, within, contains, beyond, dwithin
  • Functions: length, commify, round, tostring
  • Spatial functions: fromtext, area, distance, buffer

Security Issues

While the bulk of the work is in the bowels of MapServer any change of this magnitude could have unintended consequences. In this case I think the largest risks are buffer overflows associated with string operators in the parser and memory leaks. Care would need to be taken in developing a comprehensive set of test cases.

Todo’s

  1. The ‘’IN’’ operator is in dire need of optimization.
  2. All OGC filter operations need to be supported. Bounding box filters in particular have not been looked at.
  3. Need ‘’LIKE’’ operator code. (e.g. Dr. Dobbs, 9/08, ‘’Matching Wildcards: An Algorithm’‘, pp. 37-39)
  4. How to handle layer tolerances in msQueryByFilter()?
  5. Best way to manage tokens: array, list, tree? Bison/Yacc needs array or list, but both Frank and Paul have referred to trees. (Update: the tokens are now managed as a linked list.)
  6. Thread safety...
  7. Parser error handling, any errors have basically always been silently ignored.

Bug ID

#3613

Voting history

Passed on 12/1/2010 with a +1 from Steve L, Steve W, Perry, Assefa, Tamas.