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This chapter describes the BEA XQuery Scripting Extension (XQSE) that enables you to add procedural constructs to XQuery-based data services. The chapter describes the language extensions and includes the statement grammar along with one or more examples.
This chapter includes the following topics:
ALDSP data services are based on the XQuery language, which enables you to use the structure of XML to express queries against data. The XQuery Scripting Extension builds on this base by adding procedural constructs, including basic statements, control flow, and user-defined procedures to XQuery.
XQSE is therefore a superset of XQuery, extending it with additional features that enable you to create richer and more complex data services while working within the context of XML and XQuery. You can think of XQSE as an extension to XQuery in the same way as Oracle PL/SQL is an extension of SQL.
XQSE extends XQuery by adding procedural capabilities to the declarative query capabilities of XQuery. In XQuery, a query consists of a prolog followed by an XQuery expression. The prolog of a query sets up the environment for the expression by defining namespaces, external variables, and functions, among other information.
In XQSE, a prolog can also contain definitions of procedures and XQSE functions which are, respectively, side-effecting and non-side-effecting callable units of execution written in XQSE.
The following shows the grammar of the XQSE prolog and query body:
Prolog ::= ((DefaultNamespaceDecl | Setter | NamespaceDecl | Import)
Separator)* ((VarDecl | FunctionDecl | ProcedureDecl |
XQSEFunctionDecl | OptionDecl) Separator)*
QueryBody ::= Expr | Block
In XQSE, the body of a top-level query can be either an XQuery expression or an XQSE block. A block is a sequence of statements that are executed sequentially.
XQSE enables you to declare a procedure in the prolog of an ALDSP data service. An XQSE procedure is similar to an XQuery function, but unlike a function, a procedure can have one or more side effects. Another difference is that XQuery functions are declarative; the body of an XQuery function is an XQuery expression.
The body of an XQSE procedure, in contrast, consists of a block, which is a list of statements executed in sequential order when the procedure is called. Alternatively, as in XQuery, you can declare a procedure as external, in which case it does not have a body and is implemented by ALDSP by importing procedures from external data sources such as relational stored procedures or Web services.
The following shows the grammar of the XQSE procedure declaration:
ProcedureDecl ::= "declare" "procedure" QName "(" ParamList? ")"
("as" SequenceType)? (Block | ("external") )
A procedure may, but is not required to, return a value. Individual XQSE statements do not have return values by themselves, so a procedure returns a value only when an explicit return statement is included in the body of procedure. If the return type of a procedure is not specified, its return value is of type item()* by default.
| Note: | Since returning a value is optional, a return statement is not required in the body of a procedure. In the absence of a return statement, the return value of a procedure is the empty sequence and its return type is empty(). |
You can use recursion in procedures. This is another difference between XQuery functions and XQSE procedures in ALDSP.
(: Procedure to delete an employee given just their employee ID :)
(: Calls the default delete function on the data service after retrieving the employee object using the ID :)
declare procedure tns:deleteByEmployeeID($id as xs:string?) as empty() {
declare $emp as element(empl:Employee)? := tns:getByEmployeeID($id);
tns:delete($emp);
};
XQSE extends the XQuery function declaration syntax to enable you to declare XQSE-based functions in addition to procedures. An XQSE function is essentially a read-only procedure written in XQSE with no side effects.
As with a procedure, the body of an XQSE function consists of a block, which is a list of statements. The following shows the grammar of the XQSE function declaration:
XQSEFunctionDecl ::= "declare" xqse "function" QName "(" ParamList? ")"
("as" SequenceType)? (Block | ("external") )Since an XQSE function does not have any side-effects, you can call it from within an XQuery expression anywhere that a normal XQuery function can be called.
Since XQuery functions are declarative and therefore amenable to compile-time query optimization, you should write data service functions using XQuery instead of XQSE where possible. However, it is sometimes necessary (or at least conceptually more convenient) to express certain read-only computations procedurally. XQSE functions are appropriate in these cases.
For example, you could use an XQSE function to perform calculations that would otherwise require the use of tail recursion in XQuery. This is necessary since ALDSP does not permit the use of recursion in XQuery functions.
(: Procedure to compute the level of an employee in the org tree :)
declare xqse function tns:distanceFromTop($id as xs:string?) as xs:integer? {
declare $mgrCnt as xs:integer := 0;
declare $curEmp as element(empl:Employee)? :=
tns:getByEmployeeID($id);
declare $mgrId as xs:string? := fn:data($curEmp/ManagerID);
if (fn:empty($curEmp)) then return value ();
while (fn:not(fn:empty($mgrId))) {
set $mgrCnt := $mgrCnt + 1;
set $curEmp := tns:getByEmployeeID($mgrId);
set $mgrId := fn:data($curEmp/ManagerID);
};
return value ($mgrCnt);
};
XQSE offers the value statement and procedure call statement to distinguish between function and procedure calls.
| Note: | You can call a function wherever an expression can be used, but procedures can only be called in certain parts of XQSE because they include side effects. |
The following shows the grammar of the XQSE value statement and procedure call statement:
ValueStatement := ExprSingle | ProcedureCall
ProcedureCall ::= FunctionCall
Statement := SimpleStatement | BlockStatement
SimpleStatement ::= SetStatement | IfStatement | ReturnStatement |
ProcedureCall
An XQSE block contains a list of statements. You can use a block to declare mutable variables (using declare clauses) and manipulate those variables in subsequent statements, which are executed in sequential order.
The following shows the grammar of the XQSE block statement:
Block ::= {" ((BlockDecl ";")* StatementSequence "}"
BlockDecl ::= "declare" "$" VarName TypeDeclaration?
(":=" ValueStatement)?
StatementSequence := ((SimpleStatement ";") | (BlockStatement (";")?))*
BlockStatement := WhileStatement | IterateStatement | TryStatement |
Block
While XQuery expressions have values, statements do not (with the exception of the return statement, described in
Return Statement on page 6-7.). Therefore, a block does not have a return value since a block is itself a compound statement.
Every variable in a block must be declared before it can be used. If you declare a variable without explicitly specifying a type, the variable will have a default type of item()*.
You also need to initialize declared variables using a set statement or as part of the BlockDecl before they can be used. Referencing uninitialized variables (not appearing on the left hand side of an assignment statement) results in an error.
| Note: | Variables in a block are mutable. Unlike let and for variables that appear in XQuery expressions, which are immutable bindings of names to values, variables in an XQSE block are assignable (similar to variables in Java and C++, among other languages). |
You can define nested blocks, in which case regular scoping rules apply. For example, a variable with a specific fully-qualified name declared in an inner block will shadow (redefine and hide) variables in a containing outer block that has the identical fully-qualified name.
(: Procedure to compute the level of an employee in the org tree :)
declare xqse function tns:distanceFromTop($id as xs:string?)
as xs:integer? {
declare $mgrCnt as xs:integer := 0;
declare $curEmp as element(empl:Employee)? :=
tns:getByEmployeeID($id);
declare $mgrId as xs:string? := fn:data($curEmp/ManagerID);
if (fn:empty($curEmp)) then return value ();
while (fn:not(fn:empty($mgrId))) {
set $mgrCnt := $mgrCnt + 1;
set $curEmp := tns:getByEmployeeID($mgrId);
set $mgrId := fn:data($curEmp/ManagerID);
}
return value ($mgrCnt);
};
The XQSE set statement sets the variable VarName to the value specified by ValueStatement. The following shows the grammar of the XQSE set statement:
SetStatement ::= "set" "$" VarName ":=" ValueStatement
Before using the set statement, you must first declare the variable VarName using a declare statement. Only variables declared in this way are mutable and can therefore be changed using the set statement.
| Note: | The set statement has copy semantics. Consider the following instance: |
set $z := ($x, $y)
If $x and $y are mutable variables and $x and $y are later changed, $z retains the originally copied values of $x and $y.
(: Procedure to compute the level of an employee in the org tree :)
declare xqse function tns:distanceFromTop($id as xs:string?)
as xs:integer? {
declare $mgrCnt as xs:integer := 0;
declare $curEmp as element(empl:Employee)? :=
tns:getByEmployeeID($id);
declare $mgrId as xs:string? := fn:data($curEmp/ManagerID);
if (fn:empty($curEmp)) then return value ();
while (fn:not(fn:empty($mgrId))) {
set $mgrCnt := $mgrCnt + 1;
set $curEmp := tns:getByEmployeeID($mgrId);
set $mgrId := fn:data($curEmp/ManagerID);
};
return value ($mgrCnt);
};
The XQSE while statement loops and performs the actions in the block while the effective boolean value of the condition evaluates to true. The following shows the grammar of the XQSE while statement:
WhileStatement ::= "while" "(" Expr ")" BlockThe while statement reevaluates the condition expression before each loop. Typically, the condition depends upon a mutable variable that is manipulated in the block. The loop therefore terminates when code in the block causes the effective boolean value of the condition to cease being true.
(: Procedure to compute the level of an employee in the org tree :)
declare xqse function tns:distanceFromTop($id as xs:string?)
as xs:integer? {
declare $mgrCnt as xs:integer := 0;
declare $curEmp as element(empl:Employee)? :=
tns:getByEmployeeID($id);
declare $mgrId as xs:string? := fn:data($curEmp/ManagerID);
if (fn:empty($curEmp)) then return value ();
while (fn:not(fn:empty($mgrId))) {
set $mgrCnt := $mgrCnt + 1;
set $curEmp := tns:getByEmployeeID($mgrId);
set $mgrId := fn:data($curEmp/ManagerID);
};
return value ($mgrCnt);
};
The XQSE return statement computes the expression represented by ValueStatement and returns the resulting value while exiting from the current procedure.
The following shows the grammar of the XQSE return statement:
ReturnStatement ::= "return" "value" ValueStatement
In the special case where a block containing a return statement is the body of the main query, the return statement returns the value to the invoking environment.
(: Procedure to compute the level of an employee in the org tree :)
declare xqse function tns:distanceFromTop($id as xs:string?)
as xs:integer? {
declare $mgrCnt as xs:integer := 0;
declare $curEmp as element(empl:Employee)? :=
tns:getByEmployeeID($id);
declare $mgrId as xs:string? := fn:data($curEmp/ManagerID);
if (fn:empty($curEmp)) then return value ();
while (fn:not(fn:empty($mgrId))) {
set $mgrCnt := $mgrCnt + 1;
set $curEmp := tns:getByEmployeeID($mgrId);
set $mgrId := fn:data($curEmp/ManagerID);
};
return value ($mgrCnt);
};
XQSE offers an iterate statement that is equivalent to the XQuery for clause and enables you to perform data-driven looping over a block of XQSE statements. This enables you to iterate through the result of an XQuery expression, for example.
The following shows the grammar of the XQSE iterate statement:
IterateStatement ::= "iterate" "$" VarName PositionalVar? "over"
ValueStatement Block
The iteration variable VarName is bound to each item in the sequence produced by evaluating ValueStatement, which can be either an XQuery expression or a procedure call. Optionally, the PositionalVar variable represents the index of the current item in the sequence.
| Note: | The VarName and PositionalVar variables are both mutable, though it is not advisable that you exploit this capability. |
(: Procedure to allow only updates that don't violate the salary change business rules :)
declare procedure tns:updateChecked($changedEmps as
changed-element(empl:Employee)*) {iterate $sourceEmp over $changedEmps {tns:update($changedEmps);
if (tns:invalidSalaryChange($sourceEmp)) then
fn:error(xs:QName("INVALID_SALARY_CHANGE"), ":
Salary change exceeds the limit.");
};
};
(: Procedure to perform "lite ETL", copying and transforming data from one source to another :)
declare procedure tns:copyAllToEMP2() as xs:integer {
declare $backupCnt as xs:integer := 0;
declare $emp2 as element(emp2:EMP2)?;iterate $emp1 over ens1:getAll() {return value ($backupCnt);
set $emp2 := tns:transformToEMP2($emp1);
emp2:createEMP2($emp2);
set $backupCnt := $backupCnt + 1;
}
};
The try statement enables you to perform procedural error handling in XQSE, such as those raised by the XQuery fn:error function. The try statement works in much the same way as traditional try/catch statements in languages such as Java or C++.
The following shows the grammar of the XQSE try statement:
TryStatement ::= "try" Block CatchClauseStatement+
CatchClauseStatement ::= catch "(" NameTest ("into" VarNameExpr (("," VarNameExpr)? "," VarNameExpr)?)? ")" Block
XQSE enables you to catch all XQuery errors. XQuery errors have an associated QName, enabling you to use the XQuery NameTest to restrict the errors handled by a specific catch clause. In addition, the following variables in the catch clause work similarly to the arguments of the XQuery fn:error function:
Similar to exceptions in other languages, you can re-throw errors in XQSE using the fn:error function. When doing so, you need to ensure that all components of the error, including the name, description, and error-object, are properly passed to the new fn:error call.
(: Procedure to create a replicated employee and return an appropriately specific error message if it fails :)
declare procedure tns:create($newEmps as element(empl:Employee)*)
as element(empl:ReplicatedEmployee_KEY)* {
iterate $newEmp over $newEmps {
declare $newEmp2 as element(emp2:EMP2)? :=
bns:transformToEMP2($newEmp);
try { tns:createEmployee($newEmp); }
catch (* into $err, $msg) {
fn:error(xs:QName("PRIMARY_CREATE_FAILURE"),
fn:concat("Create failed on primary copy due to: ", $err,
$msg));
};
try { emp2:createEMP2($newEmp2); }
catch (* into $err, $msg) {
fn:error(xs:QName("SECONDARY_CREATE_FAILURE"),
fn:concat("Create failed on backup copy due to: ", $err,
$msg));
};
}
};
XQSE offers an if statement that is equivalent to the XQuery IfExpr expression. The XQSE if statement differs from the XQuery IfExpr expression in the following ways:
The following shows the grammar of the XQSE if statement:
IfStatement ::= "if" "(" Expr ")" "then" Statement ("else" Statement)?(: Procedure to compute the level of an employee in the org tree :)
declare xqse function tns:distanceFromTop($id as xs:string?)
as xs:integer? {
declare $mgrCnt as xs:integer := 0;
declare $curEmp as element(empl:Employee)? :=
tns:getByEmployeeID($id);
declare $mgrId as xs:string? := fn:data($curEmp/ManagerID);
if (fn:empty($curEmp)) then return value ();
while (fn:not(fn:empty($mgrId))) {
set $mgrCnt := $mgrCnt + 1;
set $curEmp := tns:getByEmployeeID($mgrId);
set $mgrId := fn:data($curEmp/ManagerID);
};
return value ($mgrCnt);
};
The XQSE language extends the XQuery data model with information about elements that have been updated and resubmitted to ALDSP. This enables XQSE to support SDO client updates along with associated server-side update logic.
An XML node that contains changes has the XQSE type changed-element. The following shows the grammar of the XQSE changed-element type:
ItemType := AtomicType | KindTestType | <"item" "(" ")"> |
ChangedElementTypeChangedElementType := "changed-element" "(" ElementNameOrWildcard ")"
XQSE provides two built-in functions, fn-bea:old-value and fn-bea:current-value, to access the pre-update and post-update contents of the changed XML node respectively.
| Note: | You can only pass instances of changed-element into XQSE as arguments to procedures and functions. Instances of changed-element cannot be created or incrementally modified within XQSE. |
You can also use instances of changed-element in variable declarations and assignments.
(: function to determine whether or not a given salary change is legal according to business rules :)
declare function tns:invalidSalaryChange($emp as
changed-element(empl:Employee)) as xs:boolean {
let $newSalary := fn:data(fn-bea:current-value($emp)/Salary)
let $oldSalary := fn:data(fn-bea:old-value($emp)/Salary)
return (100.0 * fn:abs($newSalary - $oldSalary) div $oldSalary)
gt 10.0
};
The following summarizes the XQSE grammar:
Prolog ::= ((DefaultNamespaceDecl | Setter | NamespaceDecl | Import) Separator)*
((VarDecl | FunctionDecl | ProcedureDecl | XQSEFunctionDecl| OptionDecl) Separator)*
XQSEFunctionDecl ::= "declare" xqse "function" QName "(" ParamList? ")" ("as" SequenceType)? (Block | ("external") )ProcedureDecl ::= "declare" "procedure" QName "(" ParamList? ")" ("as" SequenceType)? (Block | ("external") )QueryBody ::= Expr | Block
Statement := SimpleStatement | BlockStatement
SimpleStatement ::= SetStatement | IfStatement | ReturnStatement | ProcedureCall
BlockStatement := WhileStatement | IterateStatement | TryStatement | Block
ValueStatement := ExprSingle | ProcedureCall
ReturnStatement ::= "return" "value" ValueStatement
Block ::= {" ((BlockDecl ";")* StatementSequence "}"StatementSequence := ((SimpleStatement ";") | (BlockStatement (";")?))*BlockDecl ::= "declare" "$" VarName TypeDeclaration? (":=" ValueStatement)?SetStatement ::= "set" "$" VarName ":=" ValueStatement
WhileStatement ::= "while" "(" Expr ")" BlockIterateStatement ::= "iterate" "$" VarName PositionalVar? "over" ValueStatement Block
ProcedureCall ::= FunctionCall
TryStatement ::= "try" Block CatchClauseStatement+
CatchClauseStatement ::= catch "(" NameTest ("into" VarNameExpr (("," VarNameExpr)? "," VarNameExpr)?)? ")" BlockIfStatement ::= "if" "(" Expr ")" "then" Statement ("else" Statement)?ItemType := AtomicType | KindTestType | <"item" "(" ")"> | ChangedElementTypeChangedElementType := "changed-element" "(" ElementNameOrWildcard ")"
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