WebSphere Message Brokers
File: ak04980_
Writer: Bill Oppenheimer

Reference topic

This build: July 31, 2007 21:31:27

DECLARE statement

The DECLARE statement defines a variable, the data type of the variable and, optionally, its initial value.

Syntax

Notes:

  1. The SHARED keyword is not allowed within a function or procedure.
  2. You cannot specify SHARED with a DataType of REFERENCE TO. (To store a message tree in a shared variable, use the ROW data type.)
  3. EXTERNAL variables are implicitly constant.
  4. It is good programming practice to give an EXTERNAL variable an initial value.
  5. If you specify a DataType of REFERENCE TO, you must specify an initial value (of either a variable or a tree) in InitialValueExpression.
  6. When you use the NAMESPACE and NAME clauses, their values are implicitly constant and of type CHARACTER.

Types of variable

You can use the DECLARE statement to define three types of variable:
External
External variables (defined with the EXTERNAL keyword) are also known as user-defined properties (see User-defined properties in ESQL). They exist for the entire lifetime of a message flow and are visible to all messages passing through the flow. You can define external variables only at the module and schema level. You can modify their initial values (optionally set by the DECLARE statement) at design time, using the Message Flow editor, or at deployment time, using the BAR editor. You cannot later modify their values using ESQL.
Normal
"Normal" variables have a lifetime of just one message passing through a node. They are visible to that message only. To define a "normal" variable, omit both the EXTERNAL and SHARED keywords.
Shared
Shared variables can be used to implement an in-memory cache in the message flow (see Optimizing message flow response times). Shared variables have a long lifetime and are visible to multiple messages passing through a flow (see Long-lived variables). They exist for the lifetime of the execution group process, the lifetime of the flow or node, or the lifetime of the node’s SQL that declares the variable (whichever is the shortest). They are initialized when the first message passes through the flow or node after each broker startup.

See also the ATOMIC option of the BEGIN ... END statement. The BEGIN ATOMIC construct is useful when a number of changes need to be made to a shared variable and it is important to prevent other instances seeing the intermediate states of the data.

CONSTANT

Use CONSTANT to define a constant. You can declare constants within schemas, modules, routines, or compound statements (both implicit and explicit). The behavior of these cases is as follows:

A constant or variable declared within a routine overlays any parameters of the same name, and all constants and variables of the same name that are declared in a containing module or schema.

Draft comment:

Within any one schema or module, all constants occupy a namespace. A constant declared within a module overlays any constant of the same name and type declared in the containing schema.

Constants within the DECLARE statement are always referred to by a single unqualified identifier.

At the schema level the DECLARE statement supports the declaration of constants and shared variables.

An error is reported if the use of a namespace identifier that is not equal to the name of a namespace constant is found. This applies only to explicit references to elements by name.

DataType

The possible values that you can specify for DataType are:
  • BOOL
  • BOOLEAN
  • INT
  • INTEGER
  • FLOAT
  • DEC
  • DECIMAL
  • DATE
  • TIME
  • TIMESTAMP
  • GMTTIME
  • GMTTIMESTAMP
  • INTERVAL. This does not apply to external variables (EXTERNAL option specified)
  • CHAR
  • CHARACTER
  • BLOB
  • BIT
  • ROW. This does not apply to external variables (EXTERNAL option specified)
  • REF. This does not apply to external or shared variables (EXTERNAL or SHARED option specified)
  • REFERENCE TO. This does not apply to external or shared variables (EXTERNAL or SHARED option specified)

EXTERNAL

Use EXTERNAL to denote a user-defined property (UDP). A UDP is a user-defined constant whose initial value (optionally set by the DECLARE statement) can be modified, at design time, by the Message Flow editor, or overridden, at deployment time, by the Broker Archive editor. Its value cannot be modified by ESQL.

For an overview of UDPs, see the Related link about user-defined properties in ESQL.

When a UDP is given an initial value on the DECLARE statement, this becomes its default. However, any value that you specify in the Message Flow editor at design time, or in the BAR editor at deployment time (even a zero length string) overrides any initial value coded on the DECLARE statement.

All UDPs in a message flow must have a value, given either on the DECLARE statement or by the Message Flow or BAR editor; otherwise a deployment-time error occurs. At run time, after the UDP has been declared its value can be queried by subsequent ESQL statements, but cannot be modified.

The advantage of UDPs is that their values can be changed at deployment time. If, for example, you use the UDPs to hold configuration data, it means that you can configure a message flow for a particular machine, task, or environment at deployment time, without having to change the code at the node level.

You can declare UDPs only in modules or schemas, this means that you can use the DECLARE statement with the EXTERNAL keyword only at the MODULE or SCHEMA level. If you use a DECLARE statement with the EXTERNAL keyword within a PROCEDURE or FUNCTION, a BIP2402E exception is produced when you deploy the message flow.

The following types of broker node are capable of accessing UDPs:
  • Compute node
  • Database node
  • Filter node
  • Nodes derived from these node-types

Take care when specifying the data type of a UDP, because a CAST is used to change the value to the requested DataType.

Example 1

DECLARE mycolour EXTERNAL CHARACTER ‘blue'; 

Example 2

DECLARE TODAYSCOLOR EXTERNAL CHARACTER;
SET COLOR = TODAYSCOLOR;
where TODAYSCOLOR is a user-defined property that has a TYPE of CHARACTER and a VALUE set by the Message Flow Editor.

NAME

Use NAME to define an alias (an alternative name) by which a variable can be known.

Example 1

-- The following statement gives Schema1 an alias of 'Joe'.
DECLARE Schema1 NAME 'Joe';  
-- The following statement produces a field called 'Joe'.
SET OutputRoot.XMLNS.Data.Schema1 = 42; 
 
-- The following statement inserts a value into a table called Table1 
-- in the schema called 'Joe'.
INSERT INTO Database.Schema1.Table1 (Answer) VALUES 42; 

Example 2

DECLARE Schema1 EXTERNAL NAME;

CREATE FIRSTCHILD OF OutputRoot.XMLNS.TestCase.Schema1 Domain('XMLNS') 
                     NAME 'Node1' VALUE '1';

-- If Schema1 has been given the value 'red', the result would be:
<xml version="1.0"?>
<TestCase>
  <red>
    <Node1>1</Node1>
  </red>

NAMESPACE

Use NAMESPACE to define an alias (an alternative name) by which a namespace can be known.

Example

This example illustrates a namespace declaration, its use as a SpaceId in a path, and its use as a character constant in a namespace expression:

       DECLARE prefixOne NAMESPACE 'http://www.example.com/PO1';

       -- On the right hand side of the assignment a namespace constant
       -- is being used as such while, on the left hand side, one is 
       -- being used as an ordinary constant (that is, in an expression).

       SET OutputRoot.XMLNS.{prefixOne}:{'PurchaseOrder'} = 
                      InputRoot.XMLNS.prefixOne:PurchaseOrder;

SHARED

Use SHARED to define a shared variable. Shared variables are private to the flow (if declared within a schema) or node (if declared within a module), but are shared between instances of the flow (threads). No type of variable is visible beyond the flow level; for example you cannot share variables across execution groups.

You can use shared variables to implement an in-memory cache in the message flow (for further information refer to optimizing message flow response times in the related links.) Shared variables have a long lifetime and are visible to multiple messages passing through a flow (for more information about long-lived variables refer to the related links.) They exist for the lifetime of the execution group process, the lifetime of the flow or node, or the lifetime of the node’s SQL that declares the variable (whichever is the shortest). They are initialized when the first message passes through the flow or node after each broker startup.

You cannot define a shared variable within a function or procedure.

The advantages of shared variables, relative to databases, are that:
  • Write access is very much faster.
  • Read access to small data structures is faster.
  • Access is direct, that is, there is no need to use a special function (SELECT) to get data, or special statements (INSERT, UPDATE, or DELETE) to modify data. Instead, you can refer to the data directly in expressions.
The advantages of databases, relative to shared variables, are that:
  • The data is persistent.
  • The data is changed transactionally.

These read-write variables are ideal for users prepared to sacrifice the persistence and transactional advantages of databases in order to obtain better performance, because they have a longer life than only one message and perform better than a database.

Because flow-shared variables can be updated by multiple instances and multiple flows, ensure that your flows or instances do not make changes to shared variables that might cause unexpected results in other flows and instances, for example if the variable is being used as a counter. You can prevent other instances seeing the intermediate stages of the data by using a BEGIN ATOMIC construct as described in the BEGIN...END statement in the related links.

Shared row variables allow a user program to make an efficient read/write copy of an input node’s message. This is generally useful and, in particular, simplifies the technique for handling large messages.

There is a restriction that subtrees cannot be copied directly from one shared row variable to another shared row variable. Subtrees can be copied indirectly by using a non-shared row variable. Scalar values extracted from one shared row variable (using the FIELDVALUE function) can be copied to another shared row variable.

Example

Draft comment:
DECLARE SHARED tempmessage ROW InputRoot.XMLNS;
DECLARE temp SHARED ROW ROW(SELECT…..FROM ….. AS blarr[]); 
This defines temp as a shared ROW variable and constructs a ROW by means of a SELECT statement. The SELECT statement obtains the value and places it into blarr as the first layer of elements.
The following sample shows how to use both shared and external variables: You can view samples only when you use the information center that is integrated with the Message Brokers Toolkit.
Related concepts
ESQL overview
User-defined properties in ESQL
Long-lived variables
Related tasks
Developing ESQL
Creating dynamic field references
Configuring a message flow at deployment time using UDPs
Optimizing message flow response times
Related reference
Syntax diagrams: available types
ESQL statements
ESQL data types in message flows
Message Flow editor
FIELDVALUE function
Broker Archive editor
BEGIN ... END statement
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This build: July 31, 2007 21:31:27

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