DECLARE statement

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

Start of changeThe three types of variable that you can define with the DECLARE statement are:
  • External
  • Normal
  • Shared
For further information, see ESQL variables.End of change

SYNTAX

Notes:

  1. The SHARED keyword is not valid within a function or procedure.
  2. You cannot specify SHARED with at 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.

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 shown in the following list:

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

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 value does not apply to external variables (EXTERNAL keyword specified).
  • CHAR
  • CHARACTER
  • BLOB
  • BIT
  • ROW. This value does not apply to external variables (EXTERNAL keyword specified).
  • REF. This value does not apply to external or shared variables (EXTERNAL or SHARED keyword specified).
  • REFERENCE TO. This value does not apply to external or shared variables (EXTERNAL or SHARED keyword 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 (see Message Flow editor) or overridden, at deployment time, by the Broker Archive editor (see Broker Archive editor). The value of a UDP cannot be modified by 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 Broker Archive editor at deployment time (even a zero length string) overrides any initial value that was coded on the DECLARE statement.

Start of changeFor example, if you code:
       DECLARE deployEnvironment EXTERNAL CHARACTER 'Dev';
you have defined a UDP variable of deployEnvironment with an initial value Dev.End of change

Start of changeAdd the UDP to the message flow by using the UDP tab in the message flow editor. When you add the flow to the BAR file, the UDP is there as an attribute of the flow; you must name the attribute to be the same as the ESQL variable in the DECLARE statement (in this case, deployEnvironment) to ensure that the initial value that you set is unchanged.End of change

All of the UDPs in a message flow must have a value, given either on the DECLARE statement or by the Message Flow or Broker Archive 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.

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 computer, task, or environment at deployment time, without having to change the code at the node level. Start of changeUDPs can also be modified at run time using the Configuration Manager Proxy (CMP) API.End of change

You can declare UDPs only in modules or schemas, that is, 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 occurs when you deploy the message flow.

The following types of broker node can access UDPs:
  • Compute node
  • Database node
  • Filter node
  • Nodes that are 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.

For an overview of UDPs, see User-defined properties in ESQL.

Example 1

DECLARE mycolor 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; 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.

Shared variables exist for the lifetime of the:
  • Execution group process
  • Flow or node, or
  • 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. 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 who are 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.

Start of changeBecause shared variables can be updated by multiple additional instances, you must ensure that you do not make changes to shared variables that might cause unexpected results, for example, if the variable is being used as a counter.End of change

You can prevent other instances seeing the intermediate stages of the data by using a BEGIN ATOMIC construct; see BEGIN ... END statement.

Your user program can make an efficient read, or write, copy of an input node's message by using shared-row variables, which simplifies the technique for handling large messages.

Restriction:

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 (by using the FIELDVALUE function) can be copied to another shared row variable.

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 with user-defined properties
Optimizing message flow response times
Related reference
Syntax diagrams: available types
ESQL statements
ESQL variables
ESQL data types in message flows
Message Flow editor
FIELDVALUE function
Broker Archive editor
BEGIN ... END statement
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Copyright IBM Corporation 1999, 2009Copyright IBM Corporation 1999, 2009.
Last updated : 2009-01-07 15:22:17

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