Value Copy and Merging

You can copy a value from a source variable to a target variable using Form mode, and combine values from multiple sources into a single variable using Template mode.

Scenario

This guide covers two fundamental use cases:

1. Form Mode: Copy a value from a single source (header, body field, query parameter) to another variable.
2. Template Mode: Combine values from multiple sources to create a single variable.

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Example 1: Copy a Value with Form Mode

In this example, the X-Tenant-Id header value is copied to a custom variable. If the header is missing, default-tenant is assigned as the default value.

Row Configuration

Field	Value
Build Mode	Form
Source	Header → X-Tenant-Id
Target	Custom Variable → tenantId
Default Value	default-tenant
Required	No

Result

Incoming Header	tenantId Variable Value
X-Tenant-Id: acme-corp	acme-corp
(header missing)	default-tenant

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Example 2: Copy a Body Field

A userId field from the request body is read in Form mode and stored in a custom variable.

Row Configuration

Field	Value
Build Mode	Form
Source	Request Body → JSON Path $.userId
Target	Custom Variable → currentUserId
Default Value	anonymous
Required	No

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Example 3: Merge Multiple Sources with Template Mode

In this example, a username from the request header, an operation type from the body, and the system date-time are combined to create a single audit entry variable.

Template

#{header.X-User-Name} | #{body.$.operationType} | #{context.system.dateTime}

Row Configuration

Field	Value
Build Mode	Template
Template	Template above
Target	Custom Variable → auditEntry
Default Value	unknown | unknown | -

Result

For a sample request (X-User-Name: john, body {"operationType":"update"}):

john | update | 2026-02-20T10:35:48.000Z

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Example 4: Building a Composite Identifier

Combining two fields from the request body and context information to create a unique transaction ID:

Template

#{body.$.orderId}-#{body.$.customerId}-#{context.message.correlationId}

Result

ORD-001-CUST-789-8260e8b4-29d7-479a-8266-5dbb29c0d0fa

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Best Practice: Use Form mode when copying from a single source. Template mode should be used when merging or transforming values; it avoids unnecessary parsing overhead.

For the full list of variable sources, see the template syntax section on the Message Builder policy page.