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Schema Definition

The schema editor in app.html and generate.html uses a plain text format.

This page explains:

  • how field rules are written
  • how schema constraints are written
  • which operators are supported
  • how constraints affect generated data
  • copy-paste examples you can use directly in the tool

Basic schema format​

A schema is usually written as repeating two-line field definitions:

Column Name
rule definition

Example:

Status
enum("Open","In Progress","Closed")

This creates one output column called Status.

You can also use a compact inline form when you prefer a PICT-style layout:

Status: enum("Open","In Progress","Closed")

For enum-heavy schemas, a shorter name: values form is also supported:

Browser: Chrome,Firefox,Safari
Theme: Light,Dark

Both formats are supported, and you can mix them in the same schema.

Field rule examples​

Literal values​

Use a literal when every generated row should contain the same value.

Build
literal(1.0.0)

This always generates 1.0.0.

Enum values​

Use an enum when the column should be chosen from a fixed set of values.

Priority
enum("High","Medium","Low")

This generates one of High, Medium, or Low.

Regex values​

Use a regex when the value should match a pattern.

Ticket Id
[A-Z]{3}-\d{4}

This generates values such as ABC-1234.

Domain and Faker-style methods​

Use a domain method for realistic generated values.

Customer Name
person.fullName

This generates names such as Alice Smith.

Comments and blank lines​

You can use blank lines to make a schema easier to read.

Lines starting with # are treated as comments.

Example:

# Core workflow fields
Priority
enum("High","Medium","Low")

# User-facing status
Status
enum("Open","Closed")

Schema constraints​

Schema constraints restrict which combinations of values are allowed.

The currently supported constraint form is:

IF predicate THEN predicate

You can terminate a constraint with either:

  • ;
  • ENDIF

Both of these are valid:

IF [Priority] = "High" THEN [Status] = "Open";
IF [Priority] = "High" THEN [Status] = "Open" ENDIF

What constraints do​

Constraints do not assign or mutate values after generation.

Instead, they define which rows are valid.

The generator creates candidate rows, checks them against the constraints, and keeps only rows that satisfy all constraints.

For example:

Priority
enum("High","Low")
Status
enum("Open","Closed")

IF [Priority] = "High" THEN [Status] = "Open";

Meaning:

  • rows with Priority = High are only valid if Status = Open
  • rows with Priority = Low can have either Open or Closed

So High + Closed should never appear in generated output.

Parameter references​

Inside constraints, column names must be written in square brackets:

[Priority]
[Status]
[Ticket Id]

Important:

  • use the exact schema column name
  • bracketed names are required inside constraints
  • spaces are allowed inside the brackets

Example:

IF [Ticket Status] = "Open" THEN [Resolution Code] = "";

Supported operators​

Equality and comparison​

Supported comparison operators are:

  • =
  • <>
  • >
  • >=
  • <
  • <=

Example:

IF [Age] >= 18 THEN [Access Level] = "Adult";

Notes:

  • use <> for "not equal"
  • != is not supported

LIKE​

LIKE checks a wildcard pattern.

Supported wildcard characters are:

  • * for any number of characters
  • ? for a single character

Example:

IF [Code] LIKE "QA-*" THEN [Environment] = "Test";

Meaning:

  • if Code starts with QA-, then Environment must be Test

NOT LIKE​

Example:

IF [Code] NOT LIKE "QA-*" THEN [Environment] = "Production";

Meaning:

  • if Code does not start with QA-, then Environment must be Production

IN​

IN checks whether a value is one of a listed set.

The current schema format uses curly braces:

IF [Priority] IN {"High","Critical"} THEN [Escalated] = "Yes";

Meaning:

  • if Priority is either High or Critical, then Escalated must be Yes

NOT IN​

Example:

IF [Priority] NOT IN {"High","Critical"} THEN [Escalated] = "No";

Meaning:

  • if Priority is neither High nor Critical, then Escalated must be No

Logical operators​

Supported logical operators are:

  • AND
  • OR
  • NOT

You can also use parentheses for grouping.

AND example​

IF [Priority] = "High" AND [Status] = "Open" THEN [Owner] <> "";

Meaning:

  • when both conditions are true, Owner must not be blank

OR example​

IF [Region] = "UK" OR [Region] = "IE" THEN [Currency] = "GBP";

Meaning:

  • if the region is UK or IE, then currency must be GBP

NOT example​

IF NOT ([Status] = "Closed") THEN [Remaining Work] <> "0";

Meaning:

  • if status is not Closed, then remaining work must not be 0

Constraint value types​

Constraints can compare against:

  • quoted string values, e.g. "Open"
  • numeric values, e.g. 18
  • other parameters, e.g. [Min Age] <= [Max Age]

Parameter-to-parameter comparison​

Min Age
number.int({"min": 18, "max": 50})
Max Age
number.int({"min": 18, "max": 65})

IF [Min Age] > [Max Age] THEN [Max Age] >= [Min Age];

Meaning:

  • any row where Min Age is greater than Max Age is only valid if Max Age is at least Min Age
  • in practice, this prevents invalid min/max ordering

Validation rules for constraints​

Constraints are validated against the schema.

Unknown columns are rejected​

This is invalid because [Severity] is not defined in the schema:

Priority
enum("High","Low")

IF [Severity] = "Critical" THEN [Priority] = "High";

Invalid enum values are rejected​

This is invalid because Urgent is not in the enum:

Priority
enum("High","Medium","Low")

IF [Priority] = "Urgent" THEN [Priority] = "High";

Invalid regex values are rejected​

This is invalid because bob does not match the regex:

Ticket Id
[A-Z]{3}-\d{4}

IF [Ticket Id] = "bob" THEN [Status] = "Open";

Impossible literal comparisons are rejected​

This is invalid because the only literal value is Closed:

Status
literal(Closed)

IF [Status] = "Closed" THEN [Status] = "Open";

Generation-time constraint failures​

Some constraints are syntactically valid and can reference valid columns and values, but still make row generation impossible.

These cases are not always discoverable during schema parsing or validation. They may only be found when the generator starts creating rows and repeatedly fails to find a valid combination.

Example:

Status
enum("Open","Closed")

IF [Status] = "Open" THEN [Status] = "Closed";
IF [Status] = "Closed" THEN [Status] = "Open";

Illustrates:

  • a schema that parses correctly
  • constraints that use valid enum values
  • a rule set that makes every possible row invalid

Meaning:

  • if Status is Open, the row is only valid when Status is Closed
  • if Status is Closed, the row is only valid when Status is Open
  • no generated row can satisfy both possibilities

When this happens, the tool reports:

Schema Constraints are impacting row generation - generated X rows, failed to generate Y rows. Consider changing constraints to improve row generation.

Important:

  • this condition is only found and reported during generation
  • care must be taken when writing constraints, especially when combining multiple rules
  • a schema can be structurally valid but still impossible to generate data from

Pairwise and n-wise constraints​

Constraints are supported during pairwise and n-wise generation only when every referenced constrained field is an enum column.

That means:

  • constrained pairwise and n-wise generation works best for enum-only decision tables
  • if a constraint references non-enum fields such as regex, literal, or domain-generated values, normal random generation is supported, but combinatorial generation is not

Copy-paste examples​

Each example below is complete and can be pasted directly into the tool.

Example 1: Basic enum dependency​

Illustrates:

  • enum fields
  • simple equality constraint
Priority
enum("High","Medium","Low")
Status
enum("Open","Queued","Closed")

IF [Priority] = "High" THEN [Status] = "Open";

Meaning:

  • High priority rows must use Open

Example 2: Multiple allowed trigger values with IN​

Illustrates:

  • IN
  • set-based trigger conditions
Priority
enum("Critical","High","Medium","Low")
Escalated
enum("Yes","No")

IF [Priority] IN {"Critical","High"} THEN [Escalated] = "Yes";

Meaning:

  • Critical and High rows must be escalated

Example 3: NOT IN inverse rule​

Illustrates:

  • NOT IN
  • inverse constraint logic
Priority
enum("Critical","High","Medium","Low")
Escalated
enum("Yes","No")

IF [Priority] NOT IN {"Critical","High"} THEN [Escalated] = "No";

Meaning:

  • Medium and Low rows must not be escalated

Example 4: Regex-triggered routing​

Illustrates:

  • regex field definition
  • LIKE
Ticket Id
[A-Z]{3}-\d{4}
Queue
enum("Support","QA","Ops")

IF [Ticket Id] LIKE "QA-*" THEN [Queue] = "QA";

Meaning:

  • values starting with QA- must use the QA queue

Example 5: Combined conditions with AND​

Illustrates:

  • AND
  • multiple field dependency
Priority
enum("High","Low")
Status
enum("Open","Closed")
Owner
enum("Alice","Bob","")

IF [Priority] = "High" AND [Status] = "Open" THEN [Owner] <> "";

Meaning:

  • a high-priority open item must have an owner

Example 6: Grouped logic with OR and parentheses​

Illustrates:

  • OR
  • parentheses
Region
enum("UK","IE","US","CA")
Currency
enum("GBP","USD","CAD")

IF ([Region] = "UK" OR [Region] = "IE") THEN [Currency] = "GBP";

Meaning:

  • both UK and IE rows must use GBP

Example 7: Numeric comparison​

Illustrates:

  • numeric comparison
  • >=
Age
number.int({"min": 16, "max": 21})
Access
enum("Restricted","Adult")

IF [Age] >= 18 THEN [Access] = "Adult";

Meaning:

  • anyone aged 18 or older must have Adult access

Example 8: Using ENDIF instead of semicolon​

Illustrates:

  • ENDIF terminator
Priority
enum("High","Low")
Status
enum("Open","Closed")

IF [Priority] = "High" THEN [Status] = "Open" ENDIF

Meaning:

  • same as the semicolon form
  • the choice of terminator is author preference

Summary​

Use schema constraints when generated rows need business rules, dependencies, or realistic combinations.

The most important points are:

  • write schema fields as column/rule pairs
  • reference columns in constraints with [Bracketed Names]
  • use IF ... THEN ...
  • terminate constraints with ; or ENDIF
  • use <> rather than !=
  • use constraints to filter invalid rows, not to mutate rows after generation