Rules engine - Lerian

The Tracer rules engine lets you build powerful validation logic without writing code. Using CEL (Common Expression Language)—a type-safe expression language developed by Google—you can create sophisticated rules that evaluate in under 1ms, automate complex business decisions, and adapt quickly to new requirements.

Why use the rules engine

Flexibility: Create and modify rules without code deploys
Performance: Compiled expressions evaluate in under 1ms each
Type safety: Expression syntax validated at rule creation
Complete evaluation: All active rules evaluated for comprehensive audit trail
Scope-based: Apply rules to specific segments, accounts, or transaction types

By the end of this guide, you will:

Understand rule engine concepts and evaluation flow
Create and test expression-based rules
Manage the rule lifecycle (DRAFT, ACTIVE, INACTIVE, DELETED)
Apply best practices for rule management

What is the rules engine

The rules engine is the Tracer component responsible for evaluating expressions during transaction validation. It enables fraud analysts and risk managers to configure business logic that executes in real time—without requiring code deployments or engineering support.

How it works

In this flow:

Load rules fetches all active rules from cache (or database on cache miss)
Evaluate expressions runs all CEL expressions against the transaction context
Collect matches gathers all rules that matched and determines the decision

Evaluation pattern

All active rules are evaluated against every transaction. There is no priority ordering or short-circuit evaluation. This ensures:

Complete audit trail (all matching rules recorded)
No information loss (analysts can see all triggers)
Simple logic (no priority conflicts)

Decision precedence:

If any DENY rule matches, Tracer returns a DENY decision
If only ALLOW rules match, Tracer returns an ALLOW decision
If no rules match, Tracer returns the configured default (typically ALLOW for fail-open behavior)

Tracer returns decisions; it does not block transactions directly. Your system receives the decision and is responsible for taking the appropriate action (e.g., blocking, allowing, or queuing for review).

Core concepts

Before creating rules, understand the foundational elements.

Rules

A rule is a unit of business logic composed of:

Expression - A type-safe expression that evaluates to true or false
Action - What decision to return when the expression is true
Scopes - Which transactions the rule applies to
Status - The rule’s lifecycle state

Expressions

Expressions are written in CEL (Common Expression Language), a type-safe language that evaluates transaction context and returns a boolean value (true or false). CEL provides compile-time validation, so syntax errors are caught when you create the rule—not when transactions are being processed. Example expressions:

amount > 10000

account.segmentId == "high-risk-segment" && amount > 5000

merchant.category == "gambling"

Expressions have access to the full validation request context including:

amount - Transaction amount in cents (smallest currency unit)
transactionType - CARD, WIRE, PIX, CRYPTO
subType - Transaction subtype (e.g., debit, credit, international)
currency - ISO 4217 currency code
transactionTimestamp - When the transaction occurred
account - AccountId, segmentId, portfolioId
segment - SegmentId and segment metadata
portfolio - PortfolioId and portfolio metadata
merchant - MerchantId, category, country, riskLevel (optional)
metadata - Custom fields passed in the request

Expression examples by use case

Here are practical examples organized by business scenario:

Amount-based rules

// Block transactions above a threshold
amount > 10000

// Block high-value international transfers
transactionType == "WIRE" && subType == "international" && amount > 50000

// Review large cryptocurrency transactions
transactionType == "CRYPTO" && amount > 5000

Merchant-based rules

// Block gambling merchants
merchant.category == "7995"

// Block high-risk merchant categories
merchant.category in ["7995", "5967", "5966"]

// Review transactions from new merchant countries
merchant.country != "BR" && amount > 1000

Account-based rules

// Block suspended accounts
account.status == "suspended"

// Review transactions from newly created accounts
metadata.accountAgeDays < 30 && amount > 500

// Block closed accounts
account.status == "closed"

Combined conditions

// High-value transaction from high-risk segment
account.segmentId == "high-risk-segment-uuid" && amount > 5000

// International PIX above threshold
transactionType == "PIX" && subType == "international" && amount > 10000

// Large card transaction to foreign merchant
transactionType == "CARD" && merchant.country != "BR" && amount > 3000

Using metadata

// Block transactions from untrusted devices
metadata.deviceTrust == "untrusted"

// Review first-time purchases above threshold
metadata.isFirstPurchase == true && amount > 1000

// Block transactions outside business hours (using metadata)
metadata.isBusinessHours == false && amount > 5000

// VIP customers bypass certain restrictions
metadata.customerTier == "vip" && amount < 50000

Metadata fields are provided by your integration. Design your payload to include the context your rules need.

Actions

Actions determine the decision when an expression evaluates to true:

Action	Description
`ALLOW`	Allow the transaction
`DENY`	Deny the transaction
`REVIEW`	Route to manual review

Scopes

Scopes define which transactions a rule applies to. A rule only evaluates against transactions that match at least one of its scopes. Scope fields (all optional):

segmentId - Match transactions from a specific segment
portfolioId - Match transactions from a specific portfolio
accountId - Match transactions from a specific account
merchantId - Match transactions to a specific merchant
transactionType - Match specific transaction types (CARD, WIRE, PIX, CRYPTO)
subType - Match specific subtypes (debit, credit, instant, etc.)

If a scope field is not specified, it matches any value for that field.

Rule lifecycle

Rules progress through a defined lifecycle to ensure safe deployment.

States

State	Description
`DRAFT`	Not evaluated; expression can be modified freely
`ACTIVE`	Evaluated during validations; expression is immutable
`INACTIVE`	Not evaluated; preserved for audit trail; can be reactivated
`DELETED`	Permanently removed; does not appear in listings; cannot be recovered

Transitions

Transition	From	To	Description
`activate`	DRAFT, INACTIVE	ACTIVE	Start evaluation (validates expression)
`deactivate`	ACTIVE	INACTIVE	Stop evaluation
`delete`	DRAFT, INACTIVE	DELETED	Permanent removal (cannot delete ACTIVE rules)

Active rules must be deactivated before deletion. This prevents accidental removal of rules that are currently being evaluated.

Create a rule

Create rules using POST /v1/rules. Rules are created in DRAFT status by default. A rule requires:

name: A unique, descriptive name
expression: A CEL expression that evaluates to true or false
action: The decision to return when the expression matches (ALLOW, DENY, or REVIEW)
scopes (optional): Limit which transactions the rule applies to

For complete payload structure and field details, see the API reference.

Activate and deactivate rules

After creating a rule, activate it to start evaluation. Deactivate rules to stop evaluation without deleting them.

Operation	Endpoint	Description
Activate	`POST /v1/rules/{ruleId}/activate`	Start evaluating this rule
Deactivate	`POST /v1/rules/{ruleId}/deactivate`	Stop evaluating (preserves for audit)

Deactivating a rule preserves it for audit purposes. Use delete only when you want to permanently remove a rule.

List and query rules

Query rules for management and auditing using GET /v1/rules.

Query parameters

Parameter	Type	Description
`status`	string	Filter by status (DRAFT, ACTIVE, INACTIVE)
`pageSize`	integer	Items per page (default: 100, max: 1000)
`pageToken`	string	Pagination token

Get a specific rule

Use GET /v1/rules/{ruleId} to retrieve the full rule definition including expression and scopes.

Update a rule

Update rules using PATCH /v1/rules/{ruleId}. Rules can be updated in any status, with one important restriction:

The expression field cannot be updated while a rule is ACTIVE. Deactivate the rule first to modify its expression.

Delete a rule

Delete rules that are no longer needed. Only DRAFT and INACTIVE rules can be deleted. ACTIVE rules must be deactivated first.

DELETE /v1/rules/{ruleId}
X-API-Key: {api_key}

Deletion is permanent. Deleted rules cannot be recovered and do not appear in any listings.

Best practices

Follow these practices for effective, maintainable rules.

Naming

Use descriptive names - The name should clearly state what the rule does
Include context - Mention the scenario or transaction type
Avoid abbreviations - Prefer clarity over brevity

Less clear	More clear
`Rule 1`	`Block night transactions above BRL 5,000`
`Block high`	`Deny high-value weekend transactions`
`PIX rule`	`Review PIX transfers to new recipients`

Expression design

Keep expressions simple - Complex logic is harder to maintain
Use scopes for filtering - Don’t repeat scope conditions in expressions
Test edge cases - Consider boundary values and null fields

Lifecycle management

Start in DRAFT - Test before activating
Deactivate before editing - Never edit active rules
Archive unused rules - Keep audit trail intact
Delete only when certain - Deletion is permanent

Monitoring

Review matched rules - Check which rules are triggering
Monitor DENY rates - High deny rates may indicate overly aggressive rules
Audit regularly - Ensure rules still align with business requirements

Quick reference

Key endpoints, actions, and status information.

Endpoints

Operation	Method	Endpoint
Create rule	POST	`/v1/rules`
List rules	GET	`/v1/rules`
Get rule	GET	`/v1/rules/{id}`
Update rule	PATCH	`/v1/rules/{id}`
Delete rule	DELETE	`/v1/rules/{id}`
Activate rule	POST	`/v1/rules/{id}/activate`
Deactivate rule	POST	`/v1/rules/{id}/deactivate`

Actions

Action	Description
`ALLOW`	Allow the transaction
`DENY`	Deny the transaction
`REVIEW`	Route to manual review

Statuses

Status	Evaluated	Editable	Can delete
`DRAFT`	No	Yes	Yes
`ACTIVE`	Yes	Partial (expression immutable)	No (deactivate first)
`INACTIVE`	No	Yes	Yes
`DELETED`	No	No	N/A

​Why use the rules engine

​What is the rules engine

​How it works

​Evaluation pattern

​Core concepts

​Rules

​Expressions

​Expression examples by use case

​Amount-based rules

​Merchant-based rules

​Account-based rules

​Combined conditions

​Using metadata

​Actions

​Scopes

​Rule lifecycle

​States

​Transitions

​Create a rule

​Activate and deactivate rules

​List and query rules

​Query parameters

​Get a specific rule

​Update a rule

​Delete a rule

​Best practices

​Naming

​Expression design

​Lifecycle management

​Monitoring

​Quick reference

​Endpoints

​Actions

​Statuses

Why use the rules engine

What is the rules engine

How it works

Evaluation pattern

Core concepts

Rules

Expressions

Expression examples by use case

Amount-based rules

Merchant-based rules

Account-based rules

Combined conditions

Using metadata

Actions

Scopes

Rule lifecycle

States

Transitions

Create a rule

Activate and deactivate rules

List and query rules

Query parameters

Get a specific rule

Update a rule

Delete a rule

Best practices

Naming

Expression design

Lifecycle management

Monitoring

Quick reference

Endpoints

Actions

Statuses