Target flags with CEL

Common Expression Language (CEL) provides the logic for sophisticated, rule-based flag evaluations. It lets you create precise targeting rules by combining multiple user or request attributes using logical operators.

This guide shows you how to define flags with CEL rules and update evaluation specs.

Prerequisites

Before you begin, ensure you have:

1. Completed the Deploy feature flags quickstart .
2. A gcloud environment configured to manage App Lifecycle Manager resources.

Logical operators and syntax

CEL supports standard logical operators to build complex conditions:

• && logical AND
• || logical OR
• ! logical NOT
• in check if a value exists in a list

Define attributes

CEL attributes are specific data points, such as source.ip , request headers, or paths, used within boolean expressions to define security or routing policies.

Any attribute used in a CEL condition must be explicitly defined and listed in the attributes array of the EvaluationSpec . If an attribute is used in a condition but missing from the list, the API will reject the request.

Create an attribute

You can use App Lifecycle Manager to create CEL attributes.

To create a plan attribute, run the command:

 gcloud  
beta  
app-lifecycle-manager  
flags  
attributes  
create  
 "plan-attr" 
  
 \ 
  
--key = 
 "plan" 
  
 \ 
  
--attribute-value-type = 
 "STRING" 
  
 \ 
  
--location = 
global 

To create a country attribute, run the command:

 gcloud  
beta  
app-lifecycle-manager  
flags  
attributes  
create  
 "country-attr" 
  
 \ 
  
--key = 
 "country" 
  
 \ 
  
--attribute-value-type = 
 "STRING" 
  
 \ 
  
--location = 
global 

To create a platform attribute, run the command:

 gcloud  
beta  
app-lifecycle-manager  
flags  
attributes  
create  
 "platform-attr" 
  
 \ 
  
--key = 
 "platform" 
  
 \ 
  
--attribute-value-type = 
 "STRING" 
  
 \ 
  
--location = 
global 

Configure a flag with CEL rules

When creating or updating a flag, define the evaluation logic in the evaluation-spec .

For example, this snippet enables a feature for premium customers in Germany using their mobile app.

 gcloud  
beta  
app-lifecycle-manager  
flags  
create  
 "advanced-reporting-flag" 
  
 \ 
  
--key = 
 "advanced-reporting-flag" 
  
 \ 
  
--flag-value-type = 
BOOL  
 \ 
  
--unit-kind = 
 "UNIT_KIND_NAME" 
  
 \ 
  
--location = 
global  
 \ 
  
--evaluation-spec = 
 '{ 
 "rules": [{ 
 "id": "premium_mobile_users_in_germany", 
 "condition": "plan == \"premium\" && country == \"DE\" && platform == \"mobile\"", 
 "target": "enabled" 
 }], 
 "defaultTarget": "disabled", 
 "attributes": [ 
 "projects/PROJECT_ID/locations/global/flagAttributes/plan-attr", 
 "projects/PROJECT_ID/locations/global/flagAttributes/country-attr", 
 "projects/PROJECT_ID/locations/global/flagAttributes/platform-attr" 
 ] 
 }' 
 

Replace PROJECT_ID , and UNIT_KIND_NAME with your values.

Application integration

Your application must inject the attributes into the evaluation context at runtime.

Python example:

  from 
  
 openfeature.evaluation_context 
  
 import 
 EvaluationContext 
 eval_ctx 
 = 
 EvaluationContext 
 ( 
 targeting_key 
 = 
 user_id 
 , 
 attributes 
 = 
 { 
 "plan" 
 : 
 "premium" 
 , 
 "country" 
 : 
 "DE" 
 , 
 "platform" 
 : 
 "mobile" 
 } 
 ) 
 is_enabled 
 = 
 client 
 . 
 get_boolean_value 
 ( 
 "advanced-reporting-flag" 
 , 
 False 
 , 
 eval_ctx 
 ) 
 

Best practices

• Order of Evaluation:Rules are evaluated sequentially. The first rule that evaluates to true determines the flag's value.
• Safe Defaults:If a user context is missing a required attribute for a condition, the evaluation engine will skip that specific rule.
• Character Limits:CEL expressions can be up to 4,096 bytes (4kB) in size.

What's next

• Learn about Multi-Tenant Architecture .
• Learn about Experimentation and A/B Testing .