Sevro — Open-source low-code backend

Reference for the CEL expression language used in conditions, actions, validation, and filters.

Sevro uses CEL (Common Expression Language) for all logic: method conditions, actions, computed fields, validation rules, authorization policies, and record filters.

CEL is a simple, fast, and safe expression language designed for embedded use. It's not Turing-complete — no loops, no recursion — which makes it predictable and debuggable.

Context variables

Different contexts provide different variables:

Method conditions

Variable	Description
`record`	The current record's fields
`params`	The method's input parameters

record.status == "draft"
record.amount > 0
params.discount <= record.amount

Method actions

Variable	Description
`record`	The current record's fields (before mutation)
`params`	The method's input parameters

"paid"                    // set status to "paid"
params.customer_name      // copy parameter to field
now()                     // set timestamp to current time
record.amount * 0.9       // compute discounted price

Computed fields

Variable	Description
`record`	The current record's fields

record.price * record.quantity    // compute total
record.first_name + " " + record.last_name  // compute full name

Validation rules

Variable	Description
`record`	The current record's fields

record.age >= 0
record.email.contains("@")
record.start_date < record.end_date

Record filters (ListRecords)

Variable	Description
`record`	The record being filtered

record.status == "active"
record.age > 18
record.name.contains("Alice")

Authorization policies

Variable	Description
`identity`	The authenticated identity (`identity.subject`, `identity.roles`)
`action`	The action being performed
`resource`	The target resource
`record`	The record (if applicable)

identity.role == "admin"
record.creator_id == identity.subject
identity.role == "manager" || action != "approve"

Workflow expressions

Variable	Description
`$.input.<field>`	Workflow input
`$nodes.<id>.output.<field>`	Output from a previous node
`$nodes.<id>.error`	Error from a failed node

$nodes.check_inventory.output.available == true
$.input.order_id
$nodes.validate_order.output.total + $nodes.create_shipment.output.shipping_cost

Built-in functions

Function	Returns	Description
`now()`	string	Current timestamp in RFC 3339 format
`uuid()`	string	Generate a random UUID
`len(x)`	int	Length of a string, list, or map
`upper(s)`	string	Convert string to uppercase
`lower(s)`	string	Convert string to lowercase
`contains(s, sub)`	bool	Check if string contains substring
`startsWith(s, prefix)`	bool	Check if string starts with prefix
`endsWith(s, suffix)`	bool	Check if string ends with suffix
`format(fmt, args...)`	string	Format string with arguments

Type system

CEL supports the following types:

Type	Examples
`bool`	`true`, `false`
`int`	`42`, `-7`
`double`	`3.14`, `1e5`
`string`	`"hello"`, `'world'`
`null`	`null`
`list`	`[1, 2, 3]`
`map`	`{"key": "value"}`

Type checking

CEL is statically typed — type errors are caught at compile time. If a field is a string, comparing it to a number is a compile error.

Operators

Operator	Description
`==`, `!=`	Equality
`<`, `>`, `<=`, `>=`	Comparison
`&&`	Logical AND
`\|\|`	Logical OR
`!`	Logical NOT
`+`	Addition / string concatenation
`-`, `*`, `/`, `%`	Arithmetic
`in`	List/map membership

Examples

Conditions

// Only allow marking as paid if status is draft
record.status == "draft"

// Only allow cancel if not already paid
record.status != "paid"

// Only allow order if amount is positive and customer is active
record.amount > 0 && record.customer_active == true

Actions

// Set a field to a constant
"paid"

// Copy a parameter
params.customer_name

// Set timestamp
now()

// Compute a value
record.price * record.quantity

// Conditional value
record.status == "draft" ? "submitted" : record.status

Validation

// Age must be non-negative
record.age >= 0

// Email must contain @
record.email.contains("@")

// End date must be after start date
record.start_date < record.end_date

// Discount can't exceed 50%
record.discount <= 0.5

Filters

// Active users
record.is_active == true

// Created in the last 30 days
record.created_at > "2024-01-15T00:00:00Z"

// Name starts with "A"
record.name.startsWith("A")

// Status is one of several values
record.status in ["draft", "pending", "review"]

Sevro supports registering custom Go functions callable from CEL. This allows extending the expression language with domain-specific logic. Custom functions are registered at the platform level via plugins (WASM modules).

See the plugin documentation for details on registering custom functions.

CEL Expressions