Core Schema Notation (CSN)

CSN (pronounced as "Season") is a notation for compact representations of CDS models — tailored to serve as an optimized format to share and interpret models with minimal footprint and dependencies.

It's similar to JSON Schema but goes beyond JSON's abilities, in order to capture full-blown Entity-Relationship Models and Extensions. This makes CSN models a perfect source to generate target models, such as OData/EDM or OpenAPI interfaces, as well as persistence models for SQL or NoSQL databases.

Anatomy

A CSN model in JSON:

json

{
  "requires": [ "@sap/cds/common", "./db/schema" ],
  "definitions": {
    "some.type": { "type": "cds.String", "length": 11 },
    "another.type": { "type": "some.type" },
    "structured.type": { "elements": {
      "foo": { "type": "cds.Integer" },
      "bar": { "type": "cds.String" }
    }}
  },
  "extensions": [
    { "extend":"Foo", "elements":{
      "bar": { "type": "cds.String" }
    }}
  ]
}

The same model in YAML:

yaml

requires:
  - @sap/cds/common
  - ./db/schema
definitions:
  some.type: {type: cds.String, length: 11}
  another.type: {type: some.type }
  structured.type:
    elements:
      foo: {type: cds.Integer}
      bar: {type: cds.String}
extensions: [
  - extend: Foo
    elements:
      bar: {type: cds.String}
]

The same model as a plain JavaScript object:

({
  requires:[ '@sap/cds/common', './db/schema' ],
  definitions: {
    'some.type': { type:"cds.String", length:11 },
    'another.type': { type:"some.type" },
    'structured.type': { elements: {
      'foo': { type:"cds.Integer" },
      'bar': { type:"cds.String" }
    }}
  },
  extensions: [
    { extend:'Foo', elements:{
      'bar': { type:"cds.String" }
    }
  ],
})

For the remainder of this spec, you see examples in plain JavaScript representation with the following conventions:

({property:...})   // a CSN-specified property name
({'name':...})     // a definition's declared name
"value"            // a string value, including referred names
11, true           // number and boolean literal values

Properties

requires – an array listing imported models
definitions – a dictionary of named definitions
extensions – an array of unnamed aspects
i18n – a dictionary of dictionaries of text translations

All properties are optional

For example, one model could contain a few definitions, while another one only contains some extensions.

References are case-sensitive

All references in properties like type or target use exactly the same notation regarding casing as their targets' names. To avoid problems when translating models to case-insensitive environments like SQL databases, avoid case-significant names and references. For example, avoid two different definitions in the same scope whose names only differ in casing, such as foo and Foo.

Literals

There are several places where literals can show up in models, such as in SQL expressions, calculated fields, or annotations.

Standard literals are represented as in JSON:

Kind	Example
Globals	`true`, `false`, `null`
Numbers¹	`11` or `2.4`
Strings	`"foo"`
Dates²	`"2016-11-24"`
Times²	`"16:11Z"`
DateTimes²	`"2016-11-24T16:11Z"`
Records	`{"foo":<literal>, ...}`
Arrays	`[<literal>, ...]`

In addition, CSN specifies these special forms for references, expressions, and enum symbols:

Kind	Example
Unparsed Expressions	`{"=":"foo.bar < 9"}`
Enum symbols³	`{"#":"asc"}`

Remarks

¹ This is as in JSON and shares the same issues when decimals are mapped to doubles with potential rounding errors. The same applies to Integer64. Use strings to avoid that, if applicable.
² Also, as in JSON, dates, and times are represented just as strings as specified in ISO 8601; consumers are assumed to know the types and handle the values correctly.
³ As enum symbols are equal to their values, it frequently suffices to just provide them as strings. Similar to time and dates in CSN and JSON, the consumers are assumed to know the types and handle the values correctly. The {"#":...} syntax option is to serve cases where you have to distinguish the kind only based on the provided value, for example, in untyped annotations.

Definitions

Each entry in the definitions dictionary is essentially a type definition. The name is the absolute, fully qualified name of the definition, and the value is a record with the definition details.

Example

({definitions:{
  'Name':     {type:"cds.String"},
  'Currency': {type:"cds.String", length:3},
  'USD':      {type:"Currency"},
  'Amount':   {elements:{
    'value':    {type:"cds.Decimal", precision:11, scale:3},
    'currency': {type:"Currency"},
  }},
  'SortOrder':{enum:{ 'asc':{}, 'desc':{} }}
}})

The name of a definition is its key in the enclosing dictionary, like in definitions for top-level entries or in elements for structured types and entities.

Names must:

Be nonempty strings.
Neither start, nor end with . or ::.
Not contain substrings .. or :::.
Not contain the substring :: more than once.

Properties

kind – one of context, service, entity, type, action, function, or annotation
type – an optional base type that this definition is derived from
elements – optional dictionary of elements in case of structured types

Property kind is always omitted for elements and can be omitted for top-level type definitions. These examples are semantically equivalent:

Foo1 = { type:"cds.String" }
Foo2 = { type:"cds.String", kind:"type" }

Type Definitions

Custom-defined types are entries in definitions with an optional property kind="type" and the following properties.

Property	Used for
`type`	Scalar Types, Structured Types, and Associations
`elements`	Structured Types
`items`	Arrayed Types
`enum`	Enumeration Types

Example

({definitions: {
  'scalar.type':  {type:"cds.String", length:3 },
  'struct.type':  {elements:{'foo': {type:"cds.Integer"}}},
  'arrayed.type': {items:{type:"cds.Integer"}},
  'enum.type':    {enum:{ 'asc':{}, 'desc':{} }}
}})

Properties

kind – omitted or "type"
type – the base type, this definition is derived from
elements – optional element definitions for structured types.
items – optional definition of item types for arrayed types.
enum – an optional dictionary of enum members for enumeration types.
value – a constant literal value or calculation expression
default – a default value or expression
localized = true if this type was declared like foo : localized String
... – other type-specific properties, for example, a String's length

Scalar Types

Scalar types always have property type specified, plus optional type-specific parameter properties.

({definitions:{
  'scalar.type': {type:"cds.String", length:3 },
}})

See the CDL reference docs for an overview of CDS' built-in types.

While in CDS sources you can refer to these types without prefix, they always have to be specified with their fully qualified names in CSN, for example:

({definitions: {
  'Foo': { type:"cds.Integer" },
  'Bar': { type:"cds.Decimal", precision:11, scale:3 },
}})

Structured Types

Structured types are signified by the presence of an elements property. The value of elements is a dictionary of elements. The name is the local name of the element and the values in turn are Type Definitions.

The optional property includes contains a list of fully qualified entity-, aspect-, or type-names. Elements, actions, and annotations from those definitions are then copied into the structured type.

({definitions:{
  'structured.type': {elements:{
    'foo': {type:"cds.Integer"},
    'bar': {type:"cds.String"}
  }}
}})

Arrayed Types

Arrayed types are signified by the presence of a property items. The value of which is in turn a type definition that specifies the arrayed items' type.

({definitions:{
  'arrayed.type': {items:{type:"cds.Integer"}}
}})

Enumeration Types

The enum property is a dictionary of enum member elements with the name being the enum symbol and the value being a CQN literal value expression. The literal expression optionally specifies a constant val as a literal plus optional annotations. An enumeration type can specify an explicit type (for example, Decimal) but can also omit it and refer from given enumeration values, or String as default.

({definitions:{
  'Gender': {enum:{
    'male':{},
    'female':{},
    'non_binary': {
          val: 'non-binary'
        }
  }},
  'Status': {enum:{
    'submitted': {val:1},
    'fulfilled': {val:2}
  }},
  'Rating': {type:"cds.Decimal", enum:{
    'low':    {val:0},
    'medium': {val:50},
    'high':   {val:100}
  }}
}})

Entity Definitions

Entities are structured types with kind ='entity'. In addition, one or more elements usually have property key set to true, to flag the entity's primary key.

Example

({definitions:{
  'Products': {kind:"entity", elements:{
    'ID':     {type:"cds.Integer", key:true},
    'title':  {type:"cds.String", notNull:true},
    'price':  {type:"Amount", virtual:true},
  }}
}})

Properties

kind – is always "entity"
elements – as in Structured Types, optionally equipped with one or more of these boolean properties:
- key – signifies that the element is (part of) the primary key
- virtual – has this element ignored in generic persistence mapping
- notNull – the not null constraint as in SQL
includes – as in Structured Types

View Definitions

Views are entities defined as projections on underlying entities. In CSN, views are signified by the presence of property query, which captures the projection as a CQN expression.

Example

({definitions:{
  'Foo': { kind:"entity", query: {
    SELECT:{
      from: {ref:['Bar']},
      columns: [ {ref:['title']}, {ref:['price']} ]
    }
  }}
}})

Properties

kind – mandatory; always "entity"
query – the parsed query in CQN format
elements – optional elements signature, omitted and inferred
params – optional parameters

Views with Declared Signatures

Views with declared signatures have the additional property elements filled in as in entities:

({definitions:{
  'with.declared.signature': {kind:"entity",
    elements: {
      'title': {type:"cds.String"},
      'price': {type:"Amount"}
    },
    query: { SELECT:{...} },
  }
}})

Views with Parameters

Views with parameters have an additional property params – an optional dictionary of parameter type definitions:

({definitions:{
  'with.params': {kind:"entity",
    params: { 'ID': { type: 'cds.Integer' } },
    query: { SELECT:{...} },
  }
}})

Projections

Use the projection property for views if you don't need the full power of SQL. See as projection on in CDL for restrictions.

({ definitions: {
  'Foo': { kind: "entity",
    projection: {
      from: { ref: ['Bar'] },
      columns: [ '*' ]
    }
  }
}})

Properties

kind – mandatory; always "entity"
projection – the parsed query; equivalent to query.SELECT, see CQN
elements – optional elements signature, omitted and inferred

Associations

Associations are like scalar type definitions with type being cds.Association or cds.Composition plus additional properties specifying the association's target and optional information like on conditions or foreign keys.

Basic to-one Associations

The basic form of associations are to-one associations to a designated target:

({definitions:{
  'Books': { kind:"entity", elements:{
    'author': { type:"cds.Association", target:"Authors" },
  }},
  //> an association type-def
  'Currency': { type:"cds.Association", target:"Currencies" },
}})

With Specified `cardinality`

Add property cardinality to explicitly specify a to-one or to-many relationship:

({definitions:{
  'Authors': { kind:"entity", elements:{
    'books': { type:"cds.Association", target:"Books", cardinality:{max:"*"} },
  }},
}})

Property cardinality is an object {src?,min?,max} with...

src set to 1 give a hint to database optimizers, that a source entity always exists
min specifying the target's minimum cardinality – default: 0
max specifying the target's maximum cardinality – default: 1

In summary, the default cardinality is [0..1], which means to-one.

With Specified `on` Condition

So-called unmanaged associations have an explicitly specified on condition:

({definitions:{
  'Authors': { kind:"entity", elements:{
    'books': { type:"cds.Association", target:"Books", cardinality{max:"*"},
      on: [{ref:['books', 'author']}, '=', {ref:['$self']}]
    },
  }}
 }})

With Specified `keys`

Managed to-one associations automatically use the target's designated primary key elements. You can overrule this by explicitly specifying alternative target properties to be used in the keys property:

({definitions:{
  'Books': {kind:"entity", elements:{
    'genre': {type:"cds.Association", target:"Genres", keys:[
      {ref:["category"], as:"cat"},
      {ref:["name"]},
    ]},
  }},
}})

Property keys has the format and mechanisms of CQN projections.

Annotations

Annotations are represented as properties, prefixed with @. This format applies to type/entity-level annotations as well as to element-level ones.

Example

({definitions:{
  'Employees': {kind:"entity",
    '@title':"Mitarbeiter",
    '@readonly':true,
    elements:{
      'firstname': {type:"cds.String", '@title':"Vorname"},
      'surname':   {type:"cds.String", '@title':"Nachname"},
    }
  },
}})

Annotations are used to add custom information to definitions, the prefixed @ acts as a protection against conflicts with built-in/standard properties. They're flat lists of key-value pairs, with keys being fully qualified property names and values being represented as introduced in the section Literals and Expressions.

Aspects

In parsed-only models, the top-level property extensions holds an array of unapplied extensions or annotations (→ see also Aspects in CDL). The entries are of this form:

ext = { extend|annotate: <name>, <property>: <value>, … }

with:

extend or annotate referring to the definition to be extended or annotated
<property> being the property that should be extended, for example, elements if an entity should be extended with further elements

Extend with <named aspect>

The most basic form allows to express an extension of a named definition with another named definition (→ see Named Aspects):

csn = { extensions:[
  { extend:"TargetDefinition", includes:["NamedAspect"]}
]}

Extend with <anonymous aspect>

The form { extend:<target>, <property>: <value>, … } allows to add elements to an existing struct definition as well as to add or override annotations of the target definition:

csn = { extensions:[

  // extend Foo with @foo { ..., bar: String; }
  {
    extend: "Foo",
    '@foo': true,
    elements: {
      // adds a new element 'bar'
      bar: { type: "cds.String", '@bar': true },
    }
  },

]}

annotate with <anonymous aspect>

The form { annotate:<target>, <property>: <value>, … } allows to add or override annotations of the target definition as well as those of nested elements:

csn = {extensions:[

  // annotate Foo with @foo;
  { annotate:"Foo", '@foo':true },

  // annotate Foo with @foo { boo @boo }
  { annotate:"Foo", '@foo':true, elements: {
    // annotates existing element 'boo'
    boo: {'@boo':true },
  }},

]}

Services

Services are definitions with kind ='service':

({definitions:{
  'MyOrders': {kind:"service"}
}})

Actions / Functions

Entity definitions (for bound actions/functions) can have an additional property actions. The keys of these actions are the (local) names of actions/functions. Unbound actions/functions of a service are represented as top level definitions.

Example:

({definitions:{
  'OrderService': {kind:"service"},
  'OrderService.Orders': {kind:"entity", elements:{...}, actions:{
    'validate': {kind:"function",
      returns: {type: "cds.Boolean"}
    }
  }},
  'OrderService.cancelOrder': {kind:"action",
      params:{
        'orderID': {type:"cds.Integer"},
        'reason':  {type:"cds.String"},
      },
      returns: {elements:{
        'ack': {enum:{ 'succeeded':{}, 'failed':{} }},
        'msg': {type:"cds.String"},
      }}
    }
  }}
}})

Properties

kind – either "action" or "function" as in OData
params – a dictionary with the values being Type Definitions
returns – a Type Definition describing the response

Note: The definition of the response can be a reference to a declared type or the inline definition of a new (structured) type.

Imports

The requires property lists other models to import definitions from. It is the CSN equivalent of the CDL using directive.

Example

({
  requires: [ '@sap/cds/common', './db/schema' ],
  // [...]
})

As in Node.js the filenames are either absolute module names or relative filenames, starting with ./ or ../.

i18n

A CSN may optionally contain a top-level i18n property, which can contain translated texts. The expected structure is as follows:

({
  i18n: {
    'language-key': {
      'text-key': "some string"
    }
  }
})

This data must be written and handled by the application, there's no out-of-the-box support for this by CAP.

Rules Reference

Parts of 2024

All of 2023

All of 2022

All of 2021

All of 2020

Core Schema Notation (CSN) ​

Anatomy ​

Properties ​

Literals ​

Remarks ​

Definitions ​

Example ​

Properties ​

Type Definitions ​

Example ​

Properties ​

Scalar Types ​

Structured Types ​

Arrayed Types ​

Enumeration Types ​

Entity Definitions ​

Example ​

Properties ​

View Definitions ​

Example ​

Properties ​

Views with Declared Signatures ​

Views with Parameters ​

Projections ​

Properties ​

Associations ​

Basic to-one Associations ​

With Specified cardinality ​

With Specified on Condition ​

With Specified keys ​

Annotations ​

Example ​

Aspects ​

Extend with <named aspect> ​

Extend with <anonymous aspect> ​

annotate with <anonymous aspect> ​

Services ​

Actions / Functions ​

Properties ​

Imports ​

Example ​

i18n ​

Core Schema Notation (CSN)

Anatomy

Properties

Literals

Remarks

Definitions

Example

Properties

Type Definitions

Example

Properties

Scalar Types

Structured Types

Arrayed Types

Enumeration Types

Entity Definitions

Example

Properties

View Definitions

Example

Properties

Views with Declared Signatures

Views with Parameters

Projections

Properties

Associations

Basic to-one Associations

With Specified `cardinality`

With Specified `on` Condition

With Specified `keys`

Annotations

Example

Aspects

Extend with <named aspect>

Extend with <anonymous aspect>

annotate with <anonymous aspect>

Services

Actions / Functions

Properties

Imports

Example

i18n