Language Specification


Sysl is a system modelling language designed for modelling distributed web applications. Sysl allows you to specify Application behaviour and Data Models that are shared between your applications. Another related concept is of software Projects where you can document what changes happened in each project or a release. Your complete system can be described as forest of trees, where one tree represents one application or data model. Sysl uses indentation to represent parent-child or has relationships. E.g. an application has endpoints or a table has columns.

To explain these concepts, we will design an application called MobileApp which interacts with another application called Server.

Identifier Names

Identifiers name program entities such as variables and types. An identifier is a sequence of one or more letters, digits, underscores ('_') and dashes ('-').

Reserved words

Identifiers cannot be named any of the following reserved words:

any as bool bytes date
datetime decimal else float float32
float64 if int int32 int64 string


Sysl currently supports whitespace within identifiers however its use is strongly discouraged. As an alternative to using whitespace within identifiers consider adhering to standard naming conventions and use the long name of an application:

GCP "Google Cloud Platform":


An application is an independent entity that provides services via its various endpoints.

Here is how an application is defined in sysl.


MobileApp and Server are user-defined Applications that do not have any endpoints yet. We will design this app as we move along.

Notes about sysl syntax:

  • : and ... have special meaning. : followed by an indent is used to create a parent-child relationship.
    • All lines after : should be indented. The only exception to this rule is when you want to use the shortcut ....
    • The ... (aka shortcut) means that we don't have enough details yet to describe how this endpoint behaves. Sysl allows you to take an iterative approach in documenting the behaviour. You add more as you know more.

Application Names

Application names are subjected to the established rules around Identifier Names. Additionally application names must begin with either a letter or an underscore.

Mobile App:
Login: ...
Mobile App*:
Login: ...

Long Names

Shorter application names are preferred to enable shorter expressions when specifying references to the app. Sysl allows the definition of a LongName using the following syntax

Mobile App "My Awesome Mobile Application":
Endpoint: ...


Application names can also be namespaced using two colons. This allows the formation of a hierarchical structure in Sysl, so that applications can be grouped.

Payments :: CreditCard "Credit Card Payment Service":
Pay: ...

Namespaces can be nested arbitrarily deep.

Payments :: CreditCard :: Validate "Credit Card Validator":
Pay: ...

Multiple Declarations

Sysl allows you to define an application in multiple places. There is no redefinition error in sysl.

Login: ...
Register: ...

The result will be as if it was declared like so:

Login: ...
Register: ...


Endpoints are the services that an application offers. Let's add endpoints to our MobileApp.

Login: ...
Search: ...
Order: ...

Now, our MobileApp has three endpoints: Login, Search and Orders.

Notes about sysl syntax:

  • Again, ... is used to show we don't have enough details yet about each endpoint.
  • All endpoints are indented. Use a tab or spaces to indent.
  • These endpoints can also be REST APIs. See section on Rest below on how to define REST API endpoints.

Each endpoint should have statements that describe its behaviour. Before that let's look at data types and how they can be used in sysl.

You will have various kinds of data passing through your systems. Sysl allows you to express ownership, information classification and other attributes of your data in one place.

Continuing with the previous example, let's define a Server that expects LoginData for the Login Flow.

Login (request <: Server.LoginData): ...
!type LoginData:
username <: string
password <: string

In the above example, we have defined another application called Server that has an endpoint called Login. It also defines a new data type called LoginData that it expects callers to pass in the login call.

Notes about sysl syntax:

  • <: is used to define the arguments to Login endpoint.
  • !type is used to define a new data type LoginData.
    • Note the indent to create fields username and password of type string.
    • See Data Types to see what all the supported data types.
  • Data types (like LoginData) belong to the app under which it is defined.
  • Refer to the newly defined type by its fully qualified name. e.g. Server.LoginData.


Rest is a very common architectural style for defining web services. Here is how you can define a web service:

<describe behaviour using statements>




You can breakdown long URL's to reduce repetition. See below for complete example:

AccountTransactionApi [package="io.sysl.account.api"]:
/accounts [interface="Accounts"]:
BankDatabase <- GetAccount(account_number)
POST (Transaction):
BankDatabase <- WithdrawFunds(account_number)
GET ?start_date=string&end_date=string:
BankDatabase <- QueryTransactions(account_number, start_date, end_date)
!type Account:
account_number <: int?
account_type <: string?
account_status <: string?
account_balance <: int?
!type Transaction:
transaction_id <: int?
transaction_type <: string?
transaction_date_time <: date?
transaction_amount <: int?
from_account_number <: Accounts.account_number
to_account_number <: Accounts.account_number
AccountTransactionApi <- GET /accounts/{account_number}
Return balance
AccountTransactionApi <- POST /accounts/{account_number}/withdraw
Withdraw funds

Here are few things to notice

  • interface attribute allows you to specify the name of the generated interface class. This interface has all the methods of your api.
  • Query parameters start_date and end_date (of type string) that you can pass to GET /accounts/{account_number}/transactions endpoint.
  • ATM <-GetBalance makes a call to AccountTransactionApi <- GET /accounts/{account_number}.

Command to generate code:

Run this in the same directory as api.sysl:

reljam spring-rest-service api AccountTransactionApi

Parameter Types

Different types of parameters can be defined for an endpoint

REST Endpoint Specific

Path Parameters

Path parameters can be defined using curly brackets in the path name.

In the example above

AccountTransactionApi [package="io.sysl.account.api"]:
/accounts [interface="Accounts"]:
BankDatabase <- GetAccount(account_number)
Query Parameters

Query parameters can be defined in the method statement using a ?queryParamName=SomePrimitiveType Multiple query parameters can be separated using the & character When a reference to a type must be made, curly brackets must surround the type reference


GET ?depth=int&limit=int?&offset=int?:
return ok
/second :
GET ?tags={Tags}
return ok
!alias Tags:
sequence of string

Header and Body Parameters

Header and body parameters can be defined using brackets (foo <: int) To create a body parameter, we use the pattern ~body e.g (bodyParam <: int [~body]) To create a header parameter, we use the pattern ~header e.g (bodyParam <: int [~header]) Parameters are separated by commas

GET (filter <: int, offset <: int, tags <: Tags)
GET (foo <: int [~body]) ?depth=int [~bar]:
!alias Tags:
sequence of string


Sysl supports following primitive data types:


For example:

!type Person:
name <: string
age <: int

Sysl is a flexible modelling language that allows for primitive types to be modelled with an appropriate level of detail.

For example, consider the Person type above. If, for the purpose of modelling, it isn't necessary to specify the maximum length the name field can be or the number of bits used to store the age then the above model is sufficient. However, if these additional attributes are important then Sysl provides the ability to constrain certain primitive types.

int32 # int with bit width 32
int64 # int with bit width 64
float32 # float with bit width 32
float64 # float with bit width 64
decimal(p.s) # decimal with precision p and scale s e.g. decimal(5.2)
string(max) # string with maximum length e.g. string(100)
string(min..max) # string with minimum and maximum lengths e.g. string(10..12)

For example, the above Person could be adjusted to provide some additional constraints:

!type Person:
name <: string(128) # maximum length 128
age <: int32 # 32-bit integer values

We can also define our own datatypes using the !type keyword within an application.

!type response:
data <: string
type <: int


Type names follow the same rules as Identifier Names. Additionally type names must begin with either a letter or an underscore.

Special Characters

If special characters such as ':' or '.' are needed in a type or endpoint name, this can be expressed in Sysl by using their URL encoded equivalent instead.

Optional Types

We can define optional parameters and fields in sysl using a postfix ?.

The following example defines an optional token field in the type response

!type response:
data <: string
type <: int
token <: string?


You can define an enumeration using the !enum syntax to give a name to an enumeration type and list the enumerated names and their values. For example:

Login (request <: Server.LoginData):
return Server.Code
!enum Code:
success: 1
invalid: 2
tooManyAttempts: 3
!type LoginData: ...

An enumeration is a type and can be referenced in the same way that other types are referenced else where in a sysl specification.

NOTE: The syntax for enumerations will likely change from name: value to name = value in future. Limitations in the current parser prevent the second form from parsing.

Return response

An endpoint can return a response to the caller. Everything after return keyword till the end-of-line is considered response payload.

You can return:

  • An empty response with code (code could be ok, error or any HTTP status code)
    • e.g return ok
    • e.g return error
    • e.g return 200
  • A named return response with code
    • with a primitive Sysl type
      • e.g return error <: string
      • e.g return ok <: string
      • e.g return 200 <: string
    • with a Sysl type - formal type to return to the caller
      • e.g return ok <: Response
      • e.g return 200 <: OrderData
      • e.g return 200 <: AnotherApp.OrderData
    • with an expression of a Sysl Type
      • e.g return ok <: sequence of string
      • e.g return 200 <: set of SimpleObj
return ok <: string
return ok <: Server.LoginData
return ok <: sequence of string
return ok <: UserPreference
if notfound:
return 404 <: ResourceNotFoundError
else if failed:
return 500 <: ErrorResponse
return 200
!type UserPreference:
Geography <: string
!type ResourceNotFoundError:
msg <: string
!type ErrorResponse:
msg <: string
!type LoginData:
userID <: string


You can attach more metadata to your application specification. This information can be used by sysl plugins to extend the default functionality. The attributes are added inside square brackets Application [...attributes]. Sysl attributes are of two types: Patterns and Key-Value pairs:

  • Patterns

    A pattern is ~ followed by a word that means something to you. E.g. [~tag].

    Application [~rest]:

    In the above example, rest is a pattern.

  • Key-Value pair

    As the name suggests, you can associate some data with your application or an endpoint.

Application [version="1.1"]:

The value can be a string "foo", an array of strings ["foo", "bar"], array of array of strings [["foo"], ["bar"]]

A complete example:

BizApp [version="1.1", clients=["web", "daemon"]]:

Reserved Attributes

Sysl defines some internal attributes that you can use to customize the look of your diagrams.

  • Changing Application icons in Sequence Diagram

    The default icon for the app is a circle with an arrow. You can change this icon to:

    • human - App [~human]:
    • database - DataBase [~db]
    • External App - IdentityProvider [~external] - In an enterprise system, you might have some external third-party system that your app might interact with. Mark an app as ~external and sysl will place that app at extreme right of a sequence diagram.

Complete example that uses the above patterns:

User [~human]:
Check Balance:
MobileApp <- Login
MobileApp <- Check Balance
MobileApp [~ui]:
Server <- Login
Check Balance:
Server <- Read User Balance
do input validation
DB <- Save
return success or failure
Read User Balance:
DB <- Load
return balance
DB [~db]:
Save: ...
Load: ...
Project [seqtitle="Diagram"]:
User <- Check Balance

Here is the result:

Sample sequence diagram show custom icons


Our MobileApp does not have any detail yet on how it behaves. Let's use sysl statements to describe behaviour. Sysl supports following types of statements:


Use simple text to describe behaviour. See below for examples of text statements:

do input validation
"Use special characters like \n to break long text into multiple lines"
'Cannot use special characters in single quoted statements'


A standalone service that does not interact with anybody is not a very useful service. Use the call syntax to show interaction between two services.

In the below example, MobileApp makes a call to backend Server which in turn calls database layer.

Server <- Login
Login(data <: LoginData):
build query
DB <- Query
check result
return Server.LoginResponse
!type LoginData:
username <: string
password <: string
!type LoginResponse:
message <: string
lookup data
return data

Now we have all the ingredients to draw a sequence diagram. Here is one generated by sysl for the above example:

Sequence diagram for a login call

See Generate Diagrams on how to draw sequence and other types of diagrams using sysl.

Control flows


Sysl allows you to express high level of detail about your design. You can specify decisions, processing loops etc.

If, else

You can express an endpoint's critical decisions using IF/ELSE statement:

validate input
IF session exists:
use existing session
create new session
process input

IF and ELSE keywords are case-insensitive. Here is how sysl will render these statements:


For, Loop, Until, While

Express processing loop using FOR:

validate input
For each element in input:
process element

FOR keyword is case insensitive. Here is how sysl will render these statements:


You can use Loop, While, Until, Loop-N as well (all case-insensitive).

Multiple Declarations

Sysl allows you to define an application in multiple places. There is no redefinition error in sysl.

Login: ...
Register: ...

Result will be as-if it was declared like so:

Login: ...
Register: ...


Most of the changes to your system will be done as part of a well defined project or a software release.

TODO: Elaborate


To keep things modular, sysl allows you to import definitions created in other .sysl files.

E.g. server.sysl

Login: ...
Register: ...

and you use import in client.sysl

import server
Server <- Login

Above code assumes, server and client files are in the same directory. If they are in different directories, you must have at least a common root directory and import /path/from/root.

All sysl commands accept --root argument. Run sysl -h or reljam -h for more details.

Internal relative file

You have server.sysl, client.sysl and deps/deps.sysl. server.sysl and client.sysl files are in the same directory. deps.sysl file in the sub-directory. You can import server.sysl and deps.sysl files in client.sysl as below:

# client.sysl
import server
import deps/deps
Server <- Login
Deps <- Dep
# server.sysl
Login: ...
Register: ...
# deps.sysl
Dep: ...

Internal absolute file

If the imported are in the same project but outside of current folder, you must have at least a common root directory and import /path/from/root.

All sysl commands accept --root argument. Run sysl help for more details.

# <root-dir>/servers/server.sysl
Login: ...
Register: ...
# <root-dir>/clients/client.sysl
import /servers/server
Server <- Login

External file

Sysl supports importing sysl files via the web using Sysl Modules, which are based on Go Modules.

The imported sysl repository needs to be initialised with a go.mod file(run go mod init under the repository working directory). There's no need to include go code in the repository. As long as there's a go.mod file, the repository will be treated as a sysl/go module.

Import statements are prefaced with // e.g import //the/external/repo/filepath

# This file is located at
Login: ...
Register: ...
# This file is located at
import //
Server <- Login

To disable Sysl Modules, set the environment variable SYSL_MODULES=off.

Non-sysl file

When you import a sysl file, you can omit the .sysl file extension.

To import a non-sysl file like swagger file, you can import foreign_import_swagger.yaml as ~swagger.

Valid types include

  • ~sysl
  • ~swagger
  • ~openapi3


!type The type keyword is used to define a type. In the following example we define a Post type made up of multiple attributes.

!type Post:
userId <: int
id <: int
title <: string
body <: string


!alias Alias' can be used to simplify a type;

!alias Posts:
sequence of Post


!view Views are sysl's functions; we can use them in the transformation language, see [docs/transformation.html]for more info


!union Unions are a union type;

!alias TypeString:
!alias TypeInt32:
!type TypeUUID:
id <: string
!union UnionType:
!type User:
id <: UnionType

User id can be one of string, int32 or TypeUUID only.


!table Add more here


!wrap Add more here

Data Models

Sysl supports defining two types of data models, one for your database and other for your app. You can refer to these models in your app just like any other app.

Relational Data Model

Relational Data model is the most common way of persisting data in a database. You can define your data model directly in sysl. data.sysl

!table Customer:
customer_id <: int [~pk]
!table CustomerOrder:
order_id <: int [~pk]
customer_id <: Customer.customer_id

In the above example:

  • CustomerOrderModel is a user-defined top-level app that contains definitions of various tables or types in your data model.
  • Customer.customer_id is a primary key.
  • CustomerOrder has a foreign key customer_id which refers to the primary key of Customer (i.e. customer_id).

Object Model

Define a typical in-memory Object model of an application like so:

!type Address:
line_1 <: string
city <: string
!type Customer:
customer_id <: int
addresses <: set of Address
!type CustomerOrder:
order_id <: int
customer <: Customer


  • set of Address - Set is the only collection type

Events, publisher and subscriber

Sysl has support for the publisher-subscriber model. In the example below, UserService has RegisterEvent that is subscribed by EmailNotifier and SmsNotifier applications.

<-> RegisterEvent: ...
do registration
. <- RegisterEvent
UserService -> RegisterEvent:
EmailNotifier got the RegisterEvent
UserService -> RegisterEvent:
SmsNotifier got RegisterEvent


Project files use the Collector syntax to add additional layer of information on top of the definitions. Best example of this is where you want to capture how an API evolved over time.

E.g. server.sysl

do input validation
DB <- Save
return success or failure
Read User Balance:
DB <- Load
return balance

Say, in project-1.sysl you modified the call to DB <- Save, you can capture this information like so:

Server [status="modified"]:
.. * <- *:
Login [status="modified"]
DB <- Save [status="modified"]

.. * <- * is the way to tell sysl to take an existing definition of the application and merge the attributes from:

  • endpoints
  • calls

Then for diagrams related to Project-1, you can show this information in the diagrams which endpoints are getting reused and which ones are new. This requires usage of epfmt, where you can use the value of the variable status like so %(@status). See Attributes for more details.

By creating separate file for each project, you will always be able to recreate necessary documentation at that point in time. This can help you answer questions like When or why was this introduced?.