Data models

Models are optional. Every service works without any input or output declarations at all - self.request.input always holds the incoming message and self.response.payload builds responses through plain dot access, as the message building examples show. Input can also be declared as a plain tuple of field names, and declaring input and output shows both forms side by side. Declare models when you want the shape of your API pinned down, validated and documented.

• Data models for your API services are created using Python's built-in dataclasses
• Models are based on the results of analysis in your business domain and you define the models first, before your services use them
• Models allow for static typing and code completion in your IDE. If anything changes in models that your services use, you will get immediate messages in your IDE or build tools to warn you that you may be, for instance, accessing an attribute of a model that no longer exists - in this way, you can prevent runtime errors well in advance, already during development or testing.
• Models can be hot-deployed to Zato servers without any restart
• Models can be defined in-line, directly in the same Python module that your services are in or they can be defined in a separate file (e.g. model.py)
• In runtime, Zato auto-serializes models to and from JSON, Python dicts and OpenAPI
• Models are the basis for OpenAPI specification generation - Zato extracts schema information from your models to build complete API documentation
• Models are not tied to REST - you can use them with any services or systems. For instance, you can have an ElasticSearch, MongoDB or any other service and all of them can use models and OpenAPI
• Models can be generated from external tools, e.g. their canonical form can be expressed in UML and an external tool can build them in Python when needed

Usage example

In the code example below, we assume that we need to represent the following business needs:

• A client has many phones
• Each phone has a few attributes, such as its current card number (IMEI) or current owner
• We would like to have an API service that tells us what phones a given client has

Graphically, it can be expressed as such:

In code, this will be the result as below, where each request, response and model object is a dataclass subclassing a class called Model.

# -*- coding: utf-8 -*-

# stdlib
from dataclasses import dataclass

# Zato
from zato.common.typing_ import list_
from zato.server.service import Model, Service

# ###########################################################################

@dataclass(init=False)
class Phone(Model):
    imei:       str
    owner_id:   int
    owner_name: str

# ###########################################################################

@dataclass(init=False)
class GetPhoneListRequest(Model):
    client_id: int

@dataclass(init=False)
class GetPhoneListResponse(Model):
    phone_list:    list_[Phone]
    response_type: str

# ###########################################################################

class GetPhoneDetails(Service):

    # Use API models
    input  = GetPhoneListRequest
    output = GetPhoneListResponse

    def handle(self):

        # Enable type checking and type completion
        request = self.request.input # type: GetPhoneListRequest

        # Log details of our request
        self.logger.info('Processing client `%s`', request.client_id)

        # Build our response now - in a full service this information
        # would be read from an external system or database.

        # Our list of phones to return
        phone_list = []

        # Build the fist phone ..
        phone1 = Phone()
        phone1.imei = '123'
        phone1.owner_id = 456
        phone1.owner_name = 'John Doe'

        # .. the second one ..
        phone2 = Phone()
        phone2.imei = '789'
        phone2.owner_id = 999
        phone2.owner_name = 'Jane Doe'

        # .. populate the container for phones that we return ..
        phone_list.append(phone1)
        phone_list.append(phone2)

        # .. build the top-level response element ..
        response = GetPhoneListResponse()
        response.response_type = 'RZH'
        response.phone_list = phone_list

        # .. and return the response to our caller
        self.response.payload = response

# ###########################################################################

This service is ready to be invoked with:

$ curl http://pubapi:<password>@localhost:17010/zato/api/invoke/phone.get-phone-list \
    -d '{"client_id":789}'
{"response_type":"RZH",
 "phone_list": [
    {"imei":"123","owner_id":456,"owner_name":"John Doe"},
    {"imei":"789","owner_id":999,"owner_name":"Jane Doe"}]}
$

Dot access on the payload

With a model declared as output, there is no need to create the response instance yourself - self.response.payload already is an instance of that model and its fields can be assigned through dot access directly:

class GetPhoneDetails(Service):

    # Use API models
    input  = GetPhoneListRequest
    output = GetPhoneListResponse

    def handle(self):
        self.response.payload.response_type = 'RZH'
        self.response.payload.phone_list = phone_list

Assigning to a field that is not part of the model raises an error at that very line, so typos never make it to the wire.

Note the difference in depth - a string declaration such as output = 'customer' leaves the shape below the declared name open, whereas a model pins the whole shape down, field by field.

Unknown fields are rejected

This applies to every model instance, not only to the response payload. A model accepts its declared fields and nothing else - assigning any other name raises an error at the very line that assigns it, listing the fields the model does have:

phone = Phone()
phone.imei = '123'
phone.imie = '456'   # AttributeError - no such field `imie` in `Phone`

Advanced - attributes outside the model

In specific cases an object needs to travel with a model instance through your own code, without being part of the model itself. This is an advanced technique - store the attribute with object.__setattr__ and the field check does not apply:

object.__setattr__(phone, 'source_row', row)

Serialization reads declared fields only, so such attributes never appear in to_dict, to_json or responses. Names starting with an underscore can be assigned directly, with no need for object.__setattr__.

What data types can be used

• When working with models, use simple types - strings, integers, floats, lists or other dataclasses
• Do not use advanced or meta-programming techniques like generics or union fields in dataclasses
• Simply put, use only this kind of data types that can be expressed as JSON, and that will let you express 100% of any business needs just like regular JSON does

How to use optional fields

To indicate that a particular field in a model is optional, use the code as below:

# stdlib
from dataclasses import dataclass

# Zato
from zato.common.typing_ import optional # Note the underscore in typing_
from zato.server.service import Model

@dataclass(init=False)
class Customer(Model):

    # This string field is optional, its default value is None
    email: optional[str] = None

Optional fields in responses

Marking a response field as optional means that you are not required to populate it in your responses with any value, in which case the default one will be used.

The field as such will be always returned. Its value will be that which you assign to it or the default one if you do not give it any value.

For instance, note that the dict representation of the Customer object below uses the default value of None.

response = Customer()
serialized = response.to_dict() # {'email': None}

Optional fields in requests

If the optional field is part of a request, the default value will be used only if the field is missing on input.

For instance, assume that your service expects a Customer object on input and the data sent is {} (an empty dict). In this case, your service will receive {'email': None} on input because the default value of None will be used for the "email" field.

Again, if the "email" field had existed on input, no matter what its value was, the default value would not have been used.

How to validate or post-process data

• Your model can post-process the data it receives after Zato already de-serialized a request or input to a dataclass - this can be used for validation of data received or for additional post-processing, e.g. you can have an email field declared as a string and your model can further confirm that not only is it a string but it matches the expected email format or domain as well

• The post-processing is done in a method called after_created, declared as below:

  from zato.server.service import Model, ModelCtx
  
  @dataclass(init=False)
  class GetBillingListRequest(Model):
      client_id: int
      client_name: str
  
      def after_created(self, ctx:'ModelCtx') -> 'None':
          ...
  

The ctx object has three attributes of interest:

• ctx.data - a dictionary of data that has been just processed and can be post-processed
• ctx.service - the service that is being invoked
• ctx.DataClass - the dataclass that ctx.data represents

Note that the method is invoked for each JSON or dict received, not for each individual field. It means that, if the method is invoked, it can assume that the common validation and processing tasks run by Zato have completed successfully, e.g. that all the non-optional fields are sure to exist at this point.

For instance, if you send this information in a request ..

{"client_id": 123, "client_name":"John Done"}

.. the ctx.data dict will be {'client_id': 123, 'client_name':'John Done'} rather than 'client_id' and 'client_name' individually.

The ctx.service is the same service whose handle method is being invoked. Because it is a service, it means that it has access to everything that a service can do. For instance, for each dataclass received you can perform input validation using an external REST call. Or you can apply your own data masking, e.g. replacing credit card numbers with *** signs. There are no limits to what a service can do.

The after_created method should modify ctx.data in place, without returning anything when it is called.

Where to keep models

• Models can be stored in the same Python file that your services are in
• If needed, they can also be moved to another file, e.g. model.py and imported just like elsewhere in Python, e.g. from model import Client
• After you hot-deploy a model, each service that uses it will auto-redeploy as well to make sure that it uses the latest version of the model, as can be observed in the server.log file

  INFO -  Deployed 3 models from `/path/to/model.py` ->
    ['model.GetPhoneListRequest', 'model.GetPhoneListResponse', 'model.Phone']
  INFO - Deployed 1 service from `/path/to/phone.py` -> ['phone.get-phone-list']
  

• However, when deploying multiple Python files in one group (e.g. from Dashboard), make sure to deploy your models first to let services find them afterwards

Dict and JSON serialization

Each model has two convenience methods: .to_dict() and .to_json() that will serialize that model to a dict or JSON, respectively.

Thanks to these methods, models can be used to express complex dictionaries using statically typed dataclasses that may be easier to author and maintain.

For instance, when invoking ElasticSearch, you may be required to build a complex dict to express various query filters. Instead of that, create your own models that represent ElasticSearch filters and serialize them using .to_dict().

There are also many situations where a JSON string is needed - build your data using models and call .to_json() when the time comes for serialization.

Invoking other services

Models can be used when you invoke one Zato service from another, e.g.:

# -*- coding: utf-8 -*-

# stdlib
from dataclasses import dataclass

# Zato
from zato.server.service import Model, Service

# ###########################################################################

@dataclass(init=False)
class GetClientRequest(Model):
    client_id: int

@dataclass(init=False)
class GetClientResponse(Model):
    client_name: str

# ###########################################################################

class APIService(Service):
  def handle(self):

      # The service that will be invoking
      name = 'my.api.get-client'

      # Build our request ..
      request = GetClientRequest()
      request.client_id = 123

      # .. and get client details - note that we indicate in a type hint
      # .. what the actual model we have in the response.
      response = self.invoke(name, request) # type: GetClientResponse

# ###########################################################################

class GetClient(Service)

    # Use API models
    input  = GetClientRequest
    output = GetClientResponse

    def handle(self):

        # Enable type checking and type completion
        request = self.request.input # type: GetClientRequest

        # Log what we are doing ..
        self.logger.info('Returning data for %s', request.client_id)

        # .. build our response ..
        response = GetClientResponse()
        response.client_name = 'Jane Doe'

        # .. and return it to our caller.
        self.response.payload = response

# ###########################################################################

Invoking REST APIs

Models can be used to invoke REST APIs directly, without a need for serialization to dicts or JSON.

# -*- coding: utf-8 -*-

# stdlib
from dataclasses import dataclass

# Zato
from zato.server.service import Model, Service

# ###########################################################################

@dataclass(init=False)
class GetClientRequest(Model):
    client_id: int

@dataclass(init=False)
class GetClientResponse(Model):
    client_name: str

# ###########################################################################

class APIService(Service):
  def handle(self):

      # The REST endpoint
      name = 'My Client API'

      # Build our request ..
      request = GetClientRequest()
      request.client_id = 123

      # .. invoke the endpoint ..
      conn = self.rest[name]
      response = conn.post(self.cid, request)

      # .. read the response data + enable type checking and type completion ..
      response = response.data # type: GetClientResponse

      # .. and log what we have received.
      self.loger.info('Data received %s', response.client_name)

# ###########################################################################

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