This section walks you through the process of writing an XOS model and synchronizer. It does not provide exhaustive coverage, but it does guide you through the most common use cases, including:

  • How to define XOS models.
  • How to load the models into the xos-core.
  • How to write a TOSCA recipe that creates an instance of the models.
  • How to write a sync_step for the models.
  • How to write unit tests for a sync_step (TODO).


The following assumes you start with a minikube deployment of XOS (although any Kubernetes cluster will do), so that running kubectl get pods returns something similar to:

NAME                             READY     STATUS    RESTARTS   AGE
xos-chameleon-6fb76d5689-s7vxb   1/1       Running   0          21h
xos-core-58bcf4f477-79hs7        1/1       Running   0          21h
xos-db-566dd8c6f9-l24h5          1/1       Running   0          21h
xos-gui-665c5f85bc-kdmbm         1/1       Running   0          21h
xos-redis-5cf77fd49f-fcw5h       1/1       Running   0          21h
xos-tosca-69588f677c-77lll       1/1       Running   0          20h
xos-ws-748c7f9f75-cnjnh          1/1       Running   0          21h

The tutorial also assumes you have downloaded the XOS source code into directory $SRC_DIR.

Note: Instructions on how to install XOS on a Kubernetes cluster and download the XOS source code is provided elsewhere in this Guide.

Directory Structure

XOS services are located under $SRC_DIR/services. The first step is to create a new directory to store our models and synchronizer code. We will use the name hello-world for this example:

cd $SRC_DIR/services
mkdir hello-world
cd hello-world

Although empty when we start, we will end up with a directory structure that looks like the following. You can look at the corresponding directories of other services in $SRC_DIR/services for examples.

├── Dockerfile.synchronizer
├── samples
│  └── hello-world.yaml
└── xos
  ├── synchronizer
  │   ├── config.yaml
  │   ├── hello-world-synchronizer.py
  │   ├── models
  │   │   └── hello-world.xproto
  │   ├── steps
  │   │   ├── sync_hello_world_service.py
  │   │   ├── sync_hello_world_service_instance.py
  │   │   ├── test_sync_hello_world_service.py
  │   │   └── test_sync_hello_world_service_instance.py
  │   └── test_config.yaml
  └── unittest.cfg

Walking through the structure, we see the following:

  • Dockerfile.synchronizer specifies the Docker image we will build to run the synchronizer.
  • VERSION specifies the version of our code; it is reported to the core.
  • xos/synchronizer contains all the code that will be bundled in the Docker image.

Looking at some of the files, we see:

  • xos/synchronizer/models/hello-world.xproto contains the model definitions.
  • samples/hello-world.yaml is an example of a TOSCA recipe to instantiate those models.
  • xos/synchronizer/hello-world-synchronizer.py is the main synchronizer process.
  • xos/synchronizer/steps/sync_hello_world_service.py contains the operations needed to synchronize the backend component.

Create the Synchronizer Entry Point

The synchronizer entry point (main looping process) is responsible for:

  • loading the synchronizer configuration
  • loading and running the synchronizer framework

Cut-and-paste the following into hello-world-synchronizer.py:

import importlib
import os
import sys
from xosconfig import Config

config_file = os.path.abspath(os.path.dirname(os.path.realpath(__file__)) + '/config.yaml')
Config.init(config_file, 'synchronizer-config-schema.yaml')

observer_path = os.path.join(os.path.dirname(os.path.realpath(__file__)),"../../synchronizers/new_base")
mod = importlib.import_module("xos-synchronizer")

Define the Synchronizer Configuration

In xos/synchronizer/config.yaml add this content:

name: hello-world
  username: admin@opencord.org
  password: letmein
  endpoint: xos-core:50051
models_dir: "/opt/xos/synchronizers/hello-world/models"
steps_dir: "/opt/xos/synchronizers/hello-world/steps"
  - HelloWorldService
  - HelloWorldServiceInstance
  version: 1
      class: logging.StreamHandler
          - console
      level: DEBUG

This tells the synchronizer framework in a running container where to fine the configuration parameters specific to HelloWorld. By convention, we use /opt/xos.

Define Models

For our example, we are going to define the two most common models for an XOS-managed service: HelloWorldService and HelloWorldServiceInstance. These are HelloWorld-specific definitions of the two core models:

  • Service: defines service-wide parameters/fields.
  • ServiceInstance: defines subscriber-specific parameters/fields.

Open the hello-world.xproto file and add the following content:

option name = "hello-world";
option app_label = "hello-world";

message HelloWorldService (Service){
    required string hello_from = 1 [help_text = "The name of who is saying hello", null = False, db_index = False, blank = False];

message HelloWorldServiceInstance (ServiceInstance){
    option owner_class_name="HelloWorldService";
    required string hello_to = 1 [help_text = "The name of who is being greeted", null = False, db_index = False, blank = False];

This specifies two models: HelloWorldService extends the Service model, and HelloWorldServiceInstance extends the ServiceInstance model. Both of these models inherit the attributes defined in the parent classes, which you can see in file $SRC_DIR/xos-core/core/models/core.xproto.

Load the Models into the Core

Service models are pushed to the core through a mechanism referred to as dynamic onboarding or dynamic loading. In practice, when a synchronizer container runs, the first thing it does is to push its models into the core container.

But first we need to build and deploy our synchronizer container in the test environment.

Build the Synchronizer Container

We assume that you are familiar with the Docker concepts of container and image. If not, we encourage you to look here: Docker concepts

The first thing we need to do is to define a Dockerfile. To do that, open Dockerfile.synchronizer and add the following content:

FROM xosproject/xos-synchronizer-base:candidate

COPY xos/synchronizer /opt/xos/synchronizers/hello-world
COPY VERSION /opt/xos/synchronizers/hello-world/


WORKDIR "/opt/xos/synchronizers/hello-world"

CMD bash -c "python hello-world-synchronizer.py"

This file is used to build our synchronizer container image. As you might have noticed, the container image we're defining inherits FROM xosproject/xos-synchronizer-base:candidate, so we'll need to obtain that image.

We can use the following commands to do this:

eval $(minikube docker-env) # this will point our shell on the minikube docker daemon
docker pull xosproject/xos-synchronizer-base:master
docker tag xosproject/xos-synchronizer-base:master xosproject/xos-synchronizer-base:candidate

Now we can build our synchronizer image by executing the following from the $SRC_DIR/service/hello-world directory:

eval $(minikube docker-env)
docker build -t xosproject/hello-world-synchronizer:candidate -f Dockerfile.synchronizer .

Run Your Synchronizer Container

You can create a simple Kubernetes resource in a file called kb8s-hello-world.yaml:

apiVersion: v1
kind: Pod
  name: hello-world-synchronizer
    - name: hello-world-synchronizer
      image: xosproject/hello-world-synchronizer:candidate
      - name: certchain-volume
        mountPath: /usr/local/share/ca-certificates/local_certs.crt
        subPath: config/ca_cert_chain.pem
    - name: certchain-volume
        name: ca-certificates
          - key: chain
            path: config/ca_cert_chain.pem
  restartPolicy: Never

and then run it using kubectl create -f kb8s-hello-world.yaml

Check the logs of your synchronizer using:

kubetcl logs -f hello-world-synchronizer

You should see output similar to the following:

Service version is 1.0.0.dev
required_models, found:        models=HelloWorldService, HelloWorldServiceInstance
Loading sync steps             step_dir=/opt/xos/synchronizers/hello-world/steps synchronizer_name=hello-world
Loaded sync steps              steps=[] synchronizer_name=hello-world
Skipping event engine due to no event_steps dir. synchronizer_name=hello-world
Skipping model policies thread due to no model_policies dir. synchronizer_name=hello-world
No sync steps, no policies, and no event steps. Synchronizer exiting. synchronizer_name=hello-world

Check that your models are on-boarded in the XOS GUI by opening the GUI:

minikube service xos-gui

Use the default credentials admin@opencord.org/letmein to login.

Create TOSCA Recipes to Instantiate Your Models

The models you defined earlier in this tutorial are, more precisely, model schema. Once your model schema has been loaded into the XOS core, you can create one or more instances of those models. We typically use a TOSCA recipe to do this.

The XOS TOSCA engine automatically understands workflows for any models that have been loaded into the core. You can consult them at any time connecting to the TOSCA endpoint from a browser:


Note: You can find the minikube ip by executing this command on your system:minikube ip.

In this page you should find a list of all the available resources. Just search for helloworldservice and visit the corresponding page at:


You will see the TOSCA definition for the HelloWorldService model.

You can use that (and the HelloWorldServiceInstance model definition too) to create an instance of both models. For your convenience, save the following content to a file called hello-world-tosca.yaml

tosca_definitions_version: tosca_simple_yaml_1_0
  - custom_types/helloworldservice.yaml
  - custom_types/helloworldserviceinstance.yaml
  - custom_types/servicegraphconstraint.yaml

description: Create an instance of HelloWorldService and one of HelloWorldServiceInstance


      type: tosca.nodes.HelloWorldService
        name: HelloWorld
        hello_from: Jhon Snow

      type: tosca.nodes.HelloWorldServiceInstance
        name: HelloWorld Service Instance
        hello_to: Daenerys Targaryen

      type: tosca.nodes.ServiceGraphConstraint
        constraints: '["HelloWorld"]'

This TOSCA will create an instance of your service and an instance of your service instance.

You can then submit this TOSCA using this command:

curl -H "xos-username: admin@opencord.org" -H "xos-password: letmein" -X POST --data-binary @hello-world-tosca.yaml http://<minikube-ip>:30007/run
Created models: ['service', 'serviceinstance', 'serviceinstance']

Once this command has been executed, connect to the GUI at:


and see that your models have been instantiated.

Note: In the home page press the Service Instances button to display ServiceInstance models, and the navigate to the Hello World sub-menu to the left.

Create your First Synchronizer Steps

Everything up to this point (with the exception of defining the models themselves) is the boilerplate needed to run a synchronizer. It is the sync_step that is actually responsible for mapping changes in the XOS data model into some action on the backend component XOS is managing.

To keep the tutorial simple we are not going to operate on a real component, but we can demonstrate the basic idea of how a sync_step interacts with the models.

Successful sync_step

Before continuing, let's remove the container we just deployed. Do this by running:

kubectl delete pod hello-world-synchronizer

To write the sync_step we need to create two files in xos/synchronizer/sync_step. The first one synchronizes the HelloWorldService model and it is called sync_hello_world_service.py.

Every sync_step extends the SyncStep base class, and overrides two methods:

  • sync_record
  • delete_record

See the synchronizer reference for more details.

Here is an example of sync_step that simply logs changes on the HelloWorldService model:

from synchronizers.new_base.SyncInstanceUsingAnsible import SyncStep
from synchronizers.new_base.modelaccessor import HelloWorldService

from xosconfig import Config
from multistructlog import create_logger

log = create_logger(Config().get('logging'))

class SyncHelloWorldService(SyncStep):
    provides = [HelloWorldService]

    observes = HelloWorldService

    def sync_record(self, o):
        log.info("HelloWorldService has been updated!", object=str(o), hello_from=o.hello_from)

    def delete_record(self, o):
        log.info("HelloWorldService has been deleted!", object=str(o), hello_from=o.hello_from)

Let's deploy this first step and see what happens. The first thing you need to do is rebuild the synchronizer container:

eval $(minikube docker-env)
docker build -t xosproject/hello-world-synchronizer:candidate -f Dockerfile.synchronizer .

Once done, restart it as follows:

kubectl create -f kb8s-hello-world.yaml

At his point, running kubectl logs -f hello-world-synchronizer should show that your synchronizer is no longer exiting, but is now looping while waiting for changes in the models.

Every time you make a change to the model, you will see:

  • The event is logged in the synchronizer log (kubectl logs -f hello-world-synchronizer)
  • The backend_code and backend status of the model are updated
  • The model is not picked up by the synchronizer until you make some changes to it

When you make changes to the models (you can do this via the GUI or by updating the TOSCA you created before), you will see a message similar to this one in the logs:

Syncing object                            model_name=HelloWorldService pk=1 synchronizer_name=hello-world thread_id=140152420452096
HelloWorldService has been updated!       hello_from=u'Jhon Snow' object=HelloWorld
Synced object                             model_name=HelloWorldService pk=1 synchronizer_name=hello-world thread_id=140152420452096

Note: The sync_record method is triggered also when a model is created, so as soon as you start the synchronizer you will see the above message.

If you delete the model, you'll see the delete_record method being invoked.

Handling Errors in a sync_step

We are now going to trigger an error, to demonstrate how the synchronizer framework is going to help us in dealing with them.

Let's start creating the sync_step for HelloWorldServiceInstance in a file named sync_hello_world_service.py.

from synchronizers.new_base.SyncInstanceUsingAnsible import SyncStep, DeferredException
from synchronizers.new_base.modelaccessor import HelloWorldServiceInstance

from xosconfig import Config
from multistructlog import create_logger

log = create_logger(Config().get('logging'))

class SyncHelloWorldServiceInstance(SyncStep):
    provides = [HelloWorldServiceInstance]

    observes = HelloWorldServiceInstance

    def sync_record(self, o):
        log.debug("HelloWorldServiceInstance has been updated!", object=str(o), hello_to=o.hello_to)

        if o.hello_to == "Tyrion Lannister":
          raise DeferredException("Maybe later")

        if o.hello_to == "Joffrey Baratheon":
          raise Exception("Maybe not")

        log.info("%s is saying hello to %s" % (o.owner.leaf_model.hello_from, o.hello_to))

    def delete_record(self, o):
        log.debug("HelloWorldServiceInstance has been deleted!", object=str(o), hello_to=o.hello_to)

To run this code you will need to:

  • delete the running container
  • rebuild the image
  • run the container again

In this case we are emulating an error in our sync_step. In reality, this can be caused by a connection error, malformed data, or any of a number of reasons.

Go to the GUI and start playing a little bit with the models!

If you set the HelloWorldServiceInstance.hello_to property to Tyrion Lannister you will see this keep popping up:

HelloWorldServiceInstance has been updated! hello_to=u'Tyrion Lannister' object=HelloWorld Service Instance
sync step failed!              e=DeferredException('Maybe later',) model_name=HelloWorldServiceInstance pk=1 synchronizer_name=hello-world
Traceback (most recent call last):
  File "/opt/xos/synchronizers/new_base/event_loop.py", line 357, in sync_cohort
    self.sync_record(o, log)
  File "/opt/xos/synchronizers/new_base/event_loop.py", line 227, in sync_record
  File "/opt/xos/synchronizers/hello-world/steps/sync_hello_world_service_instance.py", line 18, in sync_record
    raise DeferredException("Maybe later")
DeferredException: Maybe later

Here is what happens when an error occurs. The synchronizer framework will:

  • Log the exception
  • Set the backend_code of that instance to 2
  • Set the Exception error in the backend_status
  • Keep retrying

Note: To see backend_code and backend_status in the GUI you can press d to open the debug tab while looking at a model detail view.

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