External State Adapter

Creating presistent external state for the BPTK-Server.


BPTK-Py offers a way to persist data externally. This allows simulation instances to be fully restored from a save point - one example of how we use this in practice is to persist the current state of a Beer Distribution Game session to an external database. This ensures that a game can be resumed in the case of a system failure.

Using persistent state

To persist state, an instantiation of a class inheriting BPTK_Py.ExternalStateAdapter must be passed into the BPTK-server constructor. When an adapter is provided, BPTK will call the corresponding methods in the provided ExternalStateAdapter implementation automatically.

BPTK provides BPTK_Py.FileAdapter, an implementation of the ExternalStateAdapter class that can be used to store the state locally in JSON files. Creating your own implementation for ExternalStateAdapter (for example to save the state to an external database) is trivial.

Let’s look at an example on how to add persistent state using BPTK’s FileAdapter:

from BPTK_Py import BptkServer
from BPTK_Py import FileAdapter
from model import bptk_factory
import os
import json

adapter = FileAdapter(True, os.path.join(os.getcwd(), "state"))

The code above imports all required modules and creates a new FileAdapter object. The FileAdapter init method takes two arguments: 1. Compression. When enabled, BPTK sends a compressed format of the instance state to the provided ExternalStateAdapter and automatically decompresses the instance states on load. 2. Directory path. The path to which the state will be saved and from which the state will be loaded. This directory must be empty on first start-up.

# Calling the BptkServer class
application = BptkServer(__name__, bptk_factory, external_state_adapter=adapter)

Running the code above will create a new BPTK-server. The server takes the adapter as an optional argument. When no adapter is provided, the state will not be saved.

How does it work?

To continue this tutorial, run a new BPTK server using the run_server.sh script (or run_server.bat under Windows) in the current Jupyter notebook directory.

When an ExternalStateAdapter is provided, BPTK will automatically call the methods in the adapter implementation. An ExternalStateAdapter implements the following methods: 1. _save_state: Takes a list of all instance states as an argument. This method is called when the save-state endpoint of the BPTK-server is called. 2. _load_state: Takes no arguments and returns a list of all instance states that are stored. This method is called when the load-state endpoint of the BPTK-server is called and on BPTK start-up. 3. _save_instance: Takes an instance state as an argument. This method is called when an instance step is run. 4. _load_instance: Takes an instance id as an argument. This method is called when an instance cannot be found in local storage. 5. delete_instance: Takes an instance id as an argument. This method is called when the stop-instance endpoint is called.

The implementation handles IO with the storage solution. Let’s look at an example:

import requests

req = requests.post("http://localhost:5000/start-instance")
instance_id = req.json()['instance_uuid']

The code above starts a new BPTK simulation instance and returns the instance id.

content = {
    "scenario_managers": [
    "scenarios": [
    "equations": [

req = requests.post(f'http://localhost:5000/{instance_id}/begin-session', json.dumps(content), headers={'Content-Type': 'application/json'})

The code above starts a new session for a given instance.

step = {     

req = requests.post(f'http://localhost:5000/{instance_id}/run-step', json.dumps(step), headers={'Content-Type': 'application/json'})

When run-step is called, BPTK will call the provided ExternalStateAdapter to save that instance. This way, every instance is always up to date.

You will see a JSON-File with the instance id as its name in the state directory.

Implementing your own ExternalStateAdapter

Implementing your own ExternalStateAdapter is trivial. All the logic is handled by BPTK. The adapter must only handle the IO with the storage solution. Let’s look at an example dummy implementation:

from BPTK_Py import ExternalStateAdapter
from BPTK_Py import InstanceState
import json
import datetime

class DBAdapter(ExternalStateAdapter):
    def __init__(self, compress: bool, db_client):
        self.db_client = db_client

In the first line we extend ExternalStateAdapter. Then we create a constructor, taking a boolean and a db_client as an input. - The boolean value indicates whether the state will be compressed and decompressed by BPTK. This is recommended, it can drastically reduce the size of an instance. - The db_client argument represents a dummy database client. Most database connections work using a database client, adapting this dummy class to your storage solution should therefore be simple.

    def _save_state(self, instance_states: list[InstanceState]):
        for state in instance_states:

    def _save_instance(self, state: InstanceState):
        data = { 
            "data": { 
                "state": json.dumps(state.state), 
                "instance_id": state.instance_id,
                "time": str(state.time),
                "timeout": state.timeout,
                "step": state.step
        self.db_client.save(key=state.instance_id, data=data)

    def _load_state(self) -> list[InstanceState]:    
        instances = []
        instance_paths = os.listdir(self.path)

        for instance_uuid in instance_paths:

        return instances

    def _load_instance(self, instance_uuid: str) -> InstanceState:
            instance_data = json.loads(self.db_client.read(key=instance_uuid))
            decoded_data = json.loads(instance_data["data"]["state"])
            instance_id = instance_data["data"]["instance_id"]
            timeout = instance_data["data"]["timeout"]
            step = instance_data["data"]["step"]
            return InstanceState(decoded_data, instance_id, datetime.datetime.now(), timeout, step)
        except Exception as e:
            print("Error: " + str(e))
            return None
    def delete_instance(self, instance_uuid: str):
        except Exception as e:
            print("Error: " + str(e))
            return None

The code above implements all five functions. _load_instance and _delete_instance can be called for instances which do not exist in the database, error handling code is therefore advisable.