Add a state management how to

docs/platform/how-to/state-management.md

@@ -0,0 +1,70 @@
+# State management
+
+When deploying a service, enabling state management enables the service to maintain its state between restarts by creating a storage area for data and files, ensuring persistence across service restarts.
+
+## Enabling state management
+
+Enable state management during service deployment if your code relies on state or expects files to persist across restarts:
+
+![state management](../images/how-to/state/state-management.png){width=80%}
+
+Enabling state creates a `state` folder that can be accessed by your code. 
+
+!!! important
+
+    Note that data stored in this area is retained between service restarts and is shared between all replicas of the service, but it is not available globally to other services.
+
+## Storing files in the state folder
+
+You can use this `state` folder as a place to download and store model and other files. 
+
+!!! note
+
+    While the `state` folder is not visible in the UI, it is still available for use.
+
+For example, the following code downloads a model file to the `state` folder. It subsequently loads the model file from the `state` folder when needed:
+
+``` python
+import requests
+
+model = requests.get('https://acme.com/models/model1.dat')
+f = open('./state/model1.dat', 'wb')
+f.write(model)
+f.close()
+...
+f = open('./state/model1.dat', 'rb')
+...
+```
+
+The model can be modified at run time, and its state is preserved between service restarts.
+
+!!! note
+
+    There is a 100GB storage limit for the state management storage area.
+
+## Using state from Quix Streams
+
+If you have enabled state management for a service, then you can also use the [state management features](https://quix.io/docs/client-library/state-management.html) of Quix Streams in that service. This enables you to store and retrieve data in various formats, and retain that state between service restarts. See the [documentation](https://quix.io/docs/client-library/state-management.html) for further details.
+
+## Running Quix Streams locally
+
+If you are running Quix Streams locally, rather than using it as part of a deployed service in the cloud, then the `state` folder is created automatically for you when you use state management features of the library. For example, if you ran the following code locally:
+
+``` python
+from quixstreams import LocalFileStorage
+
+storage = LocalFileStorage()
+storage.clear()
+
+storage.set("KEY1", 12.51)
+storage.set("KEY2", "str")
+storage.set("KEY3", True)
+storage.set("KEY4", False)
+```
+
+Then you would see the `state` folder and its contents:
+
+```
+$ ls state
+key1 key2 key3 key4
+```

mkdocs.yml

@@ -45,6 +45,7 @@ nav:
       - 'Add environment variables': 'platform/how-to/environment-variables.md'
       - 'Testing using Quix data store': 'platform/how-to/testing-data-store.md'
       - 'Configure deployments': 'platform/how-to/yaml-variables.md'
+      - 'State management': 'platform/how-to/state-management.md'
     - 'Tutorials':
       - 'platform/tutorials/index.md'
       - 'Computer vision':