Question posted in 
This is a wrap-up of several open questions in the vaadin forum (which is moving to stack-overflow) – see 2, 3, 4, …
The basic question is: "How can one run a (recent V14 LTS or V19/20) vaadin-flow application reliably on google app engine (GAE manual or automatic scaling) standard environment (i.e. no docker, no google compute engine), without experiencing constant refreshing of components"
- Vaadin has a tutorial for deploying vaadin applications to GAE flexible (not standard as for this question). This tutorial doesn’t mention that one might run into trouble when GAE switches the server-instance. Even GAE mentiones Vaadin as one of the supported frameworks.
- Old Vaadin 8 release notes state, that support for GAE has been dropped.
- According to the questions in the vaadin forum, vaadin-flow at GAE will lead (or at least has led in older versions of vaadin) to constant refreshing of components and/or loss of session-state.
- If one uses manual scaling, ones applications can rely on the state of the memory over time, so vaadin-flow applications should not be bothered with switches of the server-instance (which will eventually occur when an instance is shut down due to an error or maintenence reasons).
So the first question is: "When running a vaadin-flow application on GAE standard with manual scaling, will it lead to constant refreshing of componends and/or loss of session-state even when the instance is not switched?"
If that works, than vaadin-flow is fine on GAE standard when one needs neither dynamic scaling nor high availability. It that does not work, vaadin-flow on GAE standard would be a NOGO for any type of application (and one needs to switch to another provider or to GAE flexible and docker).
The next question is: "What has to be done to make vaadin-flow application on GAE standard run reliably, even when instances are scaled or switched due to maintenance?"
The following suggestions were stated in the forum – but noone ever confirmed that they work:
- One could have set
<sessions-enabled>true</sessions-enabled>for java 8. This setting no longer exists for java 11. Even when when nr. of instances is changing or instances are restarted, this could have been the solution since session data is stored in memcache which is available over all instances. - When instances are moved or shut down, google sends a shutdown notification -> one could implement a shutdown-hook and try to serialize all session-state (if vaadin provides a way to serialize it manually and automatically de-serialize it when another instance takes over).
Has anyone found a reliable solution for this?
3
Answers
My application is running since months now and I must say, that it worked just fine. BUT we never needed more than one instance (basic_scaling: max_instances: 1), so no sticky-sessions were needed and my conclusions might not be valid if one needs more than one instance:
Session-affinity can be set in app.yaml according to https://cloud.google.com/appengine/docs/flexible/java/using-websockets-and-session-affinity. So it might as well work with multiple instances
Regarding serializing all session state (eg. on a shutdown hook): this requires that all of your stateful objects, like all of your data beans, Vaadin Flow views, compositions etc., are serializable. In other words, all classes must implement
java.io.Serializable. This can be done, as all classes from Vaadin should be serializable, but it’s up to the application developer to make sure that all custom instantiable classes in the codebase (and any instantiable classes in the dependencies) can be serialized and deserialized. Based on practical experience, this is not a trivial requirement – it usually drives the application’s architecture by limiting or changing the design patterns used in the code. Making an existing codebase fully serializable will likely incur significant refactoring work. This is one of the big reasons why sticky sessions (which are not available in GAE) are the recommended approach for deploying Vaadin Flow applications in multi-node environments.I think it’s not possible.
According to https://stackoverflow.com/a/10640357/377320 GAE uses Datastore to store the session informations and only synchronizes objects set via
session.setAttribute(). However, according to https://mvysny.github.io/vaadin-14-session-replication/ Vaadin doesn’t callsetAttribute(), moreover Vaadin does that on purpose.That means that GAE won’t synchronize the state properly but the requests will land on random nodes (since session affinity/sticky sessions is not supported on GAE), causing the requests to land on nodes with obsolete Vaadin state. That leads me to believe that the most likely outcome is that Vaadin components will constantly lose state, and Vaadin will try to frequently attempt to resync the UI state by performing browser reloads.
Vaadin Flow works best with sticky sessions and long-running servers while GAE sounds like an opposite of that.
I believe your best bet is to use Vaadin Fusion with a stateless server.