When using distributed and scalable architecture, eventual consistency is often a requirement.
Graphically, how to deal with this eventual consistency?
Users are used to click save, and see the result instantaneously... with eventual consistency it's not possible.
How to deal with the GUI for such scenarios?
Please note the question applies both for desktop applications and web applications.
PS: I'm working with the Microsoft platform, but I imagine the question applies to any technology...
A Task Based UI fits this model great. You create and execute tasks from the UI. You can also have something like a task status monitor to show the user when a task has executed.
Another option is to use some kind of pooling from the client. You send the command, and pool from the client until the command completed and the new data is available. You will have a delay in some cases from when the user presses save to when he will see the new record, but in most cases it should be almost synchronous.
Another (good?) option is to assume/design commands that don't fail. This is not trivial but you can have a cache on the client and add the data from the command to that cache and display it to the user even before the command has been executed. If the command fails for some unexpected situation, well then just design a good "we are sorry" message for misleading the user for a few seconds.
You can also combine the methods above.
Usually eventual consistency is more of a business/domain problem, and you should have your domain experts handle it.
There are 2 ways:
- To trick a user (just to show that things has happened then they really hasn't happened yet)
- Show that system is processing request and use polling in background (not good) or just timer with value of your SLA.
I prefer the 1st option.
As someone has already mentioned, task based UI's fit well for this, and what I would do is employ a technique that 'buys you time' for the command to propagate.
For example, imagine we are on a list screen, where the user can perform various actions, one of which being to add a new item to the list. After choosing to add an item you could display a "What would you like to do next?" which could have 'Add another item', 'Do this task', 'Do some other task', 'Go back to list'.
By the time they have clicked on an option, the data would have hopefully been refreshed.
Also, if you're using a task based UI, you can analyse the patterns of task execution and use these "what would you like to do next" screens to streamline the UI. Similar to amazon's "other people also bought these items".
I think that other answers mix together CQRS in general and eventual consistency in particular. Task-based UI is very suitable for CQRS but it does not resolve the issue with eventually consistent read model.
First, I would like to challenge your statement:
Users are used to click save, and see the result instantaneously... with eventual consistency it's not possible.
What do you by this? Why is it not possible to see the result immediately? I think the issue here is your definition of result.
The result of any action is that that action has been performed. There are numerous of ways to show this! It depends on what kind of action do you want to complete. Examples:
Send an email: if user has entered a correct email address, it is almost guaranteed that the action will complete successfully. To prevent unexpected failures one might use durable queues since this kind of actions do not need to be done synchronously. So you just say "email sent". Typically you see this kind of response when you ask to reset your password.
Update some information in a user profile: after you have validated the new data on the client, most probably the command will succeed too since the only thing that could happen is the database error (if you use database). Again, even this can be mitigated by using durable queues. In this case you just show the updated field in the same form. The good practice for SPA is to have a comprehensive data store on the client side, like Redux does. In this case you can safely update the server by sending a command and also updating the client-side store, which will result in UI to shows the latest data. Disclaimer: some answers refer to this technique as "tricking the user", but I disagree with this definition.
If you have commands that are prone to error, you can use techniques that are already described in other answers like Websockets or Server-side events to communicate errors back. This requires quite a lot of additional work. You can also send a command and wait for reply or execute commands synchronously. Some would say "this is not CQRS" but this would be just another dogma to be challenged. Ensuring the command has completed the execution in combination with the previous point (client-side data store) will be a good solution.
I am not sure if there is any 100% bullet proof technique that allows you to always show non-stale data from the read model. I think it goes against the principles of CQRS. Even with real-time events you will only get events that indicate that you write model has been updated. Still, your projections could have failed and reacting on this is a whole other story.
However, I would not concentrate that much on this issue. The fact is that well-tested projections and almost-guaranteed commands will work very well. For error handling in 90% of situations it is enough to have some manual or half-manual process to recover from those errors. For the last 10% you can combine generic "error" messages pushed from the server saying "sorry, your action XXX has failed to execute" and the top priority actions could have some creative process behind them but in reality those situations would be very very rare.
As previously stated, it is fine to tell the user that the request (command) has been acknowledged (successfully issued). In case of some failure, the system should communicate this to the requester, by means of:
- email;
- SMS;
- custom inbox (e.g. like the SO inbox);
- whatever.
E.g., mail client / service:
- I am sending a mail to a wrong address;
- the mail service says: "email sent successfully :)";
- after few minutes, I receive a mail from the service: "email could not be delivered".
I believe a great way to inform the user about a recent failure is to present him an error panel while he's navigating through the application. A user gesture might be required in order to dismiss that alert etc.
For example:
I wouldn't go with tricking the user or blocking him from committing some other actions. I would rather go for streaming data toward UI after they are being acknowledged by a read side. Let's consider these two cases:
Users saves data and expects result. Connection is established toward server. After they are being acknowledged by a read side, they are streamed toward UI and UI is being updated.
User saves data and refreshes web page. Upon reload, data are being fetched from data store and connection for streaming is established. If read side didn't update the data store in the meantime, there's still an opened stream and UI should be updated after data reaches the read side.
Why streaming from read side and not directly from write side? Simply, that would be a confirmation that read side has been reached. From technical aspect, Server-Sent Events could be used.
Disadvantage:
Results will still not be reflected immediately by a read side. But at least, in most cases, user will be able to continue with his work without being blocked by a UI.
来源：https://stackoverflow.com/questions/7316487/gui-recommandations-for-eventual-consistency