Tersus: Demo Stage 9 – Refresh: Publish-Subscribe Improvements

Demo Stage 9 – Refresh: Publish-Subscribe ImprovementsDate: Nov 28, 2023At the end of stage 4 we noted that the updater experiences two refreshes of the table upon any update – one due to the built-in refresh process in each of the buttons ('New', 'Update' or 'Delete'), and one due to the subscription to updates. We said that to overcome this problem, user subscriptions to updates should ignore updates made by themselves.Let's do it now. Ignore Your Own UpdatesWe'll use the 'Source' field of the publish-subscribe message to identify the publisher of each change. Thus the publisher "signs" any change message he/she publishes, and the subscribers will ignore any message signed by themselves.Here is a possible way to model this behavior:

Create an identifier for your application session:

In 'Items', add a Text data item 'App Session Id'.
Zoom into 'Init' and add a process 'Init App Session Id' (to model it, drag from the palette the templates 'Sequence Number' and 'Convert Number to Text'):

Note 1: If you don't know in which section of the palette to find a template you need, press the 'Search' icon (the located at the top of the palette), and type the template's name (or a prefix of its name).

Note 2: 'App Session Id' is initialized with the view, and won't change until a refresh is performed in the browser. Therefore it is a good way to identify your application session.

Sign any change message you publish with your identifier:

Zoom into the 'Publish Change' process (from stage 4).
Add a trigger 'App Session Id'.
Add a flow from the trigger to the 'Source' element of 'Message':
Provide the proper value in each of the 4 places where 'Publish Change' is used (all 4 are inside 'Header Row/Action Header Group'):
1. In 'New/<On Click>/Enter Details/Footer/Save/<On Click>':
  - Right click, select Add Ancestor Reference, and select 'Items'.
  - Add a trigger 'App Session Id' to 'Save New'.
  - Add a flow from the 'App Session Id' element of 'Items' to the 'App Session Id' trigger of 'Save'.
  - Add a flow from this trigger to the corresponding trigger of 'Publish Change'.
2. Repeat the same in 'Update/<On Click>/Update Details/Footer/Save/<On Click>'.
3. Repeat the same in 'Delete/<On Click>/Delete from DB' (except for Add Ancestor Reference, as 'Delete/<On Click>' already contains 'Items').
4. In 'Got It/<On Click>/Mark as Got', Add Ancestor Reference of 'Items', and then a flow from its element to the 'App Session Id' trigger of 'Save Update'.

When a message arrives, ignore it if it is signed by your own identifier:

Return to the 'Init' of 'Items' and zoom into 'Subscribe to Shopping List Changes'. Further zoom into 'Subscribe' and then into '<On Message>'.
Encapsulate 'Get Current Page Number' and 'Go to Page', and rename the encapsulated action to 'Refresh Current Table Display'. Add to it a 'Control' trigger (right click, select Add Element, and select 'Control').
Back to '<On Message>', right click, select Add Ancestor Reference, and select 'Items'.
Compare the 'App Session Id' element of 'Items' with the 'Source' element of 'Message'. The 'Refresh Current Table Display' process should be activated only if the source is different from our own identifier:

Note that in order for the 'App Sessions Id' to be generated, you need to first stop the application and then restart it.

Download Model: Stage 9 (version 2.4.1)See Also:

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