Order of the events at the client: onItemUpdate, onEndOfSnapshot

[Slavko Parezanin]

Hi,

Regarding order of events we get:
1) onSubscription
2) onEndOfSnapshot
3) onItemUpdate (ADD)

After we return the initial set items in snapshot, we have a new Item that we want to listen.
We added it by:
_listener.Add(itemName, GetCurrentValues(true), false);

As soon as we changed it to:
_listener.Add(itemName, GetCurrentValues(true), true);

everything is OK.

Best regards,
Slavko

[giuseppe.corti]

Hi Slavko,

Please can you confirm that the third parameter of this method

_listener.Add(itemName, GetCurrentValues(true), true);

is the isSnapshot flag?

If this is the case, please note that it must be set to "true" only for the updates needed to realign the current state of the Item, following a Subscribe request by the Lightstreamer server, but never after the call of the EndOfSnapshot method for that Item.

Regards,
Giuseppe

[Slavko Parezanin]

Yes, third parameter is "isSnapshot".
Let us talk about cities in the region.

We want population of cities per region, so we subscribe:
new Subscription("COMMAND", "Abbruzzo", cellList);


At server we return the Snapshot


update.Add("command", "ADD");
_listener.OnEvent(Abbruzzo, Aquila, true);
_listener.OnEvent(Abbruzzo, Lanciano, true);
_listener.OnEndSnapshot(Abbruzzo);

and we modify the items, without any problem.
update.Add("command", "UPDATE");
_listener.OnEvent(Abbruzzo, Aquila, false);


But, when you build a new city in Abbruzzo , fer example Filetto, we did:


update.Add("command", "ADD");
_listener.OnEvent(Abbruzzo, Filetto, false);

we get the problem with order of the events at the client.


As soon as we changed it to:
update.Add("command", "ADD");
_listener.OnEvent(Abbruzzo, Filetto, true);

everything is ok.


Regards !

[giuseppe.corti]

Hi Slavko,

I'm sorry but I'm a bit confused about the behavior that you expect.

Please, let me consider the sequence from the point of view of the client:
1. Subscribes to the "Abbruzzo" Item in COMMAND mode;
2. receives onSubscription event;
3. receives onItemUpdate (ADD) events for the city of Aquila and Lanciano (isSnapshot return true);
4. receives the onEndOfSnapshot event;
5. receives several onItemUpdate (UPDATE) events for the changes in population count for the city of Aquila and Lanciano (isSnapshot return false);
6. receives an onItemUpdate (ADD) event for the new city of Filetto (isSnapshot return false);
7. receives several onItemUpdate (UPDATE) events for the changes in population count for the city of Aquila, Lanciano and Filetto (isSnapshot return false);

Do you expect something different in the sequence above?

[Slavko Parezanin]

Yes, it is expected and it is ok.
I just wanted to show you what caused the problem of order of events at client.

- new Subscription("COMMAND", "Abbruzzo", cellList);
- we return Snapshot.
- _listener.OnEndSnapshot(Abbruzzo);
- we updates ...

we added update.Add("command", "ADD");
_listener.OnEvent(Abbruzzo, Filetto, false); // FALSE is WRONG !!!

- we refresh the browser.
- we get wrong order of events !

But after we set:
_listener.OnEvent(Abbruzzo, Filetto, true);

everything is ok !

regards!

[giuseppe.corti]

Hi Slavko,

If this solution works in your application then it's fine.

Maybe I did not understand how this instruction

_listener.OnEvent(Abbruzzo, Filetto, true);

exactly apply in your code.

I understood that with that statement you're pushing, from the adapter to the server, an event of command type ADD with the snapshot flag set to true; but may be that I'm wrong.
By the way, in that case you should have a warning message like this

"Unexpected snapshot event for COMMAND item ..."

in the log server.

[Slavko Parezanin]

I did some investigation of log, and i have no warning messages like these
"Unexpected snapshot event for COMMAND item ..."


Before i do more investigation, i would like know your suggestion for the case.


- We start the LS
- User opens the browser after a few seconds aftre, asking for Snapshot,
new Subscription("COMMAND", "Abbruzzo", cellList);

- We return 2 cities as the Snapshot.
update.Add("command", "ADD");
_listener.OnEvent(Abbruzzo, update(Aquila), true);


update.Add("command", "ADD");
_listener.OnEvent(Abbruzzo, update(Lanciano), true);

_listener.OnEndSnapshot(Abbruzzo);


- After that we get notified that a new city is built and we send:
update.Add("command", "ADD");
_listener.OnEvent(Abbruzzo, Filetto, true);


- User does browser refresh
- User have all the cities in Snapshot and right order of events.


In case of returning the new city Lanciano like this:
update.Add("command", "ADD");
_listener.OnEvent(Abbruzzo, update(Lanciano), false);
user gets wrong order of events after browser refresh.
It seems only the first time, after browser Refresh.


regards!