[SOLVED] Questions about Persistence behavior

Setup, Configuration and Use Beginners
1

Hi,

I have configured the persistence using a mysql DB.
Now I’ve noticed, if I restart openhab service (at least) one item starts as NULL and then switches to the correct value. The item itself is a String item that fetches its value via http binding.

  1. Is it expected it starts with NULL value, followed by a change to the correct value?
    I would have expected it starts with the value from the DB, then it updates the value via http binding.

I’d like to check if the String item contains a new / changed value. In this case a notification will be sent to my smartphone. As mentioned above, the item starts as NULL and thus my rule (when item changed) triggers.

  1. How can I avoid this?
    In the rule I use

if (MyStringitem.previousState == NULL)

Unfortunately this leads to an error message:

The name ‘NULL’ cannot be resolved to an item or type

Any help is highly appreciated.

Stefan

2

Yes. Every Item state starts with NULL and then gets updated either from the binding or the persistence ( restoreOnStartup if used on the item)

I am not 100% sure about the order… I think it is: NULL->restore->binding

try:

if (MyStringitem.previousState.state == NULL)
                              ^^^^^^

Ref: After openhabe 2.3 update, homematic binding don't update item status - #4 by rlkoshak

3

Thanks, now it works.

I found out that the state NULL is NOT saved in the persistence and cannot be retrieved as previousstate.
What I did:

  1. restart openhab service, so items starts as NULL
  2. Then item changes to the real value
  3. My rule (item changed) triggers
  4. Output for MyStringitem.previousState().state.toString is: current value
  5. Output for MyStringitem.previousState(true).state.toString is: older value

As you see, neither 4 nor 5 are NULL. So all I need to do is check if 4. differs from MyStringitem.state

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4

Please mark the thread as solved, thanks

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5

very correct!
I was thinking about exactly this when posting but I was not sure (asking myself if persistence would ever store NULL…:stuck_out_tongue:)

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