How does the Strelka transport card work?

Question

NFC

Strelka is a kind of analogue of the Moscow Troika card, operating in the Moscow region to pay for public transport. When paying for the fare, the fare is debited from the card balance, the balance can be replenished remotely.
I can't understand how this card works from a technical point of view.
For example, it seems that Troika keeps the balance directly on the card itself, and the validator in the turnstile checks and changes it, the transaction itself takes place offline. Therefore, for example, when replenishing a card via the Internet, then you still need to go to the terminal in the metro and it will write a new balance to the card.
At Strelka, payment is increasingly similar to the online mode:
— terminals for payment in public transport have mobile Internet,
- they promise instant remote replenishment of the card via Internet banks, which does not need to be recorded separately on the card (in fact, it is not very instant),
- often the terminal gives an error when paying, the conductor / driver tries to pay again and again and someday it still works (it looks like poor mobile internet coverage).
At the same time, some of the signs indicate that full online payment is not carried out:
- at least, I just don’t believe that they have implemented full online, since payment is made directly on buses that travel all over the region, so it’s somehow stupid to hope to good internet
- payment always goes pretty quickly (if it goes through, of course) - not instantly, like in the subway, for example, but within 1-2 seconds (and bank terminals in stores with really online transactions often freeze for much longer),
- at least replenishment of the account and announced as instant (that is, you can replenish the balance in the application and pay immediately), in practice you should not hope for this and you will only be able to pay somewhere the next day, despite the fact that the balance is displayed immediately in the application,
- in In the mobile application, you can see the history of fare payment and the fact of travel does not appear there immediately, but about 30 minutes after payment.
Maybe someone from those present knows how this system is arranged from the inside and will tell?

Answer 1

I doubt there's anything clever in there.
The balance is stored on the card and in the central database, the account is associated with a unique card identifier.
Public transport at the final stops synchronizes the data of the central database with the validators.
To store the database of valid cards, a lot of memory is not needed - they are placed in the validator.
Permanent online is not required.