It is built on Lightning, in a similar way as other layers have done, like Cashu and Ark.
Discussion
Spark's server code is open-source with MIT licence, as of today. However, its SSP it's not.
Fees can come from 4 different sources:
- Wallet fees (possibly)
- Spark operator fee
- SSP fees
- Standard LN routing fees
Spark is no KYC, allows for unilateral exit, which makes it a better trust model with respect to custodial solutions.
It has some advantages over LN, such as better handling of small balances and dependable offline receive. Moreover, it has a better business momentum with respect to Ark, as of today.
Spark's trust model comes with some disadvantages, though.
Rogue employer could change the code to steal funds. Moreover, it is more susceptible to government interventions.
Also, it could tend to centralization (imagine LightSpark being the only operator) and lacks privacy, since all tx data is public.
Possible improvements:
- More Spark Entities with better privacy policies
- BOLT12 and BIP353 support
- Improve SSP scaling
- Improve engagement with open-source dev community
- Public proof of no forking happening