I'm sure we could argue to the end of time about what constitutes a layer 2, but it's good to read through how other people see these things.
Here's Spark's taxonomy:
- State Channels (Lightning)
- Federated Sidechains (Liquid)
- Merge-Mined Sidechains (Rootstock)
- Statechains (Spark, Mercury Layer)
- Virtual UTXO Protocols (Ark, Arkade)
- Proof-of-Transfer Chains (Stacks)
It's interesting what they exclude: notably BitVM, Rollups, and ecash -- they do mention these at another point in the article, but it's a very brief mention.
Later on under the section "Trust Assumptions Compared" they present the following table:
| Protocol | What You Trust | What You Can Verify | Failure Mode |
| Lightning | Nothing (trustless with watchtowers) | All channel states on-chain | Must monitor chain or use watchtower |
| Liquid | Federation majority is honest | L-BTC supply via audit | Federation collusion can steal funds |
| Rootstock | PowPeg federation + miners | Merge-mining attestations | Federation compromise risks peg |
| Stacks | sBTC signer set (14 signers) | All Stacks transactions on-chain | Signer collusion risks sBTC peg |
| Spark | At least 1-of-n operators is honest | Pre-signed exit transactions | All operators offline: liveness failure, not theft |
| Ark | Operator does not double-sign | VTXO branch proofs | Operator equivocation before on-chain confirmation |
In particular, I think that the statement that Spark's failure mode is only "liveness failure, not theft" is a bit strong. It seems to me that there are scenarios where a Spark operator could take your sats, although I haven't thought about it in a while and can't produce any examples at the moment.
All in all, I think Bitcoin Layers did a better job of this comparison.
with spark ...
you trust: 1-of-{ Lightspark }, because they're the only operator
you can verify: pre-signed exit transactions, but not whether they've deleted keys
failure modes: availability, censorship, privacy, griefing, and yes, potentially theft
This is NOT scaling... this is enshitification !
Spark is taking over sadly