Thread by @bkiepuszewski

By now you must have heard that Rollups derive their security from Ethereum. Even Rollups that have a centralised Sequencer. So why would you want to decentralise Sequencers ? Surely it adds complexity and can slow down the L2 block production ? 🧵👇

Let's have a look at the zkRollup as an example. They commit periodically L2 state root to Ethereum with Validity Proof. Once L2 state root is submitted to L1 all L2 transactions are valid and final so the safeness of the protocol is guaranteed /2

But security is not just safeness - it's safeness + liveness (see

https://twitter.com/bkiepuszewski/status/1539978129711587334?s=20&t=sCiC4jdb5qw2ofwYqlBGjQ

for more details). Can you have liveness with a centralised Sequencer ? /3

In practice liveness means that:
- individual users' transactions are not censored
- L2 Sequencer keeps producing non-empty blocks
/4

You can achieve basic censorship protection via several techniques, e.g. commit-reveal schemes or forcing transactions via L1. However when L2 sequencer stops producing blocks, things get more tricky /5

There must be a way to take over the responsibility of producing blocks from frozen (or malicious) Sequencer. This can be done via some sort of handover protocol, but it will most certainly mean that L2 will be down for some time /6

Decentralising Sequencers is certainly another strategy for achieving liveness and it may improve users' experience as one censoring (or down) Sequencer won't do much damage as there are others that will keep producing L2 blocks /7

There are two more reasons why you might want to decentralise Sequencers even though safety is guaranteed by L1. The first is to have faster L2 finality /8

With a single Sequencer, when it confirms the transaction (typically happens almost immediately) you need to trust it that it will submit it to a L1 block. With more Sequencers they need to reach consensus on the next L2 block /9

Once there is a consensus, L2 block with your transaction will be included in L1 as long as you have honest majority on L2. If this security assumption holds, your transaction should be treated as final ("L2-final"). If you are a coffee vendor, that should suffice /10

If you are a car vendor, or you simply don't trust L2 majority, just wait for L1 confirmation. You get to choose and you get to pick which security assumptions you are comfortable with. /11

Finally you may want to decentralise Sequencers if you don't want to have one single entity responsible for block production for regulatory reasons. This is obviously an important consideration but not for all apps/use cases. Again - app-specific Rollups have a choice here /12

There is a discussion going on on @StarkWareLtd forums regarding #StarkNet decentralisation: both "propose-and-vote" protocols like Tendermint and "longest-chain" protocols like Ouroboros are considered. Check their forum for more details: community.starknet.io /13

The TL/DR - decentralising Sequencer is not strictly necessary for the security of a Rollup but it may improve liveness, adds L2 finality and splits the responsibility (and potentially - rewards, including MEV) of producing L2 blocks amongst many participants. End /14