Alternative block producers

Offchain scaling protocols can have alternative block producers. In sidechains, they produce blocks and provide users with finality. In rollups, block production is slightly different as "sequencers" provide users with a "preconfirmation" that users' transactions will be eventually settled on the Layer 1. This is (basically) soft finality. If designed correctly, rollup users can bypass the sequencer layer and use the L1 for transaction inclusion.

Alternative block producers designs are relatively straight forward:

• Centralized: One entity controls transaction ordering and proposes blocks

• Federated: A group of nodes controls transaction ordering and proposes blocks

• Permissionless: A consensus protocol controls transaction ordering and proposes blocks

The question lies with whether users have the ability to self-sequence + self propose in the event of a liveness failure, and if the systems producing L2 blocks are permissionless.

In rollups, self-sequencing and self-proposing are also necessary for unilateral exit, along with trustless bridging.