Ethereum and Bitcoin L2s: Ordering First, Data Availability Later

L2 systems often order transactions first and rely on L1 for data availability (DA). That separation is a feature, not a bug. It enables fast UX while anchoring security to a stronger base layer.

This post explains how that works on Ethereum and Bitcoin L2s, using a simple mock example and a clear mental model.

Ordering vs data availability (decoupled by design)

• Ordering: choose the sequence of transactions (the history).
• Data availability: make the ordered data public so anyone can verify and replay it.

L2s separate them:

• Ordering happens off‑chain for speed.
• DA happens on L1 for security and auditability.

Ethereum L2 example (mock block)

Step 1: Users send transactions

TxA: Alice → Bob (fee 0.2) TxB: Bob → Charlie (fee 0.8) TxC: Alice → Dave (fee 0.5)

Step 2: Sequencer orders them (off‑chain)

Order chosen:

1. TxB
2. TxC
3. TxA

This ordering can be fee‑based or MEV‑optimized. Ethereum does not guarantee fairness — it guarantees replayable ordering once published.

Step 3: L2 executes immediately

Users see fast confirmations because execution happens right away.

Step 4: Data availability via L1

Sequencer posts calldata to Ethereum L1:

[TxB, TxC, TxA]

Now anyone can verify and replay the L2 block. The order is immutable once included in L1.

Bitcoin L2s (similar principle, slower settlement)

Many BTC L2s still follow a similar logic:

• L2 orders and executes off‑chain.
• BTC L1 acts as the availability / settlement anchor.
• Settlement is slower because BTC finality is slower.

The difference is that Bitcoin’s DA and finality are slower, so withdrawals and settlement are slower too.

Ordering is not fairness

Ethereum L2s do not guarantee fair ordering. They guarantee deterministic, replayable ordering after the order is posted to L1.

That means:

• Ordering can be fee‑based.
• Ordering can be centralized.
• Ordering can be MEV‑optimized.

The guarantee is: once posted, the order cannot be changed or hidden.

Comparison table

Key takeaway

Ethereum and Bitcoin L2s do not solve DA themselves. They order fast off‑chain, then rely on L1 to make that history public and permanent. That separation is exactly why rollups scale without re‑inventing consensus.