Legal and regulatory requirements vary by jurisdiction and structure. This guide is informational and does not state what any institution must do. Consult your own legal and regulatory advisors.
Banks and exchanges have two main options on the OP Stack: OP Mainnet, a shared public network, or a dedicated chain through OP Enterprise, the Optimism Foundation's product offered Self Managed or Fully Managed. Which fits depends on how much of the ordering and operations setup the institution wants to own.
How does transaction ordering work on an OP Stack chain?
A single party, the sequencer, builds L2 blocks, executes transactions and submits the resulting data to Ethereum. Unlike Ethereum, OP Stack chains in the standard configuration have no public mempool, so pending transactions are visible only to the sequencer. The execution client orders them by highest priority fee first. OP Stack chains have their own block production timing. OP Mainnet, for example, produces a block every two seconds.
- A user sends a transaction to the chain.
- It waits in the sequencer's private mempool.
- The execution client orders pending transactions by priority fee.
- The sequencer seals a block.
- The chain operator's batcher posts the transaction data to Ethereum.
The execution client decides ordering. The rollup node fixes only what the protocol requires, such as the Ethereum origin and deposits.
Source: OP Stack protocol overview.
How does a sequencer mempool compare with a public mempool?
A public mempool lets anyone watching it see pending transactions, which is what creates opportunities for front-running. A sequencer mempool limits that visibility to one party but doesn't remove the exposure, because that party still sees everything pending.
| Question | Public mempool (Ethereum L1) | Sequencer mempool (OP Stack chain) |
|---|---|---|
| Who sees pending transactions? | Anyone watching the pool | Only the sequencer |
| Effect on front-running strategies | Public visibility creates MEV opportunities | Limits the visibility those strategies depend on |
| What remains | Visibility is open to all observers | The sequencer still sees every pending transaction |
What does a chain operator control today?
The chain operator handles the sequencer steps, from receiving transactions to publishing data to Ethereum, while the execution client makes the ordering decisions. On a Fully Managed chain, the Optimism Foundation manages that operator role as a managed service through OP Enterprise. OP Mainnet also ran a time-boxed ordering experiment in 2026, which has concluded and is publicly documented in the Optimism blog post and the Optimism notice.
What may banks and exchanges consider about transaction ordering?
The considerations differ by audience. Treat this as a list of considerations, not a compliance standard.
| Audience | What they may consider | What exists on the OP Stack today |
|---|---|---|
| Banks | Client orders not visible to outside parties before execution. Ordering explainable to auditors. Clear support terms. | Private sequencer mempool. Documented priority-fee ordering. Support terms vary by OP Enterprise plan. |
| Exchanges | Predictable treatment of customer orders. Latency and throughput. Documented priority rules. | Documented priority-fee rule. Subblock preconfirmations on a 200 ms target interval. |
| Fintechs and large companies | Stable, documented ordering behavior before payments or settlement run on it. | Standard priority-fee ordering on OP Mainnet and on OP Enterprise chains. |
What are Subblocks, and how do they speed up confirmations?
Subblocks, formerly called Flashblocks, are partial blocks the sequencer streams while it builds an L2 block, giving a preconfirmation before the block is sealed. Fast preconfirmations matter for exchanges confirming customer orders. The stream interval is chain-configurable, typically 200 ms, and it's a target, not a guarantee.
Preconfirmations can be revoked, and Subblocks do not change the OP Stack's validity or finality rules. For how and when a transaction reaches finality, see the transaction finality documentation. Subblocks are available to OP Enterprise chains.
Sources: Enabling Subblocks and the Subblocks specification.
This guide covers the OP Stack. Other chains also offer ways to manage transaction ordering.
Frequently Asked Questions
How can a bank limit pending-transaction visibility on an OP Stack chain?
On an OP Stack chain the mempool is not public. Only the sequencer sees pending transactions, which limits the visibility that front-running strategies on public mempools depend on. The sequencer still sees every pending transaction, and no design removes MEV exposure entirely. OP Enterprise is the Optimism Foundation's product for dedicated chains, offered Self Managed or Fully Managed.
How can an exchange get predictable transaction ordering on an OP Stack chain?
Ordering follows a documented rule: the execution client orders by highest priority fee first. Subblocks add preconfirmations streamed on a target interval of about 200 ms, available to OP Enterprise chains. Preconfirmations are revocable, so they aren't final.
What ordering behavior can a fintech or large company expect?
Fintechs can use OP Mainnet, a shared public network, or a dedicated chain through OP Enterprise, and both use priority-fee ordering by the sequencer today. Support terms vary by OP Enterprise plan. The Optimism Foundation does not provide compliance or regulatory advice, and legal requirements vary by jurisdiction and structure.
Can OP Stack chains use custom ordering rules?
OP Mainnet and OP Enterprise chains use standard priority-fee ordering today. OP Mainnet ran a time-boxed ordering experiment in 2026, which has concluded and is publicly documented in the Optimism blog post. Custom ordering is not a current feature of OP Mainnet.
Does OP Mainnet have a public mempool?
No. The sequencer's mempool is private and visible only to the sequencer. That limits pending-transaction visibility for outside observers, but the sequencer still sees every pending transaction.
Glossary
- Transaction ordering: the sequence in which a chain processes transactions in a block.
- Inclusion: whether a transaction makes it into a block.
- MEV (maximal extractable value): profit a party earns by choosing how transactions are ordered or included.
- Front-running: placing a transaction ahead of someone else's after seeing it pending.
- Mempool: the waiting area for pending transactions.
- Sequencer: the single party on an OP Stack chain that builds L2 blocks, orders transactions and submits data to Ethereum.
- Subblocks: partial blocks the sequencer streams while building an L2 block, giving a revocable preconfirmation. Formerly called Flashblocks.
- OP Labs and The Optimism Foundation: The Optimism Foundation is the technology service provider for the Ethereum Layer 2 services, while OP Labs is the Foundation's primary subcontractor for these services. Labs also provides the Foundation with business development and deal structuring support for its Layer 2 commercial offerings.
Related reading
- What is the OP Stack?
- What is OP Enterprise?
- Enterprise blockchain reliability: uptime, SLA and monitoring
The Optimism Foundation provides blockchain technology. It does not provide legal, regulatory or financial advice. Legal and regulatory requirements vary by jurisdiction and structure, so consult your own legal and regulatory advisors. This page is informational. It is not an offer.
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