> For the complete documentation index, see [llms.txt](https://docs.theradius.xyz/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.theradius.xyz/overview/lighthouse/slot-ordering.md). # Slot-Ordering Lighthouse uses a **slot-based transaction ordering mechanism.** A slot is a unit of transactions, comprising searcher bundles (from Lighthouse) and user transactions (from SBB). Slot ordering is run independently of fixed block times, managing the ordering of transactions within each slot. Once slots are submitted to the mempool and included in a block, they are executed in a *predetermined order* defined by the slot. This model guarantees deterministic outcomes and eliminates risks of arbitrary reordering. *** ### **Slot Structure** Each slot is divided into two segments to ensure value capture and user protection coexist: 1. **Top-of-Slot (Priority Txs):** Arbitrage bundles from searchers via Lighthouse auctions. These transactions backrun the previous slots (user transactions) to capture market opportunities. 2. **Bottom-of-Slot (User Txs):** User transactions from SBB, which are protected from exploitation (e.g., frontrunning or sandwich attacks).

Figure 1. Slot Structure – Each slot comprises Lighthouse (searcher bundles) and SBB (user transactions), operating independently of block times.

*** ### **Why slots, not blocks?** Slot ordering solves key execution and latency limitations of standard block-based ordering: * **Network Congestion**: Fixed block intervals risk hitting gas or size limits during transaction surges, potentially preventing committed transaction orders from being included. * **Communication Delays**: Mitigates issues caused by latency between the rollup and external systems (e.g., SBB or Lighthouse) that disrupt timely block production. * **External Failures**: Prevents external system failures from cascading into the rollup's core transaction pipeline, ensuring predictable block production. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.theradius.xyz/overview/lighthouse/slot-ordering.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.