> For the complete documentation index, see [llms.txt](https://docs.xyo.network/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.xyo.network/developers/cookbook/build-a-local-indexer.md).

# Build a Local Indexer

Build a restartable read model from finalized XL1 blocks.

A reliable indexer is a finalized-block stream plus a durable checkpoint and an idempotent projection.

{% stepper %}
{% step %}

## Choose a checkpoint

Store the hash of the last block your indexer completed. A block number alone is not enough to verify continuity.
{% endstep %}

{% step %}

## Follow finalized blocks

Use `finalizedBlockStreamFromGateway` from `@xyo-network/xl1-sdk`. Do not poll raw RPC endpoints or index unfinalized state as final.
{% endstep %}

{% step %}

## Validate application payloads

Treat block and transaction envelopes as chain-validated, but validate dereferenced application payload bodies against your public schema before projecting them.
{% endstep %}

{% step %}

## Write idempotently

Key projection records by stable chain evidence, such as transaction hash plus payload position. Reprocessing the same finalized block should not duplicate data.
{% endstep %}

{% step %}

## Advance after commit

Persist the checkpoint only after the projection transaction succeeds. On restart, resume from that checkpoint.
{% endstep %}
{% endstepper %}


---

# 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 by asking a question.

Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters:

```
GET https://docs.xyo.network/developers/cookbook/build-a-local-indexer.md?ask=<question>&goal=<user_goal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case.

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.