> For the complete documentation index, see [llms.txt](https://docs.lenses.io/latest/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.lenses.io/latest/devx/6.0/user-guide/tutorials/sql-studio/accessing-headers.md).

# Accessing headers

Since version 0.11, Apache Kafka supports message headers. Headers are a typical concept in messaging systems like JMS and transport systems like TCP or HTTP.

They can be used for routing, filtering, and annotation. The header can contain pairs of `key=value`, where the `key` is always a string(text), and the `value` is an array of bytes.

Given that the actual value is an array of bytes, the developer who produces the messages knows whether this array of bytes represents a string, an int, a long, or a double. Lenses can visualize and use the record headers via SQL queries.

By default, Lenses fetches the headers of messages and displays them on the UI.

To view the value of a specific header you can run:

```sql
SELECT HEADERASSTRING("User") as user
FROM trips
LIMIT 100
```


---

# Agent Instructions
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## 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.lenses.io/latest/devx/6.0/user-guide/tutorials/sql-studio/accessing-headers.md?ask=<question>&goal=<endgoal>
```

`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.