---
metadata:
  - name: generator
    content: Diplodoc Platform v5.50.6
alternate:
  - https://ytsaurus.tech/docs/en/user-guide/problems/mapreduce-debug.md
  - https://ytsaurus.tech/docs/ru/user-guide/problems/mapreduce-debug.md
---
> **Documentation Index:** Fetch the complete configuration index at https://ytsaurus.tech/docs/en/llms.txt

# Debugging MapReduce programs

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## Emulating job startup locally

You can emulate job startup on any computer. For example, you can use your development server. To learn more about local job emulation, see [Debugging jobs locally](https://ytsaurus.tech/docs/en/user-guide/problems/jobtool.md) and [Slow job troubleshooting](https://ytsaurus.tech/docs/en/user-guide/problems/jobshell-and-slowjobs.md).
<!-- endsource: en/_includes/user-guide/problems/mapreduce-debug/local-emulation.md -->

<!-- source: en/_includes/user-guide/problems/mapreduce-debug/stderr-running.md -->
## Getting the current stderr of a running job

As an operation runs, there may be situations where there's still one incomplete job left while the rest have already finished. In YTsaurus, it's possible to get the stderr that the job has written up to a given moment.

You can do that with a special mode implemented in the `yt` command: `yt get-job-stderr`.

```bash
yt get-job-stderr --operation-id 35060d89-f9328e09-3f403e8-6f4eb4b5 --job-id fe270b54-a938652-3fc0384-2144
```

Stderr in the output is truncated according to the same rules that apply when you save files in Cypress: the first few megabytes plus the last few megabytes are shaved off.
<!-- endsource: en/_includes/user-guide/problems/mapreduce-debug/stderr-running.md -->

## Getting full stderr of all jobs of an operation

In YTsaurus, you can save full stderr of all jobs to a table. You can export stderr of those jobs that were not aborted.

To enable the described behavior:

{% list tabs %}

- In Python

  Use the `stderr_table` parameter. For example:

  ```python
  yt.wrapper.run_map_reduce( mapper, reducer, '//path/to/input', '//path/to/output', reduce_by=['some_key'], stderr_table='//path/to/stderr/table', )
  ```
- In С++

  Use the [StderrTablePath](https://github.com/ytsaurus/ytsaurus/blob/07c9c385ae116f56d8ecce9fa6765fa1a90e95cc/yt/cpp/mapreduce/interface/operation.h#L563) setting.

- In SDKs in other languages

  You can pass the `stderr_table_path` setting directly to the operation specification. For a description of this option, see [Operation options](https://ytsaurus.tech/docs/en/user-guide/data-processing/operations/operations-options.md).

{% endlist %}

<!-- source: en/_includes/user-guide/problems/mapreduce-debug/stderr-table.md -->
A stderr table has the following columns:

1. `job_id`: Job ID.
2. `part_index`: If a job has stderr that is too large, it is split into parts. The `part_index` value indicates the index of a specific stderr part.
3. `data`: Stderr data itself.

The table is sorted by `job_id`, `part_index`.

You can read such a table using the [read_blob_table](https://ytsaurus.tech/docs/en/user-guide/storage/blobtables.md#read_blob_table) command.
<!-- endsource: en/_includes/user-guide/problems/mapreduce-debug/stderr-table.md -->