psobot opened a new pull request #14706:
URL: https://github.com/apache/beam/pull/14706
This pull request fixes a subtle bug that occurs reliably for me on Google
Cloud Dataflow. When using `save_main_session`, Python SDK workers will attempt
to download a `pickled_main_session` file from Google Cloud Storage, and will
use that file to reload the `__main__` session. Pipelines that depend on this
functionality expect names defined in `__main__` to be present at runtime, and
will fail with `NameError`s if the main session is not available.
Unfortunately, roughly 0.5% of the time (in my experience), the Python SDK
worker fails to download `pickled_main_session` from GCS to the local worker
machine, and an empty file is created in its place. This causes Dill to log an
`EOFError`<sup>1</sup>, but then continues to load and run the user code, which
then fails due to missing names. For small pipelines (under 20 workers) this
almost never occurs; but larger pipelines with hundreds of workers often have
at least one machine fail to load the `__main__` session due to this bug, and
the subsequent `NameError` exceptions cause the entire batch job to fail.
This patch does three things:
- checks to see if the pickled main session file is empty
- if so, raises a new type of exception (`CorruptMainSessionException`)
- catches that exception, logs it, and then re-raises, causing the worker
to fail to boot.
Admittedly, I don't know if this is the right solution or if it will
generalize to all Beam use cases, but I've found this patch to work effectively
in my team's pipelines. If a corrupt main session is detected, the worker fails
to boot, Cloud Dataflow restarts the container, and the pickled main session
loads correctly on the second try. An alternative implementation might be to
ensure that the pickled main session's file size matches the size provided by
the remote filesystem.
R: @robertwb
R: @lukecwik
---
<sup>1</sup> Example traceback from an empty main session file:
```
Traceback (most recent call last):
File
"/usr/local/lib/python3.8/site-packages/apache_beam/runners/worker/sdk_worker_main.py",
line 131, in main
_load_main_session(semi_persistent_directory)
File
"/usr/local/lib/python3.8/site-packages/apache_beam/runners/worker/sdk_worker_main.py",
line 236, in _load_main_session
pickler.load_session(session_file)
File
"/usr/local/lib/python3.8/site-packages/apache_beam/internal/pickler.py", line
318, in load_session
return dill.load_session(file_path)
File "/usr/local/lib/python3.8/site-packages/dill/_dill.py", line 368, in
load_session
module = unpickler.load()
File "/usr/local/lib/python3.8/site-packages/dill/_dill.py", line 472, in
load
obj = StockUnpickler.load(self)
EOFError: Ran out of input
```
------------------------
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`ValidatesRunner` compliance status (on master branch)
--------------------------------------------------------
<table>
<thead>
<tr>
<th>Lang</th>
<th>ULR</th>
<th>Dataflow</th>
<th>Flink</th>
<th>Samza</th>
<th>Spark</th>
<th>Twister2</th>
</tr>
</thead>
<tbody>
<tr>
<td>Go</td>
<td>---</td>
<td>
<a
href="https://ci-beam.apache.org/job/beam_PostCommit_Go/lastCompletedBuild/";>
<img alt="Build Status"
src="https://ci-beam.apache.org/job/beam_PostCommit_Go/lastCompletedBuild/badge/icon";>
</a>
<td>
<a
href="https://ci-beam.apache.org/job/beam_PostCommit_Go_VR_Flink/lastCompletedBuild/";>
<img alt="Build Status"
src="https://ci-beam.apache.org/job/beam_PostCommit_Go_VR_Flink/lastCompletedBuild/badge/icon";>
</a>
</td>
<td>---</td>
<td>
<a
href="https://ci-beam.apache.org/job/beam_PostCommit_Go_VR_Spark/lastCompletedBuild/";>
<img alt="Build Status"
src="https://ci-beam.apache.org/job/beam_PostCommit_Go_VR_Spark/lastCompletedBuild/badge/icon";>
</a>
</td>
<td>---</td>
</tr>
<tr>
<td>Java</td>
<td>
<a
href="https://ci-beam.apache.org/job/beam_PostCommit_Java_ValidatesRunner_ULR/lastCompletedBuild/";>
<img alt="Build Status"
src="https://ci-beam.apache.org/job/beam_PostCommit_Java_ValidatesRunner_ULR/lastCompletedBuild/badge/icon";>
</a>
</td>
<td>
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<img alt="Build Status"
src="https://ci-beam.apache.org/job/beam_PostCommit_Java_ValidatesRunner_Dataflow/lastCompletedBuild/badge/icon?subject=V1";>
</a><br>
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<td>
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</td>
</tr>
<tr>
<td>Python</td>
<td>---</td>
<td>
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<td>
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</a>
</td>
<td>---</td>
<td>
<a
href="https://ci-beam.apache.org/job/beam_PostCommit_Python_VR_Spark/lastCompletedBuild/";>
<img alt="Build Status"
src="https://ci-beam.apache.org/job/beam_PostCommit_Python_VR_Spark/lastCompletedBuild/badge/icon";>
</a>
</td>
<td>---</td>
</tr>
<tr>
<td>XLang</td>
<td>
<a
href="https://ci-beam.apache.org/job/beam_PostCommit_XVR_Direct/lastCompletedBuild/";>
<img alt="Build Status"
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<td>
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<img alt="Build Status"
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</a>
</td>
<td>---</td>
<td>
<a
href="https://ci-beam.apache.org/job/beam_PostCommit_XVR_Spark/lastCompletedBuild/";>
<img alt="Build Status"
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</a>
</td>
<td>---</td>
</tr>
</tbody>
</table>
Examples testing status on various runners
--------------------------------------------------------
<table>
<thead>
<tr>
<th>Lang</th>
<th>ULR</th>
<th>Dataflow</th>
<th>Flink</th>
<th>Samza</th>
<th>Spark</th>
<th>Twister2</th>
</tr>
</thead>
<tbody>
<tr>
<td>Go</td>
<td>---</td>
<td>---</td>
<td>---</td>
<td>---</td>
<td>---</td>
<td>---</td>
<td>---</td>
</tr>
<tr>
<td>Java</td>
<td>---</td>
<td>
<a
href="https://ci-beam.apache.org/job/beam_PreCommit_Java_Examples_Dataflow_Cron/lastCompletedBuild/";>
<img alt="Build Status"
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</a><br>
</td>
<td>---</td>
<td>---</td>
<td>---</td>
<td>---</td>
<td>---</td>
</tr>
<tr>
<td>Python</td>
<td>---</td>
<td>---</td>
<td>---</td>
<td>---</td>
<td>---</td>
<td>---</td>
<td>---</td>
</tr>
<tr>
<td>XLang</td>
<td>---</td>
<td>---</td>
<td>---</td>
<td>---</td>
<td>---</td>
<td>---</td>
<td>---</td>
</tr>
</tbody>
</table>
Post-Commit SDK/Transform Integration Tests Status (on master branch)
------------------------------------------------------------------------------------------------
<table>
<thead>
<tr>
<th>Go</th>
<th>Java</th>
<th>Python</th>
</tr>
</thead>
<tbody>
<tr>
<td>
<a
href="https://ci-beam.apache.org/job/beam_PostCommit_Go/lastCompletedBuild/";>
<img alt="Build Status"
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</a>
</td>
<td>
<a
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<img alt="Build Status"
src="https://ci-beam.apache.org/job/beam_PostCommit_Java/lastCompletedBuild/badge/icon";>
</a>
</td>
<td>
<a
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<img alt="Build Status"
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</a>
</td>
</tr>
</tbody>
</table>
Pre-Commit Tests Status (on master branch)
------------------------------------------------------------------------------------------------
<table>
<thead>
<tr>
<th>---</th>
<th>Java</th>
<th>Python</th>
<th>Go</th>
<th>Website</th>
<th>Whitespace</th>
<th>Typescript</th>
</tr>
</thead>
<tbody>
<tr>
<td>Non-portable</td>
<td>
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<td>---</td>
<td>---</td>
<td>---</td>
</tr>
</tbody>
</table>
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