Add tpool proxy wrapper for DB-API calls

https://blueprints.launchpad.net/oslo-incubator/+spec/add-tpool-proxy-wrapper

During Icehouse release cycle in order to drop dependency on eventlet we removed eventlet tpool.Proxy helper and the corresponding config option (use_tpool) from oslo.db code. Projects were supposed to store those in their source trees. Since then we’ve got a lot of push back from Nova and Cinder teams based on their experience of adoption of oslo.db changes. To make things easier for oslo.db users, it’d actually be better to provide optional integration with eventlet tpool.Proxy as a separate module within oslo.db tree.

Problem description

Most OpenStack deployments nowadays use MySQL. Unfortunately, the most popular MySQL Python DBAPI driver - MySQL-Python - has a serious problem with eventlet green threads: being a C-extension it hangs the process on blocking DB queries (as eventlet can’t monkey patch it to force a green thread switch on blocking reads/writes from/to a socket).

eventlet provides a work around for this problem: tpool.Proxy helper class is meant to wrap classes/modules which methods/functions might possibly block the execution of a green thread. The call is performed in the context of a real OS thread, which knows how to deal with eventlet thread pool, on return it will switch the execution context back to a green thread, so the process doesn’t hang.

Common DB code used to provide use_tpool option which enabled tpool.Proxy proxying for all database methods calls. This was used by Nova at least. It’s worth mentioning, that in order to actually use use_tpool option one had to use a patched version of eventlet, as neither PyPI releases nor master HEAD play nicely with MySQL-Python.

During Icehouse release cycle we worked on removing extra dependencies from common DB code. eventlet was removed as one of the dependencies we thought were unnecessary (as oslo.db should neither enforce you to use a particular concurrency model, nor it can make any assumptions how the target projects process concurrent requests, whether they use eventlet green threads, real OS threads, multiple processing, etc). But the best is the enemy of the good, and it seems that it’s actually better to provide optional integration with eventlet tpool.Proxy, so the people wouldn’t need to do this in their source trees.

Proposed change

To make it possible to use oslo.db with MySQL-Python DB API driver and eventlet tpool.Proxy call proxying we need:

  1. Provide a helper that will either call DBAPI methods directly or use tpool.Proxy to wrap the calls depending on the value of use_tpool config option set.
  2. Make this proposed helper register use_tpool config option in group database.
  3. Add an additional entry point to oslo.db, that will expose use_tpool config option to projects.
  4. Even though we’ll use eventlet tpool.Proxy class at runtime, we are not going to add it to oslo.db requirements.txt (we don’t want eventlet to be an install time dependency; if you use eventlet, it will be up to you to ensure it’s installed) and test-requirements.txt (that would prevent us from running unit tests on Python 3.x versions; as we don’t need to test tpool.Proxy functionality itself, we could safely mock this class in tests).

Alternatives

The alternative way to solve this would be:

  1. Put this helper into openstack.common.concurrency module, which would depend on oslo.db.
  2. Document, that if someone wanted to use oslo.db with eventlet tpool.Proxy and MySQL-Python Python DB API driver, they would need to use a helper provided by openstack.common.concurrency module.

The advantage of this solution is that we don’t need to re-add eventlet stuff back to oslo.db.

The disadvantage of this solution is that we add more interdependencies between oslo.* libraries and make it harder to use oslo.db (it’d probably be not very clear to users why they need to use another library, if they use eventlet and MySQL together).

Impact on Existing APIs

None.

Security impact

None.

Performance Impact

Enabling of use_tpool improves performance of DB-bound services (APIs) a lot, when eventlet and MySQL-Python are used. Note: you will still need to use unreleased eventlet code.

Configuration Impact

use_tpool won’t be a new config file option, we just introduce it in the context of oslo.db. Currently, projects consume it from openstack.common.db package. No changes will be needed from deployers perspective.

Developer Impact

OpenStack projects that want to use oslo.db with eventlet tpool.Proxy call proxying will need to switch from using of DBAPI class to the new helper.

Implementation

Assignee(s)

Primary assignee:
akurilin
Other contributors:
rpodolyaka

Milestones

Target Milestone for completion:
Juno-1

Work Items

  1. Add a new module to oslo.db which will introduce tpool.Proxy helper and use_tpool config option.
  2. Add a new entry point to oslo.db exposing use_tpool config option.

Incubation

None.

Adoption

None.

Library

None.

Anticipated API Stabilization

None.

Documentation Impact

use_tpool should be marked as experimental. It should be stated in docs, that patched version of eventlet is needed to use it.

Dependencies

To work properly, this depends on the unreleased version of eventlet. Though, we’ve been providing use_tpool option so far anyway. Rackspace uses patched version of eventlet in production. We are going to leave it up to deployers to decide whether to enable this or not (default was and remains False).

References

A PoC patch on review:

https://review.openstack.org/#/c/96467/

The related ML thread:

http://lists.openstack.org/pipermail/openstack-dev/2014-April/033120.html

The Launchpad bug for tracking usage of use_tpool in projects:

https://bugs.launchpad.net/nova/+bug/1309297

eventlet tpool docs:

http://eventlet.net/doc/threading.html#module-eventlet.tpool

The patch fixing the eventlet issue:

https://bitbucket.org/eventlet/eventlet/pull-request/29/

Note

This work is licensed under a Creative Commons Attribution 3.0 Unported License. http://creativecommons.org/licenses/by/3.0/legalcode