Replace Zookeeper, Memcache servicegroup driver with Tooz drivers

https://blueprints.launchpad.net/nova/+spec/tooz-for-service-groups

Nova’s zookeeper and memcache service group drivers have fallen into disrepair. tooz (a new oslo library that has been under development for around a year) is able to provide the grouping concepts in a way that means nova would no longer need to maintain the zookeeper and memcache service group drivers.

Problem description

Nova currently has to maintain, test and support db, memcache, zookeeper drivers. The db driver is currently the only widely deployed driver.

The memcache and zookeepr drivers have since fallen into disrepair. For example, the Zookeeper driver uses evzookeeper which is no longer actively maintained and doesn’t work with eventlet >= 0.17.1.

If Nova adds a tooz service group driver, it means Nova can deprecate the existing zookeeper and memcache drivers, and eventually no longer have to maintain that code.

Longer term, its hoped users will be able to migrate away from the db driver to the more promising systems supported by tooz, but this effort it outside the scope of this specification.

Use Cases

  • In scope: Migrate existing memcache and zookeeper driver users to the new tooz service group driver

  • Out of scope: Existing db driver users migrate to the tooz driver

Project Priority

None

Proposed change

Before this effort can start, we need to fix the service group API abstraction: https://blueprints.launchpad.net/nova/+spec/servicegroup-api-control-plane

The above effort is likely to extend the amount of the data that needs to be stored by tooz, such as if the node is enabled or disabled. At the same time, this will make clear many of the current limitations of the existing non-DB service group drivers that need to be avoided in the tooz driver.

A ToozServiceGroupDriver will be added that implements the existing ServiceGroupDriver interface. Although the above work may adjust the interface slightly.

It will simply wrap calls into the tooz library, and raise Nova specific exceptions for errors that are hit.

Alternatives

We could do nothing, but that is unlikely to get the broken drivers fixed, leading to lots of confusion for users, and painful upgrades.

We could just drop support for memcache and zookeeper service group drivers all together, but there appears to be a community that wants to support them via tooz and it would be good to support that community.

We could attempt a live-upgrade from Nova’s memcache and zookeeper drivers to the new tooz driver, but given those drivers appear to be currently broken, that seems like wasted effort. User feedback during the deprecation cycle can be used to decide if this is needed before we remove the drivers from the Nova code base.

Data model impact

The DB service group driver will be unaffected by this change.

The memcache and zookeeper drivers do not store information in those system in the same way that tooz stores information. But Nova is just a user of the tooz library, so we can skip over the details of that change here.

REST API impact

None

Security impact

None

Notifications impact

None

Other end user impact

None

Performance Impact

None expected, there will still need to be periodic heartbeats into memcache to ensure the memcache key is not expired, zookeeper clients (even using kazoo also need to periodically check-in with zookeeper) so nothing should be drastically different in this arena.

Other deployer impact

There will be a new configuration value for the tooz driver, using a URL. A few examples of this URL format:

servicegroup_tooz_url = "kazoo://127.0.0.1:2181?timeout=5"
servicegroup_tooz_url = "memcached://localhost:11211?timeout=5"
servicegroup_tooz_url = "redis://localhost:6379?timeout=5"

Given the current state of Nova’s memcache and zookeeper drivers, we are adopting a very simple transition approach. It will be something like:

  • Upgrade to liberty, using existing service group driver

  • Restart nova-compute first.

  • Update configuration to point at new service group driver

  • Restart nova-api after updating configuration.

We will look for feedback from users on this approach during the deprecation window for the memcache and zookeeper drivers, to assess if the simpler approach will be sufficient.

Developer impact

Nova developers may have to interact more with the oslo community (and the tooz subcommunity) when learning, understanding, and integrating with tooz.

Implementation

Assignee(s)

Primary assignee: vilobhmm

Work Items

  • Implement the new tooz servicegroup driver.

  • Document the migration from existing drivers to the tooz driver, including the implementation of any required data migration.

  • Add deprecation warnings for memcache and zookeeper drivers.

  • Complete testing work detailed below.

Dependencies

This spec adds a dependency on tooz

This work also depends on the work to tidy up the service group API: https://blueprints.launchpad.net/nova/+spec/servicegroup-api-control-plane

Testing

  • Work with infra to ensure one of the gate jobs starts using the tooz driver

Documentation Impact

  • Describe the new tooz driver and its configuration options

  • Describe how to migrate from memcacahe or zookeeper to tooz

  • Communicate the deprecation of the memcache and zookeeper drivers