Incorporate Tooz as service coordination!/story/2001842

This spec is part of the ironic-inspector HA work. To further split the inspector service, this spec proposes to introduce tooz as the base service for the coordination between ironic-inspector API and conductor services.

Problem description

Based on the previous work, the single process ironic-inspector is logically splitted into two services both running under oslo.service, namely ironic_inspector and ironic-inspector-conductor.

Currently the functional test uses fake messaging driver which only works within a single process, To split two services into two processes, we need to address it before we can split two services into respective executables. we can either add rabbitmq support for functional test environment or introduce another messaging mechanism like json-rpc. Since etcd is already an OpenStack base service, the community has a preference on the later one.

Even when services are splitted, we are facing the challenge of service coordination, for multiple inspector conductor services, we need a way to prevent racing of concurrent operation on the same node, or choosing which inspector conductor should the request be delivered to.

Proposed change

Includes tooz 1 as project requirement for the service coordination. tooz supports various backends via drivers 2 which provides features like distributed lock, group member management.

etcd 3 is a distributed key value store that provides a reliable way to store data across a cluster of machines. etcd is a backend driver supported by tooz, and it’s also a base service for the OpenStack platform 4. Note that tooz only supports ETCD API v3 for group management, to use etcd as a backend driver, libraries such as python-etcd3 or python-etcd3gw is needed.

All proposed work is implemented via tooz interfaces. Each service will create a coordinator and keep heartbeating, the example workflow for ironic-inspector API service is listed below:

  1. Create a coordinator with hostname

  2. Create a predefined group, bypass if the group already exists. By default, the group name is ironic_inspector.service_group.

  3. Upon each API request, the API service will query members from the group, randomly pick one conductor and deliver the request to it through rpc mechanism.

The example workflow for ironic-inspector conductor service is listed below:

  1. Create a coordinator with hostname

  2. Join the predefined group as a member, the group name is ironic_inspector.service_group by default. If the group does not exist, it will be created.

  3. Leave group when service is shutdown.

The spec will add a console script ironic-inspector-conductor for executing the ironic-inspector conductor service, add a wsgi wrapper for using the API service under WSGI containers, there will be an automatically created ironic-inspector-api-wsgi script for console.

To keep backwards compatibility, ironic-inspector continues to serve the single process mode, and sticks to use the fake messaging driver for now. It will be removed when gets obsolete. We expect to add json-rpc in the future.

Adds a boolean configuration option [DEFAULT]standalone to specify the executing mode of ironic-inspector. The ironic-inspector script only runs when standalone is True, while the script for splitted service of ironic-inspector API and conductor only runs when standalone is False.

There is no distributed locking support for ironic-inspector, this spec will introduce an abstract lock layer, and implement locking support based on tooz. After the spec is implemented, there will be two kind of locks: internal and tooz. The using of different locking types is decided internally and not exposed to end users at the moment. ironic-inspector running as a single process will adopt semaphore based internal lock, otherwise tooz lock will be used.


Though it’s totally workable to utilize database as the coordination source just like ironic, it would be much lighter if implemented with tooz. tooz also supports multiple backends, which brings more flexibility in deployement.

Data model impact


HTTP API impact


Client (CLI) impact


Ironic python agent impact


Ironic impact


Performance and scalability impact

Running ironic-inspector services distributedly is expected to have performance gaining, as well as the ability of horizontal scalablility.

Security impact


Deployer impact

A new configuration option [coordination]backend_url will be added to support the configuration of tooz backend driver. This is only used when [DEFAULT]standalone is False.

Developer impact


Upgrades and Backwards Compatibility

The single process ironic-inspector service is unchanged. However, for installations adopt distributed ironic-inspector services, corresponding libraries need to be installed for the tooz backend driver configured to be used by tooz. For example, python3-etcd3gw for interaction with etcd, python3-pymemcache for interaction with memcached.



Primary assignee:

kaifeng -

Other contributors:


Work Items

Implement proposed work.


tooz are required library support for ironic-inspector running as separated services. The dependency of tooz backend driver should be satisfied as well.


Will be covered by unittest and bifrost.