Base services are services that OpenStack components can assume will be present in any OpenStack deployment. OpenStack components may therefore leverage advanced features exposed by those base services without fear of increasing the overall operational complexity for OpenStack deployers.
There are two related reasons behind the existence of this list. First and most straightforward, as stated by the Definition above, OpenStack components are allowed to rely on a solution listed here without needing to provide an alternative or fallback mechanism to account for situations when the solution is not present. The other and perhaps more subtle reason is to drive consistency of implementation between different OpenStack components who may have similar needs but would otherwise likely end up embedding their own varied implementations.
When someone developing a new feature is aware that there is a standard solution provided by this list, they are likely (and encouraged) to use it instead of designing something from scratch. This helps mitigate the risk that multiple components might otherwise independently provide similar but divergent solutions to the same basic problem space. It's also intended to encourage more useful base functionality by default across OpenStack components, because the perceived cost (to performance or complexity) of including this one extra dependency is lessened by each other component of the system which may also benefit from using it.
- A Castellan-compatible key store
- OpenStack components may keep secrets in a key store, using Oslo's Castellan library as an indirection layer. While OpenStack provides a Castellan-compatible key store service, Barbican, other key store backends are also available for Castellan. Note that in the context of the base services set Castellan is intended only to provide an interface for services to interact with a key store, and it should not be treated as a means to proxy API calls from users to that key store. In order to reduce unnecessary exposure risks, any user interaction with secret material should be left to a dedicated API instead (preferably as provided by Barbican).
- An oslo.db-compatible database
- OpenStack components store data in a database, using oslo.db as an indirection layer. While most OpenStack deployments use MySQL, other databases are supported.
- An oslo.messaging-compatible message queue
- Some inter-process and inter-service communication in OpenStack components is accomplished using message queues through oslo.messaging as an indirection layer. While most OpenStack deployments use RabbitMQ, other message queues are supported.
- Etcd
- OpenStack components may use Etcd, a distributed reliable key-value store for distributed key locking, storing configuration, keeping track of service live-ness and other scenarios.
- Keystone
- Keystone handles AuthN/AuthZ for OpenStack components. Deployments can assume that Keystone will be present to perform that role.
Leveraging features from a base service (rather than working around limitations or badly reinventing the wheel) is key to reaching acceptable levels of stability, performance and scaling. However, since they will likely have to be deployed in most OpenStack deployments, base services increase the operational complexity of running OpenStack. It is therefore very important to balance those two sides and conservatively consider proposed additions to the base services list, especially when those additions introduce a whole new class of operational challenges.
Once services start to make use of advanced features in a base service, it is difficult to remove it from the list and make it a specific dependency instead. Removals from the base service list should therefore be a rare and carefully considered event.
Proposed modifications to this document require a formal vote from the Technical Committee membership.