Currently, the pagination support for Nova does not allow the caller to specify the sort order and direction of the data set. This blueprint enhances the pagination support for the /servers and /servers/detail APIs so that multiple sort keys and sort directions can be supplied on the request.
There is no support for retrieving server data in a specific order, it is defaulted to descending sort order by the "created date" and "id" keys. In order to retrieve data in any sort order and direction, the REST APIs need to accept multiple sort keys and directions.
Use Case: A UI that displays a table with only the page of data that it has retrieved from the server. The items in this table need to be sorted by status first and by display name second. In order to retrieve data in this order, the APIs must accept multiple sort keys/directions.
The /servers and /servers/detail APIs will support the following parameters being repeated on the request:
The caller can specify these parameters multiple times in order to generate a list of sort keys and sort directions. The first key listed is the primary key, the next key is the secondary key, etc.
For example: /servers?sort_key=status&sort_dir=desc&sort_key=display_name& &sort_dir=desc&sort_key=created_at&sort_dir=desc
Note: The "created_at" and "id" sort keys are always appended at the end of the key list if they are not already specified on the request.
The database layer already supports multiple sort keys and directions. This blueprint will update the API layer to retrieve the sort information from the API request and pass that information down to the database layer.
All sorting is handled in the sqlalchemy.utils.paginate_query function. This function accepts an ORM model class as an argument and the only valid sort keys are attributes on the given model class. Therefore, the valid sort keys are limited to the model attributes on the models.Instance class.
For the v2 API a new 'os-server-sort-keys' API extension will be created to signify that the new sorting parameters proposed in this blueprint should be honored. The v3 API will support the new sorting parameters by default.
Repeating parameter key/values was chosen because glance already did it:
However, the list of sort keys and directions could be built by splitting the associated parameter values.
For example: /servers?sort_key=status,display_name,created_at&sort_dir=desc,desc,desc
The downside of this approach is that it would require pre-defined token characters. I'm open to this solution if it is deemed better.
A new v2 API extension will be created to signify that the new sorting parameters should be honored. Extension details:
Note that this API extension will behave in the same manner as the current 'os-user-data' extension. For example, the extension is defined as:
And it is checked in the v2 server API here:
The following existing v2 GET APIs will support the new sorting parameters if the 'os-server-sort-keys' API extenstion is loaded:
The following existing v3 GET APIs will support the new sorting parameters by default:
Note that the design described in this blueprint could be applied to other GET REST APIs but this blueprint is scoped to only those listed above. Once this design is finalized, then the same approach could be applied to other APIs.
The existing API documentation needs to be updated to include the following new Request Parameters:
|sort_key||query||string||Sort key (repeated for multiple), keys default to 'created_at' and 'id'|
|sort_dir||query||string||Sort direction, either 'asc' or 'desc' (repeated for multiple), defaults to 'desc'|
Neither the API response format nor the return codes will be modified, only the order of the servers that are returned.
In the event that an invalid sort key is specifed then a "badRequest" error response (code 400) will be returned with a message like "Invalid input received: Invalid sort key".
The novaclient should be updated to accept sort keys and sort directions, new parameters:
|--sort-keys||Comma-separated list of sort keys used to specify server ordering. Each key must be paired with a sort direction value.|
|--sort-dirs||Comma-separated list of sort directions used to specify server ordering. Each key Must be paired with a sort key value. Valid values are 'asc' and 'desc'.|
All sorting will be done in the database. The choice of sort keys is limited to attributes on the models.Instance ORM class -- not every attribute key returned from a detailed query is a valid sort key.
Performance data was gathered by running on a simple devstack VM with 2GB memory. 5000 instances were inserted into the DB. The data shows that the sort time on the main data table is dwarfed (see first table below) when running a detailed query -- most of the time is spent querying the other tables for each item; therefore, the impact of the sort key on a detailed query is minimal.
For example, the data below compares the processing time of a GET request for a non-detailed query to a detailed query with various limits using the default sort keys. The purpose of this table is to show the processing time for a detailed query is dominated by getting the additional details for each item.
|Limit||Non-Detailed (sec)||Detailed (sec)||Non-Detailed / Detailed %|
Non-detailed query data was also gathered. The table below compares the processing time using default sort keys to the processing using display_name as the sort key. Items were added with a 40 character display_name that was generated in an out-of-alphabetical sort order.
|Limit||Default keys (sec)||display_name key (sec)||Slowdown %|
In conclusion, the sort processing on the main data table has minimal impact on the overall processing time. For a detailed query, the sort time is dwarfed by other processing -- even if the sort time when up 3x it would only represent 4.8% of the total processing time for a detailed query with a limit of 1000 (and only increase the processing time by .11 sec with a limit of 50).
The choice of sort keys has a minimal impact on data retrieval performance (see performance data above). Therefore, the user should be allowed to retrieve data in whatever order they need to for creating their views (see use case in the Problem Description).
Ideally the logic for processing the sort parameters would be common to all components and would be done in oslo; a similar blueprint is also being proposed in cinder: https://blueprints.launchpad.net/cinder/+spec/cinder-pagination
Therefore, I see the following work items:
Both unit and Tempest tests need to be created to ensure that the data is retrieved in the specified sort order. Tests should also verify that the default sort keys ("created_at" and "id") are always appended to the user supplied keys (if the user did not already specify them).
Testing should be done against multiple backend database types.
The /servers and /servers/detail API documentation will need to be updated to:
The documentation could also note that query performance will be affected by the choice of the sort key, noting which keys are indexed.