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CHAINING.md

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Chaining

A feature that the query library adds is the ability to chain queries together.

Chaining, in this context means taking the results of a query, and using them to configure and run another query - be it on the same resource, or on another resource.

How then() works

The diagram below shows how we can use then() to perform chaining between two different Openstack Resources. then() will return the new query, shunting any results into new query if keep_previous_results = True see API.md for usage details

Diagram showing how then() works when going from ServerQuery to UserQuery

Here, then() finds all unique values of user_id from the results of the first ServerQuery query. Using these, it sets up a UserQuery with a pre-defined where() call - selecting for user_ids match what was returned in the first query

get_chain_mappings holds how to map the query to other queries. Which we look for in ServerMapping

How append_from() works

The diagram below show how we can use append_from() to perform chaining between two different Openstack Resources. append_from() will run a separate query internally, and shunt the results into original query, it will return the original query see API.md for usage details

Diagram showing how append_from() works when going from ServerQuery to UserQuery

Here, append_from() works almost identically to then(), only this time, it runs a select selecting for specified properties - e.g user_name that belong to UserQuery. This is then returned and concatenated into results via ResultContainer class as forwarded_props.

Shared common properties

A shared common property is are pairs of properties that hold the same information between two different openstack resources. These properties can either:

  1. Map one-to-Many. e.g. UserQuery to ServerQuery
  • the shared common property is user_id
  • A User can have many associated servers - so in this case one user_id can map to multiple servers
  1. Map Many-to-one. e.g. going backwards from ServerQuery to UserQuery
  • The shared common property is still user_id

Query Library supports both instances.

Successive Chaining and Multi-step chaining

The query library has functionality to allow multi-step chaining - this is currently left for the user to implement - via successive then() chaining.

If we find that there are workflows that require successive chaining - we can add it as a pattern - this is a WIP

Keeping Previous Results

A common workflow may want to keep certain results from previous chained queries (see API.md for how to do that)

  • append_from will automatically forward selected values as forwarded_props to results
  • then gives the user the option to

Note Many-to-one property chaining can lead to duplicates - this is left to the user to sort out