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Combined Status from WECs#

Note on terminology: the general idea that we wish to address is returning reported state about a workload object to its WDS. At the current level of development, we equate reported state with the status section of an object --- while anticipating a more general treatment in the future.

There are two methods of returning reported state: a general one and a special case. The general method returns reported state from any number of WECs. The special case applies when the number of WECs is exactly 1, and returns the reported state into the original object in the WDS. It may make sense to expand the latter into a general feature in the future, accommodating features such as scheduling.

Introduction to the General Technique#

The general technique for combining reported state from WECs is built upon the following ideas:

  1. The way that reported state is combined is specified by the user, in a simple but powerful way modeled on SQL. This is chosen because it is a well worked out set of ideas, is widely known, and is something that we may someday want to use in our implementation. We do not need to support anything like full SQL (for any version of SQL). This proposal only involves one particular pattern of relatively simple SELECT statement, and a different expression language (CEL, which is the most prominent expression language in the Kubernetes milieu).

  2. The expressions may reference the content of the workload object as it sits in the WDS as well as the state returned from the WECs.

  3. The specification of how to combine reported state is defined in StatusCollector objects. These objects are referenced in the BindingPolicy objects right next to the criteria for selecting workload objects. This saves users the trouble of having to write selection criteria twice. With the specification being separate rather than embedded, it is possible to have a library of StatusCollectors that can be reused across different BindingPolicy objects. In the future KubeStellar would provide a library of StatusCollector objects that cover convenient use-cases for kubernetes built-in resources such as deployments.

  4. The Binding objects also hold references to StatusCollector objects. Each reference to a workload object is paired with references to all the StatusCollectors mentioned in all the DownsyncObjectTestAndStatusCollection structs that matched the workload object.

  5. The combined reported state appears in a new kind of object, one per (workload object, Binding object) pair.

  6. A user can request a list without aggregation, possibly after filtering, but certainly with a limit on list length. The expectation is that such a list makes sense only if the length of the list will be modest. For users that want access to the full reported state from each WEC for a large number of WECs, KubeStellar should have an abstraction that gives the users access --- in a functional way, not by making another copy --- to that state (which is already in the mailbox namespaces).

  7. The reported state for a given workload object from a given WEC is implicitly augmented with metadata about the WEC and about the end-to-end propagation from WDS to that WEC. This extra information is available just like the regular contents of the object, for use in combining reported state.

  8. The specifics of queryable objects and implicit augmentations can be found in types.go and is specified in Queryable Objects.

Relation with SQL#

Overview of Relation with SQL#

To a given workload object, and in the context of a given Binding object, the user has bound some StatusCollector objects. The meaning of a StatusCollector in the context of a (workload object, Binding object) pair is analogous to an SQL SELECT statement that does the following things.

  1. The SELECT statement has one input table, which has a row per WEC that the Binding says the workload object should go to.

  2. The SELECT statement can have a WHERE clause that filters out some of the rows.

  3. The SELECT statement either does aggregation or does not. In the case of not doing aggregation, the SELECT statement simply has a collection of named expressions defining the columns of its output.

  4. In the case of aggregation, the SELECT statement has the following.

    • An optional GROUP BY clause saying how the rows (WECs) are grouped to form the inputs for aggregation, in terms of named expressions. For convenience here, each of these named expressions is implicitly included in the output columns.

    • A collection of named expressions using aggregation functions to define additional output columns.

  5. The SELECT statement has a LIMIT on the number of rows that it will yield.

Detailed Relation with SQL#

For a given workload object, Binding, and StatusCollector: start with a table named PerWEC. This table has one primary key column and it holds the name of the WEC that the reported state is from. The dependent columns hold the workload object content from the WDS, the workload object content returned from the WEC, and the augmentations.

There are three forms of StatusCollector, equivalent to three forms of SQL statement.

Plain selection#

When the StatusCollector has selection but no "GROUP BY" and no aggregation, this is equivalent to the following form of SELECT statement. The List of stale WECs example below is an example of this form.

SELECT <selected columns>
FROM PerWEC WHERE <filter condition>
LIMIT <limit>
Aggregation without GROUP BY#

When there is aggregation but no plain selection and no GROUP BY, this is equivalent to the following form of SELECT statement. The Number of WECs example below is an example of this form.

SELECT <aggregation columns>
FROM PerWEC WHERE <filter condition>
LIMIT <limit>
Aggregation with GROUP BY#

When there is GROUP BY and aggregation but no plain selection, this is equivalent to the following form of SELECT statement. The Histogram of Pod phase example below is an example of this form.

SELECT <group-by column names>, <aggregation columns>
FROM (SELECT <group-by column 1 expr> AS <group-by column 1 name>,
             ...
             <group-by column N expr> AS <group-by column N name>,
             *
      FROM PerWEC WHERE <filter condition>)
GROUP BY <group-by column names>
LIMIT <limit>

When there are N GROUP BY columns, the result has a row for each tuple of values (v1, v2, ... vN) such that there exists a WEC for which (v1, v2, ... vN) are the values of the GROUP BY columns. The result has no more rows than that.

Specification of the general technique#

See types.go.

Queryable Objects#

A CEL expression within a StatusCollector can reference the following objects:

  1. inventory: The inventory object for the workload object:

  2. inventory.name: The name of the inventory object.

  3. obj: The workload object from the WDS:

  4. All fields of the workload object except the status subresource.

  5. returned: The reported state from the WEC:

  6. returned.status: The status section of the object returned from the WEC.

  7. propagation: Metadata about the end-to-end propagation process:

  8. propagation.lastReturnedUpdateTimestamp: metav1.Time of last update to any returned state.

Examples of using the general technique#

Number of WECs#

The StatusCollectorSpec would look like the following.

combinedFields:
   - name: count
     type: COUNT

The analogous SQL statement would look something like the following.

SELECT COUNT(*) AS count FROM PerWEC LIMIT <something>

The table resulting from this would have one column and one row. The one value in this table would be the number of WECs.

Histogram of Pod phase#

The StatusCollectorSpec would look like the following.

groupBy:
   - name: phase
     def: returned.status.phase
combinedFields:
   - name: count
     type: COUNT

The analogous SQL statement would look something like the following.

SELECT phase, COUNT(*) AS count
FROM (SELECT <SQL expression for returned.status.phase> AS phase, *
      FROM PerWEC)
GROUP BY phase
LIMIT <something>

The result would have two columns, holding a phase value and a count. The number of rows equals the number of different values of returned.status.phase that appear among the WECs. For each row (P, N): P is a phase value that appears in at least one WEC, and N is the number of WECs where the phase value is P.

Histogram of number of available replicas of a Deployment#

This reports, for each number of available replicas, how many WECs have that number.

groupBy:
   - name: numAvailable
     def: returned.status.availableReplicas
combinedFields:
   - name: count
     type: COUNT

List of WECs where the Deployment is not as available as desired#

filter: "obj.spec.replicas != returned.status.availableReplicas"
select:
   - name: wec
     def: inventory.name

Full status from each WEC with information retrieval time#

This produces a listing of object status paired with inventory object name.

select:
   - name: wec
     def: inventory.name
   - name: status
     def: returned.status
   - name: retrievalTime
     def: propagation.lastReturnedUpdateTimestamp

Special case for 1 WEC#

This proposal refines the meaning of BindingPolicySpec.WantSingletonReportedState to request the special case when it applies.