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Dependency resolution - Understanding Operators | Operators | OKD 4.11
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Operator Lifecycle Manager dependency resolution

About dependency resolution

Operator Lifecycle Manager (OLM) manages the dependency resolution and upgrade lifecycle of running Operators. In many ways, the problems OLM faces are similar to other system or language package managers, such as yum and rpm.

However, there is one constraint that similar systems do not generally have that OLM does: because Operators are always running, OLM attempts to ensure that you are never left with a set of Operators that do not work with each other.

As a result, OLM must never create the following scenarios:

  • Install a set of Operators that require APIs that cannot be provided

  • Update an Operator in a way that breaks another that depends upon it

This is made possible with two types of data:

Properties

Typed metadata about the Operator that constitutes the public interface for it in the dependency resolver. Examples include the group/version/kind (GVK) of the APIs provided by the Operator and the semantic version (semver) of the Operator.
Constraints or dependencies

An Operator’s requirements that should be satisfied by other Operators that might or might not have already been installed on the target cluster. These act as queries or filters over all available Operators and constrain the selection during dependency resolution and installation. Examples include requiring a specific API to be available on the cluster or expecting a particular Operator with a particular version to be installed.

OLM converts these properties and constraints into a system of Boolean formulas and passes them to a SAT solver, a program that establishes Boolean satisfiability, which does the work of determining what Operators should be installed.

Operator properties

All Operators in a catalog have the following properties:

olm.package

Includes the name of the package and the version of the Operator

olm.gvk

A single property for each provided API from the cluster service version (CSV)

Additional properties can also be directly declared by an Operator author by including a properties.yaml file in the metadata/ directory of the Operator bundle.

Example arbitrary property
properties:
- type: olm.kubeversion
  value:
    version: "1.16.0"

Arbitrary properties

Operator authors can declare arbitrary properties in a properties.yaml file in the metadata/ directory of the Operator bundle. These properties are translated into a map data structure that is used as an input to the Operator Lifecycle Manager (OLM) resolver at runtime.

These properties are opaque to the resolver as it does not understand the properties, but it can evaluate the generic constraints against those properties to determine if the constraints can be satisfied given the properties list.

Example arbitrary properties
properties:
  - property:
      type: color
      value: red
  - property:
      type: shape
      value: square
  - property:
      type: olm.gvk
      value:
        group: olm.coreos.io
        version: v1alpha1
        kind: myresource

This structure can be used to construct a Common Expression Language (CEL) expression for generic constraints.

Additional resources

Operator dependencies

The dependencies of an Operator are listed in a dependencies.yaml file in the metadata/ folder of a bundle. This file is optional and currently only used to specify explicit Operator-version dependencies.

The dependency list contains a type field for each item to specify what kind of dependency this is. The following types of Operator dependencies are supported:

olm.package

This type indicates a dependency for a specific Operator version. The dependency information must include the package name and the version of the package in semver format. For example, you can specify an exact version such as 0.5.2 or a range of versions such as >0.5.1.

olm.gvk

With this type, the author can specify a dependency with group/version/kind (GVK) information, similar to existing CRD and API-based usage in a CSV. This is a path to enable Operator authors to consolidate all dependencies, API or explicit versions, to be in the same place.

olm.constraint

This type declares generic constraints on arbitrary Operator properties.

In the following example, dependencies are specified for a Prometheus Operator and etcd CRDs:

Example dependencies.yaml file
dependencies:
  - type: olm.package
    value:
      packageName: prometheus
      version: ">0.27.0"
  - type: olm.gvk
    value:
      group: etcd.database.coreos.com
      kind: etcdCluster
      version: v1beta2

Generic constraints

An olm.constraint property declares a dependency constraint of a particular type, differentiating non-constraint and constraint properties. Its value field is an object containing a failureMessage field holding a string-representation of the constraint message. This message is surfaced as an informative comment to users if the constraint is not satisfiable at runtime.

The following keys denote the available constraint types:

gvk

Type whose value and interpretation is identical to the olm.gvk type

package

Type whose value and interpretation is identical to the olm.package type

cel

A Common Expression Language (CEL) expression evaluated at runtime by the Operator Lifecycle Manager (OLM) resolver over arbitrary bundle properties and cluster information

all, any, not

Conjunction, disjunction, and negation constraints, respectively, containing one or more concrete constraints, such as gvk or a nested compound constraint

Common Expression Language (CEL) constraints

The cel constraint type supports Common Expression Language (CEL) as the expression language. The cel struct has a rule field which contains the CEL expression string that is evaluated against Operator properties at runtime to determine if the Operator satisfies the constraint.

Example cel constraint
type: olm.constraint
value:
  failureMessage: 'require to have "certified"'
  cel:
    rule: 'properties.exists(p, p.type == "certified")'

The CEL syntax supports a wide range of logical operators, such as AND and OR. As a result, a single CEL expression can have multiple rules for multiple conditions that are linked together by these logical operators. These rules are evaluated against a dataset of multiple different properties from a bundle or any given source, and the output is solved into a single bundle or Operator that satisfies all of those rules within a single constraint.

Example cel constraint with multiple rules
type: olm.constraint
value:
  failureMessage: 'require to have "certified" and "stable" properties'
  cel:
    rule: 'properties.exists(p, p.type == "certified") && properties.exists(p, p.type == "stable")'

Compound constraints (all, any, not)

Compound constraint types are evaluated following their logical definitions.

The following is an example of a conjunctive constraint (all) of two packages and one GVK. That is, they must all be satisfied by installed bundles:

Example all constraint
schema: olm.bundle
name: red.v1.0.0
properties:
- type: olm.constraint
  value:
    failureMessage: All are required for Red because...
    all:
      constraints:
      - failureMessage: Package blue is needed for...
        package:
          name: blue
          versionRange: '>=1.0.0'
      - failureMessage: GVK Green/v1 is needed for...
        gvk:
          group: greens.example.com
          version: v1
          kind: Green

The following is an example of a disjunctive constraint (any) of three versions of the same GVK. That is, at least one must be satisfied by installed bundles:

Example any constraint
schema: olm.bundle
name: red.v1.0.0
properties:
- type: olm.constraint
  value:
    failureMessage: Any are required for Red because...
    any:
      constraints:
      - gvk:
          group: blues.example.com
          version: v1beta1
          kind: Blue
      - gvk:
          group: blues.example.com
          version: v1beta2
          kind: Blue
      - gvk:
          group: blues.example.com
          version: v1
          kind: Blue

The following is an example of a negation constraint (not) of one version of a GVK. That is, this GVK cannot be provided by any bundle in the result set:

Example not constraint
schema: olm.bundle
name: red.v1.0.0
properties:
- type: olm.constraint
  value:
  all:
    constraints:
    - failureMessage: Package blue is needed for...
      package:
        name: blue
        versionRange: '>=1.0.0'
    - failureMessage: Cannot be required for Red because...
      not:
        constraints:
        - gvk:
            group: greens.example.com
            version: v1alpha1
            kind: greens

The negation semantics might appear unclear in the not constraint context. To clarify, the negation is really instructing the resolver to remove any possible solution that includes a particular GVK, package at a version, or satisfies some child compound constraint from the result set.

As a corollary, the not compound constraint should only be used within all or any constraints, because negating without first selecting a possible set of dependencies does not make sense.

Nested compound constraints

A nested compound constraint, one that contains at least one child compound constraint along with zero or more simple constraints, is evaluated from the bottom up following the procedures for each previously described constraint type.

The following is an example of a disjunction of conjunctions, where one, the other, or both can satisfy the constraint:

Example nested compound constraint
schema: olm.bundle
name: red.v1.0.0
properties:
- type: olm.constraint
  value:
    failureMessage: Required for Red because...
    any:
      constraints:
      - all:
          constraints:
          - package:
              name: blue
              versionRange: '>=1.0.0'
          - gvk:
              group: blues.example.com
              version: v1
              kind: Blue
      - all:
          constraints:
          - package:
              name: blue
              versionRange: '<1.0.0'
          - gvk:
              group: blues.example.com
              version: v1beta1
              kind: Blue

The maximum raw size of an olm.constraint type is 64KB to limit resource exhaustion attacks.

Dependency preferences

There can be many options that equally satisfy a dependency of an Operator. The dependency resolver in Operator Lifecycle Manager (OLM) determines which option best fits the requirements of the requested Operator. As an Operator author or user, it can be important to understand how these choices are made so that dependency resolution is clear.

Catalog priority

On OKD cluster, OLM reads catalog sources to know which Operators are available for installation.

Example CatalogSource object
apiVersion: "operators.coreos.com/v1alpha1"
kind: "CatalogSource"
metadata:
  name: "my-operators"
  namespace: "operators"
spec:
  sourceType: grpc
  image: example.com/my/operator-index:v1
  displayName: "My Operators"
  priority: 100

A CatalogSource object has a priority field, which is used by the resolver to know how to prefer options for a dependency.

There are two rules that govern catalog preference:

  • Options in higher-priority catalogs are preferred to options in lower-priority catalogs.

  • Options in the same catalog as the dependent are preferred to any other catalogs.

Channel ordering

An Operator package in a catalog is a collection of update channels that a user can subscribe to in an OKD cluster. Channels can be used to provide a particular stream of updates for a minor release (1.2, 1.3) or a release frequency (stable, fast).

It is likely that a dependency might be satisfied by Operators in the same package, but different channels. For example, version 1.2 of an Operator might exist in both the stable and fast channels.

Each package has a default channel, which is always preferred to non-default channels. If no option in the default channel can satisfy a dependency, options are considered from the remaining channels in lexicographic order of the channel name.

Order within a channel

There are almost always multiple options to satisfy a dependency within a single channel. For example, Operators in one package and channel provide the same set of APIs.

When a user creates a subscription, they indicate which channel to receive updates from. This immediately reduces the search to just that one channel. But within the channel, it is likely that many Operators satisfy a dependency.

Within a channel, newer Operators that are higher up in the update graph are preferred. If the head of a channel satisfies a dependency, it will be tried first.

Other constraints

In addition to the constraints supplied by package dependencies, OLM includes additional constraints to represent the desired user state and enforce resolution invariants.

Subscription constraint

A subscription constraint filters the set of Operators that can satisfy a subscription. Subscriptions are user-supplied constraints for the dependency resolver. They declare the intent to either install a new Operator if it is not already on the cluster, or to keep an existing Operator updated.

Package constraint

Within a namespace, no two Operators may come from the same package.

Additional resources

CRD upgrades

OLM upgrades a custom resource definition (CRD) immediately if it is owned by a singular cluster service version (CSV). If a CRD is owned by multiple CSVs, then the CRD is upgraded when it has satisfied all of the following backward compatible conditions:

  • All existing serving versions in the current CRD are present in the new CRD.

  • All existing instances, or custom resources, that are associated with the serving versions of the CRD are valid when validated against the validation schema of the new CRD.

Additional resources

Dependency best practices

When specifying dependencies, there are best practices you should consider.

Depend on APIs or a specific version range of Operators

Operators can add or remove APIs at any time; always specify an olm.gvk dependency on any APIs your Operators requires. The exception to this is if you are specifying olm.package constraints instead.

Set a minimum version

The Kubernetes documentation on API changes describes what changes are allowed for Kubernetes-style Operators. These versioning conventions allow an Operator to update an API without bumping the API version, as long as the API is backwards-compatible.

For Operator dependencies, this means that knowing the API version of a dependency might not be enough to ensure the dependent Operator works as intended.

For example:

  • TestOperator v1.0.0 provides v1alpha1 API version of the MyObject resource.

  • TestOperator v1.0.1 adds a new field spec.newfield to MyObject, but still at v1alpha1.

Your Operator might require the ability to write spec.newfield into the MyObject resource. An olm.gvk constraint alone is not enough for OLM to determine that you need TestOperator v1.0.1 and not TestOperator v1.0.0.

Whenever possible, if a specific Operator that provides an API is known ahead of time, specify an additional olm.package constraint to set a minimum.

Omit a maximum version or allow a very wide range

Because Operators provide cluster-scoped resources such as API services and CRDs, an Operator that specifies a small window for a dependency might unnecessarily constrain updates for other consumers of that dependency.

Whenever possible, do not set a maximum version. Alternatively, set a very wide semantic range to prevent conflicts with other Operators. For example, >1.0.0 <2.0.0.

Unlike with conventional package managers, Operator authors explicitly encode that updates are safe through channels in OLM. If an update is available for an existing subscription, it is assumed that the Operator author is indicating that it can update from the previous version. Setting a maximum version for a dependency overrides the update stream of the author by unnecessarily truncating it at a particular upper bound.

Cluster administrators cannot override dependencies set by an Operator author.

However, maximum versions can and should be set if there are known incompatibilities that must be avoided. Specific versions can be omitted with the version range syntax, for example > 1.0.0 !1.2.1.

Additional resources

Dependency caveats

When specifying dependencies, there are caveats you should consider.

No compound constraints (AND)

There is currently no method for specifying an AND relationship between constraints. In other words, there is no way to specify that one Operator depends on another Operator that both provides a given API and has version >1.1.0.

This means that when specifying a dependency such as:

dependencies:
- type: olm.package
  value:
    packageName: etcd
    version: ">3.1.0"
- type: olm.gvk
  value:
    group: etcd.database.coreos.com
    kind: etcdCluster
    version: v1beta2

It would be possible for OLM to satisfy this with two Operators: one that provides etcdCluster and one that has version >3.1.0. Whether that happens, or whether an Operator is selected that satisfies both constraints, depends on the ordering that potential options are visited. Dependency preferences and ordering options are well-defined and can be reasoned about, but to exercise caution, Operators should stick to one mechanism or the other.

Cross-namespace compatibility

OLM performs dependency resolution at the namespace scope. It is possible to get into an update deadlock if updating an Operator in one namespace would be an issue for an Operator in another namespace, and vice-versa.

Example dependency resolution scenarios

In the following examples, a provider is an Operator which "owns" a CRD or API service.

Example: Deprecating dependent APIs

A and B are APIs (CRDs):

  • The provider of A depends on B.

  • The provider of B has a subscription.

  • The provider of B updates to provide C but deprecates B.

This results in:

  • B no longer has a provider.

  • A no longer works.

This is a case OLM prevents with its upgrade strategy.

Example: Version deadlock

A and B are APIs:

  • The provider of A requires B.

  • The provider of B requires A.

  • The provider of A updates to (provide A2, require B2) and deprecate A.

  • The provider of B updates to (provide B2, require A2) and deprecate B.

If OLM attempts to update A without simultaneously updating B, or vice-versa, it is unable to progress to new versions of the Operators, even though a new compatible set can be found.

This is another case OLM prevents with its upgrade strategy.