The guide describes the process of making a resource (usually defined by a CustomResourceDefinition)
of one Kubernetes cluster (the “service cluster” or “local cluster”) available for use in the KDP
platform (the “platform cluster” or “KDP workspaces”). This involves setting up a KDP Service and
then installing the api-syncagent and defining PublishedResources in the local cluster.
All of the documentation and API types are worded and named from the perspective of a service owner, the person(s) who own a service and want to make it available to consumers in the KDP platform.
A “service” in KDP comprises a set of resources within a single Kubernetes API group. It doesn’t need to be all of the resources in that group, service owners are free and encouraged to only make a subset of resources (i.e. a subset of CRDs) available for use in the platform.
For each of the CRDs on the service cluster that should be published, the service owner creates a
PublishedResource object, which will contain both which CRD to publish, as well as numerous other
important settings that influence the behaviour around handling the CRD.
When publishing a resource (CRD), exactly one version is published. All others are ignored from the standpoint of the resource synchronization logic.
All published resources together form the KDP Service. When a service is enabled in a workspace
(i.e. it is bound to it), users can manage objects for the projected resources described by the
published resources. These objects will be synced from the workspace onto the service cluster,
where they are meant to be processed in whatever way the service owners desire. Any possible
status information (in the status subresource) will in turn be synced back up into the workspace
where the user can inspect it.
Additionally, a published resource can describe additional so-called “related resources”. These usually originate on the service cluster and could be for example connection detail secrets created by Crossplane, but could also originate in the user workspace and just be additional, auxiliary resources that need to be synced down to the service cluster.
PublishedResourceIn its simplest form (which is rarely practical) a PublishedResource looks like this:
apiVersion: syncagent.kcp.io/v1alpha1
kind: PublishedResource
metadata:
name: publish-certmanager-certs # name can be freely chosen
spec:
resource:
kind: Certificate
apiGroup: cert-manager.io
version: v1
However, you will most likely apply more configuration and use features described below.
The api-syncagent can be instructed to only work on a subset of resources in the KDP platform. This can be restricted by namespace and/or label selector.
apiVersion: syncagent.kcp.io/v1alpha1
kind: PublishedResource
metadata:
name: publish-certmanager-certs # name can be freely chosen
spec:
resource: ...
filter:
namespace: my-app
resource:
matchLabels:
foo: bar
Warning: The actual CRD schema is always copied verbatim. All projections etc. have to take into account that the resource contents must be expressible without changes to the schema, so you cannot define entirely new fields in an object that are not defined by the original CRD.
For stronger separation of concerns and to enable whitelabelling of services, the type meta for
can be projected, i.e. changed between the local service cluster and the KDP platform. You could
for example rename Certificate from cert-manager to Zertifikat inside the platform.
Note that the API group of all published resources is always changed to the one defined in the
KDP Service object (meaning 1 api-syncagent serves all the selected published resources under the
same API group). That is why changing the API group cannot be configured in the projection.
Besides renaming the Kind and Version, dependent fields like Plural, ShortNames and Categories
can be adjusted to fit the desired naming scheme in the platform. The Plural name is computed
automatically, but can be overridden. ShortNames and Categories are copied unless overwritten in the
PublishedResource.
It is also possible to change the scope of resources, i.e. turning a namespaced resource into a cluster-wide. This should be used carefully and might require extensive mutations.
apiVersion: syncagent.kcp.io/v1alpha1
kind: PublishedResource
metadata:
name: publish-certmanager-certs # name can be freely chosen
spec:
resource: ...
projection:
version: v1beta1
kind: Zertifikat
plural: Zertifikate
shortNames: [zerts]
# categories: [management]
# scope: Namespaced # change only when you know what you're doing
Consumers (end users) in the platform would then ultimately see projected names only. Note that GVK projection applies only to the synced object itself and has no effect on the contents of these objects. To change the contents, use external solutions like Crossplane to transform objects.
Since the api-syncagent ingests resources from many different Kubernetes clusters (workspaces) and combines them onto a single cluster, resources have to be renamed to prevent collisions and also follow the conventions of whatever tooling ultimately processes the resources locally.
The renaming is configured in spec.naming. In there, renaming patterns are configured, where
pre-defined placeholders can be used, for example foo-$placeholder. The following placeholders
are available:
$remoteClusterName – the KDP workspace’s cluster name (e.g. “1084s8ceexsehjm2”)$remoteNamespace – the original namespace used by the consumer inside the KDP workspace$remoteNamespaceHash – first 20 hex characters of the SHA-1 hash of $remoteNamespace$remoteName – the original name of the object inside the KDP workspace (rarely used to construct
local namespace names)$remoteNameHash – first 20 hex characters of the SHA-1 hash of $remoteNameIf nothing is configured, the default ensures that no collisions will happen: Each workspace in the platform will create a namespace on the local cluster, with a combination of namespace and name hashes used for the actual resource names.
apiVersion: syncagent.kcp.io/v1alpha1
kind: PublishedResource
metadata:
name: publish-certmanager-certs # name can be freely chosen
spec:
resource: ...
naming:
# This is the implicit default configuration.
namespace: "$remoteClusterName"
name: "cert-$remoteNamespaceHash-$remoteNameHash"
Besides projecting the type meta, changes to object contents are also nearly always required.
These can be configured in a number of way in the PublishedResource.
Configuration happens spec.mutation and there are two fields:
spec contains the mutation rules when syncing the desired state (often in spec, but can also
be other top-level fields) from the remote side to the local side. Use this to apply defaulting,
normalising, and enforcing rules.status contains the mutation rules when syncing the status subresource back from the local
cluster up into the platform. Use this to normalize names and values (e.g. if you rewrote
.spec.secretName from "foo" to "dfkbssbfh", make sure the status does not “leak” this name
by accident).Mutation is always done as a series of steps. Each step does exactly one thing and only one must be configured per step.
apiVersion: syncagent.kcp.io/v1alpha1
kind: PublishedResource
metadata:
name: publish-certmanager-certs # name can be freely chosen
spec:
resource: ...
mutation:
spec:
# choose one per step
- regex: ...
template: ...
delete: ...
regex:
path: "json.path[expression]"
pattern: "(.+)"
replacement: "foo-\\1"
This mutation applies a regular expression to a single value inside the document. JSON path is the usual path, without a leading dot.
template:
path: "json.path[expression]"
template: "{{ .LocalObject.ObjectMeta.Namespace }}"
This mutation applies a Go template expression to a single value inside the document. JSON path is the usual path, without a leading dot.
delete:
path: "json.path[expression]"
This mutation simply removes the value at the given path from the document. JSON path is the usual path, without a leading dot.
The processing of resources on the service cluster often leads to additional resources being
created, like a Secret for each cert-manager Certificate or a connection detail secret created
by Crossplane. These need to be made available to the user in their workspaces.
Likewise it’s possible for auxiliary resources having to be created by the user, for example when the user has to provide credentials.
To handle these cases, a PublishedResource can define multiple “related resources”. Each related
resource currently represents exactly one object to synchronize between user workspace and service
cluster (i.e. you cannot express “sync all Secrets”). While the main published resource sync is
always workspace->service cluster, related resources can originate on either side and so either can
work as the source of truth.
At the moment, only ConfigMaps and Secrets are allowed related resource kinds.
For each related resource, the api-syncagent needs to be told the name/namespace. This is done by
selecting a field in the main resource (for a Certificate this would mean spec.secretName).
Both name and namespace need to be part of the main object (or be fixed values, like a hardcoded
kube-system namespace).
The path expressions for name and namespace are evaluated against the main object on either side
to determine their values. So if you had a Certificate in your workspace with
spec.secretName = "my-cert" and after syncing it down, the copy on the service cluster has a
rewritten/mutated spec.secretName = "jk23h4wz47329rz2r72r92-cert" (e.g. to prevent naming
collisions), the expression spec.secretName would yield "my-cert" for the name in the workspace
and "jk...." as the name on the service cluster. Once the object exists with that name on the
originating side, the api-syncagent will begin to sync it to the other side.
apiVersion: syncagent.kcp.io/v1alpha1
kind: PublishedResource
metadata:
name: publish-certmanager-certs
spec:
resource:
kind: Certificate
apiGroup: cert-manager.io
version: v1
naming:
# this is where our CA and Issuer live in this example
namespace: kube-system
# need to adjust it to prevent collions (normally clustername is the namespace)
name: "$remoteClusterName-$remoteNamespaceHash-$remoteNameHash"
related:
# unique name for this related resource. The name must be unique within one
# PublishedResource and is the key by which consumers (end users) can identify and consume the
# related resource. Common names are "connection-details" or "credentials".
- identifier: tls-secret
origin: service # service or platform
kind: Secret # for now, only "Secret" and "ConfigMap" are supported;
# there is no GVK projection for related resources
# configure where in the parent object we can find
# the name/namespace of the related resource (the child)
reference:
name:
# This path is evaluated in both the local and remote objects, to figure out
# the local and remote names for the related object. This saves us from having
# to remember mutated fields before their mutation (similar to the last-known
# annotation).
path: spec.secretName
# namespace part is optional; if not configured,
# Sync Agent assumes the same namespace as the owning resource
#
# namespace:
# path: spec.secretName
# regex:
# pattern: '...'
# replacement: '...'
#
# to inject static values, select a meaningless string value
# and leave the pattern empty
#
# namespace:
# path: metadata.uid
# regex:
# replacement: kube-system
This combination of Service and PublishedResource make cert-manager certificates available in
kcp. The Service needs to be created in a workspace, most likely in an organization workspace.
The PublishedResource is created wherever the api-syncagent and cert-manager are running.
apiVersion: core.kdp.k8c.io/v1alpha1
kind: Service
metadata:
name: certificate-management
spec:
apiGroup: certificates.example.corp
catalogMetadata:
title: Certificate Management
description: Acquire certificates signed by Example Corp's internal CA.
apiVersion: syncagent.kcp.io/v1alpha1
kind: PublishedResource
metadata:
name: publish-certmanager-certs
spec:
resource:
kind: Certificate
apiGroup: cert-manager.io
version: v1
naming:
# this is where our CA and Issuer live in this example
namespace: kube-system
# need to adjust it to prevent collions (normally clustername is the namespace)
name: "$remoteClusterName-$remoteNamespaceHash-$remoteNameHash"
related:
- origin: service # service or kcp
kind: Secret # for now, only "Secret" and "ConfigMap" are supported;
# there is no GVK projection for related resources
# configure where in the parent object we can find
# the name/namespace of the related resource (the child)
reference:
name:
# This path is evaluated in both the local and remote objects, to figure out
# the local and remote names for the related object. This saves us from having
# to remember mutated fields before their mutation (similar to the last-known
# annotation).
path: spec.secretName
# namespace part is optional; if not configured,
# Sync Agent assumes the same namespace as the owning resource
# namespace:
# path: spec.secretName
# regex:
# pattern: '...'
# replacement: '...'
The following sections go into more details of the behind the scenes magic.
Even though the whole configuration is written from the standpoint of the service owner, the actual
synchronization logic considers the kcp side as the canonical source of truth. The Sync Agent
continuously tries to make the local objects look like the ones in kcp, while pushing status updates
back into kcp (if the given PublishedResource (i.e. CRD) has a status subresource enabled).
The Sync Agent tries to keep sync-related metadata on the service cluster, away from the consumers. This is both to prevent vandalism and to hide implementation details.
To ensure stability against future changes, once the Sync Agent has determined how a local object should be named, it will remember this decision in the object’s metadata. This is so that on future reconciliations, the (potentially costly, but probably not) renaming logic does not need to be applied again. This allows the Sync Agent to change defaults and also allows the service owner to make changes to the naming rules without breaking existing objects.
Since we do not want to store metadata on the kcp side, we instead rely on label selectors on the local objects. Each object on the service cluster has a label for the remote cluster name, namespace and object name, and when trying to find the matching local object, the Sync Agent simply does a label-based search.
There is currently no sync-related metadata available on source objects (in kcp workspaces), as this would either be annotations (untyped strings…) or require schema changes to allow additional fields in basically random CRDs.
Note that fields like generation or resourceVersion are not relevant for any of the sync logic.
The sync loop can be divided into 5 parts:
For this, as mentioned in the connection chapter above, the Sync Agent tries to follow label selectors on the service cluster. This helps prevent cluttering with consumer workspaces with sync metadata. If no object is found to match the labels, that’s fine and the loop will continue with phase 2, in which a possible Conflict error (if labels broke) is handled gracefully.
The remote object in the workspace becomes the source object and its local equivalent on the
service cluster is called the destination object.
A finalizer is used in the kcp workspaces to prevent orphans in the service cluster side. This is the only real evidence in the kcp side that the Sync Agent is even doing things. When a remote (source) object is deleted, the corresponding local object is deleted as well. Once the local object is gone, the finalizer is removed from the source object.
We have a source object and now need to create the destination. This chart shows what’s happening.
graph TB
A(source object):::state --> B([cleanup if in deletion]):::step
B --> C([ensure finalizer on source object]):::step
C --> D{exists local object?}
D -- yes --> I("continue with next phase…"):::state
D -- no --> E([apply projection]):::step
subgraph "ensure dest object exists"
E --> G([ensure resulting namespace exists]):::step
G --> H([create local object]):::step
H --> H_err{Errors?}
H_err -- Conflict --> J([attempt to adopt existing object]):::step
end
H_err -- success --> I
J --> I
classDef step color:#77F
classDef state color:#F77
After we followed through with these steps, both the source and destination objects exists and we can continue with phase 4.
Resource adoption happens when creation of the initial local object fails. This can happen when labels get mangled. If such a conflict happens, the Sync Agent will “adopt” the existing local object by adding / fixing the labels on it, so that for the next reconciliation it will be found and updated.
Content synchronization is rather simple, really.
First the source “spec” is used to patch the local object. Note that this step is called “spec”, but
should actually be called “all top-level elements besides apiVersion, kind, status and
metadata, but still including some labels and annotations”; so if you were to publish RBAC objects,
the syncer would include roleRef field, for example).
To allow proper patch generation, the last known state is kept on the local object, similar to how
kubectl creates an annotation for it. This is required for the Sync Agent to properly detect changes
made by mutation webhooks on the service cluster.
If the published resource (CRD) has a status subresource enabled (not just a status field in its
scheme, it must be a real subresource), then the Sync Agent will copy the status from the local object
back up to the remote (source) object.
The same logic for synchronizing the main published resource applies to their related resources as well. The only difference is that the source side can be either remote (workspace) or local (service cluster).
Since the Sync Agent tries its best to keep sync-related data out of kcp workspaces, the last known state for related resources is not kept together with the destination object in the kcp workspaces. Instead all known states (from the main object and all related resources) is kept in a single Secret on the service cluster side.