---
title: Improved and extensible Certificate controller
authors:
  - "@munnerz"
reviewers:
  - "@joshvanl"
  - "@meyskens"
approvers:
  - "@joshvanl"
  - "@meyskens"
creation-date: 2020-02-19
last-updated: 2020-02-19
status: implementable
---

# Improved and extensible Certificate controller

<!-- toc -->
- [Summary](#summary)
- [Motivation](#motivation)
  - [Goals](#goals)
  - [Non-Goals](#non-goals)
- [Proposal](#proposal)
  - [API Changes](#api-changes)
  - [Controller Behaviour](#controller-behaviour)
  - [Internal API Resource Behaviour](#internal-api-resource-behaviour)
  - [Test Plan](#test-plan)
  - [Risks and Mitigations](#risks-and-mitigations)
  - [Graduation Criteria](#graduation-criteria)
      - [Alpha](#alpha)
      - [Alpha -&gt; Beta Graduation](#alpha---beta-graduation)
      - [Beta -&gt; GA Graduation](#beta---ga-graduation)
      - [Removing a deprecated flag](#removing-a-deprecated-flag)
  - [Version Skew Strategy](#version-skew-strategy)
<!-- /toc -->

## Summary

The Certificate controller is one of the most commonly used controllers in the project.
It represents the 'full lifecycle' of an x509 private key and certificate, including
private key management and renewal.

Internally, the controller is implemented in a fairly straightforward way. We have a single
controller which is responsible for:

* ensuring the stored private key matches the 'requirements' on the Certificate resource
* ensuring the stored certificate matches the 'requirements' on the Certificate resource
* handling renewals when a certificate is nearing expiry (as per `spec.renewBefore`)
* managing the lifecycle of a CertificateRequest resource in order to 'issue' a certificate
* exposing Prometheus metrics

The above list is non-exhaustive.

This document proposes an alternate way of structuring this controller, to improve reliability,
testability and extensibility.

## Motivation

As the project is maturing, more requirements around this controller are starting to become
apparent.

We have outstanding feature requests that are currently difficult to implement with the existing
design:

* Allow private key rotation when renewing certificates [#2402](https://github.com/cert-manager/cert-manager/issues/2402)
* Allowing alternative Secret output formats (e.g., single .pem file priv/cert output) [#843](https://github.com/cert-manager/cert-manager/issues/843)
* Add support for JKS, PKCS12 and PEM files [#586](https://github.com/cert-manager/cert-manager/issues/586)
* Make certificate renewal easier to test [#2578](https://github.com/cert-manager/cert-manager/issues/2578)

This proposal aims to facilitate the above features, as well as make it easier to develop individual
areas of the controller over time and continue to make improvements.

### Goals

* Make it easier to maintain the Certificates controller
* Make it easier to *extend* the Certificates controller
* Make it possible to 'hook in' to the state of the controller (e.g., manually triggering renewal)

### Non-goals

* We must not make backwards incompatible changes to the CRD schema

## Proposal

As noted above, the existing logic for the Certificates controller is a single loop which is
responsible for reconciling all of the aforementioned areas of the Certificate resource.

Instead, the Certificates controller will split into a number of distinct controllers, each
with their own well-defined responsibilities, that communicate via the Certificate resource's
`status` field.

* `keymanager` - generates and stores private keys when an issuance is required.
  Manages the `status.nextPrivateKeySecretName` field.
* `requestmanager` - creates and manages CertificateRequest resources for Certificates when
  an issuance is required.
* `issuing` - issues the signed x509 certificate and 'next private key' into the `spec.secretName`
  when the CertificateRequest is valid. Manages the `status.revision` field.
  Responsible for removing the `Issuing` condition.
* `trigger` - monitors the Secret resource and `certificate.spec` and adds the `Issuing`
  condition when issuance is required.

### API changes

In order to facilitate communication and cooperation between these controllers, some API changes
are required to contain computed state to be consumed by other controllers.
These additional fields will be encompassed in the `certificate.status` stanza.

```go
package v1alpha3

type CertificateStatus struct {
    // EXISTING FIELDS HERE

    // ADDITIONAL FIELDS

    // The current 'revision' of the certificate as issued.
    //
    // When a CertificateRequest resource is created, it will have the
    // `cert-manager.io/certificate-revision` set to one greater than the
    // current value of this field.
    //
    // Upon issuance, this field will be set to the value of the annotation
    // on the CertificateRequest resource used to issue the certificate.
    //
    // Persisting the value on the CertificateRequest resource allows the
    // certificates controller to know whether a request is part of an old
    // issuance or if it is part of the ongoing revision's issuance by
    // checking if the revision value in the annotation is greater than this
    // field.
    //
    // A CertificateRequest with no annotation that is owned by a Certificate
    // resource will be automatically deleted in order to not complicate the
    // 'renewal required' issuance logic. This means that any requests in user
    // clusters will be deleted upon upgrade, and in cases where a re-issuance
    // is required, another will be created with the appropriate `revision`.
    //
    // +optional
    Revision *int `json:"revision,omitempty"`

    // The name of the Secret resource containing the private key to be used
    // for the next certificate iteration.
    // The keymanager controller will automatically set this field if the
    // `Issuing` condition is set to `True`.
    // It will automatically unset this field when Issuing is not set or False.
    // +optional
    NextPrivateKeySecretName *string `json:"nextPrivateKeySecretName,omitempty"`
}

type CertificateCondition string

var (
    // A condition added to Certificate resources when an issuance is required.
    // This condition will be automatically added and set to true if:
    //   * No keypair data exists in the target Secret
    //   * The data stored in the Secret cannot be decoded
    //   * The private key and certificate do not have matching public keys
    //   * If a CertificateRequest for the current revision exists and the
    //     certificate data stored in the Secret does not match the
    //    `status.certificate` on the CertificateRequest.
    //   * If no CertificateRequest resource exists for the current revision,
    //     the options on the Certificate resource are compared against the
    //     x509 data in the Secret, similar to what's done in earlier versions.
    //     If there is a mismatch, an issuance is triggered.
    //
    // The final case above where no CertificateRequest resource exists is
    // essential for backwards compatibility, as older CertificateRequest
    // resources will not have a revision assigned so we cannot compare against
    // them. In these cases, we fall back to the behaviour we use today of
    // comparing the Certificate spec to the issued x509 certificate in the
    // Secret.
    //
    // This condition may also be added by external API consumers to trigger
    // a re-issuance manually for any other reason.
    //
    // It will be removed by the 'issuing' controller upon complete issuance.
    CertificateConditionIssuing CertificateCondition = "Issuing"
)
```

At the core of this proposal is the addition of the `status.iteration` field (an integer).
This field indicates the current 'version' of the certificate.
When a certificate is first created, the iteration is set to `1`.

###
#### Keymanager API fields

We will add a new field to the `CertificateStatus` structure:

```go
package v1alpha3

type CertificateStatus struct {
    ...

    // The name of the Secret resource 
    // +optional
    NextPrivateKeySecretName string `json:"nextPrivateKeySecretName,omitempty"`
}
```


### Controller behaviour

Implementing this proposal will require a complete replacement of the current
`certificates` controller. Almost all areas of code will be replaced.

The 'compare a `certificate.spec` with an x509 certificate' logic can and
should be preserved to help maintain some semblance of backward compatibility
for users upgrading from previous releases.

#### keymanager

The `keymanager` controller will be responsible for maintaining the
`status.nextPrivateKeySecretName` field and any 'next private key' Secret
resources that are owned by Certificates.

* If the `Issuing` condition is `True`:
    * If the `status.nextPrivateKeySecretName` field is not set:
        * Check for existing 'next private key' Secret resources:
            * If one matches, set the `status.nextPrivateKeySecretName` field.
              This handles cache inconsistencies when we observe Secret
              creation before updating `status.nextPrivateKeySecretName`.
            * Otherwise, generate a new private key according to
              `spec.privateKey` and store it in a new Secret resource.
              Persist the name of the Secret as `status.nextPrivateKeySecretName`.
    * If the `status.nextPrivateKeySecretName` field is set:
        * If the Secret contains key data that matches `spec.privateKey`:
            * do nothing
        * If the Secret is not 'owned' by the Certificate, do nothing and log
          an Event to inform the user.
        * If the Secret is not labelled as a 'next private key', do nothing and
          log an Event to inform the user.
        * If the Secret does not exist:
            * Generate a new private key according to `spec.privateKey` and
              create a Secret resource with the given name.
        * If the secret contains key data that does not match `spec.privateKey`:
            * Generate a new private key according to `spec.privateKey` and
              store it in the Secret resource.

* If the `Issuing` condition is `False` or not set:
    * Delete all owned Secret resources with the `cert-manager.io/next-private-key: "true"`
    * Ensure `status.nextPrivateKeySecretName` is unset - we may want to
      consider not doing this in case a user has manually specified this field
      and pointed it at an 'un-owned' Secret. This depends on whether we want to
      support this as a mode of operation.

When creating a 'next private key' Secret resource, the
`cert-manager.io/next-private-key: "true"` annotation is added as well as an
OwnerReference to the Certificate resource.

Private keys generated by the key manager will be encoded in PKCS#8 format for
ease of interoperability. The `issuing` controller will encode the resulting
key-pair into the format requested by the user once the request has been
completed.

The introduction of this dedicated controller also means we can implement
private key rotation when a certificate is re-issued/renewed.
This is a welcome new feature, but in some cases a user may want to pin the
private key used for a key-pair (as is the default and only supported behaviour
prior to implementing this design).
To continue to enable this, the `spec.privateKey.rotationPolicy` field
controls how the next private key should be sourced. It supports two values:

* `Always`: a new private key will be generated on every re-issuance.
  This includes renewals as well as changes to the `spec.privateKey` and other
  `spec` fields.
* `Never`: private keys will never been regenerated and must be provided by the
  user.

> TODO: We may be better to have `spec.privateKey.pinnedSecretName` instead, to
> name a Secret that contains the private key to use. We could then require
> this key to be provided in a specific format.
> With the `rotationPolicy` policy design, a user must pre-create the Secret
> resource containing their private key ahead of time, which creates a conflict
> of ownership and confusion.

If the `spec.privateKey` options change during an issuance, the key will be
regenerated and the same Secret resource will be reused to store the updated
private key. Consumers reading this Secret (i.e. the `requestmanager`) must
compare the public key of the named 'next private key' and if the public key
does not match with the CertificateRequest being managed, the
CertificateRequest should be recreated.

#### requestmanager

The `requestmanager` is responsible for managing a CertificateRequest resource
for a Certificate. If a Certificate has the `Issuing` condition, it will ensure
a CertificateRequest signed by the `status.nextPrivateKeySecretName` exists and
matches the specification for the Certificate in `certificate.spec`.

This is the controller that most closely resembles the bulk of the logic in the
existing `certificates` controller.

It will behave as follows:

* If the `Issuing` condition is `True`:
    * If the `status.nextPrivateKeySecretName` field is not set:
        * Do nothing
    * List all owned CertificateRequest resources, delete all that have a
      'revision' not equal to `status.revision` + 1 (or `1` if
      `status.revision` is not set).
        * If multiple have the same current revision, delete any that do not 'match'
          as per the two verification steps below. If multiple still exist, log an
          event and delete all but one.
    * If no CertificateRequest for the current revision exists:
        Create a CertificateRequest based on the `certificate.spec`.
    * Verify the public key of the certificate request matches the public key
      of the stored `status.nextPrivateKeySecretName`
        * If not, delete the CertificateRequest & log an Event
    * Verify the CSR options match what is requested in `certificate.spec`
        * If not, delete the CertificateRequest & log an Event
* If the `Issuing` condition is `False` or not set:
    * Delete any CertificateRequest resources that do not have a 'revision'
      equal to `status.revision`.

### issuing

This controller will copy the `status.certificate` and `status.ca` fields from
valid CertificateRequest resources and the `tls.key` from the
`status.nextPrivateKeySecretName` Secret resource into the `spec.secretName`.

It is responsible for encoding the private key and certificate data into the
appropriate format.

Once the key-pair has been written to `spec.secretName`, it will set the
`status.revision` field to that of the CertificateRequest and remove the
`Issuing` status condition (in the same Update call).

It will behave as follows:

* If the 'Issuing' condition is `True`:
    * If the `status.nextPrivateKeySecretName` field is not set:
        * Do nothing
    * Find the CertificateRequest for `status.revision` + 1
        * If none exist, do nothing.
        * If multiple exist, do nothing (`requestmanager` will handle this)
    * If the CertificateRequest is not in a 'final' state, do nothing.
    * If it is 'valid':
        * Verify the public key of the certificate request matches the public key
          of the stored `status.nextPrivateKeySecretName`
            * If not, do nothing (`requestmanager` will handle this)
        * Verify the CSR options match what is requested in `certificate.spec`
            * If not, do nothing (`requestmanager` will handle this)
        * Encode and issue the key-pair (store it in the Secret)
        * Set `status.revision` to revision of the CertificateRequest
        * Remove `Issuing` status condition
        * Clear `status.lastFailureTime` (if set)
        * Log an Event
    * If not 'valid':
        * Set `status.lastFailureTime` (if not equal to CertificateRequest)
        * Set `Issuing` status condition to `False` with reason explaining when
          the request will be retried
* If the 'Issuing' condition is `False` or not set:
    * Do nothing

#### trigger

The trigger plugin is responsible for observing the state of the currently
issued `spec.secretName` and the rest of the `certificate.spec` fields to
determine whether a re-issuance is required.
It triggers re-issuance by adding the `Issuing` status condition when a new
certificate is required.

These conditions will cause a request to be triggered:

* `spec.secretName` does not exist, or contains data that cannot be decoded.
* Issuer name/kind annotations on Secret resource do not match current `spec.issuerRef`.
* The public key of the `tls.key` and `tls.crt` do not match.
* A CertificateRequest for the current `status.revision` exists and the
  current requested `certificate.spec` does not match the options on the CSR.
* If the stored x509 certificate's `NotAfter` is within `spec.renewBefore` of
  the current time.

Additionally, the controller is responsible for implement a 'back-off' if
CertificateRequest resources persistently fail to complete.
For now, this will be a continuation of the current behaviour of backing off
by 1 hour after a request fails.
Even if any of the above conditions are true, a request will not be triggered
unless the current time is at least 1 hour after the `status.lastFailureTime`,
if set.
In future we can extend this logic to back-off exponentially by storing a
longer history of CertificateRequest resource's failure times, but this is out
of scope of this proposal.

Another actor may choose to manually trigger an issuance by setting the
`Issuing` condition themselves (i.e. with a cert-manager CLI tool, or their own
controller). The `trigger` controller will not interfere in this case and
will take no extra action.
