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Add proposal for service externalName
This is a proposal to address: #13748.
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Rodrigo Campos
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docs/proposals/service-external-name.md
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docs/proposals/service-external-name.md
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<!-- BEGIN MUNGE: UNVERSIONED_WARNING -->
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<!-- BEGIN STRIP_FOR_RELEASE -->
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<h2>PLEASE NOTE: This document applies to the HEAD of the source tree</h2>
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If you are using a released version of Kubernetes, you should
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refer to the docs that go with that version.
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Documentation for other releases can be found at
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[releases.k8s.io](http://releases.k8s.io).
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</strong>
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--
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# Service externalName
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Author: Tim Hockin (@thockin), Rodrigo Campos (@rata)
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Date: July 2016
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Status: Waiting LGTM
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# Goal
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Allow a service to have a CNAME record in the cluster internal DNS service. For
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example, a `db` service can have a CNAME to `something.rds.aws.amazon.com`
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pointing to an RDS resource.
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# Motivation
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There were tons of issues about this, I'd try to summarize motivation here. More
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info is on github issues/PRs: #13748, #11838, #13358, #23921
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One motivation is to present as cluster services, services that are hosted by
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some provider. Some cloud providers, like AWS, give a hostname (IPs are not
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static) and the user wants to refer using regular kubernetes tools to these
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services. This was asked in bugs, at least for AWS in RedShift, RDS,
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Elasticsearch Service, ELB, etc.
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Some others just want an external service, for example "oracle", with dns name
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"oracle-1.testdev.mycompany.com", without having to keep DNS in sync, and just
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want a CNAME.
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Another use case is to "integrate" some services to local development. For
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example, I have a search service running in kubernetes in staging, let's say
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`search-1.stating.mycompany.com`. Let's say it's on AWS, so it's behind an ELB
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(which doesn't have a static IP, it has a hostname). I'm building an app that
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consumes `search-1` and I don't want to run it on my local PC (before kubernetes
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I didn't). I can just create a service that has a CNAME to the `search-1`
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endpoint in staging and be happy like I was before.
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Also, openshift needs this for "service refs". Service ref is really just the
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three use cases mentioned above, but in the future a way to automatically inject
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"service ref"s into namespaces via "service catalog"[1] might be considered. And
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service ref is the natural way to integrate an external service, since it takes
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advantage of native DNS capabilities already in wide use.
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[1]: https://github.com/kubernetes/kubernetes/pull/17543
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# Alternatives considered
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In the issues linked above, there is also some alternatives considered. I will
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try to sum them up here, but the list might not be complete or have all the
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discussion.
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One option is to add the hostname to endpoints. This was proposed in:
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https://github.com/kubernetes/kubernetes/pull/11838. This is problematic as
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endpoints are used in tons of places and users assume the required fields (like
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IP, for example) are always present and a valid IP (and check that). If the
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field is not required anymore, they can break, or if there is a hostname instead
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of the IP, they can break too. But assuming that can be solved, it was also
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discussed that the hostname will have to be resolved, with a timeout, sync/async
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and the DNS entry has a TTL and presents other problems. One option, not
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perfect, was to only resolve the hostname on creation. But this was considered
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not a good idea. The best thing was to do this at a higher level, maybe a
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service type.
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There are more ideas on how to approach this problem on #13748, but all pointed
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to some problem. Ranging from using another upstream DNS server to creating a
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Name object assoaciated with DNSs.
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# Proposed solution
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The proposed solution is to add this at the service layer by adding a new
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`externalName` type to the service. This will create a CNAME record in the
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internal cluster DNS service.
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Using a CNAME avoids having to do the lookup, decide a timeout for it, and
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having to lookup for it when the TTL expires. It's way simpler to implement,
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while solving the right problem. And doing it at the service layer avoids all
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the problem discussed with doing the change at the endpoints layer.
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The proposed solution is the one by Tim Hockin here:
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https://github.com/kubernetes/kubernetes/issues/13748#issuecomment-230397975
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Currently a ServiceSpec looks like this (comments stripped down):
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```
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type ServiceSpec struct {
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Ports []ServicePort
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// If not specified, the associated Endpoints object is not automatically managed
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Selector map[string]string
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// "", a real IP, or "None". If not specified, this is default allocated. If "None", this Service is not load-balanced
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ClusterIP string
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// ClusterIP, NodePort, LoadBalancer. Only applies if clusterIP != "None"
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Type ServiceType
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// Only applies if clusterIP != "None"
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ExternalIPs []string
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SessionAffinity ServiceAffinity
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// Only applies to type=LoadBalancer
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LoadBalancerIP string
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LoadBalancerSourceRanges []string
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```
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So, the proposal is to change it to:
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```
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type ServiceSpec struct {
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Ports []ServicePort
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// If not specified, the associated Endpoints object is not automatically managed
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+ // Only applies if type is ClusterIP, NodePort, or LoadBalancer. If type is ExternalName, this is ignored.
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Selector map[string]string
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// "", a real IP, or "None". If not specified, this is default allocated. If "None", this Service is not load-balanced.
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+ // Only applies if type is ClusterIP, NodePort, or LoadBalancer. If type is ExternalName, this is ignored.
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ClusterIP string
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- // ClusterIP, NodePort, LoadBalancer. Only applies if clusterIP != "None"
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+ // ExternalName, ClusterIP, NodePort, LoadBalancer. Only applies if clusterIP != "None"
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Type ServiceType
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+ // Only applies if type is ExternalName
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+ ExternalName string
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// Only applies if clusterIP != "None"
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ExternalIPs []string
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SessionAffinity ServiceAffinity
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// Only applies to type=LoadBalancer
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LoadBalancerIP string
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LoadBalancerSourceRanges []string
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```
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So, for example, it can be used like this:
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```
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apiVersion: v1
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kind: Service
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metadata:
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name: my-rds
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spec:
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ports:
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- port: 12345
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type: ExternalName
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externalName: myapp.rds.whatever.aws.says
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```
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There is one thing to take into account (that no other alternative considered
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fixes either): TLS. If the service is a CNAME for an endpoint that uses TLS,
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connecting with another name may fail cert validation. This is acknowledged and
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left for future consideration.
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<!-- BEGIN MUNGE: GENERATED_ANALYTICS -->
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[]()
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<!-- END MUNGE: GENERATED_ANALYTICS -->
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