323 lines
16 KiB
Markdown
323 lines
16 KiB
Markdown
---
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---
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* TOC
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{:toc}
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__Terminology__
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Throughout this doc you will see a few terms that are sometimes used interchangeably elsewhere, that might cause confusion. This section attempts to clarify them.
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* Node: A single virtual or physical machine in a Kubernetes cluster.
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* Cluster: A group of nodes in a single failure domain, unless mentioned otherwise.
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* Persistent Volume Claim (PVC): A request for storage, typically a [persistent volume](/docs/user-guide/persistent-volumes/walkthrough/).
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* Host name: The hostname attached to the UTS namespace of the pod, i.e the output of `hostname` in the pod.
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* DNS/Domain name: A *cluster local* domain name resolvable using standard methods (eg: [gethostbyname](http://linux.die.net/man/3/gethostbyname)).
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* Ordinality: the proprety of being "ordinal", or occupying a position in a sequence.
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* Pet: a single member of a Pet Set; more generally, a stateful application.
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* Peer: a process running a server, capable of communicating with other such processes.
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__Prerequisites__
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This doc assumes familiarity with the following Kubernetes concepts:
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* [Pods](/docs/user-guide/pods/single-container/)
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* [Cluster DNS](/docs/admin/dns/)
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* [Headless Services](/docs/user-guide/services/#headless-services)
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* [Persistent Volumes](/docs/user-guide/volumes/)
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* [Dynamic volume provisioning](http://releases.k8s.io/{{page.githubbranch}}/examples/experimental/persistent-volume-provisioning/README.md)
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You need a working Kubernetes cluster at version >= 1.3, with a healthy DNS [cluster addon](http://releases.k8s.io/{{page.githubbranch}}/cluster/addons/README.md) at version >= 15. You cannot use Pet Set on a hosted Kubernetes provider that has disabled `alpha` resources.
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## What is a Pet Set?
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In Kubernetes, most pod management abstractions group them into disposable units of work that compose a micro service. Replication controllers for example, are designed with a weak guarantee - that there should be N replicas of a particular pod template. The pods are treated as stateless units, if one of them is unhealthy or superseded by a newer version, the system just disposes it.
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```
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foo.default.svc.cluster.local
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|service|
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/ \
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| pod-asdf | | pod-zxcv |
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```
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A Pet Set, in contrast, is a group of stateful pods that require a stronger notion of identity. The document refers to these as "clustered applications".
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```
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*.foo.default.svc.cluster.local
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| mysql-0 | <-> | mysql-1 |
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[pv 0] [pv 1]
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```
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The co-ordinated deployment of clustered applications is notoriously hard. They require stronger notions of identity and membership, which they use in opaque internal protocols, and are especially prone to race conditions and deadlock. Traditionally administrators have deployed these applications by leveraging nodes as stable, long-lived entities with persistent storage and static ips.
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The goal of Pet Set is to decouple this dependency by assigning identities to individual instances of an application that are not anchored to the underlying physical infrastructure. For the rest of this document we will refer to these entities as "Pets". Our use of this term is predated by the "Pets vs Cattle" analogy.
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__Relationship between Pets and Pods__: PetSet requires there be {0..N-1} Pets. Each Pet has a deterministic name - PetSetName-Ordinal, and a unique identity. Each Pet has at most one pod, and each Pet Set has at most one Pet with a given identity.
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## When to use Pet Set?
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A Pet Set ensures that a specified number of "pets" with unique identities are running at any given time. The identity of a Pet is comprised of:
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* a stable hostname, available in DNS
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* an ordinal index
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* stable storage: linked to the ordinal & hostname
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These properties are useful in deploying stateful applications. However most stateful applications are also clustered, meaning they form groups with strict membership requirements that rely on stored state. Pet Set also helps with the 2 most common problems encountered managing such clustered applications:
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* discovery of peers for quorum
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* startup/teardown ordering
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Only use Pet Set if your application requires some or all of these properties. Managing pods as stateless replicas is vastly easier.
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Example workloads for Pet Set:
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* Databases like MySQL or PostgreSQL that require a single instance attached to a NFS persistent volume at any time
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* Clustered software like Zookeeper, Etcd, or Elasticsearch that require stable membership.
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## Alpha limitations
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Before you start deploying applications as Pet Sets, there are a few limitations you should understand.
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* Pet Set is an *alpha* resource, not available in any Kubernetes release prior to 1.3.
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* As with all alpha/beta resources, it can be disable through the `--runtime-config` option passed to the apiserver, and in fact most likely will be disabled on hosted offerings of Kubernetes.
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* The only updatable field on a Pet Set is `replicas`
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* The storage for a given pet must either be provisioned by a [dynamic storage provisioner](http://releases.k8s.io/{{page.githubbranch}}/examples/experimental/persistent-volume-provisioning/README.md) based on the requested `storage class`, or pre-provisioned by an admin. Note that dynamic volume provisioning is also currently in alpha.
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* Deleting the Pet Set *will not* delete any pets. You will either have to manually scale it down to 0 pets first, or delete the pets yourself.
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* Deleting and/or scaling a Pet Set down will *not* delete the volumes associated with the Pet Set. This is done to ensure safety first, your data is more valuable than an auto purge of all related Pet Set resources. **Deleting the Persistent Volume Claims will result in a deletion of the associated volumes**.
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* All Pet Sets currently require a "governing service", or a Service responsible for the network identity of the pets. The user is responsible for this Service.
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* Updating an existing Pet Set is currently a manual process, meaning you either need to deploy a new Pet Set with the new image version, or orphan Pets one by one, update their image, and join them back to the cluster.
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## Example Pet Set
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We'll create a basic Pet Set to demonstrate how Pets are assigned unique and "sticky" identities.
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{% include code.html language="yaml" file="petset.yaml" ghlink="/docs/user-guide/petset.yaml" %}
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Saving this config into `petset.yaml` and submitting it to a Kubernetes cluster should creae the defined Pet Set and Pets it manages:
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```shell
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$ kubectl create -f petset.yaml
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service "nginx" created
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petset "nginx" created
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```
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## Pet Identity
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The identity of a Pet sticks to it, regardless of which node it's (re) scheduled on. We can examine the identity of the pets we just created.
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### Ordinal index
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you should see 2 pods with predictable names formatted thus: `$(petset name)-$(ordinal index assigned by petset controller)`
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```shell
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$ kubectl get po
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NAME READY STATUS RESTARTS AGE
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web-0 1/1 Running 0 10m
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web-1 1/1 Running 0 10m
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```
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### Stable storage
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2 persistent volumes, one per pod. This is auto created by the Pet Set based on the `volumeTemplate` field
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```shell
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$ kubectl get pv
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NAME CAPACITY ACCESSMODES STATUS CLAIM REASON AGE
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pvc-90234946-3717-11e6-a46e-42010af00002 1Gi RWO Bound default/www-web-0 11m
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pvc-902733c2-3717-11e6-a46e-42010af00002 1Gi RWO Bound default/www-web-1 11m
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```
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### Network identity
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The network identity has 2 parts. First, we created a headless Service that controls the domain within which we create Pets. The domain managed by this Service takes the form: `$(service name).$(namespace).svc.cluster.local`, where "cluster.local" is the [cluster domain](http://releases.k8s.io/{{page.githubbranch}}/build/kube-dns/README.md#how-do-i-configure-it). As each pet is created, it gets a matching DNS subdomain, taking the form: `$(petname).$(governing service domain)`, where the governing service is defined by the `serviceName` field on the Pet Set.
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Here are some examples of choices for Cluster Domain, Service name, Pet Set name, and how that affects the DNS names for the Pets and the hostnames in the Pet's pods:
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Cluster Domain | Service (ns/name) | Pet Set (ns/name) | Pet Set Domain | Pet DNS | Pet Hostname |
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-------------- | ----------------- | ----------------- | -------------- | ------- | ------------ |
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cluster.local | default/nginx | default/web | nginx.default.svc.cluster.local | web-{0..N-1}.nginx.default.svc.cluster.local | web-{0..N-1} |
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cluster.local | foo/nginx | foo/web | nginx.foo.svc.cluster.local | web-{0..N-1}.nginx.foo.svc.cluster.local | web-{0..N-1} |
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kube.local | foo/nginx | foo/web | nginx.foo.svc.kube.local | web-{0..N-1}.nginx.foo.svc.kube.local | web-{0..N-1} |
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Note that Cluster Domain will be set to `cluster.local` unless [otherwise configured](http://releases.k8s.io/{{page.githubbranch}}/build/kube-dns/README.md#how-do-i-configure-it).
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Lets verify our assertion with a simple test.
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```shell
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$ kubectl get svc
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NAME CLUSTER-IP EXTERNAL-IP PORT(S) AGE
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nginx None <none> 80/TCP 12m
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...
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```
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First, the PetSet gives provides a stable hostname:
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```shell
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$ for i in 0 1; do kubectl exec web-$i -- sh -c 'hostname'; done
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web-0
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web-1
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```
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And the hostname is linked to the in-cluster DNS address:
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```shell
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$ kubectl run -it --image busybox dns-test --restart=Never /bin/sh
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dns-test # nslookup web-0.nginx
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Server: 10.0.0.10
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Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local
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Name: web-0.nginx
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Address 1: 10.180.3.5
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dns-test # nslookup web-1.nginx
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Server: 10.0.0.10
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Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local
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Name: web-1.nginx
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Address 1: 10.180.0.9
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```
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The containers are running nginx webservers, which by default will look for an index.html file in `/usr/share/nginx/html/index.html`. That directory is backed by a `PersistentVolume` created by the Pet Set. So lets write our hostname there:
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```shell
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$ for i in 0 1; do
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kubectl exec web-$i -- sh -c 'echo $(hostname) > /usr/share/nginx/html/index.html';
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done
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```
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And verify each webserver serves its own hostname:
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```shell
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$ for i in 0 1; do kubectl exec -it web-$i -- curl localhost; done
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web-0
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web-1
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```
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Now delete all pods in the petset:
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```shell
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$ kubectl delete po -l app=nginx
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pod "web-0" deleted
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pod "web-1" deleted
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```
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Wait for them to come back up, and try to retrieve the previously written hostname through the DNS name of the peer. They match, because the storage, DNS name, and hostname stick to the Pet no matter where it gets scheduled:
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```shell
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$ kubectl exec -it web-1 -- curl web-0.nginx
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web-0
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$ kubectl exec -it web-0 -- curl web-1.nginx
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web-1
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```
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## Peer discovery
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A pet can piece together its own identity:
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1. Use the [downward api](/docs/user-guide/downward-api/) to find its pod name
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2. Run `hostname` to find its DNS name
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3. Run `mount` or `df` to find its volumes (usually this is unnecessary)
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It's not necessary to "discover" the governing Service of a Pet Set, since it's known at creation time you can simply pass it down through an [environment variable](/docs/user-guide/environment-guide).
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Usually pets also need to find their peers. In the previous nginx example, we just used `kubectl` to get the names of existing pods, and as humans, we could tell which ones belonged to a given Pet Set. Another way to find peers is by contacting the API server, just like `kubectl`, but that has several disadvantages (you end up implementing a Kubernetes specific init system that runs as pid 1 in your application container).
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Pet Set gives you a way to disover your peers using DNS records. To illustrate this we can use the previous example (note: one usually doesn't `apt-get` in a container).
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```shell
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$ kubectl exec -it web-0 /bin/sh
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web-0 # apt-get update && apt-get install -y dnsutils
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...
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web-0 # nslookup -type=srv nginx.default
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Server: 10.0.0.10
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Address: 10.0.0.10#53
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nginx.default.svc.cluster.local service = 10 50 0 web-1.ub.default.svc.cluster.local.
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nginx.default.svc.cluster.local service = 10 50 0 web-0.ub.default.svc.cluster.local.
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```
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## Updating a Pet Set
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You cannot update any field of the PetSet except `spec.replicas`. You can update the replicas field using standard kubectl update commands like [patch](/docs/user-guide/kubectl/kubectl_patch) and [edit](/docs/user-guide/kubectl/kubectl_edit). Pet Set currently *does not* support image upgrade as noted in the section on [limitations](#alpha-limitations).
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## Scaling a Pet Set
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You can scale a Pet Set by updating the "replicas" field. Note however that the controller will only:
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1. Create one pet at a time, in order from {0..N-1}, and wait till each one is in [Running and Ready](/docs/user-guide/pod-states) before creating the next
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2. Delete one pet at a time, in reverse order from {N-1..0}, and wait till each one is completely shutdown (past its [terminationGracePeriodSeconds](/docs/user-guide/pods/index#termination-of-pods)) before deleting the next
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## Deleting a Pet Set
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Cleaning up a Pet Set is somewhat manual, as noted in the [limitations section](#alpha-limitations). You can delete a Pet Set using Kubectl, but this will *not* scale it down to 0:
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```shell
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$ kubectl delete -f petset.yaml
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petset "web" deleted
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$ kubectl get po -l app=nginx
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NAME READY STATUS RESTARTS AGE
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web-0 1/1 Running 0 21h
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web-1 1/1 Running 0 21h
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$ kubectl delete po -l app=nginx
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pod "web-0" deleted
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pod "web-1" deleted
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```
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Deleting the pods will *not* delete the volumes. Until we finalize the recycle policy for these volumes they will have to get cleaned up by an admin. This is to ensure that you have the chance to copy data off the volume before deleting it. Simply deleting the PVC after the pods have left the [terminating state](/docs/user-guide/pods/index#termination-of-pods) should trigger deletion of the backing Persistent Volumes.
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**Note: you will lose all your data once the PVC is deleted, do this with caution.**
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```shell
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$ kubectl get po -l app=nginx
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$ kubectl get pvc -l app=nginx
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NAME STATUS VOLUME CAPACITY ACCESSMODES AGE
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www-web-0 Bound pvc-62d271cd-3822-11e6-b1b7-42010af00002 0 21h
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www-web-1 Bound pvc-62d6750e-3822-11e6-b1b7-42010af00002 0 21h
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$ kubectl delete pvc -l app=nginx
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$ kubectl get pv
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```
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If you simply want to clean everything:
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```shell{% raw %}
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$ grace=$(k get po web-0 --template '{{.spec.terminationGracePeriodSeconds}}')
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$ kubectl delete petset,po -l app=nginx
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$ sleep $grace
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$ kubectl delete pvc -l app=nginx
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{% endraw %}```
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## Troubleshooting
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You might have noticed an `annotations` field in all the Pet Sets shown above.
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```yaml
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annotations:
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pod.alpha.kubernetes.io/initialized: "true"
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```
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This field is a debugging hook. It pauses any scale up/down operations on the entire Pet Set. If you'd like to pause a petset after each pet, set it to `false` in the template, wait for each pet to come up, verify it has initialized correctly, and then set it to `true` using `kubectl edit` on the pet (setting it to `false` on *any pet* is enough to pause the Pet Set). If you don't need it, create the Pet Set with it set to `true` as shown. This is surprisingly useful in debugging bootstrapping race conditions.
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## Future Work
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There are a LOT of planned improvements since Pet Set is still in alpha.
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* Data gravity and local storage
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* Richer notification events
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* Public network identities
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* WAN cluster deployments (multi-AZ/region/cloud provider)
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* Image and node upgrades
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This list goes on, if you have examples, ideas or thoughts, please contribute.
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## Alternatives
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Deploying one RC of size 1/Service per pod is a popular alternative, as is simply deploying a DaemonSet that utilizes the identity of a Node.
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## Next steps
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The deployment and maintenance of stateful applications is a vast topic. The next step is to explore cluster bootstrapping and initialization, [here](/docs/user-guide/petset/bootstrapping/).
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