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Merge pull request #4033 from kargakis/docs

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Refine services doc
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Alex Robinson 2015-02-02 14:30:01 -08:00
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@ -3,7 +3,7 @@
## Overview ## Overview
Kubernetes [`Pods`](pods.md) are ephemeral. They can come and go over time, especially when Kubernetes [`Pods`](pods.md) are ephemeral. They can come and go over time, especially when
driven by things like [ReplicationControllers](replication-controller.md). driven by things like [`ReplicationControllers`](replication-controller.md).
While each `pod` gets its own IP address, those IP addresses can not be relied While each `pod` gets its own IP address, those IP addresses can not be relied
upon to be stable over time. This leads to a problem: if some set of `pods` upon to be stable over time. This leads to a problem: if some set of `pods`
(let's call them backends) provides functionality to other `pods` (let's call (let's call them backends) provides functionality to other `pods` (let's call
@ -28,8 +28,8 @@ enables this decoupling.
## Defining a service ## Defining a service
A `service` in Kubernetes is a REST object, similar to a `pod`. Like a `pod` a A `service` in Kubernetes is a REST object, similar to a `pod`. Like a `pod`, a
`service` definitions can be POSTed to the apiserver to create a new instance. `service` definition can be POSTed to the apiserver to create a new instance.
For example, suppose you have a set of `pods` that each expose port 9376 and For example, suppose you have a set of `pods` that each expose port 9376 and
carry a label "app=MyApp". carry a label "app=MyApp".
@ -121,7 +121,7 @@ terms of the portal IP and port. We will be adding DNS support for
`services`, too. `services`, too.
As an example, consider the image processing application described above. As an example, consider the image processing application described above.
when the backend `services` is created, the Kubernetes master assigns a portal When the backend `service` is created, the Kubernetes master assigns a portal
IP address, for example 10.0.0.1. Assuming the `service` port is 1234, the IP address, for example 10.0.0.1. Assuming the `service` port is 1234, the
portal is 10.0.0.1:1234. The master stores that information, which is then portal is 10.0.0.1:1234. The master stores that information, which is then
observed by all of the `service proxy` instances in the cluster. When a proxy observed by all of the `service proxy` instances in the cluster. When a proxy
@ -141,18 +141,18 @@ being aware of which `pods` they are accessing.
![Services detailed diagram](services_detail.png) ![Services detailed diagram](services_detail.png)
## External Services ## External Services
For some parts of your application (e.g. your frontend) you want to expose a service on an external (publically visible) IP address. For some parts of your application (e.g. frontend) you want to expose a service on an external (publically visible) IP address.
If you want your service to be exposed on an external IP address, you can optionally supply a list of "publicIPs" If you want your service to be exposed on an external IP address, you can optionally supply a list of `publicIPs`
which the service should respond to. These IP address will be combined with the Service's port and will also be which the `service` should respond to. These IP address will be combined with the `service`'s port and will also be
mapped to the set of pods selected by the service. You are then responsible for ensuring that traffic to that mapped to the set of `pods` selected by the `service`. You are then responsible for ensuring that traffic to that
external IP address gets sent to one or more kubernetes worker nodes. An IPTables rules on each host that maps external IP address gets sent to one or more Kubernetes worker nodes. An IPTables rules on each host that maps
packets from the specified public IP address to the service proxy in the same manner as internal service IP packets from the specified public IP address to the service proxy in the same manner as internal service IP
addresses. addresses.
On cloud providers which support external load balancers, there is a simpler way to achieve the same thing. On such On cloud providers which support external load balancers, there is a simpler way to achieve the same thing. On such
providers (e.g. GCE) you can leave ```publicIPs``` empty, and instead you can set the providers (e.g. GCE) you can leave ```publicIPs``` empty, and instead you can set the
```createExternalLoadBalancer``` flag on the service. This sets up a cloud provider specific load balancer ```createExternalLoadBalancer``` flag on the service. This sets up a cloud-provider-specific load balancer
(assuming that it is supported by your cloud provider) and populates the Public IP field with the appropriate value. (assuming that it is supported by your cloud provider) and populates the Public IP field with the appropriate value.
## Shortcomings ## Shortcomings