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How selectors interact with headless services 

There was no information on how the system handles headless services differently.
pull/630/head
Joshua Perry 2016-06-07 15:33:26 -06:00
parent d049bc38a4
commit 07d137ee1c
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docs/user-guide/services/index.md
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@ -317,18 +317,27 @@ Sometimes you don't need or want load-balancing and a single service IP. In
this case, you can create "headless" services by specifying `"None"` for the this case, you can create "headless" services by specifying `"None"` for the
cluster IP (`spec.clusterIP`). cluster IP (`spec.clusterIP`).
For such `Services`, a cluster IP is not allocated. DNS is configured to return
multiple A records (addresses) for the `Service` name, which point directly to
the `Pods` backing the `Service`. Additionally, the kube proxy does not handle
these services and there is no load balancing or proxying done by the platform
for them. The endpoints controller will still create `Endpoints` records in
the API.
This option allows developers to reduce coupling to the Kubernetes system, if This option allows developers to reduce coupling to the Kubernetes system, if
they desire, but leaves them freedom to do discovery in their own way. they desire, but leaves them freedom to do discovery in their own way.
Applications can still use a self-registration pattern and adapters for other Applications can still use a self-registration pattern and adapters for other
discovery systems could easily be built upon this API. discovery systems could easily be built upon this API.
For such `Services` a cluster IP is not allocated, the kube proxy does not handle
these services, and there is no load balancing or proxying done by the platform
for them. How DNS is automatically configured depends on if the service has
selectors or not.
### With selectors
For headless services that define selectors, the endpoints controller will still
create `Endpoints` records in the API, and DNS is configured to return multiple A
records (addresses) for the `Service` name which point directly to the `Pods`
backing the `Service`.
### Without selectors
For headless services that do not define selectors, the endpoints controller does
not create `Endpoints` records. However, the DNS system looks for and configures
A records for any `Endpoints` that share a name with the service.
## Publishing services - service types ## Publishing services - service types
For some parts of your application (e.g. frontends) you may want to expose a For some parts of your application (e.g. frontends) you may want to expose a