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plugin/etcd: update documention (#4049)

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* plugin/etcd: update documention

Make clear(er) how this plugin find records.

Fixes: #4048

Signed-off-by: Miek Gieben <miek@miek.nl>

* Update plugin/etcd/README.md

Co-authored-by: Chris O'Haver <cohaver@infoblox.com>

* Update plugin/etcd/README.md

Co-authored-by: Chris O'Haver <cohaver@infoblox.com>

Co-authored-by: Chris O'Haver <cohaver@infoblox.com>
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plugin/etcd/README.md
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@ -8,14 +8,15 @@
The *etcd* plugin implements the (older) SkyDNS service discovery service. It is *not* suitable as The *etcd* plugin implements the (older) SkyDNS service discovery service. It is *not* suitable as
a generic DNS zone data plugin. Only a subset of DNS record types are implemented, and subdomains a generic DNS zone data plugin. Only a subset of DNS record types are implemented, and subdomains
and delegations are not handled at all. and delegations are not handled at all. The plugin will also recursively descend the tree and return
all records found, see "Special Behavior" below for details.
The data in the etcd instance has to be encoded as The data in the etcd instance has to be encoded as
a [message](https://github.com/skynetservices/skydns/blob/2fcff74cdc9f9a7dd64189a447ef27ac354b725f/msg/service.go#L26) a [message](https://github.com/skynetservices/skydns/blob/2fcff74cdc9f9a7dd64189a447ef27ac354b725f/msg/service.go#L26)
like [SkyDNS](https://github.com/skynetservices/skydns). It works just like SkyDNS. like [SkyDNS](https://github.com/skynetservices/skydns). It works just like SkyDNS.
The etcd plugin makes extensive use of the *forward* plugin to forward and query other servers in the The *etcd* plugin makes extensive use of the *forward* plugin to forward and query other servers in the
network. network - if that plugin has been enabled as well.
## Syntax ## Syntax
@ -28,7 +29,6 @@ etcd [ZONES...]
The path will default to `/skydns` the local etcd3 proxy (http://localhost:2379). If no zones are The path will default to `/skydns` the local etcd3 proxy (http://localhost:2379). If no zones are
specified the block's zone will be used as the zone. specified the block's zone will be used as the zone.
If you want to `round robin` A and AAAA responses look at the `loadbalance` plugin.
~~~ ~~~
etcd [ZONES...] { etcd [ZONES...] {
@ -61,16 +61,18 @@ etcd [ZONES...] {
The *etcd* plugin leverages directory structure to look for related entries. For example The *etcd* plugin leverages directory structure to look for related entries. For example
an entry `/skydns/test/skydns/mx` would have entries like `/skydns/test/skydns/mx/a`, an entry `/skydns/test/skydns/mx` would have entries like `/skydns/test/skydns/mx/a`,
`/skydns/test/skydns/mx/b` and so on. Similarly a directory `/skydns/test/skydns/mx1` will have all `/skydns/test/skydns/mx/b` and so on. Similarly a directory `/skydns/test/skydns/mx1` will have all
`mx1` entries. `mx1` entries. Note this plugin will search through the entire (sub)tree for records. In case of the
first example, a query for `mx.skydns.text` will return both the contents of the `a` and `b` records.
If the directory extends deeper those records are returned as well.
With etcd3, support for [hierarchical keys are With etcd3, support for [hierarchical keys are
dropped](https://coreos.com/etcd/docs/latest/learning/api.html). This means there are no directories dropped](https://coreos.com/etcd/docs/latest/learning/api.html). This means there are no directories
but only flat keys with prefixes in etcd3. To accommodate lookups, etcdv3 plugin now does a lookup but only flat keys with prefixes in etcd3. To accommodate lookups, the *etcd* plugin now does a lookup
on prefix `/skydns/test/skydns/mx/` to search for entries like `/skydns/test/skydns/mx/a` etc, and on prefix `/skydns/test/skydns/mx/` to search for entries like `/skydns/test/skydns/mx/a` etc, and
if there is nothing found on `/skydns/test/skydns/mx/`, it looks for `/skydns/test/skydns/mx` to if there is nothing found on `/skydns/test/skydns/mx/`, it looks for `/skydns/test/skydns/mx` to
find entries like `/skydns/test/skydns/mx1`. find entries like `/skydns/test/skydns/mx1`.
This causes two lookups from CoreDNS to etcdv3 in certain cases. This causes two lookups from CoreDNS to etcd in certain cases.
## Examples ## Examples
@ -226,3 +228,7 @@ If you query the zone name for `TXT` now, you will get the following response:
% dig +short skydns.local TXT @localhost % dig +short skydns.local TXT @localhost
"this is a random text message." "this is a random text message."
~~~ ~~~
## Also See
If you want to `round robin` A and AAAA responses look at the *loadbalance* plugin.