feat(zookeeper): support zookeeper service discovery

docs/en/latest/config.json

@@ -207,6 +207,7 @@
             "discovery/consul_kv",
             "discovery/nacos",
             "discovery/eureka",
+            "discovery/zookeeper",
             "discovery/kubernetes"
           ]
         },

docs/en/latest/discovery/zookeeper.md

@@ -0,0 +1,144 @@
+---
+title: nacos
+---
+
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+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+-->
+
+## Service Discovery Via Zookeeper
+
+`Zookeeper` service discovery needs to rely on the [apisix-seed](https://github.com/api7/apisix-seed) project.
+
+### How `apisix-seed` Works
+
+![APISIX-SEED](../../../assets/images/apisix-seed.png)
+
+`apisix-seed` completes data exchange by watch the changes of `etcd` and `zookeeper` at the same time.
+
+The process is as follows:
+
+1. `APISIX` registers a route and specifies the service discovery type as `zookeeper` to `etcd`.
+2. `apisix-seed` watch the resource changes of `APISIX` in `etcd` and filters the discovery type and obtains the service name.
+3. `apisix-seed` binds the service to the `etcd` resource and starts watch the service in zookeeper.
+4. The client registers the service with `zookeeper`.
+5. `apisix-seed` gets the service changes in `zookeeper`.
+6. `apisix-seed` queries the bound `etcd` resource information through the service name, and writes the updated service node to `etcd`.
+7. The `APISIX` worker watch etcd changes and refreshes the service node information to the memory.
+
+### Setting `apisix-seed` and Zookeeper
+
+The configuration steps are as follows:
+
+1. Start the Zookeeper service
+
+```bash
+docker run -itd --rm --name=dev-zookeeper -p 2181:2181 zookeeper:3.7.0
+```
+
+2. Download and compile the `apisix-seed` project.
+
+```bash
+git clone https://github.com/api7/apisix-seed.git
+cd apisix-seed
+go build
+```
+
+3. Modify the `apisix-seed` configuration file, config path `conf/conf.yaml`.
+
+```bash
+etcd:                            # APISIX ETCD Configure
+  host:
+    - "http://127.0.0.1:2379"
+  prefix: /apisix
+  timeout: 30
+
+discovery:
+  zookeeper:                     # Zookeeper Service Discovery
+    hosts:
+      - "127.0.0.1:2181"         # Zookeeper service address
+    prefix: /zookeeper
+    weight: 100                  # default weight for node
+    timeout: 10                  # default 10s
+```
+
+4. Start `apisix-seed` to monitor service changes
+
+```bash
+./apisix-seed
+```
+
+### Setting `APISIX` Route and Upstream
+
+Set a route, the request path is `/zk/*`, the upstream uses zookeeper as service discovery, and the service name
+is `APISIX-ZK`.
+
+```shell
+curl http://127.0.0.1:9080/apisix/admin/routes/1 \
+-H 'X-API-KEY: edd1c9f034335f136f87ad84b625c8f1' -X PUT -i -d '
+{
+    "uri": "/zk/*",
+    "upstream": {
+        "service_name": "APISIX-ZK",
+        "type": "roundrobin",
+        "discovery_type": "zookeeper"
+    }
+}'
+```
+
+### Register Service and verify Request
+
+1. Service registration using Zookeeper CLI
+
+- Register Service
+
+```bash
+# Login Container
+docker exec -it ${CONTAINERID} /bin/bash
+# Login Zookeeper Client
+oot@ae2f093337c1:/apache-zookeeper-3.7.0-bin# ./bin/zkCli.sh
+# Register Service
+[zk: localhost:2181(CONNECTED) 0] create /zookeeper/APISIX-ZK '{"host":"127.0.0.1:1980","weight":100}'
+```
+
+- Successful Response
+
+```bash
+Created /zookeeper/APISIX-ZK
+```
+
+2. Verify Request
+
+- Request
+
+```bash
+curl -i http://127.0.0.1:9080/zk/hello
+```
+
+- Response
+
+```bash
+HTTP/1.1 200 OK
+Connection: keep-alive
+Content-Type: text/html; charset=utf-8
+Date: Tue, 29 Mar 2022 08:51:28 GMT
+Server: APISIX/2.12.0
+Transfer-Encoding: chunked
+
+hello
+```