net.LookupSRV()查詢k8s無頭服務

(金慶的專欄 2021.2)

如下創建 StatefulSet 和 Headless Service: test.yaml

```
apiVersion: v1
kind: Service
metadata:
  name: headless-svc
  labels:
    app: headless-svc
spec:
  ports:
  - port: 80
    name: aaaa
  - port: 20080
    name: bbbb
  selector:
    app: headless-pod
  clusterIP: None
 
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: statefulset-test
spec:
  serviceName: headless-svc
  replicas: 3
  selector:
    matchLabels:
      app: headless-pod
  template:
    metadata:
      labels:
        app: headless-pod
    spec:
      containers:
      - name: myhttpd
        image: httpd
        ports:
        - containerPort: 80
        - containerPort: 20080
```

部署：
```
kubectl apply -f test.yaml
```

然后開一個 shell:
```
kubectl run mygolang -it --image golang -- bash
If you don't see a command prompt, try pressing enter.
root@mygolang:/go#
```

先裝個 vim:
```
apt install vim
```

然后寫個 golang 測試程序，向 DNS 查詢 bbbb 服務的地址與端口：
```
root@mygolang:/jinqing# cat main.go
package main

import (
    "fmt"
    "github.com/davecgh/go-spew/spew"
    "net"
)

func main() {
    cname, addresses, err := net.LookupSRV("bbbb", "tcp", "headless-svc.default.svc.cluster.local")
    if err != nil {
        fmt.Printf("failed: %s\n", err)
    }
    fmt.Printf("cname: %s\n", cname)
    spew.Dump(addresses)
}
root@mygolang:/jinqing#
```

運行結果為：
```
cname: _bbbb._tcp.headless-svc.default.svc.cluster.local.
([]*net.SRV) (len=3 cap=4) {
 (*net.SRV)(0xc00000e220)({
  Target: (string) (len=58) "statefulset-test-0.headless-svc.default.svc.cluster.local.",
  Port: (uint16) 20080,
  Priority: (uint16) 0,
  Weight: (uint16) 33
 }),
 (*net.SRV)(0xc00000e1e0)({
  Target: (string) (len=58) "statefulset-test-2.headless-svc.default.svc.cluster.local.",
  Port: (uint16) 20080,
  Priority: (uint16) 0,
  Weight: (uint16) 33
 }),
 (*net.SRV)(0xc00000e200)({
  Target: (string) (len=58) "statefulset-test-1.headless-svc.default.svc.cluster.local.",
  Port: (uint16) 20080,
  Priority: (uint16) 0,
  Weight: (uint16) 33
 })
}
```

多次運行發現結果項次序固定，并沒有按權重隨機。

Service yaml 定義中，必須為每個端口命名，不然沒法查詢。

```
    cname, addresses, err := net.LookupSRV("bbbb", "tcp", "headless-svc.default.svc.cluster.local")
```
可利用默認域名后綴簡寫為
```
    cname, addresses, err := net.LookupSRV("bbbb", "tcp", "headless-svc")
```
輸出相同。