example.go

// external.go
package external

import (
    "context"
    "encoding/json"
    "errors"
    "fmt"
    "net/http"
    "time"
)

var (
    ErrResponseNotOK error = errors.New("response not ok")
)

type (
    Data struct {
        ID   string `json:"id"`
        Name string `json:"name"`
    }

    External interface {
        FetchData(ctx context.Context, id string) (*Data, error)
    }

    v1 struct {
        baseURL string
        client  *http.Client
        timeout time.Duration
    }
)

func New(baseURL string, client *http.Client, timeout time.Duration) *v1 {
    return &v1{
        baseURL: baseURL,
        client:  client,
        timeout: timeout,
    }
}

func (v *v1) FetchData(ctx context.Context, id string) (*Data, error) {
    url := fmt.Sprintf("%s/?id=%s", v.baseURL, id)

    ctx, cancel := context.WithTimeout(ctx, v.timeout)
    defer cancel()

    req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
    if err != nil {
        return nil, err
    }

    resp, err := v.client.Do(req)
    if err != nil {
        return nil, err
    }
    defer resp.Body.Close()

    if resp.StatusCode != http.StatusOK {
        return nil, fmt.Errorf("%w. %s", ErrResponseNotOK, http.StatusText(resp.StatusCode))
    }

    var d *Data
    return d, json.NewDecoder(resp.Body).Decode(&d)
}