How do you handle HTTP client caching in Go?

package main

import (
    "net/http"
    "time"
)

func main() {
    client := &http.Client{}

    // Create a new HTTP request with cache headers
    req, _ := http.NewRequest("GET", "http://example.com", nil)
    req.Header.Set("If-Modified-Since", time.Now().UTC().Format(http.TimeFormat))
    req.Header.Set("If-None-Match", "etag")

    // Send the HTTP request
    res, _ := client.Do(req)

    // Handle the HTTP response
    if res.StatusCode == http.StatusNotModified {
        // The resource is already cached, so we don't need to retrieve it again
    } else {
        // The resource has changed or is not cached, so we need to retrieve it
        // and update the cache headers for future requests
    }
}

In this example, we create a new HTTP request with the If-Modified-Since and If-None-Match headers set to indicate the time the resource was last modified and the entity tag for the resource, respectively. When the HTTP response is received, we check the status code to see if the resource has changed since it was last cached. If the status code is http.StatusNotModified, we know that the resource is already cached and we don't need to retrieve it again. Otherwise, we retrieve the resource and update the cache headers for future requests.

You can modify the cache headers in the HTTP request to suit your caching requirements. Additionally, you can use a caching library such as github.com/gregjones/httpcache to implement more advanced caching strategies such as in-memory caching or disk caching. The httpcache library provides an easy-to-use API for storing and retrieving cached data with configurable TTLs and eviction policies.

package main

import (
    "fmt"
    "io/ioutil"
    "net/http"
    "time"

    "github.com/gregjones/httpcache"
    "github.com/gregjones/httpcache/diskcache"
)

func main() {
    // Create a new disk cache with a maximum size of 100MB and a TTL of 1 hour
    cache := diskcache.New("cache")
    cache.MaxSize(100 * 1024 * 1024)
    cache.TTL(time.Hour)

    // Create a new HTTP transport with caching enabled
    transport := httpcache.NewTransport(cache)
    client := &http.Client{Transport: transport}

    // Send a GET request to the HTTP server
    req, _ := http.NewRequest("GET", "http://localhost:8080", nil)
    res, err := client.Do(req)
    if err != nil {
        fmt.Println(err)
        return
    }
    defer res.Body.Close()

    // Read the response body
    body, _ := ioutil.ReadAll(res.Body)
    fmt.Println(string(body))
}

In this example, we first create a new disk cache with a maximum size of 100MB and a TTL of 1 hour using the diskcache.New function from the httpcache/diskcache package. We then create a new HTTP transport with caching enabled using the httpcache.NewTransport function from the httpcache package, and we use this transport to create a new HTTP client.

Next, we send a GET request to the HTTP server using the caching HTTP client. If the response is already cached, the client will return the cached response from the cache without sending a request to the server. If the response is not cached, the client will send the HTTP request to the server and cache the response.

Finally, we read the response body and print it to the console. If the response was cached, the body will be "Cached response: " followed by the cached content. If the response was not cached, the body will be the content returned by the server.

Note that this is just an example and you may need to modify the cache headers and caching strategies to suit your application's needs. Additionally, you may want to use a different caching library or caching strategy depending on your requirements.