转到http侦听器，每秒钟更新一次数据

Question

I'm trying to build a little website in Go containing a report based on data collected from a web service. It uses an API to query the service for data. The service can only be queried once every few seconds.

However, I have to query it a number of times to get the complete report data. Right now I just hammer the API to update my whole data structure each time the http handler (http.HandleFunc) is called. Of course, this is bad because it triggers lots of queries to the external API that are throttled. So, my report comes up very, very, very, slowly.

What I want to do instead is to have a function to updateReportData in a non-blocking way and store that data in some variable that the http.HandleFunc() can just ingest without calling the external API.

But, I'm very new to Go (and things like closures, semaphores, concurrency, etc) and so I'm not really sure how to build this. Should I be using channels? Should I use timers? How can I get the updateReportData to not block the http.HandleFunc, but still run on a fixed interval?

To sum up, I want to have a background routine update a data structure on a fixed interval and I want to be able to use http.HandleFunc to serve whatever data is in the data structure any time i make an http request to the program. I just have no idea how to start. Any advice would be appreciated.

Answer 1

There are a few things you need to do:

1. Create a background service that polls for the data. This service can run as a goroutine that periodically checks for new data.
2. Create a channel that the background service uses to send new data to a centralized place to store the values. The background service should write data to this channel whenever it finds something new. Another option would be to protect the centralized data store with a mutex. Depending on the way the data is written and read, one option will be a better choice.
3. Create a HTTP handler that returns the current contents of the centralized data store.

Here is a simplified example showing how to use a goroutine and a sync.RWMutext to accomplish what you want:

package main

import (
    "fmt"
    "net/http"
    "sync"
    "time"
)

var (
    timeSumsMu sync.RWMutex
    timeSums   int64
)

func main() {
    // Start the goroutine that will sum the current time
    // once per second.
    go runDataLoop()
    // Create a handler that will read-lock the mutext and
    // write the summed time to the client
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        timeSumsMu.RLock()
        defer timeSumsMu.RUnlock()
        fmt.Fprint(w, timeSums)
    })
    http.ListenAndServe(":8080", nil)
}

func runDataLoop() {
    for {
        // Within an infinite loop, lock the mutex and
        // increment our value, then sleep for 1 second until
        // the next time we need to get a value.
        timeSumsMu.Lock()
        timeSums += time.Now().Unix()
        timeSumsMu.Unlock()
        time.Sleep(1 * time.Second)
    }
}