Reading a Go map safely

My last post explained how I wrote to a Go map from multiple goroutines (safely!). To recap, I setup a goroutine to handle the writes, and passed the write requests (the key and the value) to it over a channel from the goroutines that handled the http requests. Here’s how I handle concurrent reads!

The goroutine that handled writes to the map now also listens (and is now called a listener instead!) on a read channel, through which the http handlers can pass a key as well as a channel that *they *will now listen on. The listener now looks like:

func (db *db) listener() {
    for {
        select {
        case writeReq := <-db.writeChan:
            key := writeReq["key"]
            value := writeReq["value"]
            db.dataMap[key] = value
            b, err := json.Marshal(db.dataMap)
            if err != nil {
                fmt.Println("Error marshalling db: ", err)
            }
            if len(b) > len(db.data) {
                db.extend(len(b))
            }
            copy(db.data, b)
        case readReq := <-db.readChan:
            key := readReq.key
            returnChan := readReq.returnChan
            if value, ok := db.dataMap[key]; ok {
                returnChan <- value
            } else {
                returnChan <- "ERROR: NOT FOUND"
            }
        }
    }
}

type readChanMessage struct {
    key        string
    returnChan chan string
}

func (db *db) handler(w http.ResponseWriter, r *http.Request, ps httprouter.Params) {
    switch r.Method {
    case "GET":
        key := ps.ByName("key")
        c := make(chan string)
        m := readChanMessage{key, c}
        db.readChan <- m
        value := <-c
        close(c)
        if value == "ERROR: NOT FOUND" {
            http.NotFound(w, r)
        } else {
            fmt.Fprint(w, value)
        }
    case "POST":
        m := make(map[string]string)
        m["key"] = ps.ByName("key")
        m["value"] = r.FormValue("value")
        db.writeChan <- m
        fmt.Fprint(w, r.FormValue("value"))
    }
}

The other option was to pass the http response writer over the read channel instead of using a return channel. Willa Drengwitz (thanks!) pointed out that there is a disadvantage to this— this will block all other reads/writes if the loop ends up waiting to write the http response.

Arpith Siromoney 💬

Reading a Go map safely