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fileio3.go
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fileio3.go
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// File Transfer model #3
//
// In which the client requests each chunk individually, using
// command pipelining to give us a credit-based flow control.
package main
import (
zmq "github.com/pebbe/zmq4"
"fmt"
"os"
"strconv"
)
const (
CHUNK_SIZE = 250000
PIPELINE = 10
)
func client_thread(pipe chan<- string) {
dealer, _ := zmq.NewSocket(zmq.DEALER)
dealer.Connect("tcp://127.0.0.1:6000")
// Up to this many chunks in transit
credit := PIPELINE
total := 0 // Total bytes received
chunks := 0 // Total chunks received
offset := 0 // Offset of next chunk request
for {
for credit > 0 {
// Ask for next chunk
dealer.SendMessage("fetch", offset, CHUNK_SIZE)
offset += CHUNK_SIZE
credit--
}
chunk, err := dealer.RecvBytes(0)
if err != nil {
break // Shutting down, quit
}
chunks++
credit++
size := len(chunk)
total += size
if size < CHUNK_SIZE {
break // Last chunk received; exit
}
}
fmt.Printf("%v chunks received, %v bytes\n", chunks, total)
pipe <- "OK"
}
// The rest of the code is exactly the same as in model 2, except
// that we set the HWM on the server's ROUTER socket to PIPELINE
// to act as a sanity check.
// The server thread waits for a chunk request from a client,
// reads that chunk and sends it back to the client:
func server_thread() {
file, err := os.Open("testdata")
if err != nil {
panic(err)
}
router, _ := zmq.NewSocket(zmq.ROUTER)
router.SetRcvhwm(PIPELINE * 2)
router.SetSndhwm(PIPELINE * 2)
router.Bind("tcp://*:6000")
for {
msg, err := router.RecvMessage(0)
if err != nil {
break // Shutting down, quit
}
// First frame in each message is the sender identity
identity := msg[0]
// Second frame is "fetch" command
if msg[1] != "fetch" {
panic("command != \"fetch\"")
}
// Third frame is chunk offset in file
offset, _ := strconv.ParseInt(msg[2], 10, 64)
// Fourth frame is maximum chunk size
chunksz, _ := strconv.Atoi(msg[3])
// Read chunk of data from file
chunk := make([]byte, chunksz)
n, _ := file.ReadAt(chunk, offset)
// Send resulting chunk to client
router.SendMessage(identity, chunk[:n])
}
file.Close()
}
// The main task is just the same as in the first model.
func main() {
pipe := make(chan string)
// Start child threads
go server_thread()
go client_thread(pipe)
// Loop until client tells us it's done
<-pipe
}