1//  File Transfer model #3
2//
3//  In which the client requests each chunk individually, using
4//  command pipelining to give us a credit-based flow control.
5
6package main
7
8import (
9	zmq "github.com/pebbe/zmq4"
10
11	"fmt"
12	"os"
13	"strconv"
14)
15
16const (
17	CHUNK_SIZE = 250000
18	PIPELINE   = 10
19)
20
21func client_thread(pipe chan<- string) {
22	dealer, _ := zmq.NewSocket(zmq.DEALER)
23	dealer.Connect("tcp://127.0.0.1:6000")
24
25	//  Up to this many chunks in transit
26	credit := PIPELINE
27
28	total := 0  //  Total bytes received
29	chunks := 0 //  Total chunks received
30	offset := 0 //  Offset of next chunk request
31
32	for {
33		for credit > 0 {
34			//  Ask for next chunk
35			dealer.SendMessage("fetch", offset, CHUNK_SIZE)
36			offset += CHUNK_SIZE
37			credit--
38		}
39		chunk, err := dealer.RecvBytes(0)
40		if err != nil {
41			break //  Shutting down, quit
42		}
43		chunks++
44		credit++
45		size := len(chunk)
46		total += size
47		if size < CHUNK_SIZE {
48			break //  Last chunk received; exit
49		}
50	}
51	fmt.Printf("%v chunks received, %v bytes\n", chunks, total)
52	pipe <- "OK"
53}
54
55//  The rest of the code is exactly the same as in model 2, except
56//  that we set the HWM on the server's ROUTER socket to PIPELINE
57//  to act as a sanity check.
58
59//  The server thread waits for a chunk request from a client,
60//  reads that chunk and sends it back to the client:
61
62func server_thread() {
63	file, err := os.Open("testdata")
64	if err != nil {
65		panic(err)
66	}
67
68	router, _ := zmq.NewSocket(zmq.ROUTER)
69	router.SetRcvhwm(PIPELINE * 2)
70	router.SetSndhwm(PIPELINE * 2)
71	router.Bind("tcp://*:6000")
72	for {
73		msg, err := router.RecvMessage(0)
74		if err != nil {
75			break //  Shutting down, quit
76		}
77		//  First frame in each message is the sender identity
78		identity := msg[0]
79
80		//  Second frame is "fetch" command
81		if msg[1] != "fetch" {
82			panic("command != \"fetch\"")
83		}
84
85		//  Third frame is chunk offset in file
86		offset, _ := strconv.ParseInt(msg[2], 10, 64)
87
88		//  Fourth frame is maximum chunk size
89		chunksz, _ := strconv.Atoi(msg[3])
90
91		//  Read chunk of data from file
92		chunk := make([]byte, chunksz)
93		n, _ := file.ReadAt(chunk, offset)
94
95		//  Send resulting chunk to client
96		router.SendMessage(identity, chunk[:n])
97	}
98	file.Close()
99}
100
101//  The main task is just the same as in the first model.
102
103func main() {
104	pipe := make(chan string)
105
106	//  Start child threads
107	go server_thread()
108	go client_thread(pipe)
109	//  Loop until client tells us it's done
110	<-pipe
111}
112