watermint.org - Takayuki Okazaki's note

Go: Bandwidth limit for multiple Reader and Writer

Bandwidth limit is really important for stabilizing systems. Core business logic stops if low priority job consume entire bandwidth of the system. When writing bandwidth limit for Reader or Writer in Go, I found go-flowrate. This library enables easy way to limit bandwidth like the code below.

func main() {
    f, _ := os.Open("data.dat")

    // Create wrapper with 100 bytes per second
    f2 := flowrate.NewReader(f, 100)

    // Biz logic
    // ...

APIs are pretty simple and reliable. But go-flowrate can limit bandwidth only for single Reader or Writer. In my case, I was working on file uploader for Dropbox which has concurrent upload feature. This code require limiting bandwidth for multiple I/O. So I decided create new small library for bandwidth limit.


bwlimit is the name of my library. You can see or clone from github bwlimit. Here is code example of bwlimit.

func main() {
    // Limit 100 bytes per second
    bwlimit := NewBwlimit(100, false)

    // Prepare multiple readers
    f1, _ := os.Open("data1.dat")
    f2, _ := os.Open("data2.dat")

    // Create wrapper
    fr1 := bwlimit.Reader(f1)
    fr2 := bwlimit.Reader(f2)

    // Biz logic
    // ...

    // Wait for all Reader to be closed or reach EOF

1) Prepare bandwidth limiting object (bwlimit in above code) first. Second argument is the flag for block(true) or unblock(false) for Read or Write operation. 2) Then, create wrapper for each Reader or Writer.


The concept of bwlimit comes from Toyota’s Production System (TPS). In TPS, Takt time is the core concept for leveling production. Define time unit of ship defined quantity of products.

For example; 1) Takt time is 100ms, 2) bandwidth limit is 1,000 bytes per second. 100 bytes is maximum transferrable data size per takt time. If bwlimit object has two Readers, that mean 50 bytes per Reader per takt time.

Flow control

bwlimit does not carry bandwidth window to next takt time. If the Reader have 50 bytes window per takt time, and the Reader read nothing. Then, window size of next takt time will be 50 bytes. This is because if bwlimit carry unused window to next takt time. bwlimit may allow burst IO for certain takt time. That sometime cause buffer overflow of routers. To prevent incident caused by burst IO.