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Interface Pitfalls and Harnessing io.Reader

When Go was first announced I remember looking over the list of its key features and feeling astonished that a new language would omit the classes and inheritance that I had come to depend on so heavily. My interest faded quickly.

Fast forward a few years and our team has fully embraced Go for its speed, tooling, standard library, concurrency support and all the other things we know and love about Go. If you’re interested in learning more about how we use Go at zvelo, we’ve recently published a blog post.

The concept of interfaces, while certainly not new to us, seemed more like an afterthought in our embrace of the language. We had used interfaces in C++, and they were useful but tedious. Despite hearing so many great things about implicitly satisfied interfaces, it still took us quite a while to really internalize what the implications of this simple concept were.

Let’s walk through the process that a newcomer to Go might follow in developing a simple text processor that replaces instances of “hodor” with “hold the door”. We will start with a naïve implementation and refactor it over several steps.


Naïve Implementation

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func process(text string) string {
	return regexp.MustCompile(`hodor`).
		ReplaceAllString(text, `hold the door`)
}

https://play.golang.org/p/wQ8UUjNi7u

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string length: 130B
BenchmarkNaive-8  200000  13003 ns/op  41985 B/op  45 allocs/op

string length: 130000B
BenchmarkNaive-8  200  6584315 ns/op  1494456 B/op  10063 allocs/op

Let’s ignore for the duration of the exercise the simplicity of the function — it simply represents anything that has to modify data. There are several obvious problems. First of all, the most glaring issue is that the regular expression is being compiled every time process is executed. Additionally, while there is no risk here (since the regex is fine), if the regular expression were to fail compilation (if, say, it was loaded dynamically), it would cause the application to panic during runtime. This would be unacceptable for production systems, and proper error handling should have been used instead (with regexp.Compile), but I digress.


Precompiled Regular Expressions

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var re = regexp.MustCompile(`hodor`)

func process(text string) string {
	return re.ReplaceAllString(text, `hold the door`)
}

https://play.golang.org/p/D4mx4gHpoz

We keep using regexp.MustCompile, but because compilation occurs during initialization, any errors in the regex are exposed immediately on application startup. Benchmarking the updated function yields significantly better performance for small strings, but as strings get larger the benefit approaches zero.

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string length: 130B
BenchmarkCRegex-8  300000  3724 ns/op  880 B/op  16 allocs/op

string length: 130000B
BenchmarkCRegex-8  200  5937989 ns/op  1492260 B/op  10032 allocs/op

Avoiding Regular Expressions

Regular expressions are excellent tools that have many valid uses, but are often abused when simple text processing will suffice.

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func process(text string) string {
	return strings.Replace(text, "hodor", "hold the door", -1)
}

https://play.golang.org/p/JB-ozcVTrt

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string length: 130B
BenchmarkAvoidRegex-8  2000000  689 ns/op  448 B/op  2 allocs/op

string length: 130000B
BenchmarkAvoidRegex-8  3000  527535 ns/op  425999 B/op  3 allocs/op

By using strings.Replace instead of a regular expression, we improve the performance by an order of magnitude for both short and long strings. We also minimize the number of memory allocations. Keep an eye on the B/op though, it scales with string size and that may become an issue for very large strings. Also, what if we want to operate on large files or even a network socket?


Using io.Reader

Let’s see if we can make this a bit more generic by using io.Reader instead of string. We’ve seen io.Reader used with things like os.File and figure that we can make that work somehow. But how do we return the processed data to the caller? Let’s just return another io.Reader.

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func process(r io.Reader) (io.Reader, error) {
	data, err := ioutil.ReadAll(r)
	if err != nil {
		return nil, err
	}
	return strings.NewReader(strings.Replace(string(data), "hodor", "hold the door", -1)), nil
}

https://play.golang.org/p/tZSjg7van_

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string length: 130B
BenchmarkBadIface-8  1000000  2111 ns/op  4720 B/op  8 allocs/op

string length: 130000B
BenchmarkBadIface-8  2000  675796 ns/op  1339467 B/op  24 allocs/op

Great! Now we are using interfaces, we’re golden right? Well… not so much. We aren’t using less memory since we are using ioutil.ReadAll (which is almost always incorrect and only works with readers that return io.EOF). Further, we are just wastefully turning the result into an io.Reader from the string. To add insult to injury, our performance has dropped significantly across all metrics too.


Streaming Data

It now occurs to us that if we streamed the data, byte by byte, we can avoid the large memory allocations. This does introduce a new knob that will affect processing performance. We will have to choose how much data to buffer at a time before running Replace. The larger the chunk size, the higher the B/op. The smaller the chunk size, the greater the number of times data has to be copied. There is no right answer for every situation.

It should be noted that there is a bit of a bug in this implementation in that the chunk could read until the middle of a hodor and it wouldn’t get replaced properly. Since this code is for demonstration only, fixing it is an exercise left to the reader.

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// DefaultChunkSize is the default amount of data read from an io.Reader
const DefaultChunkSize = 1024 * 16

// Process starts to stream data
func Process(r io.Reader) (io.Reader, error) {
	var buf, rbuf, result bytes.Buffer

	for {
		_, err := io.CopyN(&buf, r, DefaultChunkSize)
		if err != nil && err != io.EOF {
			return nil, err
		}

		if rerr := Replace(buf.Bytes(), []byte("hodor"), []byte("hold the door"), -1, &rbuf); rerr != nil {
			return nil, rerr
		}

		buf.Reset()

		if _, werr := result.Write(rbuf.Bytes()); werr != nil {
			return nil, werr
		}

		if err == io.EOF {
			return &result, nil
		}
	}
}

https://play.golang.org/p/NWBTaYI6Fe

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string length: 130B, chunk size: 16384B
BenchmarkBadStream-8  500000  2886 ns/op  4912 B/op  10 allocs/op

string length: 130000B, chunk size: 16384B
BenchmarkBadStream-8  2000  884182 ns/op  1314940 B/op  35 allocs/op

This is definitely a step in the right direction as we are truly streaming the data now. However, because we are also managing the output buffer, we still require more memory and allocations than necessary. Don’t worry about the performance loss, things are about to get much better.


A Quick Side Note about Replace

Astute readers will see the as yet undefined Replace in the above code. In effect, it is only bytes.Replace.

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// Replace reimplements bytes.Replace in a way that can reuse the buffer
func Replace(s, old, new []byte, n int, buf *bytes.Buffer) error {
	m := 0

	if n != 0 {
		// Compute number of replacements.
		m = bytes.Count(s, old)
	}

	if buf == nil {
		buf = &bytes.Buffer{}
	}

	buf.Reset()

	if m == 0 {
		// Just return a copy.
		_, err := buf.Write(s)
		return err
	}

	if n < 0 || m < n {
		n = m
	}

	// Apply replacements to buffer.
	buf.Grow(len(s) + n*(len(new)-len(old)))

	start := 0
	for i := 0; i < n; i++ {
		j := start

		if len(old) == 0 {
			if i > 0 {
				_, wid := utf8.DecodeRune(s[start:])
				j += wid
			}
		} else {
			j += bytes.Index(s[start:], old)
		}

		if _, err := buf.Write(s[start:j]); err != nil {
			return err
		}

		if _, err := buf.Write(new); err != nil {
			return err
		}

		start = j + len(old)
	}

	_, err := buf.Write(s[start:])
	return err
}

https://golang.org/pkg/bytes/#Replace

The difference between Replace and bytes.Replace is that Replace is passed a bytes.Buffer. The benefit of this is not fully realized yet, but it allows an already allocated buffer to be used instead of requiring new allocations every time it is called. This is the same strategy that io.CopyBuffer uses. Since there is no bytes.ReplaceBuffer it had to be copied and modified.


Pushing Memory Allocation to the Caller

Let’s look at one more possibility for a Process func. Rather than handling the memory allocation ourselves, with bytes.Buffer, let’s let the caller decide how to handle memory by allowing a passed in io.Writer.

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func Process(w io.Writer, r io.Reader) error {
	var buf, rbuf bytes.Buffer
	for {
		_, err := io.CopyN(&buf, r, DefaultChunkSize)
		if err != nil && err != io.EOF {
			return err
		}

		if rerr := Replace(buf.Bytes(), []byte("hodor"), []byte("hold the door"), -1, &rbuf); rerr != nil {
			return rerr
		}

		buf.Reset()

		if _, werr := w.Write(rbuf.Bytes()); werr != nil {
			return werr
		}

		if err == io.EOF {
			return nil
		}
	}
}

https://play.golang.org/p/Fkq2ObxIuy

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string length: 130B, chunk size: 16384B
BenchmarkMalloc-8  1000000  1897 ns/op  2528 B/op  6 allocs/op

string length: 130000B, chunk size: 16384B
BenchmarkMalloc-8  2000  624815 ns/op  92387 B/op  17 allocs/op

This is certainly cleaner, and is a bit more performant and memory conscious. What if there was a way for us to prevent the need for any write buffer?


Implementing our own io.Reader

By writing a Processor that implements the io.Reader interface itself, we can essentially create a pipeline for data to flow while minimizing data allocations. As an io.Reader our Processor is usable by any number of packages in the standard library and third party packages.

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// Processor implements an io.Reader
type Processor struct {
	Src       io.Reader
	Old, New  []byte
	ChunkSize int
	buf, rbuf bytes.Buffer
}

// Reset the processor
func (m *Processor) Reset(src io.Reader) {
	m.Src = src
	m.buf.Reset()
}

func (m *Processor) Read(p []byte) (int, error) {
	if m.ChunkSize == 0 {
		m.ChunkSize = DefaultChunkSize
	}

	// first flush any buffered data to p
	n, err := m.buf.Read(p)
	if n == len(p) || (err != nil && err != io.EOF) {
		return n, err
	}

	// m.buf must be empty now
	_, err = io.CopyN(&m.buf, m.Src, int64(m.ChunkSize))
	if err != nil && err != io.EOF {
		return n, err
	}

	rerr := Replace(m.buf.Bytes(), m.Old, m.New, -1, &m.rbuf)
	m.buf.Reset()

	if rerr != nil {
		return n, rerr
	}

	copied, rerr := m.rbuf.Read(p[n:])
	n += copied
	if rerr != nil && rerr != io.EOF {
		return n, rerr
	}

	// copy anything not put in p, back to the buffer
	if _, rerr := m.buf.ReadFrom(&m.rbuf); rerr != nil {
		return n, rerr
	}

	return n, err
}

https://play.golang.org/p/SY-W32GJ3L

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# STANDARD CHUNK SIZE

string length: 130B, chunk size: 16384B
BenchmarkProcReadAll-8  2000000   899 ns/op  32 B/op  1 allocs/op
BenchmarkProcRead-8     1000000  1056 ns/op  32 B/op  1 allocs/op

string length: 130000B, chunk size: 16384B
BenchmarkProcReadAll-8   3000  674009 ns/op  256 B/op  8 allocs/op
BenchmarkProcRead-8     30000   54461 ns/op   21 B/op  0 allocs/op

# LARGER CHUNK SIZE

string length: 130B, chunk size: 131072B
BenchmarkProcReadAll-8  2000000  909 ns/op  32 B/op  1 allocs/op
BenchmarkProcRead-8     2000000  985 ns/op  32 B/op  1 allocs/op

string length: 130000B, chunk size: 131072B
BenchmarkProcReadAll-8  2000  642961 ns/op  32 B/op  1 allocs/op
BenchmarkProcRead-8     2000  634125 ns/op  32 B/op  1 allocs/op

# SMALLER CHUNK SIZE

string length: 130B, chunk size: 1024B
BenchmarkProcReadAll-8  2000000  889 ns/op  32 B/op  1 allocs/op
BenchmarkProcRead-8     2000000  910 ns/op  32 B/op  1 allocs/op

string length: 130000B, chunk size: 1024B
BenchmarkProcReadAll-8  2000  623435 ns/op  4065 B/op  127 allocs/op
BenchmarkProcRead-8   500000    3140 ns/op    19 B/op    0 allocs/op

This is what we were looking for. It is nearly as performant as the strings.Replace version but uses a fraction of the memory and causes very little garbage collector thrashing.

The ReadAll metrics consider reading all of the io.Reader as a single operation, whereas the Read metrics consider one r.Read function as a single operation.

It now becomes much clearer how different chunk sizes affect things. Smaller chunks result in more allocations, but much faster individual r.Read operations. Larger chunks work much like strings.Replace since they are getting most of the data at once and their performance and memory requirements (though not allocations) approach it as well.

One thing to note is that the B/op metric is a bit deceiving since we are reusing the buffers so heavily. They indicate an average across many calls. The first few calls will do all of the allocations and then all subsequent ones can reuse them without requiring new allocations. The actual memory used by each call corresponds more closely with the chunk size.

For reference, here is the same test as the previous one, but using bytes.Replace instead of our buffered Replace.

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string length: 130B, chunk size: 16384B
BenchmarkProcReadAll-8 2000000 784 ns/op 256 B/op 2 allocs/op
BenchmarkProcRead-8    2000000 775 ns/op 256 B/op 2 allocs/op

string length: 130000B, chunk size: 16384B
BenchmarkProcReadAll-8  3000 482607 ns/op 224007 B/op 16 allocs/op
BenchmarkProcRead-8    20000  66704 ns/op  28000 B/op  2 allocs/op

The primary difference is that the B/op metric is 3 orders of magnitude larger for long strings.

If you are more of a visual learner, here is the data in a few charts, and here is its source data.


Conclusion

I’ve attempted to illustrate the learning process that someone coming from languages like Perl or Python might follow. Starting with regular expressions and ending with implementing an io.Reader, is certainly a possible, if unlikely, progression. While for this simple example strings.Replace certainly would have sufficed, more complicated algorithms might justify the additional complexity.

Remember, here, as always, write clean, maintainable code first, test it, then measure its performance. Only then optimize the parts where the performance is required.