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io.Reader and io.Writer with limit.

go get

Rationale and Usage

There are times when a limited reader or writer comes in handy.

  1. wrap upstream so that reading is metered and limited:
// request is an incoming http.Request
request.Body = limitio.NewReadCloser(c.Request.Body, maxRequestBodySize, false)

// deal with the body now with easy mind. It's maximum size is assured.

Yes, io.LimitReader works the same way, but throws EOF on exceeding limit, which is confusing.

LimitIO provides error that can be identified.

decoder := json.NewDecoder(request.Body)
err := decoder.Decode(&myStruct)
if err != nil {
    if errors.Is(err, limitio.ErrThresholdExceeded) {
        // oops, we reached the limit

    err = fmt.Errorf("other error happened: %w", err)
  1. wrap downstream so that writing is metered and limited(or instead, just pretending writing):
// request is an incoming http.Request.
// Say, we want to record its body somewhere in the middleware,
// but feeling uneasy since its body might be HUGE, which may
// result in OOM and a successful DDOS...

var reqBuf bytes.buffer

// a limited writer comes to rescue!
// `true` means after reaching `RequestBodyMaxLength`,
// `limitedReqBuf` will start pretending writing so that
// io.TeeReader continues working while reqBuf stays unmodified.
limitedReqBuf := limitio.NewWriter(&reqBuf, RequestBodyMaxLength, true)

request.Body = &readCloser{
    Reader: io.TeeReader(request.Body, limitedReqBuf), 
    Closer: c.Request.Body,

LimitIO provides Reader, Writer and their Closer versions, for details, see docs.

Status: Beta

LimitIO is battling under production environment.

APIs are subjected to change in backward compatible way.


MIT License Copyright (c) 2021 LI Zhennan


Golang io.Reader and io.Writer but with limits