[1]Stephen's Tech Blog

  • [2]Webhook Wizard 🧙‍♂️

Gopher Wrangling. Effective error handling in Go

June 19, 2023 · 4 min · Stephen Nancekivell
Table of Contents

      □ [3]Guiding principle
  • [4]1. Always handle errors
  • [5]2. Log errors in one layer
  • [6]3. Returning async errors
  • [7]4. Wrapping errors
  • [8]5. Downgrade errors Warnings

When programming in Go, the amount of error handling is something that slaps
you in the face. Most API’s you deal with will expose errors. It can become
overwhelming, but with a few tips and a guiding principle we can make handling
errors easy, keep our code clean and give you the confidence that nothing is
breaking in production.

A cartoon of a crazy stressed programmer pulling their hair out in front of
lots of screens showing a error exclamation marks

    A cartoon of a crazy stressed programmer pulling their hair out in front of
    lots of screens showing a error exclamation marks

Guiding principle[9]#

The goal for our error handling strategy is that it should require minimal
effort and provide an easy way to debug any errors that do occur.

We wont cover strategies like retrying because they are less common and also
expose errors.

1. Always handle errors[10]#

Always handle errors. Sometimes it’s tempting to skip one, you might not expect
that error to ever happen. But that’s why it’s an exception! You need to handle
it so that you can find out clearly if it ever does happen.

If you don’t handle the error, the expected value will be something else and
just lead to another error that will be harder to debug, or worse it could lead
to data corruption.

In most cases to handle the error all you need to do is return it to the caller
of your method, where they can log it.

For example, when refreshing some data you might load it, then save it. If you
skip the error handling it could overwrite potentially useful data with corrupt
data.

👎 Bad error handling

func refresh() {
    bytes, _ := loadData()
    saveData(bytes)
}

👍 Good error handling

func refresh() error {
    bytes, err := loadData()
    if err != nil {
        return err
    }
    saveData(bytes)
}

2. Log errors in one layer[11]#

You always want to log your errors, ideally to something that will notify you
about the error, so you can fix it. There is no point logging the error
multiple times at every layer. Make it the top layer’s responsibility and don’t
log in any services or lower level code.

Make sure your logging framework is including stack traces so you can trace the
error to its cause.

For example in a web app you would log the error in the http handler when
returning the Internal Server status code.

👍 Good error handling

func refresh() error {
    bytes, err := loadData()
    if err != nil {
        return err
    }
    saveData(bytes)
}

func (h *handlers) handleRefreshRequest(w http.ResponseWriter, r *http.Request) {
    err := refresh()
    if err != nil {
        log.Error("unexpected error processing request %w", err)
        w.WriteHeader(http.StatusInternalServerError)
        return
    }

    w.WriteHeader(http.StatusOK)
}

3. Returning async errors[12]#

When processing data concurrently using a go-func’s, it can be annoying to
return the error. But if you don’t your app will be less maintainable. To
handle async errors, return them via a channel to the calling thread.

👎 Bad error handling

func refreshManyConcurrently() {
    go func(){
        refresh(1)
    }()

    go func(){
        refresh(2)
    }()
}

👍 Good error handling

func refreshManyConcurrently() error {
    errors := make(chan error, 2)
    go func(){
        errors <- refresh(1)
    }()

    go func(){
        errors <- refresh(2)
    }()
    return multierror.Combine(<-errors, <- errors)
}

When calling functions that return a value and a possible error using a type
like Result[T], to wrap the response to pass on the channel.

type Result[T any] struct {
    Value T
    Error error
}

4. Wrapping errors[13]#

Sometimes you want to add additional context to an error message. Eg to include
the id of the request that caused the error. You can use fmt.error for this.

err := saveToDb(user)
if err != nil {
    return fmt.errorf("unexpected error saving user. userId=%v error=%w", user.Id, err)
}

Usually this isn’t necessary and its better to just return the error unwrapped.

5. Downgrade errors Warnings[14]#

There are types of errors that regularly occur during normal operation. The
system might not be able to prevent them all the time, but they don’t need to
investigate every time. It is better to treat them as warnings rather than
errors. These might be for things like timeouts or intermittent connection
errors.

👍 Good error handling

func (h *handlers) handleRefreshRequest(w http.ResponseWriter, r *http.Request) {
    err := refresh()
    if err != nil {
        if err == context.DeadlineExceeded {
            log.Warn("Timeout error processing request %w", err)
        } else {
            log.Error("unexpected error processing request %w", err)
        }

        w.WriteHeader(http.StatusInternalServerError)
        return
    }

    w.WriteHeader(http.StatusOK)
}

[15]« Prev
PDFs on the Fly: Programmatically Transforming Webpages into PDFs [16]Next »
How to Serve Web Sockets with Http4s
[17][18][19][20][21][22]
[23]© 2023 [24]Stephen's Tech Blog
[25][26][27][28]

References:

[1] https://stephenn.com/
[2] https://webhookwizard.com/
[3] https://stephenn.com/2023/06/gopher-wrangling.-effective-error-handling-in-go/#guiding-principle
[4] https://stephenn.com/2023/06/gopher-wrangling.-effective-error-handling-in-go/#1-always-handle-errors
[5] https://stephenn.com/2023/06/gopher-wrangling.-effective-error-handling-in-go/#2-log-errors-in-one-layer
[6] https://stephenn.com/2023/06/gopher-wrangling.-effective-error-handling-in-go/#3-returning-async-errors
[7] https://stephenn.com/2023/06/gopher-wrangling.-effective-error-handling-in-go/#4-wrapping-errors
[8] https://stephenn.com/2023/06/gopher-wrangling.-effective-error-handling-in-go/#5-downgrade-errors-warnings
[9] https://stephenn.com/2023/06/gopher-wrangling.-effective-error-handling-in-go/#guiding-principle
[10] https://stephenn.com/2023/06/gopher-wrangling.-effective-error-handling-in-go/#1-always-handle-errors
[11] https://stephenn.com/2023/06/gopher-wrangling.-effective-error-handling-in-go/#2-log-errors-in-one-layer
[12] https://stephenn.com/2023/06/gopher-wrangling.-effective-error-handling-in-go/#3-returning-async-errors
[13] https://stephenn.com/2023/06/gopher-wrangling.-effective-error-handling-in-go/#4-wrapping-errors
[14] https://stephenn.com/2023/06/gopher-wrangling.-effective-error-handling-in-go/#5-downgrade-errors-warnings
[15] https://stephenn.com/2023/06/pdfs-on-the-fly-programmatically-transforming-webpages-into-pdfs/
[16] https://stephenn.com/2022/07/web-sockets-with-http4s/
[17] https://twitter.com/intent/tweet/?text=Gopher%20Wrangling.%20Effective%20error%20handling%20in%20Go&url=https%3a%2f%2fstephenn.com%2f2023%2f06%2fgopher-wrangling.-effective-error-handling-in-go%2f&hashtags=
[18] https://www.linkedin.com/shareArticle?mini=true&url=https%3a%2f%2fstephenn.com%2f2023%2f06%2fgopher-wrangling.-effective-error-handling-in-go%2f&title=Gopher%20Wrangling.%20Effective%20error%20handling%20in%20Go&summary=Gopher%20Wrangling.%20Effective%20error%20handling%20in%20Go&source=https%3a%2f%2fstephenn.com%2f2023%2f06%2fgopher-wrangling.-effective-error-handling-in-go%2f
[19] https://reddit.com/submit?url=https%3a%2f%2fstephenn.com%2f2023%2f06%2fgopher-wrangling.-effective-error-handling-in-go%2f&title=Gopher%20Wrangling.%20Effective%20error%20handling%20in%20Go
[20] https://facebook.com/sharer/sharer.php?u=https%3a%2f%2fstephenn.com%2f2023%2f06%2fgopher-wrangling.-effective-error-handling-in-go%2f
[21] https://api.whatsapp.com/send?text=Gopher%20Wrangling.%20Effective%20error%20handling%20in%20Go%20-%20https%3a%2f%2fstephenn.com%2f2023%2f06%2fgopher-wrangling.-effective-error-handling-in-go%2f
[22] https://telegram.me/share/url?text=Gopher%20Wrangling.%20Effective%20error%20handling%20in%20Go&url=https%3a%2f%2fstephenn.com%2f2023%2f06%2fgopher-wrangling.-effective-error-handling-in-go%2f
[23] https://stephenn.com/2023/06/gopher-wrangling.-effective-error-handling-in-go/#top
[24] https://stephenn.com/
[25] https://github.com/stephennancekivell
[26] https://twitter.com/hi_stephen_n
[27] https://www.linkedin.com/in/stephen-nancekivell-77003039
[28] https://stackoverflow.com/users/893854/stephen
