The Mystery of Missing Records: Debugging a JSON-to-CSV Transformation in Go

[Lomanu4]

During my work on building a utility for data transformation on one of my side projects, I needed to convert a JSON-formatted file into CSV format. I ran into a tricky issue that took nearly an hour to debug before identifying the root cause.

The process should have been simple, consisting of three main steps:

1. Open the JSON file
2. Parse that JSON file into a specific struct
3. Write the data to a CSV file First, to give you an idea, the JSON is an array with 65,342 elements.

func JsonToCSV(data *SrcSheet) {
// Create file name in a format like "email_241030172647.csv" (email_yymmddhhmmss.csv)
fName := fileName()

// Create file
f, err := os.Create(fName)
if err != nil {
log.Println("Unable to create file", err)
return
}
defer f.Close() // Closing to release resources
w := csv.NewWriter(f) // Initializing CSV writer

// Add header
header := []string{"email", "provider", "added_on"}
if err = w.Write(header); err != nil {
log.Println("Unable to write header", err)
return
}

count := 0
for domain, elm := range data.Email {
if err := w.Write(newRecord(domain, elm)); err != nil {
log.Println("Unable to add new record", domain, err)
return
} else {
count++
}
}

log.Println("Number of records written =", count)
}

func newRecord(email string, e *SrcElements) []string {
if e == nil {
return nil
}

DBFormat := "2006-01-02 15:04:05.000"
addedOn := time.Now().UTC().Format(DBFormat)

r := []string{email, e.Provider, addedOn}
return r
}

The code is straightforward: create a new file with a specific name format, defer its closing, initialize the CSV writer, and start writing to the file. Super simple, right?

Steps 1 and 2 worked well, so omitted them. Let’s shift focus to step 3, where something unexpected happened: the CSV output contained only 65,032 records, meaning 310 records were missing.

To troubleshoot, I tried the code with just 7 JSON elements instead of 65,032. Surprisingly, nothing was written to the CSV file at all!

I double-checked for simple mistakes, like missing file closure, but everything looked fine. I then retried with the full 65,032 elements, hoping to get more clues. That’s when I noticed that not only were 310 records missing, but the last record was incomplete as well.


65030 adam@gmail.com, gmail, 2023-03-17 15:04:05.000
65031 jac@hotmail.com, hotmail, 2023-03-17 15:04:05.000
65032 nancy@xyz.com, hotmail, 2023-03-

This was progress—I could now narrow down the issue and focus on w.Write(newRecord(domain, elm)), specifically the w.Write(...) method. I checked the documentation and found the reason:

... Writes are buffered, so [Writer.Flush] must eventually be called to ensure that the record is written to the underlying io.Writer ...

I had forgotten to call w.Flush(). This made sense since, from a performance perspective, the CSV writer buffers writes instead of executing I/O operations every time w.Write() is called. By buffering data, it reduces the I/O load, and calling w.Flush() at the end ensures any remaining data in the buffer is written to the file.

Here’s the corrected code:


...
f, err := os.Create(fName)
if err != nil {
log.Println("Unable to create file", err)
return
}
defer f.Close()
w := csv.NewWriter(f)
defer w.Flush()

// Add header
header := []string{"email", "provider", "added_on"}
...

To confirm, I checked the

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source code and found that the default buffer size is 4K. In the WriteRune(...) method, you’ll see that it calls Flush whenever the buffer reaches its limit.

That’s all! I hope you enjoyed reading. I tend to learn a lot from mistakes—whether mine or others’. Even if there’s no immediate fix, discovering a wrong approach helps me avoid similar pitfalls in the future. That’s why I wanted to share this experience!

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