excelstream 0.3.0

High-performance streaming Excel library - Read/write large XLSX files with memory-efficient streaming
Documentation

excelstream

🦀 High-performance Rust library for Excel import/export with streaming support

Rust License: MIT CI

✨ What's New in v0.3.0:

  • ✨ Cell Formatting - 14 predefined styles: bold, italic, highlights, borders, number formats!
  • 🎨 Easy Styling API - write_header_bold(), write_row_styled(), write_row_with_style()
  • 💰 Number Formats - Currency, percentage, decimal, integer formats
  • 🎨 Text Styles - Bold, italic, yellow/green/red highlights
  • 📅 Date Formats - MM/DD/YYYY and timestamp formats

✨ Features

  • 🚀 Streaming Read - Process large Excel files without loading entire file into memory
  • 💾 Streaming Write - Write millions of rows with constant ~80MB memory usage
  • ⚡ High Performance - 21-47% faster than rust_xlsxwriter baseline
  • 🎨 Cell Formatting - 14 predefined styles (bold, currency, %, highlights, borders)
  • 📐 Formula Support - Write Excel formulas (=SUM, =AVERAGE, =IF, etc.)
  • 🎯 Typed Values - Strong typing with Int, Float, Bool, DateTime, Formula
  • 🔧 Memory Efficient - Configurable flush intervals for memory-limited environments
  • ❌ Better Errors - Context-rich error messages with available sheets list
  • 📊 Multi-format Support - Read XLSX, XLS, ODS formats
  • 🔒 Type-safe - Leverage Rust's type system for safety
  • 📝 Multi-sheet - Support multiple sheets in one workbook
  • 🗄️ Database Export - PostgreSQL integration examples
  • ✅ Production Ready - 47+ tests, CI/CD, zero unsafe code

📦 Installation

Add to your Cargo.toml:

[dependencies]
excelstream = "0.2"

🚀 Quick Start

Reading Excel Files (Streaming)

use excelstream::reader::ExcelReader;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut reader = ExcelReader::open("data.xlsx")?;
    
    // List all sheets
    for sheet_name in reader.sheet_names() {
        println!("Sheet: {}", sheet_name);
    }
    
    // Read rows one by one (streaming)
    for row_result in reader.rows("Sheet1")? {
        let row = row_result?;
        println!("Row {}: {:?}", row.index, row.to_strings());
    }
    
    Ok(())
}

Writing Excel Files (Streaming - v0.2.0)

use excelstream::writer::ExcelWriter;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut writer = ExcelWriter::new("output.xlsx")?;
    
    // Configure streaming behavior (optional)
    writer.set_flush_interval(500);  // Flush every 500 rows
    writer.set_max_buffer_size(512 * 1024);  // 512KB buffer
    
    // Write header (note: no bold formatting in v0.2.0)
    writer.write_header(&["ID", "Name", "Email"])?;
    
    // Write millions of rows with constant memory usage!
    for i in 1..=1_000_000 {
        writer.write_row(&[
            &i.to_string(),
            &format!("User{}", i),
            &format!("user{}@example.com", i)
        ])?;
    }
    
    // Save file (closes ZIP and finalizes)
    writer.save()?;
    
    Ok(())
}

Key Benefits:

  • ✅ Constant ~80MB memory usage regardless of dataset size
  • ✅ 21-47% faster than rust_xlsxwriter baseline
  • ✅ Direct ZIP streaming - data written to disk immediately
  • ⚠️ Note: Bold formatting and column width not yet implemented in streaming mode

Writing with Typed Values (Recommended)

For better Excel compatibility and performance, use typed values:

use excelstream::writer::ExcelWriter;
use excelstream::types::CellValue;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut writer = ExcelWriter::new("typed_output.xlsx")?;

    writer.write_header(&["Name", "Age", "Salary", "Active"])?;

    // Typed values: numbers are numbers, formulas work in Excel
    writer.write_row_typed(&[
        CellValue::String("Alice".to_string()),
        CellValue::Int(30),
        CellValue::Float(75000.50),
        CellValue::Bool(true),
    ])?;

    writer.save()?;
    Ok(())
}

Benefits of write_row_typed():

  • ✅ Numbers are stored as numbers (not text)
  • ✅ Booleans display as TRUE/FALSE
  • ✅ Excel formulas work correctly
  • ✅ Better type safety
  • ✅ 40% faster than rust_xlsxwriter

Writing Excel Formulas

use excelstream::writer::ExcelWriter;
use excelstream::types::CellValue;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut writer = ExcelWriter::new("formulas.xlsx")?;

    // Header row
    writer.write_header(&["Value 1", "Value 2", "Sum", "Average"])?;

    // Data with formulas
    writer.write_row_typed(&[
        CellValue::Int(10),
        CellValue::Int(20),
        CellValue::Formula("=A2+B2".to_string()),      // Sum
        CellValue::Formula("=AVERAGE(A2:B2)".to_string()), // Average
    ])?;

    writer.write_row_typed(&[
        CellValue::Int(15),
        CellValue::Int(25),
        CellValue::Formula("=A3+B3".to_string()),
        CellValue::Formula("=AVERAGE(A3:B3)".to_string()),
    ])?;

    // Total row with SUM formula
    writer.write_row_typed(&[
        CellValue::String("Total".to_string()),
        CellValue::Empty,
        CellValue::Formula("=SUM(C2:C3)".to_string()),
        CellValue::Formula("=AVERAGE(D2:D3)".to_string()),
    ])?;

    writer.save()?;
    Ok(())
}

Supported Formulas:

  • ✅ Basic arithmetic: =A1+B1, =A1*B1, =A1/B1
  • ✅ SUM, AVERAGE, COUNT, MIN, MAX
  • ✅ Cell ranges: =SUM(A1:A10)
  • ✅ Complex formulas: =IF(A1>100, "High", "Low")
  • ✅ All standard Excel functions

Cell Formatting and Styling

New in v0.3.0: Apply 14 predefined cell styles including bold headers, number formats, highlights, and borders!

Bold Headers

use excelstream::writer::ExcelWriter;

let mut writer = ExcelWriter::new("report.xlsx")?;

// Write bold header
writer.write_header_bold(&["Name", "Amount", "Status"])?;

// Regular data rows
writer.write_row(&["Alice", "1,234.56", "Active"])?;
writer.write_row(&["Bob", "2,345.67", "Pending"])?;

writer.save()?;

Styled Cells

Apply different styles to individual cells:

use excelstream::types::{CellValue, CellStyle};
use excelstream::writer::ExcelWriter;

let mut writer = ExcelWriter::new("report.xlsx")?;

writer.write_header_bold(&["Item", "Amount", "Change %"])?;

// Mix different styles in one row
writer.write_row_styled(&[
    (CellValue::String("Revenue".to_string()), CellStyle::TextBold),
    (CellValue::Float(150000.00), CellStyle::NumberCurrency),
    (CellValue::Float(0.15), CellStyle::NumberPercentage),
])?;

writer.write_row_styled(&[
    (CellValue::String("Profit".to_string()), CellStyle::HighlightGreen),
    (CellValue::Float(55000.00), CellStyle::NumberCurrency),
    (CellValue::Float(0.22), CellStyle::NumberPercentage),
])?;

writer.save()?;

Uniform Row Styling

Apply the same style to all cells in a row:

// All cells bold
writer.write_row_with_style(&[
    CellValue::String("IMPORTANT".to_string()),
    CellValue::String("READ THIS".to_string()),
    CellValue::String("URGENT".to_string()),
], CellStyle::TextBold)?;

// All cells highlighted yellow
writer.write_row_with_style(&[
    CellValue::String("Warning".to_string()),
    CellValue::String("Check values".to_string()),
    CellValue::String("Need review".to_string()),
], CellStyle::HighlightYellow)?;

Available Cell Styles

Style Format Code Example Use Case
CellStyle::Default None Plain text Regular data
CellStyle::HeaderBold Bold Header Column headers
CellStyle::NumberInteger #,##0 1,234 Whole numbers
CellStyle::NumberDecimal #,##0.00 1,234.56 Decimals
CellStyle::NumberCurrency $#,##0.00 $1,234.56 Money amounts
CellStyle::NumberPercentage 0.00% 95.00% Percentages
CellStyle::DateDefault MM/DD/YYYY 01/15/2024 Dates
CellStyle::DateTimestamp MM/DD/YYYY HH:MM:SS 01/15/2024 14:30:00 Timestamps
CellStyle::TextBold Bold Bold text Emphasis
CellStyle::TextItalic Italic Italic text Notes
CellStyle::HighlightYellow Yellow bg 🟨 Text Warnings
CellStyle::HighlightGreen Green bg 🟩 Text Success
CellStyle::HighlightRed Red bg 🟥 Text Errors
CellStyle::BorderThin Thin borders ▭ Text Boundaries

Complete Example

use excelstream::writer::ExcelWriter;
use excelstream::types::{CellValue, CellStyle};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut writer = ExcelWriter::new("quarterly_report.xlsx")?;

    // Bold header
    writer.write_header_bold(&["Quarter", "Revenue", "Expenses", "Profit", "Margin %"])?;

    // Q1 - Green highlight for good performance
    writer.write_row_styled(&[
        (CellValue::String("Q1 2024".to_string()), CellStyle::Default),
        (CellValue::Float(500000.00), CellStyle::NumberCurrency),
        (CellValue::Float(320000.00), CellStyle::NumberCurrency),
        (CellValue::Float(180000.00), CellStyle::NumberCurrency),
        (CellValue::Float(0.36), CellStyle::NumberPercentage),
    ])?;

    // Q2 - Yellow highlight for warning
    writer.write_row_styled(&[
        (CellValue::String("Q2 2024".to_string()), CellStyle::Default),
        (CellValue::Float(450000.00), CellStyle::NumberCurrency),
        (CellValue::Float(380000.00), CellStyle::NumberCurrency),
        (CellValue::Float(70000.00), CellStyle::HighlightYellow),
        (CellValue::Float(0.16), CellStyle::NumberPercentage),
    ])?;

    // Total row with formulas and bold
    writer.write_row_styled(&[
        (CellValue::String("Total".to_string()), CellStyle::TextBold),
        (CellValue::Formula("=SUM(B2:B3)".to_string()), CellStyle::NumberCurrency),
        (CellValue::Formula("=SUM(C2:C3)".to_string()), CellStyle::NumberCurrency),
        (CellValue::Formula("=SUM(D2:D3)".to_string()), CellStyle::NumberCurrency),
        (CellValue::Formula("=AVERAGE(E2:E3)".to_string()), CellStyle::NumberPercentage),
    ])?;

    writer.save()?;
    Ok(())
}

See also: Run cargo run --example cell_formatting to see all 14 styles in action!

Direct FastWorkbook Usage (Maximum Performance)

For maximum performance, use FastWorkbook directly:

use excelstream::fast_writer::FastWorkbook;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut workbook = FastWorkbook::new("large_output.xlsx")?;
    workbook.add_worksheet("Sheet1")?;
    
    // Write header
    workbook.write_row(&["ID", "Name", "Email", "Age"])?;
    
    // Write 1 million rows efficiently (40K rows/sec)
    for i in 1..=1_000_000 {
        workbook.write_row(&[
            &i.to_string(),
            &format!("User{}", i),
            &format!("user{}@example.com", i),
            &(20 + (i % 50)).to_string(),
        ])?;
    }
    
    workbook.close()?;
    Ok(())
}

Performance (v0.2.0):

  • ExcelWriter.write_row(): 36,870 rows/sec (+21% vs rust_xlsxwriter)
  • ExcelWriter.write_row_typed(): 42,877 rows/sec (+40% vs rust_xlsxwriter)
  • FastWorkbook direct: 44,753 rows/sec (+47% vs rust_xlsxwriter)

Memory-Constrained Writing (For Kubernetes Pods)

In v0.2.0, all writers use streaming with constant memory:

use excelstream::writer::ExcelWriter;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut writer = ExcelWriter::new("output.xlsx")?;
    
    // For pods with < 512MB RAM
    writer.set_flush_interval(500);       // Flush more frequently
    writer.set_max_buffer_size(256 * 1024); // 256KB buffer
    
    writer.write_header(&["ID", "Name", "Email"])?;
    
    // Write large dataset without OOMKilled
    for i in 1..=1_000_000 {
        writer.write_row(&[
            &i.to_string(),
            &format!("User{}", i),
            &format!("user{}@example.com", i),
        ])?;
    }
    
    writer.save()?;
    Ok(())
}

Memory usage in v0.2.0:

  • Constant ~80MB regardless of dataset size
  • Configurable flush interval and buffer size
  • Suitable for Kubernetes pods with limited resources

Multi-sheet workbook

use excelstream::writer::ExcelWriterBuilder;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut writer = ExcelWriterBuilder::new("multi.xlsx")
        .with_sheet_name("Sales")
        .build()?;
    
    // Sheet 1: Sales
    writer.write_header(&["Month", "Revenue"])?;
    writer.write_row(&["Jan", "50000"])?;
    
    // Sheet 2: Employees
    writer.add_sheet("Employees")?;
    writer.write_header(&["Name", "Department"])?;
    writer.write_row(&["Alice", "Engineering"])?;
    
    writer.save()?;
    Ok(())
}

📚 Examples

The examples/ directory contains detailed examples:

Basic Usage:

  • basic_read.rs - Basic Excel file reading
  • basic_write.rs - Basic Excel file writing
  • streaming_read.rs - Reading large files with streaming
  • streaming_write.rs - Writing large files with streaming

Performance Comparisons:

  • three_writers_comparison.rs - Compare all 3 writer types (recommended!)
  • write_row_comparison.rs - String vs typed value writing
  • writer_comparison.rs - Standard vs fast writer comparison
  • fast_writer_test.rs - Fast writer performance benchmarks

Advanced Features:

  • memory_constrained_write.rs - Memory-limited writing for pods
  • auto_memory_config.rs - Auto memory configuration demo
  • csv_to_excel.rs - CSV to Excel conversion
  • multi_sheet.rs - Creating multi-sheet workbooks

PostgreSQL Integration:

  • postgres_to_excel.rs - Basic PostgreSQL export
  • postgres_streaming.rs - Streaming PostgreSQL export
  • postgres_to_excel_advanced.rs - Advanced async with connection pooling

Running examples:

# Create sample data first
cargo run --example basic_write

# Read Excel file
cargo run --example basic_read

# Streaming with large files
cargo run --example streaming_write
cargo run --example streaming_read

# Performance comparisons (RECOMMENDED)
cargo run --release --example three_writers_comparison  # Compare all writers
cargo run --release --example write_row_comparison      # String vs typed
cargo run --release --example writer_comparison         # Standard vs fast

# Memory-constrained writing
cargo run --release --example memory_constrained_write
MEMORY_LIMIT_MB=512 cargo run --release --example auto_memory_config

# Multi-sheet workbooks
cargo run --example multi_sheet

# PostgreSQL examples (requires database setup)
cargo run --example postgres_to_excel --features postgres
cargo run --example postgres_streaming --features postgres
cargo run --example postgres_to_excel_advanced --features postgres-async

🔧 API Documentation

ExcelReader

  • open(path) - Open Excel file for reading
  • sheet_names() - Get list of sheet names
  • rows(sheet_name) - Iterator for streaming row reading
  • read_cell(sheet, row, col) - Read specific cell
  • dimensions(sheet_name) - Get sheet dimensions (rows, cols)

ExcelWriter (Streaming)

  • new(path) - Create new writer
  • write_row(data) - Write row with strings
  • write_row_typed(cells) - Write row with typed values (recommended)
  • write_header(headers) - Write header row
  • add_sheet(name) - Add new sheet
  • set_flush_interval(rows) - Configure flush frequency (default: 1000)
  • set_max_buffer_size(bytes) - Configure buffer size (default: 1MB)
  • set_column_width(col, width) - Not yet implemented in streaming mode
  • save() - Save and finalize workbook

FastWorkbook (Direct Access)

  • new(path) - Create fast writer
  • add_worksheet(name) - Add worksheet
  • write_row(data) - Write row (optimized)
  • set_flush_interval(rows) - Set flush frequency
  • set_max_buffer_size(bytes) - Set buffer limit
  • close() - Finish and save file

Types

  • CellValue - Enum: Empty, String, Int, Float, Bool, DateTime, Error, Formula
  • Row - Row with index and cells vector
  • Cell - Cell with position (row, col) and value

🎯 Use Cases

Processing Large Excel Files (100MB+)

// Streaming ensures only small portions are loaded into memory
let mut reader = ExcelReader::open("huge_file.xlsx")?;
let mut total = 0.0;

for row_result in reader.rows("Sheet1")? {
    let row = row_result?;
    if let Some(val) = row.get(2).and_then(|c| c.as_f64()) {
        total += val;
    }
}

Exporting Database to Excel

let mut writer = ExcelWriter::new("export.xlsx")?;
writer.write_header(&["ID", "Name", "Created"])?;

// Fetch from database and write directly
for record in database.query("SELECT * FROM users")? {
    writer.write_row(&[
        &record.id.to_string(),
        &record.name,
        &record.created_at.to_string(),
    ])?;
}

writer.save()?;

Converting CSV to Excel

use std::fs::File;
use std::io::{BufRead, BufReader};

let csv = BufReader::new(File::open("data.csv")?);
let mut writer = ExcelWriter::new("output.xlsx")?;

for (i, line) in csv.lines().enumerate() {
    let fields: Vec<&str> = line?.split(',').collect();
    if i == 0 {
        writer.write_header(fields)?;
    } else {
        writer.write_row(fields)?;
    }
}

writer.save()?;

⚡ Performance

Tested with 1 million rows × 30 columns (mixed data types):

Writer Type Speed (rows/s) vs Baseline Memory Usage
rust_xlsxwriter 30,525 baseline ~300MB (grows)
ExcelWriter.write_row() 36,870 +21% ⚡ ~80MB constant ✅
ExcelWriter.write_row_typed() 42,877 +40% ⚡ ~80MB constant ✅
FastWorkbook 44,753 +47% ⚡ ~80MB constant ✅

Key Advantages:

  • ✅ 21-47% faster than rust_xlsxwriter
  • ✅ Constant memory - doesn't grow with dataset size
  • ✅ True streaming - data written directly to disk
  • ✅ No memory spikes - predictable resource usage

Recommendations

Use Case Recommended Writer Why
General use ExcelWriter.write_row_typed() Best balance of speed + features
Simple exports ExcelWriter.write_row() Easy API, good performance
Maximum speed FastWorkbook Fastest, lowest-level API
Advanced formatting rust_xlsxwriter Full Excel features

📖 Documentation

  • README.md (this file) - Complete guide with examples
  • CONTRIBUTING.md - How to contribute
  • CHANGELOG.md - Version history
  • Examples in /examples directory
  • Doc tests in source code

🛠️ Development

Build

cargo build --release

Test

cargo test

Run examples

cargo run --example basic_write
cargo run --example streaming_read

Benchmark

cargo bench

📋 Requirements

  • Rust 1.70 or higher
  • Dependencies:
    • calamine - Reading Excel files
    • zip - ZIP compression for writing
    • thiserror - Error handling

🚀 Production Ready

  • ✅ Battle-tested - Handles 1M+ row datasets
  • ✅ High performance - 21-47% faster than alternatives
  • ✅ Memory efficient - Constant ~80MB usage, works in K8s pods
  • ✅ Reliable - 47 comprehensive tests covering edge cases
  • ✅ Safe - Zero unsafe code, full Rust memory safety
  • ✅ Compatible - Excel, LibreOffice, Google Sheets
  • ✅ Unicode support - Special characters, emojis, CJK
  • ✅ CI/CD - Automated testing on every commit

🤝 Contributing

Contributions welcome! Please see CONTRIBUTING.md for guidelines.

Areas for Contribution:

  • Cell formatting and styling (Phase 3)
  • Conditional formatting
  • Charts and images support
  • More examples and documentation
  • Performance optimizations

All contributions must:

  • Pass cargo fmt --check
  • Pass cargo clippy -- -D warnings
  • Pass all tests cargo test --all-features
  • Include tests for new features

📄 License

MIT License - see LICENSE file for details.

🙏 Credits

This library uses:

  • calamine - Excel reader
  • Custom FastWorkbook - High-performance streaming writer

📧 Contact

For questions or suggestions, please create an issue on GitHub.

Made with ❤️ and 🦀 by the Rust community