Chapter 1: RESP Protocol

Every Redis command travels over the network as RESP (Redis Serialization Protocol). In this chapter, we’ll build a complete RESP parser and serializer — the foundation for our Redis clone. Why start with the protocol instead of jumping straight to data storage? Because the protocol is the contract between every client and server. If you get this wrong, nothing else works — no redis-cli, no application library, no monitoring tool will be able to talk to your server. Think of RESP as the shared language: before two people can have a productive conversation, they need to agree on a language. RESP is that language for Redis, and it is intentionally simple enough to implement in an afternoon, yet powerful enough to carry binary data of any size.

Prerequisites: Go basics, understanding of TCP
Further Reading: Networking Fundamentals
Time: 2-3 hours
Outcome: Working RESP parser and writer

What is RESP?

RESP is a simple, efficient text-based protocol. Every piece of data starts with a type indicator:

┌─────────────────────────────────────────────────────────────────────────────┐
│                        RESP DATA TYPES                                       │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                              │
│   TYPE          PREFIX    EXAMPLE                  MEANING                   │
│   ────          ──────    ───────                  ───────                   │
│                                                                              │
│   Simple String   +      +OK\r\n                   "OK"                      │
│   Error           -      -ERR unknown\r\n          Error message             │
│   Integer         :      :1000\r\n                 1000                      │
│   Bulk String     $      $5\r\nhello\r\n           "hello" (length prefix)   │
│   Array           *      *2\r\n$3\r\nfoo\r\n       ["foo", "bar"]            │
│                          $3\r\nbar\r\n                                        │
│   Null            $      $-1\r\n                   nil (null bulk string)    │
│                                                                              │
│   ALL LINES END WITH \r\n (CRLF)                                            │
│                                                                              │
└─────────────────────────────────────────────────────────────────────────────┘

Why RESP?

Human-readable for debugging — you can telnet to a Redis server and type commands by hand. Try doing that with a binary protocol like gRPC.
Simple to implement — the entire spec fits on a single page. This is deliberate: Salvatore Sanfilippo (antirez) designed RESP to be implementable by any developer in any language in a few hours.
Efficient to parse (O(n)) — length prefixes on bulk strings mean the parser knows exactly how many bytes to read, with no scanning for delimiters inside the data.
Binary-safe — bulk strings can contain any byte, including null bytes and newlines, because the length is specified upfront rather than relying on a terminator.

Real Examples

Client Request (SET command)

*3\r\n        ← Array of 3 elements
$3\r\n        ← Bulk string, 3 bytes
SET\r\n       ← "SET"
$4\r\n        ← Bulk string, 4 bytes
name\r\n      ← "name"
$5\r\n        ← Bulk string, 5 bytes
Alice\r\n     ← "Alice"

Server Response

+OK\r\n       ← Simple string "OK"

GET Response with Value

$5\r\n        ← Bulk string, 5 bytes
Alice\r\n     ← "Alice"

GET Response for Missing Key

$-1\r\n       ← Null bulk string

Project Setup

mkdir myredis
cd myredis
go mod init myredis

Project Structure

myredis/
├── cmd/
│   └── server/
│       └── main.go
├── internal/
│   └── protocol/
│       ├── types.go
│       ├── parser.go
│       └── writer.go
├── go.mod
└── README.md

Implementation

Step 1: Define RESP Types

internal/protocol/types.go

package protocol

import (
    "errors"
    "strconv"
)

// RESP type prefixes
const (
    SimpleString = '+'
    Error        = '-'
    Integer      = ':'
    BulkString   = '$'
    Array        = '*'
)

// Common errors
var (
    ErrInvalidSyntax    = errors.New("invalid RESP syntax")
    ErrUnexpectedEOF    = errors.New("unexpected end of input")
    ErrInvalidInteger   = errors.New("invalid integer")
    ErrInvalidBulkLen   = errors.New("invalid bulk string length")
    ErrInvalidArrayLen  = errors.New("invalid array length")
)

// Value represents any RESP value
type Value struct {
    Type  byte     // +, -, :, $, *
    Str   string   // For SimpleString and Error
    Num   int64    // For Integer
    Bulk  []byte   // For BulkString
    Array []Value  // For Array
    Null  bool     // True if null bulk string or null array
}

// Common responses as package-level variables for reuse
var (
    OK       = Value{Type: SimpleString, Str: "OK"}
    PONG     = Value{Type: SimpleString, Str: "PONG"}
    NullBulk = Value{Type: BulkString, Null: true}
    NullArray = Value{Type: Array, Null: true}
    Zero     = Value{Type: Integer, Num: 0}
    One      = Value{Type: Integer, Num: 1}
)

// NewError creates an error response
func NewError(msg string) Value {
    return Value{Type: Error, Str: msg}
}

// NewErrorf creates a formatted error response
func NewErrorf(format string, args ...interface{}) Value {
    return Value{Type: Error, Str: fmt.Sprintf(format, args...)}
}

// NewSimpleString creates a simple string response
func NewSimpleString(s string) Value {
    return Value{Type: SimpleString, Str: s}
}

// NewInteger creates an integer response
func NewInteger(n int64) Value {
    return Value{Type: Integer, Num: n}
}

// NewBulkString creates a bulk string response
func NewBulkString(s string) Value {
    return Value{Type: BulkString, Bulk: []byte(s)}
}

// NewBulkBytes creates a bulk string from bytes
func NewBulkBytes(b []byte) Value {
    return Value{Type: BulkString, Bulk: b}
}

// NewArray creates an array response
func NewArray(vals []Value) Value {
    return Value{Type: Array, Array: vals}
}

// IsNull returns true if this is a null value
func (v Value) IsNull() bool {
    return v.Null
}

// AsString returns the string representation of the value
func (v Value) AsString() string {
    switch v.Type {
    case SimpleString, Error:
        return v.Str
    case BulkString:
        return string(v.Bulk)
    case Integer:
        return strconv.FormatInt(v.Num, 10)
    default:
        return ""
    }
}

// AsBytes returns the value as bytes
func (v Value) AsBytes() []byte {
    switch v.Type {
    case SimpleString, Error:
        return []byte(v.Str)
    case BulkString:
        return v.Bulk
    default:
        return nil
    }
}

// AsInt returns the value as int64
func (v Value) AsInt() (int64, error) {
    switch v.Type {
    case Integer:
        return v.Num, nil
    case BulkString:
        return strconv.ParseInt(string(v.Bulk), 10, 64)
    case SimpleString:
        return strconv.ParseInt(v.Str, 10, 64)
    default:
        return 0, errors.New("cannot convert to integer")
    }
}

Step 2: Build the Parser

internal/protocol/parser.go

package protocol

import (
    "bufio"
    "fmt"
    "io"
    "strconv"
)

// Parser reads RESP values from a buffered reader.
//
// Why bufio.Reader? TCP delivers data as a stream of bytes with no message
// boundaries. Bytes may arrive in arbitrary chunks -- half a command in one
// read, two and a half commands in the next. bufio.Reader handles the
// buffering so our parsing logic can think in terms of lines and exact
// byte counts rather than worrying about partial reads.
type Parser struct {
    reader *bufio.Reader
}

// NewParser creates a new RESP parser.
// The 64KB buffer size is a practical sweet spot: large enough to hold
// most Redis commands in a single read call (reducing syscall overhead),
// small enough to not waste memory across thousands of connections.
func NewParser(r io.Reader) *Parser {
    return &Parser{
        reader: bufio.NewReaderSize(r, 64*1024), // 64KB buffer
    }
}

// Parse reads and returns the next RESP value
func (p *Parser) Parse() (Value, error) {
    // Read the type indicator
    typeByte, err := p.reader.ReadByte()
    if err != nil {
        return Value{}, err
    }

    switch typeByte {
    case SimpleString:
        return p.parseSimpleString()
    case Error:
        return p.parseError()
    case Integer:
        return p.parseInteger()
    case BulkString:
        return p.parseBulkString()
    case Array:
        return p.parseArray()
    default:
        // Handle inline commands (commands without RESP framing)
        p.reader.UnreadByte()
        return p.parseInline()
    }
}

// parseSimpleString reads: +OK\r\n
func (p *Parser) parseSimpleString() (Value, error) {
    line, err := p.readLine()
    if err != nil {
        return Value{}, err
    }
    return Value{Type: SimpleString, Str: line}, nil
}

// parseError reads: -ERR message\r\n
func (p *Parser) parseError() (Value, error) {
    line, err := p.readLine()
    if err != nil {
        return Value{}, err
    }
    return Value{Type: Error, Str: line}, nil
}

// parseInteger reads: :1000\r\n
func (p *Parser) parseInteger() (Value, error) {
    line, err := p.readLine()
    if err != nil {
        return Value{}, err
    }

    num, err := strconv.ParseInt(line, 10, 64)
    if err != nil {
        return Value{}, ErrInvalidInteger
    }

    return Value{Type: Integer, Num: num}, nil
}

// parseBulkString reads: $5\r\nhello\r\n or $-1\r\n (null)
//
// This is the most important RESP type for real-world usage. Almost all
// command arguments and return values are bulk strings. The length prefix
// is what makes RESP binary-safe: instead of scanning for a delimiter, we
// know exactly how many bytes to read. This means a bulk string can contain
// literal \r\n bytes, null bytes, or any binary payload without ambiguity.
func (p *Parser) parseBulkString() (Value, error) {
    // Read length line
    line, err := p.readLine()
    if err != nil {
        return Value{}, err
    }

    length, err := strconv.Atoi(line)
    if err != nil {
        return Value{}, ErrInvalidBulkLen
    }

    // Handle null bulk string ($-1\r\n).
    // This is Redis's way of representing "no value" -- e.g., GET on a
    // nonexistent key returns null rather than an empty string.
    if length < 0 {
        return Value{Type: BulkString, Null: true}, nil
    }

    // Read exactly 'length' bytes + 2 bytes for the trailing \r\n.
    // io.ReadFull blocks until all bytes arrive or an error occurs,
    // which is essential because TCP may deliver partial data.
    buf := make([]byte, length+2)
    _, err = io.ReadFull(p.reader, buf)
    if err != nil {
        return Value{}, err
    }

    // Verify CRLF terminator. If this check fails, the stream is corrupted
    // or out of sync -- a common symptom of mismatched length values.
    if buf[length] != '\r' || buf[length+1] != '\n' {
        return Value{}, ErrInvalidSyntax
    }

    return Value{Type: BulkString, Bulk: buf[:length]}, nil
}

// parseArray reads: *3\r\n followed by 3 elements
func (p *Parser) parseArray() (Value, error) {
    // Read count line
    line, err := p.readLine()
    if err != nil {
        return Value{}, err
    }

    count, err := strconv.Atoi(line)
    if err != nil {
        return Value{}, ErrInvalidArrayLen
    }

    // Handle null array
    if count < 0 {
        return Value{Type: Array, Null: true}, nil
    }

    // Parse each element
    array := make([]Value, count)
    for i := 0; i < count; i++ {
        val, err := p.Parse()
        if err != nil {
            return Value{}, err
        }
        array[i] = val
    }

    return Value{Type: Array, Array: array}, nil
}

// parseInline handles commands sent without RESP framing
// e.g., "PING\r\n" or "SET foo bar\r\n"
func (p *Parser) parseInline() (Value, error) {
    line, err := p.readLine()
    if err != nil {
        return Value{}, err
    }

    // Split by spaces
    parts := splitInline(line)
    if len(parts) == 0 {
        return Value{}, ErrInvalidSyntax
    }

    // Convert to array of bulk strings
    array := make([]Value, len(parts))
    for i, part := range parts {
        array[i] = Value{Type: BulkString, Bulk: []byte(part)}
    }

    return Value{Type: Array, Array: array}, nil
}

// readLine reads until \r\n and returns the line without CRLF
func (p *Parser) readLine() (string, error) {
    line, err := p.reader.ReadString('\n')
    if err != nil {
        return "", err
    }

    // Remove \r\n
    if len(line) < 2 || line[len(line)-2] != '\r' {
        return "", ErrInvalidSyntax
    }

    return line[:len(line)-2], nil
}

// splitInline splits an inline command respecting quotes
func splitInline(line string) []string {
    var parts []string
    var current []byte
    inQuote := false
    quoteChar := byte(0)

    for i := 0; i < len(line); i++ {
        c := line[i]

        if !inQuote && (c == '"' || c == '\'') {
            inQuote = true
            quoteChar = c
        } else if inQuote && c == quoteChar {
            inQuote = false
        } else if !inQuote && c == ' ' {
            if len(current) > 0 {
                parts = append(parts, string(current))
                current = nil
            }
        } else {
            current = append(current, c)
        }
    }

    if len(current) > 0 {
        parts = append(parts, string(current))
    }

    return parts
}

Step 3: Build the Writer

internal/protocol/writer.go

package protocol

import (
    "bufio"
    "io"
    "strconv"
)

// Writer serializes RESP values to a buffered writer
type Writer struct {
    writer *bufio.Writer
}

// NewWriter creates a new RESP writer
func NewWriter(w io.Writer) *Writer {
    return &Writer{
        writer: bufio.NewWriter(w),
    }
}

// Write serializes a Value and writes it to the underlying writer
func (w *Writer) Write(v Value) error {
    if err := w.writeValue(v); err != nil {
        return err
    }
    return w.writer.Flush()
}

// WriteValues writes multiple values
func (w *Writer) WriteValues(vals ...Value) error {
    for _, v := range vals {
        if err := w.writeValue(v); err != nil {
            return err
        }
    }
    return w.writer.Flush()
}

func (w *Writer) writeValue(v Value) error {
    switch v.Type {
    case SimpleString:
        return w.writeSimpleString(v.Str)
    case Error:
        return w.writeError(v.Str)
    case Integer:
        return w.writeInteger(v.Num)
    case BulkString:
        return w.writeBulkString(v)
    case Array:
        return w.writeArray(v)
    default:
        return ErrInvalidSyntax
    }
}

func (w *Writer) writeSimpleString(s string) error {
    w.writer.WriteByte(SimpleString)
    w.writer.WriteString(s)
    w.writer.WriteString("\r\n")
    return nil
}

func (w *Writer) writeError(s string) error {
    w.writer.WriteByte(Error)
    w.writer.WriteString(s)
    w.writer.WriteString("\r\n")
    return nil
}

func (w *Writer) writeInteger(n int64) error {
    w.writer.WriteByte(Integer)
    w.writer.WriteString(strconv.FormatInt(n, 10))
    w.writer.WriteString("\r\n")
    return nil
}

func (w *Writer) writeBulkString(v Value) error {
    w.writer.WriteByte(BulkString)
    
    if v.Null {
        w.writer.WriteString("-1\r\n")
        return nil
    }
    
    w.writer.WriteString(strconv.Itoa(len(v.Bulk)))
    w.writer.WriteString("\r\n")
    w.writer.Write(v.Bulk)
    w.writer.WriteString("\r\n")
    return nil
}

func (w *Writer) writeArray(v Value) error {
    w.writer.WriteByte(Array)
    
    if v.Null {
        w.writer.WriteString("-1\r\n")
        return nil
    }
    
    w.writer.WriteString(strconv.Itoa(len(v.Array)))
    w.writer.WriteString("\r\n")
    
    for _, elem := range v.Array {
        if err := w.writeValue(elem); err != nil {
            return err
        }
    }
    
    return nil
}

// Flush ensures all buffered data is written
func (w *Writer) Flush() error {
    return w.writer.Flush()
}

Testing the Parser

internal/protocol/parser_test.go

package protocol

import (
    "strings"
    "testing"
)

func TestParseSimpleString(t *testing.T) {
    input := "+OK\r\n"
    parser := NewParser(strings.NewReader(input))
    
    val, err := parser.Parse()
    if err != nil {
        t.Fatalf("unexpected error: %v", err)
    }
    
    if val.Type != SimpleString || val.Str != "OK" {
        t.Errorf("expected SimpleString 'OK', got %v", val)
    }
}

func TestParseBulkString(t *testing.T) {
    input := "$5\r\nhello\r\n"
    parser := NewParser(strings.NewReader(input))
    
    val, err := parser.Parse()
    if err != nil {
        t.Fatalf("unexpected error: %v", err)
    }
    
    if val.Type != BulkString || string(val.Bulk) != "hello" {
        t.Errorf("expected BulkString 'hello', got %v", val)
    }
}

func TestParseArray(t *testing.T) {
    // *2\r\n$3\r\nfoo\r\n$3\r\nbar\r\n
    input := "*2\r\n$3\r\nfoo\r\n$3\r\nbar\r\n"
    parser := NewParser(strings.NewReader(input))
    
    val, err := parser.Parse()
    if err != nil {
        t.Fatalf("unexpected error: %v", err)
    }
    
    if val.Type != Array || len(val.Array) != 2 {
        t.Errorf("expected Array of 2 elements, got %v", val)
    }
    
    if string(val.Array[0].Bulk) != "foo" || string(val.Array[1].Bulk) != "bar" {
        t.Errorf("expected ['foo', 'bar'], got %v", val)
    }
}

func TestParseNullBulkString(t *testing.T) {
    input := "$-1\r\n"
    parser := NewParser(strings.NewReader(input))
    
    val, err := parser.Parse()
    if err != nil {
        t.Fatalf("unexpected error: %v", err)
    }
    
    if val.Type != BulkString || !val.Null {
        t.Errorf("expected null BulkString, got %v", val)
    }
}

Run tests:

go test ./internal/protocol/...

Exercises

Exercise 1: Add RESP3 support

RESP3 (Redis 6+) adds more types:

,3.14\r\n              # Double
#t\r\n                 # Boolean true
#f\r\n                 # Boolean false
!21\r\n                # Blob error
SYNTAX error here\r\n

Extend your parser to handle these types.

Exercise 2: Benchmark the parser

Create benchmarks for your parser:

func BenchmarkParseBulkString(b *testing.B) {
    input := "$1000\r\n" + strings.Repeat("x", 1000) + "\r\n"
    
    for i := 0; i < b.N; i++ {
        parser := NewParser(strings.NewReader(input))
        parser.Parse()
    }
}

Compare with different buffer sizes.

Exercise 3: Stream parsing

Implement a channel-based parser that streams values:

func (p *Parser) ParseStream(ctx context.Context) <-chan Value {
    ch := make(chan Value)
    go func() {
        defer close(ch)
        for {
            val, err := p.Parse()
            if err != nil {
                return
            }
            select {
            case ch <- val:
            case <-ctx.Done():
                return
            }
        }
    }()
    return ch
}

Key Takeaways

Type Prefixes

Every RESP value starts with a type indicator: +, -, :, $, or *

Length-Prefixed

Bulk strings and arrays include their length for efficient parsing

Binary Safe

Bulk strings can contain any bytes, including nulls

CRLF Terminated

All lines end with \r\n for cross-platform compatibility

What’s Next?

In Chapter 2: TCP Server, we’ll:

Build a TCP server that accepts connections
Handle multiple clients concurrently with goroutines
Integrate our RESP parser

Next: TCP Server

Build the network layer for your Redis clone

Overview

Testing & Code Quality

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Documentation Index

​Chapter 1: RESP Protocol

​What is RESP?

​Real Examples

​Client Request (SET command)

​Server Response

​GET Response with Value

​GET Response for Missing Key

​Project Setup

​Project Structure

​Implementation

​Step 1: Define RESP Types

​Step 2: Build the Parser

​Step 3: Build the Writer

​Testing the Parser

​Exercises

​Key Takeaways

Type Prefixes

Length-Prefixed

Binary Safe

CRLF Terminated

​What’s Next?

Next: TCP Server

Chapter 1: RESP Protocol

What is RESP?

Real Examples

Client Request (SET command)

Server Response

GET Response with Value

GET Response for Missing Key

Project Setup

Project Structure

Implementation

Step 1: Define RESP Types

Step 2: Build the Parser

Step 3: Build the Writer

Testing the Parser

Exercises

Key Takeaways

What’s Next?