path: root/cmd/bolt/main.go

package main

import (
	"bytes"
	"encoding/binary"
	"errors"
	"flag"
	"fmt"
	"io"
	"io/ioutil"
	"math/rand"
	"os"
	"runtime"
	"runtime/pprof"
	"strconv"
	"strings"
	"time"

	"github.com/boltdb/bolt"
)

var (
	// ErrCommandRequired is returned when a CLI command is not specified.
	ErrCommandRequired = errors.New("command required")

	// ErrUnknownCommand is returned when a CLI command is not specified.
	ErrUnknownCommand = errors.New("unknown command")

	// ErrPathRequired is returned when the path to a Bolt database is not specified.
	ErrPathRequired = errors.New("path required")

	// ErrFileNotFound is returned when a Bolt database does not exist.
	ErrFileNotFound = errors.New("file not found")

	// ErrInvalidValue is returned when a benchmark reads an unexpected value.
	ErrInvalidValue = errors.New("invalid value")

	// ErrCorrupt is returned when a checking a data file finds errors.
	ErrCorrupt = errors.New("invalid value")

	// ErrNonDivisibleBatchSize is returned when the batch size can't be evenly
	// divided by the iteration count.
	ErrNonDivisibleBatchSize = errors.New("number of iterations must be divisible by the batch size")
)

func main() {
	m := NewMain()
	if err := m.Run(os.Args[1:]...); err != nil {
		fmt.Println(err.Error())
		os.Exit(1)
	}
}

// Main represents the main program execution.
type Main struct {
	Stdin  io.Reader
	Stdout io.Writer
	Stderr io.Writer
}

// NewMain returns a new instance of Main connect to the standard input/output.
func NewMain() *Main {
	return &Main{
		Stdin:  os.Stdin,
		Stdout: os.Stdout,
		Stderr: os.Stderr,
	}
}

// Run executes the program.
func (m *Main) Run(args ...string) error {
	// Require a command at the beginning.
	if len(args) == 0 || strings.HasPrefix(args[0], "-") {
		return ErrCommandRequired
	}

	// Execute command.
	switch args[0] {
	case "bench":
		return newBenchCommand(m).Run(args[1:]...)
	case "check":
		return newCheckCommand(m).Run(args[1:]...)
	case "info":
		return newInfoCommand(m).Run(args[1:]...)
	case "pages":
		return newPagesCommand(m).Run(args[1:]...)
	case "stats":
		return newStatsCommand(m).Run(args[1:]...)
	default:
		return ErrUnknownCommand
	}
}

// CheckCommand represents the "check" command execution.
type CheckCommand struct {
	Stdin  io.Reader
	Stdout io.Writer
	Stderr io.Writer
}

// NewCheckCommand returns a CheckCommand.
func newCheckCommand(m *Main) *CheckCommand {
	return &CheckCommand{
		Stdin:  m.Stdin,
		Stdout: m.Stdout,
		Stderr: m.Stderr,
	}
}

// Run executes the command.
func (cmd *CheckCommand) Run(args ...string) error {
	// Parse flags.
	fs := flag.NewFlagSet("", flag.ContinueOnError)
	if err := fs.Parse(args); err != nil {
		return err
	}

	// Require database path.
	path := fs.Arg(0)
	if path == "" {
		return ErrPathRequired
	} else if _, err := os.Stat(path); os.IsNotExist(err) {
		return ErrFileNotFound
	}

	// Open database.
	db, err := bolt.Open(path, 0666, nil)
	if err != nil {
		return err
	}
	defer db.Close()

	// Perform consistency check.
	return db.View(func(tx *bolt.Tx) error {
		var count int
		ch := tx.Check()
	loop:
		for {
			select {
			case err, ok := <-ch:
				if !ok {
					break loop
				}
				fmt.Fprintln(cmd.Stdout, err)
				count++
			}
		}

		// Print summary of errors.
		if count > 0 {
			fmt.Fprintf(cmd.Stdout, "%d errors found\n", count)
			return ErrCorrupt
		}

		// Notify user that database is valid.
		fmt.Fprintln(cmd.Stdout, "OK")
		return nil
	})
}

// InfoCommand represents the "info" command execution.
type InfoCommand struct {
	Stdin  io.Reader
	Stdout io.Writer
	Stderr io.Writer
}

// NewInfoCommand returns a InfoCommand.
func newInfoCommand(m *Main) *InfoCommand {
	return &InfoCommand{
		Stdin:  m.Stdin,
		Stdout: m.Stdout,
		Stderr: m.Stderr,
	}
}

// Run executes the command.
func (cmd *InfoCommand) Run(args ...string) error {
	// Parse flags.
	fs := flag.NewFlagSet("", flag.ContinueOnError)
	if err := fs.Parse(args); err != nil {
		return err
	}

	// Require database path.
	path := fs.Arg(0)
	if path == "" {
		return ErrPathRequired
	} else if _, err := os.Stat(path); os.IsNotExist(err) {
		return ErrFileNotFound
	}

	// Open the database.
	db, err := bolt.Open(path, 0666, nil)
	if err != nil {
		return err
	}
	defer db.Close()

	// Print basic database info.
	info := db.Info()
	fmt.Fprintf(cmd.Stdout, "Page Size: %d\n", info.PageSize)

	return nil
}

// PagesCommand represents the "pages" command execution.
type PagesCommand struct {
	Stdin  io.Reader
	Stdout io.Writer
	Stderr io.Writer
}

// NewPagesCommand returns a PagesCommand.
func newPagesCommand(m *Main) *PagesCommand {
	return &PagesCommand{
		Stdin:  m.Stdin,
		Stdout: m.Stdout,
		Stderr: m.Stderr,
	}
}

// Run executes the command.
func (cmd *PagesCommand) Run(args ...string) error {
	// Parse flags.
	fs := flag.NewFlagSet("", flag.ContinueOnError)
	if err := fs.Parse(args); err != nil {
		return err
	}

	// Require database path.
	path := fs.Arg(0)
	if path == "" {
		return ErrPathRequired
	} else if _, err := os.Stat(path); os.IsNotExist(err) {
		return ErrFileNotFound
	}

	// Open database.
	db, err := bolt.Open(path, 0666, nil)
	if err != nil {
		return err
	}
	defer func() { _ = db.Close() }()

	// Write header.
	fmt.Fprintln(cmd.Stdout, "ID       TYPE       ITEMS  OVRFLW")
	fmt.Fprintln(cmd.Stdout, "======== ========== ====== ======")

	return db.Update(func(tx *bolt.Tx) error {
		var id int
		for {
			p, err := tx.Page(id)
			if err != nil {
				return &PageError{ID: id, Err: err}
			} else if p == nil {
				break
			}

			// Only display count and overflow if this is a non-free page.
			var count, overflow string
			if p.Type != "free" {
				count = strconv.Itoa(p.Count)
				if p.OverflowCount > 0 {
					overflow = strconv.Itoa(p.OverflowCount)
				}
			}

			// Print table row.
			fmt.Fprintf(cmd.Stdout, "%-8d %-10s %-6s %-6s\n", p.ID, p.Type, count, overflow)

			// Move to the next non-overflow page.
			id += 1
			if p.Type != "free" {
				id += p.OverflowCount
			}
		}
		return nil
	})
}

// StatsCommand represents the "stats" command execution.
type StatsCommand struct {
	Stdin  io.Reader
	Stdout io.Writer
	Stderr io.Writer
}

// NewStatsCommand returns a StatsCommand.
func newStatsCommand(m *Main) *StatsCommand {
	return &StatsCommand{
		Stdin:  m.Stdin,
		Stdout: m.Stdout,
		Stderr: m.Stderr,
	}
}

// Run executes the command.
func (cmd *StatsCommand) Run(args ...string) error {
	// Parse flags.
	fs := flag.NewFlagSet("", flag.ContinueOnError)
	if err := fs.Parse(args); err != nil {
		return err
	}

	// Require database path.
	path, prefix := fs.Arg(0), fs.Arg(1)
	if path == "" {
		return ErrPathRequired
	} else if _, err := os.Stat(path); os.IsNotExist(err) {
		return ErrFileNotFound
	}

	// Open database.
	db, err := bolt.Open(path, 0666, nil)
	if err != nil {
		return err
	}
	defer db.Close()

	return db.View(func(tx *bolt.Tx) error {
		var s bolt.BucketStats
		var count int
		if err := tx.ForEach(func(name []byte, b *bolt.Bucket) error {
			if bytes.HasPrefix(name, []byte(prefix)) {
				s.Add(b.Stats())
				count += 1
			}
			return nil
		}); err != nil {
			return err
		}

		fmt.Fprintf(cmd.Stdout, "Aggregate statistics for %d buckets\n\n", count)

		fmt.Fprintln(cmd.Stdout, "Page count statistics")
		fmt.Fprintf(cmd.Stdout, "\tNumber of logical branch pages: %d\n", s.BranchPageN)
		fmt.Fprintf(cmd.Stdout, "\tNumber of physical branch overflow pages: %d\n", s.BranchOverflowN)
		fmt.Fprintf(cmd.Stdout, "\tNumber of logical leaf pages: %d\n", s.LeafPageN)
		fmt.Fprintf(cmd.Stdout, "\tNumber of physical leaf overflow pages: %d\n", s.LeafOverflowN)

		fmt.Fprintln(cmd.Stdout, "Tree statistics")
		fmt.Fprintf(cmd.Stdout, "\tNumber of keys/value pairs: %d\n", s.KeyN)
		fmt.Fprintf(cmd.Stdout, "\tNumber of levels in B+tree: %d\n", s.Depth)

		fmt.Fprintln(cmd.Stdout, "Page size utilization")
		fmt.Fprintf(cmd.Stdout, "\tBytes allocated for physical branch pages: %d\n", s.BranchAlloc)
		var percentage int
		if s.BranchAlloc != 0 {
			percentage = int(float32(s.BranchInuse) * 100.0 / float32(s.BranchAlloc))
		}
		fmt.Fprintf(cmd.Stdout, "\tBytes actually used for branch data: %d (%d%%)\n", s.BranchInuse, percentage)
		fmt.Fprintf(cmd.Stdout, "\tBytes allocated for physical leaf pages: %d\n", s.LeafAlloc)
		percentage = 0
		if s.LeafAlloc != 0 {
			percentage = int(float32(s.LeafInuse) * 100.0 / float32(s.LeafAlloc))
		}
		fmt.Fprintf(cmd.Stdout, "\tBytes actually used for leaf data: %d (%d%%)\n", s.LeafInuse, percentage)

		fmt.Fprintln(cmd.Stdout, "Bucket statistics")
		fmt.Fprintf(cmd.Stdout, "\tTotal number of buckets: %d\n", s.BucketN)
		percentage = int(float32(s.InlineBucketN) * 100.0 / float32(s.BucketN))
		fmt.Fprintf(cmd.Stdout, "\tTotal number on inlined buckets: %d (%d%%)\n", s.InlineBucketN, percentage)
		percentage = 0
		if s.LeafInuse != 0 {
			percentage = int(float32(s.InlineBucketInuse) * 100.0 / float32(s.LeafInuse))
		}
		fmt.Fprintf(cmd.Stdout, "\tBytes used for inlined buckets: %d (%d%%)\n", s.InlineBucketInuse, percentage)

		return nil
	})
}

var benchBucketName = []byte("bench")

// BenchCommand represents the "bench" command execution.
type BenchCommand struct {
	Stdin  io.Reader
	Stdout io.Writer
	Stderr io.Writer
}

// NewBenchCommand returns a BenchCommand using the
func newBenchCommand(m *Main) *BenchCommand {
	return &BenchCommand{
		Stdin:  m.Stdin,
		Stdout: m.Stdout,
		Stderr: m.Stderr,
	}
}

// Run executes the "bench" command.
func (cmd *BenchCommand) Run(args ...string) error {
	// Parse CLI arguments.
	options, err := cmd.ParseFlags(args)
	if err != nil {
		return err
	}

	// Remove path if "-work" is not set. Otherwise keep path.
	if options.Work {
		fmt.Fprintf(cmd.Stdout, "work: %s\n", options.Path)
	} else {
		defer os.Remove(options.Path)
	}

	// Create database.
	db, err := bolt.Open(options.Path, 0666, nil)
	if err != nil {
		return err
	}
	db.NoSync = options.NoSync
	defer db.Close()

	// Write to the database.
	var results BenchResults
	if err := cmd.runWrites(db, options, &results); err != nil {
		return fmt.Errorf("write: ", err)
	}

	// Read from the database.
	if err := cmd.runReads(db, options, &results); err != nil {
		return fmt.Errorf("bench: read: %s", err)
	}

	// Print results.
	fmt.Fprintf(os.Stderr, "# Write\t%v\t(%v/op)\t(%v op/sec)\n", results.WriteDuration, results.WriteOpDuration(), results.WriteOpsPerSecond())
	fmt.Fprintf(os.Stderr, "# Read\t%v\t(%v/op)\t(%v op/sec)\n", results.ReadDuration, results.ReadOpDuration(), results.ReadOpsPerSecond())
	fmt.Fprintln(os.Stderr, "")
	return nil
}

// ParseFlags parses the command line flags.
func (cmd *BenchCommand) ParseFlags(args []string) (*BenchOptions, error) {
	var options BenchOptions

	// Parse flagset.
	fs := flag.NewFlagSet("", flag.ContinueOnError)
	fs.StringVar(&options.ProfileMode, "profile-mode", "rw", "")
	fs.StringVar(&options.WriteMode, "write-mode", "seq", "")
	fs.StringVar(&options.ReadMode, "read-mode", "seq", "")
	fs.IntVar(&options.Iterations, "count", 1000, "")
	fs.IntVar(&options.BatchSize, "batch-size", 0, "")
	fs.IntVar(&options.KeySize, "key-size", 8, "")
	fs.IntVar(&options.ValueSize, "value-size", 32, "")
	fs.StringVar(&options.CPUProfile, "cpuprofile", "", "")
	fs.StringVar(&options.MemProfile, "memprofile", "", "")
	fs.StringVar(&options.BlockProfile, "blockprofile", "", "")
	fs.StringVar(&options.BlockProfile, "blockprofile", "", "")
	fs.Float64Var(&options.FillPercent, "fill-percent", bolt.DefaultFillPercent, "")
	fs.BoolVar(&options.NoSync, "no-sync", false, "")
	fs.BoolVar(&options.Work, "work", false, "")
	fs.StringVar(&options.Path, "path", "", "")
	fs.SetOutput(cmd.Stderr)
	if err := fs.Parse(args); err != nil {
		return nil, err
	}

	// Set batch size to iteration size if not set.
	// Require that batch size can be evenly divided by the iteration count.
	if options.BatchSize == 0 {
		options.BatchSize = options.Iterations
	} else if options.Iterations%options.BatchSize != 0 {
		return nil, ErrNonDivisibleBatchSize
	}

	// Generate temp path if one is not passed in.
	if options.Path == "" {
		f, err := ioutil.TempFile("", "bolt-bench-")
		if err != nil {
			return nil, fmt.Errorf("temp file: %s", err)
		}
		f.Close()
		os.Remove(f.Name())
		options.Path = f.Name()
	}

	return &options, nil
}

// Writes to the database.
func (cmd *BenchCommand) runWrites(db *bolt.DB, options *BenchOptions, results *BenchResults) error {
	// Start profiling for writes.
	if options.ProfileMode == "rw" || options.ProfileMode == "w" {
		cmd.startProfiling(options)
	}

	t := time.Now()

	var err error
	switch options.WriteMode {
	case "seq":
		err = cmd.runWritesSequential(db, options, results)
	case "rnd":
		err = cmd.runWritesRandom(db, options, results)
	case "seq-nest":
		err = cmd.runWritesSequentialNested(db, options, results)
	case "rnd-nest":
		err = cmd.runWritesRandomNested(db, options, results)
	default:
		return fmt.Errorf("invalid write mode: %s", options.WriteMode)
	}

	// Save time to write.
	results.WriteDuration = time.Since(t)

	// Stop profiling for writes only.
	if options.ProfileMode == "w" {
		cmd.stopProfiling()
	}

	return err
}

func (cmd *BenchCommand) runWritesSequential(db *bolt.DB, options *BenchOptions, results *BenchResults) error {
	var i = uint32(0)
	return cmd.runWritesWithSource(db, options, results, func() uint32 { i++; return i })
}

func (cmd *BenchCommand) runWritesRandom(db *bolt.DB, options *BenchOptions, results *BenchResults) error {
	r := rand.New(rand.NewSource(time.Now().UnixNano()))
	return cmd.runWritesWithSource(db, options, results, func() uint32 { return r.Uint32() })
}

func (cmd *BenchCommand) runWritesSequentialNested(db *bolt.DB, options *BenchOptions, results *BenchResults) error {
	var i = uint32(0)
	return cmd.runWritesWithSource(db, options, results, func() uint32 { i++; return i })
}

func (cmd *BenchCommand) runWritesRandomNested(db *bolt.DB, options *BenchOptions, results *BenchResults) error {
	r := rand.New(rand.NewSource(time.Now().UnixNano()))
	return cmd.runWritesWithSource(db, options, results, func() uint32 { return r.Uint32() })
}

func (cmd *BenchCommand) runWritesWithSource(db *bolt.DB, options *BenchOptions, results *BenchResults, keySource func() uint32) error {
	results.WriteOps = options.Iterations

	for i := 0; i < options.Iterations; i += options.BatchSize {
		if err := db.Update(func(tx *bolt.Tx) error {
			b, _ := tx.CreateBucketIfNotExists(benchBucketName)
			b.FillPercent = options.FillPercent

			for j := 0; j < options.BatchSize; j++ {
				key := make([]byte, options.KeySize)
				value := make([]byte, options.ValueSize)

				// Write key as uint32.
				binary.BigEndian.PutUint32(key, keySource())

				// Insert key/value.
				if err := b.Put(key, value); err != nil {
					return err
				}
			}

			return nil
		}); err != nil {
			return err
		}
	}
	return nil
}

func (cmd *BenchCommand) runWritesNestedWithSource(db *bolt.DB, options *BenchOptions, results *BenchResults, keySource func() uint32) error {
	results.WriteOps = options.Iterations

	for i := 0; i < options.Iterations; i += options.BatchSize {
		if err := db.Update(func(tx *bolt.Tx) error {
			top, err := tx.CreateBucketIfNotExists(benchBucketName)
			if err != nil {
				return err
			}
			top.FillPercent = options.FillPercent

			// Create bucket key.
			name := make([]byte, options.KeySize)
			binary.BigEndian.PutUint32(name, keySource())

			// Create bucket.
			b, err := top.CreateBucketIfNotExists(name)
			if err != nil {
				return err
			}
			b.FillPercent = options.FillPercent

			for j := 0; j < options.BatchSize; j++ {
				var key = make([]byte, options.KeySize)
				var value = make([]byte, options.ValueSize)

				// Generate key as uint32.
				binary.BigEndian.PutUint32(key, keySource())

				// Insert value into subbucket.
				if err := b.Put(key, value); err != nil {
					return err
				}
			}

			return nil
		}); err != nil {
			return err
		}
	}
	return nil
}

// Reads from the database.
func (cmd *BenchCommand) runReads(db *bolt.DB, options *BenchOptions, results *BenchResults) error {
	// Start profiling for reads.
	if options.ProfileMode == "r" {
		cmd.startProfiling(options)
	}

	t := time.Now()

	var err error
	switch options.ReadMode {
	case "seq":
		switch options.WriteMode {
		case "seq-nest", "rnd-nest":
			err = cmd.runReadsSequentialNested(db, options, results)
		default:
			err = cmd.runReadsSequential(db, options, results)
		}
	default:
		return fmt.Errorf("invalid read mode: %s", options.ReadMode)
	}

	// Save read time.
	results.ReadDuration = time.Since(t)

	// Stop profiling for reads.
	if options.ProfileMode == "rw" || options.ProfileMode == "r" {
		cmd.stopProfiling()
	}

	return err
}

func (cmd *BenchCommand) runReadsSequential(db *bolt.DB, options *BenchOptions, results *BenchResults) error {
	return db.View(func(tx *bolt.Tx) error {
		t := time.Now()

		for {
			var count int

			c := tx.Bucket(benchBucketName).Cursor()
			for k, v := c.First(); k != nil; k, v = c.Next() {
				if v == nil {
					return errors.New("invalid value")
				}
				count++
			}

			if options.WriteMode == "seq" && count != options.Iterations {
				return fmt.Errorf("read seq: iter mismatch: expected %d, got %d", options.Iterations, count)
			}

			results.ReadOps += count

			// Make sure we do this for at least a second.
			if time.Since(t) >= time.Second {
				break
			}
		}

		return nil
	})
}

func (cmd *BenchCommand) runReadsSequentialNested(db *bolt.DB, options *BenchOptions, results *BenchResults) error {
	return db.View(func(tx *bolt.Tx) error {
		t := time.Now()

		for {
			var count int
			var top = tx.Bucket(benchBucketName)
			if err := top.ForEach(func(name, _ []byte) error {
				c := top.Bucket(name).Cursor()
				for k, v := c.First(); k != nil; k, v = c.Next() {
					if v == nil {
						return ErrInvalidValue
					}
					count++
				}
				return nil
			}); err != nil {
				return err
			}

			if options.WriteMode == "seq-nest" && count != options.Iterations {
				return fmt.Errorf("read seq-nest: iter mismatch: expected %d, got %d", options.Iterations, count)
			}

			results.ReadOps += count

			// Make sure we do this for at least a second.
			if time.Since(t) >= time.Second {
				break
			}
		}

		return nil
	})
}

// File handlers for the various profiles.
var cpuprofile, memprofile, blockprofile *os.File

// Starts all profiles set on the options.
func (cmd *BenchCommand) startProfiling(options *BenchOptions) {
	var err error

	// Start CPU profiling.
	if options.CPUProfile != "" {
		cpuprofile, err = os.Create(options.CPUProfile)
		if err != nil {
			fmt.Fprintf(cmd.Stderr, "bench: could not create cpu profile %q: %v\n", options.CPUProfile, err)
			os.Exit(1)
		}
		pprof.StartCPUProfile(cpuprofile)
	}

	// Start memory profiling.
	if options.MemProfile != "" {
		memprofile, err = os.Create(options.MemProfile)
		if err != nil {
			fmt.Fprintf(cmd.Stderr, "bench: could not create memory profile %q: %v\n", options.MemProfile, err)
			os.Exit(1)
		}
		runtime.MemProfileRate = 4096
	}

	// Start fatal profiling.
	if options.BlockProfile != "" {
		blockprofile, err = os.Create(options.BlockProfile)
		if err != nil {
			fmt.Fprintf(cmd.Stderr, "bench: could not create block profile %q: %v\n", options.BlockProfile, err)
			os.Exit(1)
		}
		runtime.SetBlockProfileRate(1)
	}
}

// Stops all profiles.
func (cmd *BenchCommand) stopProfiling() {
	if cpuprofile != nil {
		pprof.StopCPUProfile()
		cpuprofile.Close()
		cpuprofile = nil
	}

	if memprofile != nil {
		pprof.Lookup("heap").WriteTo(memprofile, 0)
		memprofile.Close()
		memprofile = nil
	}

	if blockprofile != nil {
		pprof.Lookup("block").WriteTo(blockprofile, 0)
		blockprofile.Close()
		blockprofile = nil
		runtime.SetBlockProfileRate(0)
	}
}

// BenchOptions represents the set of options that can be passed to "bolt bench".
type BenchOptions struct {
	ProfileMode   string
	WriteMode     string
	ReadMode      string
	Iterations    int
	BatchSize     int
	KeySize       int
	ValueSize     int
	CPUProfile    string
	MemProfile    string
	BlockProfile  string
	StatsInterval time.Duration
	FillPercent   float64
	NoSync        bool
	Work          bool
	Path          string
}

// BenchResults represents the performance results of the benchmark.
type BenchResults struct {
	WriteOps      int
	WriteDuration time.Duration
	ReadOps       int
	ReadDuration  time.Duration
}

// Returns the duration for a single write operation.
func (r *BenchResults) WriteOpDuration() time.Duration {
	if r.WriteOps == 0 {
		return 0
	}
	return r.WriteDuration / time.Duration(r.WriteOps)
}

// Returns average number of write operations that can be performed per second.
func (r *BenchResults) WriteOpsPerSecond() int {
	var op = r.WriteOpDuration()
	if op == 0 {
		return 0
	}
	return int(time.Second) / int(op)
}

// Returns the duration for a single read operation.
func (r *BenchResults) ReadOpDuration() time.Duration {
	if r.ReadOps == 0 {
		return 0
	}
	return r.ReadDuration / time.Duration(r.ReadOps)
}

// Returns average number of read operations that can be performed per second.
func (r *BenchResults) ReadOpsPerSecond() int {
	var op = r.ReadOpDuration()
	if op == 0 {
		return 0
	}
	return int(time.Second) / int(op)
}

type PageError struct {
	ID  int
	Err error
}

func (e *PageError) Error() string {
	return fmt.Sprintf("page error: id=%d, err=%s", e.ID, e.Err)
}