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ldetool means line data extraction tool

ldetool is a command line utility to generate Go code for fast log files parsing.

go install
  1. Contributors
  2. Rationale
  3. Typical operations and formal set of rules
  4. Performance comparison against regexp and Ragel
  5. Usage examples

How it works.

  1. First write extraction script, we usually name it <something>.lde
  2. Generate go code with ldetool <something.lde> --package main. Of course you can use your own package name, not only main
  3. Use it via the generated extraction method Parse(line []byte).

It turned out we like using it even for non-performant tasks, where we are dealing with strings, not slices of bytes and it would be handy to use it for strings as well without manual type casting. There's an option to generate code that use string, just put an option --go-string

CLI utility options
  1. --go-string generates code that uses string everywhere instead of []byte. You better not to use it for log processing as it may lead to excessive memory allocations.
  2. --yaml-dict or --json-dict sets translation rules for names. For instance, if we have YAML file with
    http: HTTP
    and feed this file to the ldetool then every name (of field or rule itself) like GetHttpHandle or get_http_handle will be translated into GetHTTPHandle
  3. --package <pkg name> name of the package to use in generated code. If a directory of *.lde file has other Go files package name will automatically setup with these files' package name.
  4. --big-endian or --little-endian sets the target architecture to be either big or little endian. This enables prefix check optimization


Take a look at these two lines

[2017-09-02T22:48:13] FETCH first[1] format[JSON] hidden[0] userAgent[Android App v1.0] rnd[21341975] country[MA]
[2017-09-02T22:48:14] FETCH first[0] format[JSON] userAgent[Android App v1.0] rnd[10000000] country[LC]

We likely need a time, value of parameter first, format, hidden, userAgent and country. We obviously don't need rnd

Extraction script syntax

See more details on extraction rules

# filename: line.lde
Line =                                   # Name of the extraction object' type
  ^'[' Time(string) ']'                  # The line must start with [, then take everything as a struct field Time string right to ']' character
  ^" FETCH "                             # Current rest must starts with " FETCH " string
  ^"first[" First(uint8) ']'[1]          # The rest must starts with "first[" characters, then take the rest until ']' as uint8. It is
                                         # known First is the single character, thus the [1] index.
                                         # under the name of First
  ^" format[" Format(string) ~']'        # Take format id. Format is a short word: XML, JSON, BIN. ~ before lookup oobject suggests
                                         # generator to use for loop scan rather than IndexByte, which is although fast
                                         # has call overhead as it cannot be inlined by Go compiler.
  ?Hidden (^" hidden[" Value(uint8) ']') # Optionally look for " hidden[\d+]"
  ^" user_agent[" UserAgent(string) ']'  # User agent data
  _ "country[" Country(string)  ']'      # Look for the piece starting with country[
Code generation

The recommended way is to put something like //go:generate ldetool --package main Line.lde in generate.go of a package and then generate a code with

go generate <project path>

It will be written into line_lde.go file in the same directory. It will look like this

Now, we have

  1. Data extractor type
    // Line autogenerated parser
    type Line struct {
        Rest   []byte
        Time   []byte
        First  uint8
        Format []byte
        Hidden struct {
            Valid bool
            Value uint8
        UserAgent []byte
        Country   []byte
  2. Parse method
    // Extract autogenerated method of Line
    func (p *Line) Extract(line []byte) (bool, error) {
    Take a look at return data. First bool signals if the data was successfully matched and error that is not nil signals if there were any error. String to numeric failures are always treated as errors, you can put ! into extraction script and all mismatches after the sign will be treated as errors
  3. Helper to access optional Hidden area returning default Go value if the the area was not matched
    // GetHiddenValue retrieves optional value for HiddenValue.Name
    func (p *Line) GetHiddenValue() (res uint8) {
        if !p.Hidden.Valid {
        return p.Hidden.Value
Generated code usage

It is easy: put

l := &Line{}

before and then feed Extract method with lines:

scanner := bufio.NewScanner(reader)
for scanner.Scan() {
    ok, err := l.Extract(scanner.Bytes())
    if !ok {
        if err != nil {
            return err

custom types

Special thanks to Matt Hook ( who proposed this feature

It is possible to use custom types in generated structure. You should declare them first via

type pkg.Type from "pkgpath";

for external types and

type typeName;

for local types before all rules definitions and you can use them as field types. The parsing to be done via

p.unmarshal<FieldName>([]byte) (Type, error)



type time.Time from "time";
type net.IP from "net";

Custom = Time(time.Time) ' ' ?Addr(^"addr: " IP(ip.IP) ' ');

Now, two parsing functions will be needed to parse this (they are to be written manually):

func (p *Custom) unmarshalTime(s string) (time.Time, error) { … }

func (p *Custom) unmarshalAddrIP(s string) (net.IP, error) { … }