Amazon Ion Go

This package is for parsing and writing text and binary based Ion data.

This package is considered experimental, under active early development, and the API is subject to change.

http://amzn.github.io/ion-docs/docs/spec.html

It aims to be efficient by only evaluating values when they are accessed, and retaining the original text or binary representation where possible for efficient re-serialization of the same form.

Git Setup

This repository contains a git submodule called ion-tests, which holds test data used by ion-go's unit tests.

The easiest way to clone the ion-go repository and initialize its ion-tests submodule is to run the following command.

$ git clone --recursive https://github.com/amzn/ion-go.git ion-go

Alternatively, the submodule may be initialized independently from the clone by running the following commands.

$ git submodule init
$ git submodule update

Development

This package uses Go Modules to model its dependencies.

Assuming the go command is in your path, building the module can be done as:

$ go build -v ./...

Running all the tests can be executed with:

$ go test -v ./...

We use goimports to format our imports and files in general. Running this before commit is advised:

$ goimports -w .

It is recommended that you hook this in your favorite IDE (Tools > File Watchers in Goland, for example).

Usage

Import github.com/amzn/ion-go/ion and you're off to the races.

Marshaling and Unmarshaling

Similar to GoLang's built-in json package, you can marshal and unmarshal Go types to Ion. Marshaling requires you to specify whether you'd like text or binary Ion. Unmarshaling is smart enough to do the right thing. Both respect json name tags, and Marshal honors omitempty.

type T struct {
  A string
  B struct {
    RenamedC int   `json:"C"`
    D        []int `json:",omitempty"`
  }
}

func main() {
  t := T{}

  err := ion.Unmarshal([]byte(`{A:"Ion!",B:{C:2,D:[3,4]}}`), &t)
  if err != nil {
    panic(err)
  }
  fmt.Printf("--- t:\n%v\n\n", t)

  text, err := ion.MarshalText(&t)
  if err != nil {
    panic(err)
  }
  fmt.Printf("--- text:\n%s\n\n", string(text))

  binary, err := ion.MarshalBinary(&t)
  if err != nil {
    panic(err)
  }
  fmt.Printf("--- binary:\n%X\n\n", binary)
}

Encoding and Decoding

To read or write multiple values at once, use an Encoder or Decoder:

func main() {
  dec := ion.NewTextDecoder(os.Stdin)
  enc := ion.NewBinaryEncoder(os.Stdout)

  for {
    // Decode one Ion whole value from stdin.
    val, err := dec.Decode()
    if err == ion.ErrNoInput {
      break
    } else if err != nil {
      panic(err)
    }

    // Encode it to stdout.
    if err := enc.Encode(val); err != nil {
      panic(err)
    }
  }

  if err := enc.Finish(); err != nil {
    panic(err)
  }
}

Reading and Writing

For low-level streaming read and write access, use a Reader or Writer.

func copy(in ion.Reader, out ion.Writer) {
  for in.Next() {
    name := in.FieldName()
    if name != "" {
      out.FieldName(name)
    }

    annos := in.Annotations()
    if len(annos) > 0 {
      out.Annotations(annos...)
    }

    switch in.Type() {
    case ion.BoolType:
      val, err := in.BoolValue()
      if err != nil {
        panic(err)
      }
      out.WriteBool(val)

    case ion.IntType:
      val, err := in.Int64Value()
      if err != nil {
        panic(err)
      }
      out.WriteInt(val)

    case ion.StringType:
      val, err := in.StringValue()
      if err != nil {
        panic(err)
      }
      out.WriteString(val)

    case ion.ListType:
      in.StepIn()
      out.BeginList()
      copy(in, out)
      in.StepOut()
      out.EndList()

    case ion.StructType:
      in.StepIn()
      out.BeginStruct()
      copy(in, out)
      in.StepOut()
      out.EndStruct()
    }
  }

  if in.Err() != nil {
    panic(in.Err())
  }
}

func main() {
  in := ion.NewReader(os.Stdin)
  out := ion.NewBinaryWriter(os.Stdout)

  copy(in, out)

  if err := out.Finish(); err != nil {
    panic(err)
  }
}

Symbol Tables

By default, when writing binary Ion, a local symbol table is built as you write values (which are buffered in memory until you call Finish so the symbol table can be written out first). You can optionally provide one or more SharedSymbolTables to the writer, which it will reference as needed rather than directly including those symbols in the local symbol table.

type Item struct {
  ID          string    `json:"id"`
  Name        string    `json:"name"`
  Description string    `json:"description"`
}

var ItemSharedSymbols = ion.NewSharedSymbolTable("item", 1, []string{
  "item",
  "id",
  "name",
  "description",
})

type SpicyItem struct {
  Item
  Spiciness   int       `json:"spiciness"`
}

func WriteSpicyItemsTo(out io.Writer, items []SpicyItem) error {
  writer := ion.NewBinaryWriter(out, ItemSharedSymbols)

  for _, item := range items {
    writer.Annotation("item")
    if err := ion.EncodeTo(writer, item); err != nil {
      return err
    }
  }

  return writer.Finish()
}

You can alternatively provide the writer with a complete, pre-built local symbol table. This allows values to be written without buffering, however any attempt to write a symbol that is not included in the symbol table will result in an error:

func WriteItemsToLST(out io.Writer, items []SpicyItem) error {
  lst := ion.NewLocalSymbolTable([]SharedSymbolTable{ItemSharedSymbols}, []string{
    "spiciness",
  })

  writer := ion.NewBinaryWriterLST(out, lst)

  for _, item := range items {
    writer.Annotation("item")
    if err := ion.EncodeTo(writer, item); err != nil {
      return err
    }
  }

  return writer.Finish()
}

When reading binary Ion, shared symbol tables are provided by a Catalog. A basic catalog can be constructed by calling NewCatalog; a smarter implementation may load shared symbol tables from a database on demand.

func ReadItemsFrom(in io.Reader) ([]Item, error) {
  item := Item{}
  items := []Item{}

  cat := ion.NewCatalog(ItemSharedSymbols)
  dec := ion.NewDecoder(ion.NewReaderCat(in, cat))

  for {
    err := dec.DecodeTo(&item)
    if err == ion.ErrNoInput {
      return items, nil
    }
    if err != nil {
      return nil, err
    }

    items = append(items, item)
  }
}

Notes

• This package only supports text as UTF-8. It does not support the UTF-16 or UTF-32 forms.
• The Float type is limited to the minimum and maximum values that Go is able to handle with float64. These values are approximately 1.797693e+308 and 4.940656e-324. Values that are too large will round to infinity and values that are too small will round to zero.

TODO

• Symbol table construction and verification.
• Define the external interfaces for marshalling and unmarshalling.
• Serializing Values to the textual and binary forms.
• Unmarshal into a struct.
• Profiling.
• Make the Decimal type keep track of precision, so trailing zeros won't be lost when translating from the textual to binary form.
• Make the Timestamp type handle the case of unknown local offsets.
• Make the Float and Decimal types recognize negative zero.