# lowmemjson `lowmemjson` is a mostly-compatible alternative to the standard library's [`encoding/json`][] that has dramatically lower memory requirements for large data structures. `lowmemjson` is not targeting extremely resource-constrained environments, but rather targets being able to efficiently stream gigabytes of JSON without requiring gigabytes of memory overhead. ## Compatibility `encoding/json`'s APIs are designed around the idea that it can buffer the entire JSON document as a `[]byte`, and as intermediate steps it may have a fragment buffered multiple times while encoding; encoding a gigabyte of data may consume several gigabytes of memory. In contrast, `lowmemjson`'s APIs are designed around streaming (`io.Writer` and `io.RuneScanner`), trying to have the memory overhead of encode and decode operations be as close to O(1) as possible. `lowmemjson` offers a high level of compatibility with the `encoding/json` APIs, but for best memory usage (avoiding storing large byte arrays inherent in `encoding/json`'s API), it is recommended to migrate to `lowmemjson`'s own APIs. ### Callee API (objects to be encoded-to/decoded-from JSON) `lowmemjson` supports `encoding/json`'s `json:` struct field tags, as well as the `encoding/json.Marshaler` and `encoding/json.Unmarshaler` interfaces; you do not need to adjust your types to successfully migrate from `encoding/json` to `lowmemjson`. That is: Given types that decode as desired with `encoding/json`, those types should decode identically with `lowmemjson`. Given types that encode as desired with `encoding/json`, those types should encode identically with `lowmemjson` (assuming an appropriately configured `ReEncoder` to match the whitespace-handling and special-character escaping; a `ReEncoderConfig` with `Compact=true` and all other settings left as zero will match the behavior of `json.Marshal`). For better memory usage: - Instead of implementing [`json.Marshaler`][], consider implementing [`lowmemjson.Encodable`][] (or implementing both). - Instead of implementing [`json.Unmarshaler`][], consider implementing [`lowmemjson.Decodable`][] (or implementing both). ### Caller API `lowmemjson` offers a [`lowmemjson/compat/json`][] package that is a (mostly) drop-in replacement for `encoding/json` (see the package's documentation for the small incompatibilities). For better memory usage, avoid using `lowmemjson/compat/json` and instead use `lowmemjson` directly: - Instead of using [json.Marshal][`json.Marshal`](val), consider using [lowmemjson.NewEncoder][`lowmemjson.NewEncoder`](w).[Encode][`lowmemjson.Encoder.Encode`](val). - Instead of using [json.Unmarshal][`json.Unmarshal`](dat, &val), consider using [lowmemjson.NewDecoder][`lowmemjson.NewDecoder`](r).[DecodeThenEOF][`lowmemjson.Decoder.DecodeThenEOF`](&val). - Instead of using [`json.Compact`][], [`json.HTMLEscape`][], or [`json.Indent`][]; consider using a [`lowmemjson.ReEncoder`][]. - Instead of using [`json.Valid`][], consider using a [`lowmemjson.ReEncoder`][] with `io.Discard` as the output. The error types returned from `lowmemjson` are different from the error types returned by `encoding/json`, but `lowmemjson/compat/json` translates them back to the types returned by `encoding/json`. ## Overview ### Caller API There are 3 main types that make up the caller API for producing and handling streams of JSON, and each of those types has some associated types that go with it: 1. `type Decoder` + `type DecodeArgumentError` + `type DecodeError` * `type DecodeReadError` * `type DecodeSyntaxError` * `type DecodeTypeError` 2. `type Encoder` + `type EncodeTypeError` + `type EncodeValueError` + `type EncodeMethodError` 3. `type ReEncoder` + `type ReEncoderConfig` + `type ReEncodeSyntaxError` + `type BackslashEscaper` * `type BackslashEscapeMode` A `*Decoder` handles decoding a JSON stream into Go values; the most common use of it will be `lowmemjson.NewDecoder(r).DecodeThenEOF(&val)` or `lowmemjson.NewDecoder(bufio.NewReader(r)).DecodeThenEOF(&val)`. A `*ReEncoder` handles transforming a JSON stream; this is useful for prettifying, minifying, sanitizing, and/or validating JSON. A `*ReEncoder` wraps an `io.Writer`, itself implementing `io.Writer`. The most common use of it will be something along the lines of `out = lowmemjson.NewReEncoder(out, lowmemjson.ReEncoderConfig{…})`. An `*Encoder` handles encoding Go values into a JSON stream. `*Encoder` doesn't take much care in to making its output nice; so it is usually desirable to have the output stream of an `*Encoder` be a `*ReEncoder`; the most common use of it will be `lowmemjson.NewEncoder(lowmemjson.NewReEncoder(out, lowmemjson.ReEncoderConfig{…})).Encode(val)`. `*Encoder` and `*ReEncoder` both tend to make many small writes; if writes are syscalls, you may want to wrap their output in a `bufio.Writer`. ### Callee API For defining Go types with custom JSON representations, `lowmemjson` respects all of the `json:` struct field tags of `encoding/json`, as well as respecting the same "marshaler" and "unmarshaler" interfaces as `encoding/json`. In addition to those interfaces, `lowmemjson` adds two of its own interfaces, and some helper functions to help with implementing those interfaces: 1. `type Decodable` + `func DecodeArray` + `func DecodeObject` 2. `type Encodable` These are streaming variants of the standard `json.Unmarshaler` and `json.Marshaler` interfaces. [`lowmemjson`]: https://pkg.go.dev/git.lukeshu.com/go/lowmemjson [`lowmemjson/compat/json`]: https://pkg.go.dev/git.lukeshu.com/go/lowmemjson/compat/json [`encoding/json`]: https://pkg.go.dev/encoding/json@go1.18 [`json.Marshaler`]: https://pkg.go.dev/encoding/json@go1.18#Marshaler [`json.Unmarshaler`]: https://pkg.go.dev/encoding/json@go1.18#Unmarshaler [`json.Marshal`]: https://pkg.go.dev/encoding/json@go1.18#Marshal [`json.Unmarshal`]: https://pkg.go.dev/encoding/json@go1.18#Unmarshal [`json.Compact`]: https://pkg.go.dev/encoding/json@go1.18#Compact [`json.HTMLEscape`]: https://pkg.go.dev/encoding/json@go1.18#HTMLEscape [`json.Indent`]: https://pkg.go.dev/encoding/json@go1.18#Indent [`json.Valid`]: https://pkg.go.dev/encoding/json@go1.18#Valid [`lowmemjson.Encodable`]: https://pkg.go.dev/git.lukeshu.com/go/lowmemjson#Encodable [`lowmemjson.Decodable`]: https://pkg.go.dev/git.lukeshu.com/go/lowmemjson#Decodable [`lowmemjson.NewEncoder`]: https://pkg.go.dev/git.lukeshu.com/go/lowmemjson#NewEncoder [`lowmemjson.Encoder.Encode`]: https://pkg.go.dev/git.lukeshu.com/go/lowmemjson#Encoder.Encode [`lowmemjson.NewDecoder`]: https://pkg.go.dev/git.lukeshu.com/go/lowmemjson#NewDecoder [`lowmemjson.Decoder.DecodeThenEOF`]: https://pkg.go.dev/git.lukeshu.com/go/lowmemjson#Decoder.DecodeThenEOF [`lowmemjson.ReEncoder`]: https://pkg.go.dev/git.lukeshu.com/go/lowmemjson#ReEncoder