diff --git a/doc/spec/json.md b/doc/spec/json.md new file mode 100644 index 000000000..596b218f2 --- /dev/null +++ b/doc/spec/json.md @@ -0,0 +1,77 @@ +# Docker Distribution JSON Canonicalization + +## Introduction + +To provide consistent content hashing of JSON objects throughout Docker +Distribution APIs, a canonical JSON format has been defined. Adopting such a +canonicalization also aids in caching JSON responses. + +## Rules + +Compliant JSON should conform to the following rules: + +1. All generated JSON should comply with [RFC + 7159](http://www.ietf.org/rfc/rfc7159.txt). +2. Resulting "JSON text" shall always be encoded in UTF-8. +3. Unless a canonical key order is defined for a particular schema, object + keys shall always appear in lexically sorted order. +4. All whitespace between tokens should be removed. +5. No "trailing commas" are allowed in object or array definitions. + +## Examples + +The following is a simple example of a canonicalized JSON string: + +```json +{"asdf":1,"qwer":[],"zxcv":[{},true,1000000000,"tyui"]} +``` + +## Reference + +### Other Canonicalizations + +The OLPC project specifies [Canonical +JSON](http://wiki.laptop.org/go/Canonical_JSON). While this is used in +[TUF](http://theupdateframework.com/), which may be used with other +distribution-related protocols, this alternative format has been proposed in +case the original source changes. Specifications complying with either this +specification or an alternative should explicitly call out the +canonicalization format. Except for key ordering, this specification is mostly +compatible. + +### Go + +In Go, the [`encoding/json`](http://golang.org/pkg/encoding/json/) library +will emit canonical JSON by default. Simply using `json.Marshal` will suffice +in most cases: + +```go +incoming := map[string]interface{}{ + "asdf": 1, + "qwer": []interface{}{}, + "zxcv": []interface{}{ + map[string]interface{}{}, + true, + int(1e9), + "tyui", + }, +} + +canonical, err := json.Marshal(incoming) +if err != nil { + // ... handle error +} +``` + +To apply canonical JSON format spacing to an existing serialized JSON buffer, one +can use +[`json.Indent`](http://golang.org/src/encoding/json/indent.go?s=1918:1989#L65) +with the following arguments: + +```go +incoming := getBytes() +var canonical bytes.Buffer +if err := json.Indent(&canonical, incoming, "", ""); err != nil { + // ... handle error +} +```