JSON Streaming

Introduction

JSON is a popular format for exchanging object data between systems. Frequently there's a need for a stream of objects to be sent over a single connection, such as a stock ticker or application log records. In these cases there's a need to identify where one JSON encoded object ends and the next begins. Technically this is known as framing.

There are three common ways to achieve this:

Line delimited JSON

Line delimited JSON streaming makes use of the fact that the JSON format does not allow newline characters within values (they have to be escaped as ` `) and that most JSON formatters default to not including any whitespace, including newlines. These features allows the newline characters to be used as a delimiter.

This example shows two JSON objects (the implicit newline characters at the end of each line are not shown):

The use of a newline as a delimiter enables this format to work very well with traditional line-oriented UNIX tools.

Concatenated JSON

Concatenated JSON streaming allows the sender to simply write each JSON object into the stream with no delimiters. It relies on the receiver using a parser that can recognize and emit each JSON object as the terminating character is parsed. Concatenated JSON isn't a new format, it's simply a name for streaming multiple JSON objects without any delimiters.

The advantage of this format is that it can handle JSON objects that have been formatted with embedded newline characters, e.g., pretty-printed for human readability. For example, these two inputs are both valid and produce the same output:

Implementations that rely on line-based input may require a newline character after each JSON object in order for the object to be emitted by the parser in a timely manner. (Otherwise the line may remain in the input buffer without being passed to the parser.) This is rarely recognised as an issue because terminating JSON objects with a newline character is very common.

Length-prefixed JSON

Length-prefixed or framed JSON streaming allows the sender to explicitly state the length of each message. It relies on the receiver using a parser that can recognize each length n and then read the following n bytes to parse as JSON.

The advantage of this format is that it can speed up parsing due to the fact that the exact length of each message is explicitly stated, rather than forcing the parser to search for delimiters. Length-prefixed JSON is also well-suited for TCP applications, where a single "message" may be divided into arbitrary chunks, because the prefixed length tells the parser exactly how many bytes to expect before attempting to parse a JSON string.

This example shows two length-prefixed JSON objects (with each length being the byte-length of the following JSON string):

Comparison

Line delimited JSON works very well with traditional line-oriented tools.

Concatenated JSON works with pretty-printed JSON but requires more effort and complexity to parse. It doesn't work well with traditional line-oriented tools. Concatenated JSON streaming is a superset of line delimited JSON streaming.

Length-prefixed JSON works with pretty-printed JSON. It doesn't work well with traditional line-oriented tools, but may offer performance advantages over line delimited or concatenated streaming. It can also be simpler to parse.

Compatibility

Line delimited JSON can be read by a parser that can handle concatenated JSON. Concatenated JSON that contains newlines within a JSON object can't be read by a line delimited JSON parser.

The terms "line delimited JSON" and "newline delimited JSON" are often used without clarifying if embedded newlines are supported.

There's also a format known as NDJ ("Newline delimited JSON") which allows comments to be embedded if the first two characters of a given line are "//". This can't be used with standard JSON parsers if comments are included.

Concatenated JSON can be converted into Line delimited JSON by a suitable JSON utility such as jq. For example

Line delimited JSON

Concatenated JSON

Length-prefixed JSON