ozonep-bson-to-json

Directly and quickly converts a BSON buffer to a JSON string stored in a Buffer. Useful for sending MongoDB database query results to a client over JSON+HTTP.

Benchmark with a ~2500-element array of medium objects (9MB BSON):

Method	Time (ms)
JSON.stringify(BSON.deserialize(arr))1	226.0
this, JS	39.7
this, portable C++	20.6
this, SSE2	15.2
this, SSE4.2	11.5
this, AVX2	10.6

¹ BSON.deserialize is the official MongoDB js-bson implementation.

Installation

The C++ implementations require a C++ compiler. See instructions here. If you do not have a C++ compiler, the slower JS version will be used.

yarn add zbjornson/bson-to-json
# or
npm install zbjornson/bson-to-json

Usage

bsonToJson

const {bsonToJson} = require("bson-to-json");
bsonToJson(bson: Uint8Array, isArray?: boolean = true): Buffer
// (note that Buffers extend Uint8Arrays, so `bson` can be a Buffer)

Transcodes a BSON document to a JSON string stored in a Buffer.

isArray specifies if the input is an array or not. BSON doesn't differentiate between arrays and objects at the top level, so this must be provided if bson is an array.

The output should be identical to JSON.stringify(BSON.deserialize(v)), with two exceptions:

1. This module writes full-precision (64-bit signed) BSON Longs to the JSON buffer. This is valid because JSON does not specify a maximum numeric precision, but js-bson instead writes an object with low and high bits.
2. This module does more/better input bounds checking than js-bson, so this module may throw different errors. (js-bson seems to rely, intentionally or not, on indexing past the end of a typed array returning undefined.)

send

const {send} = require("bson-to-json");
send(cursor: MongoDbCursor, ostr: Stream.Writable): Promise<void>

Efficiently sends the contents of a MongoDB cursor to a writable stream (e.g. an HTTP response). The returned Promise resolves when the cursor is drained, or rejects in case of an error.

Example usage in an HTTP handler

const {send} = require("bson-to-json");
async function (req, res) {
  const cursor = await db.collection("mycol").find({name: "Zach"}, {raw: true});
  res.setHeader("Content-Type", "application/json");
  await send(cursor, res);
}

This is the fastest way to transfer results from MongoDB to a client. MongoDB's cursor.forEach or for await (const doc of cursor) both have much higher CPU and memory overhead.

ISE

const {ISE} = require("bson-to-json");
ISE: string

A constant indicating what instruction set extension was used (based on your CPU's available features). One of "AVX512", "AVX2", "SSE4.2", "SSE2", "Baseline" (portable C) or "JavaScript".

Performance notes

Major reasons it's fast

• Direct UTF8 to JSON-escaped string transcoding.
• No waste temporary objects created for the GC to clean up.
• SSE2, SSE4.2 or AVX2-accelerated JSON string escaping.
• AVX2-accelerated ObjectId hex string encoding, using the technique from zbjornson/fast-hex.
• Fast integer encoding, using the method from fmtlib/fmt.
• Fast double encoding, using the same double-conversion library used in v8.
• Skips decoding array keys (which BSON stores as ASCII numbers) and instead advances by the known number of bytes in the key.
• The send method has a tight call stack and avoids allocating a Promise for each document (compared to for await...of).

Benchmarks by BSON type (ops/sec):

Type	js-bson	this, JS	this, CPP (AVX2)
long	1,760	1,236	28,031
int	1,503	1,371	17,264
ObjectId	1,048	13,322	37,079
date	445	663	10,686
number	730	1,228	1,929
boolean	444	4,839	9,283
null	482	7,487	14,709
string<len=1000, esc=0.00>1	12,304	781	55,502
string<len=1000, esc=0.01>	12,720	748	56,145
string<len=1000, esc=0.05>	12,320	756	43,867

¹String transcoding performance depends on the length of the string (len) and the number of characters that must be escaped in the JSON output (esc, a fraction from 0 to 1).

Future Plans

• Iterator-based (streaming) interface. It's mostly working in the C++ version, but crashes on gc. See documentation in the iterator branch. I also experimented with C++20 coroutines in the coroutines branch.
• Drop long dependency when Node 10 support is dropped.
• Consider adding an option to prepend a comma to the output so it can be used with MongoDB cursors more efficiently.