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# primus-backpressure [](https://github.com/davedoesdev/primus-backpressure/actions/workflows/ci.yml) [](https://coveralls.io/r/davedoesdev/primus-backpressure?branch=master) [](http://badge.fury.io/js/primus-backpressure)
Node [streams2](http://nodejs.org/api/stream.html) over [Primus](https://github.com/primus/primus): added back-pressure!
- Pass in a Primus client or spark, get back a [`stream.Duplex`](http://nodejs.org/api/stream.html#stream_class_stream_duplex_1). Do this on both sides of a Primus connection.
- [`write`](http://nodejs.org/api/stream.html#stream_writable_write_chunk_encoding_callback) returns `true` when the receiver is full.
- Use [`read`](http://nodejs.org/api/stream.html#stream_readable_read_size) and [`readable`](http://nodejs.org/api/stream.html#stream_event_readable) to exert back-pressure on the sender.
- Unit tests with 100% coverage.
- Browser unit tests using [webpack](http://webpack.github.io/) and [PhantomJS](http://phantomjs.org/).
The API is described [here](#api).
## Example
Exerting backpressure over Primus:
```javascript
var Primus = require('primus'),
PrimusDuplex = require('primus-backpressure').PrimusDuplex,
primus = Primus.createServer({ port: 9000 }),
Socket = primus.Socket,
assert = require('assert'),
read_a = false;
primus.once('connection', function (spark)
{
var spark_duplex = new PrimusDuplex(spark,
{
highWaterMark: 2
});
assert.equal(spark_duplex.write('ab'), false);
spark_duplex.on('drain', function ()
{
assert(read_a);
});
});
var client_duplex = new PrimusDuplex(new Socket('http://localhost:9000'),
{
highWaterMark: 1
});
client_duplex.once('readable', function ()
{
assert.equal(this.read().toString(), 'a');
read_a = true;
assert.equal(this.read(), null);
this.once('readable', function ()
{
assert.equal(this.read().toString(), 'b');
primus.end();
console.log('done')
});
});
```
## Another Example
Piping data over Primus:
```javascript
var Primus = require('primus'),
PrimusDuplex = require('primus-backpressure').PrimusDuplex,
primus = Primus.createServer({ port: 9000 }),
Socket = primus.Socket,
assert = require('assert'),
crypto = require('crypto'),
tmp = require('tmp'),
fs = require('fs');
primus.once('connection', function (spark)
{
var spark_duplex = new PrimusDuplex(spark);
tmp.tmpName(function (err, random_file)
{
assert.ifError(err);
var random_buf = crypto.randomBytes(1024 * 1024);
fs.writeFile(random_file, random_buf, function (err)
{
assert.ifError(err);
tmp.tmpName(function (err, out_file)
{
assert.ifError(err);
var random_stream = fs.createReadStream(random_file),
out_stream = fs.createWriteStream(out_file);
out_stream.on('finish', function ()
{
fs.readFile(out_file, function (err, out_buf)
{
assert.ifError(err);
assert.deepEqual(out_buf, random_buf);
fs.unlink(random_file, function (err)
{
assert.ifError(err);
fs.unlink(out_file, function (err)
{
assert.ifError(err);
primus.end();
console.log('done');
});
});
});
});
spark_duplex.pipe(out_stream);
random_stream.pipe(spark_duplex);
});
});
});
});
var client_duplex = new PrimusDuplex(new Socket('http://localhost:9000'));
client_duplex.pipe(client_duplex);
```
## Installation
```shell
npm install primus-backpressure
```
## Licence
[MIT](LICENCE)
## Test
Node client to Node server:
```shell
grunt test
```
Browser client to Node server (requires [PhantomJS](http://phantomjs.org/)):
```shell
grunt test-browser
```
## Code Coverage
```shell
grunt coverage
```
[c8](https://github.com/bcoe/c8) results are available [here](http://rawgit.davedoesdev.com/davedoesdev/primus-backpressure/master/coverage/lcov-report/index.html).
Coveralls page is [here](https://coveralls.io/r/davedoesdev/primus-backpressure).
## Lint
```shell
grunt lint
```
# API
[`PrimusDuplex`](#primusduplexmsg_stream-options) inherits from [`stream.Duplex`](http://nodejs.org/api/stream.html#stream_class_stream_duplex_1) so you can call any method from [`stream.Readable`](http://nodejs.org/api/stream.html#stream_class_stream_readable) and [`stream.Writable`](http://nodejs.org/api/stream.html#stream_class_stream_writable).
Extra constructor options and an additional parameter to [`readable.read`](http://nodejs.org/api/stream.html#stream_readable_read_size) are described below.
*/
/*eslint-env node */
"use strict";
var util = require('util'),
stream = require('stream'),
crypto = require('crypto'),
max_seq = Math.pow(2, 32);
/**
Creates a new `PrimusDuplex` object which exerts back-pressure over a [Primus](https://github.com/primus/primus) connection.
Both sides of a Primus connection must use `PrimusDuplex` — create one for your Primus client and one for your spark as soon as you have them.
@constructor
@extends stream.Duplex
@param {Object} msg_stream The Primus client or spark you wish to exert back-pressure over.
@param {Object} [options] Configuration options. This is passed onto `stream.Duplex` and can contain the following extra properties:
- `{Function} [encode_data(chunk, encoding, start, end, internal)]` Optional encoding function for data passed to [`writable.write`](http://nodejs.org/api/stream.html#stream_writable_write_chunk_encoding_callback). `chunk` and `encoding` are as described in the `writable.write` documentation. The difference is that `encode_data` is synchronous (it must return the encoded data) and it should only encode data between the `start` and `end` positions in `chunk`. Defaults to a function which does `chunk.toString('base64', start, end)`. Note that `PrimusDuplex` may also pass some internal data through this function (always with `chunk` as a `Buffer`, `encoding=null` and `internal=true`).
- `{Function} [decode_data(chunk, internal)]` Optional decoding function for data received on the Primus connection. The type of `chunk` will depend on how the peer `PrimusDuplex` encoded it. Defaults to a function which does `Buffer.from(chunk, 'base64')`. If the data can't be decoded, return `null` (and optionally call `this.emit` to emit an error). Note that `PrimusDuplex` may also pass some internal data through this function (always with `internal=true`) — in which case you must return a `Buffer`.
- `{Integer} [max_write_size]` Maximum number of bytes to write onto the Primus connection at once, regardless of how many bytes the peer is free to receive. Defaults to 0 (no limit).
- `{Boolean} [check_read_overflow]` Whether to check if more data than expected is being received. If `true` and the high-water mark for reading is exceeded then the `PrimusDuplex` object emits an `error` event. This should not normally occur unless you add data yourself using [`readable.unshift`](http://nodejs.org/api/stream.html#stream_readable_unshift_chunk) — in which case you should set `check_read_overflow` to `false`. Defaults to `true`.
*/
function PrimusDuplex(msg_stream, options)
{
stream.Duplex.call(this, Object.assign(
{
autoDestroy: true
}, options));
options = options || {};
this._max_write_size = options.max_write_size || 0;
this._check_read_overflow = options.check_read_overflow !== false;
this.msg_stream = msg_stream;
this._seq = 0;
this._secret = crypto.randomBytes(64);
this._remote_free = 0;
this._data = null;
this._encoding = null;
this._cb = null;
this._index = 0;
this._finished = false;
this._sending = false;
this._send_requested = false;
this._encode_data = options.encode_data || function (chunk, encoding, start, end)
{
return chunk.toString('base64', start, end);
};
this._decode_data = options.decode_data || function (chunk)
{
if (typeof chunk !== 'string')
{
this.emit('error', new Error('encoded data is not a string: ' + chunk));
return null;
}
return Buffer.from(chunk, 'base64');
};
var ths = this;
function expect_handshake(data)
{
if (!data)
{
return ths.emit('error', new Error('invalid data: ' + data));
}
if (data.type === 'end')
{
ths.push(null);
return ths.msg_stream.end();
}
if (data.type !== 'handshake')
{
return ths.emit('error', new Error('expected handshake, got: ' + data.type));
}
ths._remote_free = ths._max_write_size > 0 ?
Math.min(data.free, ths._max_write_size) : data.free;
msg_stream.removeListener('data', expect_handshake);
msg_stream.on('data', function (data)
{
var ddata, buf, free, seq, hmac, digest, i;
if (!data)
{
return ths.emit('error', new Error('invalid data: ' + data));
}
if (data.type === 'data')
{
ths._remote_seq = data.seq;
// work around https://github.com/primus/primus/issues/263
/* istanbul ignore else */
if (!ths._readableState.ended)
{
ddata = ths._decode_data(data.data);
if (!ddata) { return; }
if (ths._check_read_overflow &&
((ths._readableState.length + ddata.length) >
ths._readableState.highWaterMark))
{
ths.emit('error', new Error('too much data'));
}
else
{
ths.push(ddata);
}
}
}
else if (data.type === 'status')
{
buf = ths._decode_data(data.seq, true);
if (!buf) { return; }
if (buf.length !== 36)
{
return ths.emit('error', new Error('invalid sequence length: ' + buf.length));
}
hmac = crypto.createHmac('sha256', ths._secret);
hmac.update(buf.slice(0, 4));
digest = hmac.digest();
for (i = 0; i < 32; i += 1)
{
if (digest[i] !== buf[4 + i])
{
return ths.emit('error', new Error('invalid sequence signature'));
}
}
free = ths._max_write_size > 0 ?
Math.min(data.free, ths._max_write_size) : data.free;
seq = buf.readUInt32BE(0, true);
ths._remote_free = seq + free - ths._seq;
if (ths._seq < seq)
{
ths._remote_free -= max_seq;
}
ths._send();
}
else if (data.type === 'end')
{
ths.push(null);
if (ths._finished)
{
ths.msg_stream.end();
}
}
else
{
ths.emit('error', new Error('unknown type: ' + data.type));
}
});
ths._send();
}
msg_stream.on('data', expect_handshake);
this.on('finish', function ()
{
this._finished = true;
if (this.allowHalfOpen)
{
this.msg_stream.write(
{
type: 'end'
});
}
else
{
this.msg_stream.end();
}
});
this.msg_stream.on('end', function ()
{
ths.push(null);
if (ths.allowHalfOpen)
{
// primus streams can't be half open
ths.end();
}
});
this.msg_stream.on('error', function (err)
{
ths.emit('error', err);
});
if (!options._delay_handshake)
{
this._send_handshake();
}
}
util.inherits(PrimusDuplex, stream.Duplex);
PrimusDuplex.prototype._send_handshake = function ()
{
this.msg_stream.write(
{
type: 'handshake',
free: Math.max(this._readableState.highWaterMark - this._readableState.length, 0)
});
};
PrimusDuplex.prototype._send_status = function ()
{
// Note: Status messages are sent regardless of remote_free
// (if the remote peer isn't doing anything it could never be sent and it
// could be waiting for a status update).
// This means every time the app calls read(), a status message wlll be sent
// to the remote peer. If you want to control when status messages are
// sent then use the second parameter of read(), send_status.
// To force sending a status message without actually reading any data,
// call read(0).
if (this._remote_seq === undefined)
{
return;
}
var free = Math.max(this._readableState.highWaterMark - this._readableState.length, 0),
msg;
if (this._prev_status &&
(this._prev_status.free === free) &&
(this._prev_status.seq === this._remote_seq))
{
return;
}
msg = {
type: 'status',
free: free,
seq: this._remote_seq
};
this._prev_status = msg;
this.msg_stream.write(msg);
};
PrimusDuplex.prototype._read = function () { return undefined; };
/**
See [`readable.read`](http://nodejs.org/api/stream.html#stream_readable_read_size). `PrimusDuplex` adds an extra optional parameter, `send_status`.
@param {Number} [size] Optional argument to specify how much data to read. Defaults to `undefined` (you can also specify `null`) which means return all the data available.
@param {Boolean} [send_status] Every time you call `read`, a status message is sent to the peer `PrimusDuplex` indicating how much space is left in the internal buffer to receive new data. To prevent deadlock, these status messages are always sent — they aren't subject to back-pressure. Normally this is fine because status messages are small. However, if your application reads data one byte at a time, for example, you may wish to control when status messages are sent. To stop a status message being sent when you call `read`, pass `send_status` as `false`. `send_status` defaults to `true`. To force a status message to be sent without reading any data, call `read(0)`.
*/
PrimusDuplex.prototype.read = function (size, send_status)
{
var r = stream.Duplex.prototype.read.call(this, size);
if (send_status !== false)
{
this._send_status();
}
return r;
};
PrimusDuplex.prototype.__send = function ()
{
if ((this._data === null) ||
(this._remote_free <= 0))
{
return;
}
var size = Math.min(this._remote_free, this._data.length - this._index),
buf = Buffer.alloc(4),
hmac = crypto.createHmac('sha256', this._secret),
cb;
this._seq = (this._seq + size) % max_seq;
buf.writeUInt32BE(this._seq, 0, true);
hmac.update(buf);
this.msg_stream.write(
{
type: 'data',
data: this._encode_data(this._data, this._encoding, this._index, this._index + size),
seq: this._encode_data(Buffer.concat([buf, hmac.digest()], 36), null, 0, 36, true)
});
this._remote_free -= size;
this._index += size;
if (this._index === this._data.length)
{
this._data = null;
this._encoding = null;
this._index = 0;
cb = this._cb;
this._cb = null;
cb();
}
};
PrimusDuplex.prototype._send = function ()
{
this._send_requested = true;
if (this._sending)
{
return;
}
this._sending = true;
while (this._send_requested)
{
this._send_requested = false;
this.__send();
}
this._sending = false;
};
PrimusDuplex.prototype._write = function (data, encoding, cb)
{
if (data.length === 0)
{
return cb();
}
this._data = data;
this._encoding = encoding;
this._cb = cb;
this._send();
};
PrimusDuplex.prototype._destroy = function (err, cb)
{
this.msg_stream.end();
cb(err);
};
exports.PrimusDuplex = PrimusDuplex;
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