CFStreamCreateBoundPair streams lose data with small buffer size - objective-c

I am attempting to create a streaming html parser with libxml2 in Objective-C. I have a NSURLConnection that downloads the data, and I have created in NSInputStream and NSOutputstream with CFStreamCreateBoundPairwith a small buffer size of 10 bytes. As data is received from the NSURLConnection I write it to the output stream. It appears when the amount of data received is larger than the buffer size the left over data is lost. Is this supposed to happen? From my understanding I thought the data would be queued and written in chunks the size of the buffer to the input stream.
CFStreamCreateBoundPair Reference

You need to ensure that all data from the received chunk is eventually written into the stream.
You might do this with a simple loop in the delegate method where you continuously write a portion of the received chunk until it is completely written into the stream. However, this may cause the thread where the delegate is running to block undeterminably: if the consumer is not ready to consume more bytes, the output stream will block when attempting to write more data.
Alternatively, you might dispatch the NSData object asynchronously to a queue where a block is doing the loop and writes all the data before it completes. However, this may cause your system to run out of memory if the consumer is slow and the data is large - since all NSData live on the dispatch queue until the block finished.
Both approaches have pros and cons. I tend to prefer the first, since there is no memory issue and the connection will buffer the incoming bytes up to a certain upper limit anyway - before it stops acknowledging more bytes.

Related

Clear WebRTC Data Channel queue

I have been trying to use WebRTC Data Channel for a game, however, I am unable to consistently send live player data without hitting the queue size limit (8KB) after 50-70 secs of playing.
Sine the data is required to be real-time, I have no use for data that comes out of order. I have initialized the data channel with the following attributes:
negotiated: true,
id: id,
ordered: true,
maxRetransmits: 0,
maxPacketLifetime: 66
The MDN Docs said that the buffer cannot be altered in any way.
Is there anyway I can consistently send data without exceeding the buffer space? I don't mind purging the buffer space as it only contains data that has been clogged up over time.
NOTE: The data is transmitting until the buffer size exceeds the 8KB space.
EDIT: I forgot to add that this issue is only occurring when the two sides are on different networks. When both are within the same LAN, there is no buffering (since higher bandwidth, I presume). I tried to add multiple Data Channels (8 in parallel). However, this only increased the time before the failure occurred again. All 8 buffers were full. I also tried creating a new channel each time the buffer was close to being full and switched to the new DC while closing the previous one that was full, but I found out the hard way (reading Note in MDN Docs) that the buffer space is not released immediately, rather tries to transmit all data in the buffer taking away precious bandwidth.
Thanks in advance.
The maxRetransmits value is ignored if the maxPacketLifetime value is set; thus, you've configured your channel to resend packets for up to 66ms. For your application, it is probably better to use a pure unreliable channel by setting maxPacketLifetime to 0.
As Sean said, there is no way to flush the queue. What you can do is to drop packets before sending them if the channel is congested:
if(dc.bufferedAmount > 0)
return;
dc.send(data);
Finally, you should realise that buffering may happen in the network as well as at the sender: any router can buffer packets when it is congested, and many routers have very large buffers (this is called BufferBloat). The WebRTC stack should prevent you from buffering too much data in the network, but if WebRTC's behaviour is not aggressive enough for your needs, you will need to add explicit feedback from the sender to the receiver in order to avoid having too many packets in flight.
I don't believe you can flush the outbound buffer, you will probably need to watch the bufferedAmount and adjust what you are sending if it grows.
Maybe handle the retransmissions yourselves and discard old data if needed? WebRTC doesn't surface the SACKs from SCTP. So I think you will need to implement something yourself.
It's an interesting problem. Would love to hear the WebRTC W3C WorkGroup takes on it if exposing more info would make things easier for you.

What does the hasSpaceAvailable property on NSOutputStream mean?

I'm trying to wrap my head around the logic behind hasSpaceAvailable on NSOutputStream.
In my app, I'm sending large amounts of data (100MB) broken up into 4080byte chunks (hard limit) over a CFSocket managed by NSInput/output streams.
When I start writing the data, about a quarter way through hasSpaceAvailable suddenly becomes NO, and so I add the data to a queue. However, if I ignore that and try to write the data anyways, the write seems to work as the return value of write:maxLength: matches the maxLength parameter (4080).
What does the output stream have space for? As far as I can tell, when using UNIX/Berkley sockets there is no logic available to determine if the socket can be written to, you just write and determine if all of the data was written.
The documentation for the property states:
A boolean value that indicates whether the receiver can be written to. (read-only)
YES if the receiver can be written to or if a write must be attempted in order to determine if space is available, NO otherwise.
In my example where I'm seeing a NO, what factor is causing this result when I can still write to that socket.
I think the hasSpaceAvailable property just returns YES if the stream has sent a "space available" stream event since the last time you called the write method. You shouldn't poll that property, and it arguably shouldn't even exist. Instead, you should wait for a stream event on the output stream that says that there's space available for writing instead.
When that stream event occurs, it means that the outgoing packet queue has at least one byte fewer than the maximum number of bytes that the socket is configured to allow you to queue up. In other words, a send() or write() system call on the socket is guaranteed to write at least one byte without blocking, and the socket is guaranteed to be in a nonblocking mode.
Note that after you write data, the stream will send another space available event immediately if the stream's buffer can take more data (or after it has sent some data if the buffer is full).

Recording Audio on iPhone and Sending Over Network with NSOutputStream

I am writing an iPhone application that needs to record audio from the built-in microphone and then send that audio data to a server for processing.
The application uses a socket connection to connect to the server and Audio Queue Services to do the recording. What I am unsure of is when to actually send the data. Audio Queue Services fires a callback each time it has filled a buffer with some audio data. NSOutputStream fires an event each time it has space available.
My first thought was to send the data to the server on the Audio Queue callback. But it seems like this would run into a problem if the NSOutputStream does not have space available at that time.
Then I thought about buffering the data as it comes back from the Audio Queue and sending some each time the NSOutputStream fires a space available event. But this would seem to have a problem if the sending to the server gets ahead of the audio recording then there will be a situation where there is nothing to write on the space available event, so the event will not be fired again and the data transfer will effectivly be stalled.
So what is the best way to handle this? Should I have a timer that fires repeatedly and see if there is space available and there is data that needs to be sent? Also, I think I will need to do some thread synchronization so that I can take chunks of data out of my buffer to send across the network but also add chunks of data to the buffer as the recording proceeds without risking mangling my buffer.
You could use a ring buffer to hold a certain number of audio frames and drop frames if the buffer exceeds a certain size. When your stream-has-space-available callback gets called, pull a frame off the ring buffer and send it.
CHDataStructures provides a few ring-buffer (which it calls “circular buffer”) classes.

Can I use a smaller buffer for NSFileHandleReadCompletionNotification?

I'm reading NSFileHandleReadCompletionNotification messages from the NSNotificationCenter to receive messages from an NSTask. The problem is that the command line program I'm calling is relatively slow to output lines, and it seems that the NSFileHandleReadCompletionNotification message gets posted relatively infrequently (I guess when the buffer fills up).
Is there another Notification message I can use that would post a notification or every line, or is there a way to make the buffer smaller?
Edit: To be clear, I read that the buffer size is "imited to the buffer size of the underlying operating system" in the NSFileHandle documentation, so, I'm hoping there's some other trick.
If you read from NSFileHandle by
- (void)readInBackgroundAndNotify
method (right?), and parse data in the NSFileHandleReadCompletionNotification handler - so, the buffer size isn't limited - all "availableData" will be read in the background before you receive the notification, then you call the readInBackgroundAndNotify again to fetch next portion...
However I believe your issue happens due to well known I/O buffering technology.
You should turn buffering Off at the task-command side.
Example, if you call a Perl script, just put the line
$|=1;
or
use IO::Handle;
STDOUT->autoflush(1);
STDERR->autoflush(1);
near to the top of the script.
For a C-program - use setvbuf function to set buffer size to zero.

How to signal an NSStreamEventEndEncountered on an NSInputStream reading from NSMutableData

In Cocoa, I've setup two NSThreads, one producer and one consumer.
The producer appends data to an NSMutableData, and the receiver opens an NSInputStream from that data and reads in chunks.
The producer thread writes a lot faster than the consumer processes, which is OK. But the producer only produces a finite amount of work, then exits. I'd like for the consumer thread to handle the NSStreamEventEndEncountered event. How can I signal this when the NSInputStream comes from an NSMutableData?
Should I just make the producer send a series of bytes with a magical number signifying an end of stream?
The producer appends data to an NSMutableData, and the receiver opens an NSInputStream from that data and reads in chunks.
That won't work. From the -initWithData: documentation:
The contents of data are copied.
Meaning, it will copy out whatever's in the data object at that time, not follow the contents of the data object as you add more data to it.
(The same goes for +inputStreamWithData:.)
Try a pipe instead.