Java NIO: How to know when SocketChannel read() is complete with non-blocking I/O?

If it can keep sending bytes even after a read() has returned 0 bytes (after previous successful reads) then I have no idea how to tell when the server is done talking to me and in-fact am confused how java.io. * style communications would even know when the server is "done" either. Read and follow the protocol: redis.io/topics/protocol The spec describes the possible types of replies and how to recognize them.

Some are line terminated, while multi-line responses include a prefix count. Replies Redis will reply to commands with different kinds of replies. It is possible to check the kind of reply from the first byte sent by the server: With a single line reply the first byte of the reply will be "+" With an error message the first byte of the reply will be "-" With an integer number the first byte of the reply will be ":" With bulk reply the first byte of the reply will be "$" With multi-bulk reply the first byte of the reply will be "*" Single line reply A single line reply is in the form of a single line string starting with "+" terminated by "\r\n".... ... Multi-bulk replies Commands like LRANGE need to return multiple values (every element of the list is a value, and LRANGE needs to return more than a single element).

This is accomplished using multiple bulk writes, prefixed by an initial line indicating how many bulk writes will follow. Is it ever possible for read() to return bytes, then on a subsequent call return no bytes, but on another subsequent call again return some bytes? Basically, can I trust that the server is done responding to me if I have received at least 1 byte and eventually read() returns 0 then I know I'm done, or is it possible the server was just busy and might sputter back some more bytes if I wait and keep trying?

Yes, that's possible. Its not just due to the server being busy, but network congestion and downed routes can cause data to "pause". The data is a stream that can "pause" anywhere in the stream without relation to the application protocol.

Keep reading the stream into a buffer. Peek at the first character to determine what type of response to expect. Examine the buffer after each successful read until the buffer contains the full message according to the specification.

I originally thought a blocking SocketChannel would be the way to go with this as I don't really want a caller to do anything until I process their command and give them back a reply anyway. I think you're right. Based on my quick-look at the spec, blocking reads wouldn't work for this protocol.

Since it looks line-based, BufferedReader may help, but you still need to know how to recognize when the response is complete.

1 Bert, exactly right; for some reason I got it stuck in my head that I would "read until done" then convert the whole mess to char's then try and determine the reply type based on the protocol (I have it printed here in front of me all marked up). It wasn't until Erick's reply above that I realized my code had to be cognisant of the bytes it was reading and bounding values which leads me peek at the bytes and follow the spec. Thank you.

– Riyad Kalla Feb 7 at 21:34.

Look to your Redis specifications for this answer. It's not against the rules for a call to .read() to return 0 bytes on one call and 1 or more bytes on a subsequent call. This is perfectly legal.

If anything were to cause a delay in delivery, either because of network lag or slowness in the Redis server, this could happen. The answer you seek is the same answer to the question: "If I connected manually to the Redis server and sent a command, how could I know when it was done sending the response to me so that I can send another command? " The answer must be found in the Redis specification.

If there's not a global token that the server sends when it is done executing your command, then this may be implemented on a command-by-command basis. If the Redis specifications do not allow for this, then this is a fault in the Redis specifications. They should tell you how to tell when they have sent all their data.

This is why shells have command prompts. Redis should have an equivalent. In the case that Redis does not have this in their specifications, then I would suggest putting in some sort of timer functionality.

Code your thread handling the socket to signal that a command is completed after no data has been received for a designated period of time, like five seconds. Choose a period of time that is significantly longer than the longest command takes to execute on the server.

Since the Redis protocol actually does specify delimiters or message lengths, you should really use those. This is the best way to create a robust solution. If you're looking for a quick hack, you may be able to get away with the timer functionality I mentioned.

I wouldn't recommend it, however, since the answer is provided in the Regis protocol. – Erick Robertson Feb 7 at 21:12 1 Erick, you have saved what little hair I have left. It sounds like I was misunderstanding responsibilities in this code; attributing behaviors to SocketChannel that didn't exist.

I'll take a hack at looking for the delimiting information in the reply and causing the connection to respond appropriately. Thank you for the help. – Riyad Kalla Feb 7 at 21:16.

I am using a standard while(socket.read() > 0) {//append bytes} loop That is not a standard technique in NIO. You must store the result of the read in a variable, and test it for (a) -1, indicating EOS, meaning you should close the channel, (b) zero, meaning there was no data to read, meaning you should return to the select() loop, and (c) a positive value, meaning you have read that many bytes, which you should then extract and remove from the ByteBuffer (get()/compact()) before continuing.

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