What do a benefit from changing from blocking to non-blocking sockets?_问答_开发者

We have an application server developed with Delphi 2010 and Indy 10. This server receives more than 50 requ开发者_开发知识库ests per second and it works well. But in some cases, it seems to me that Indy is very obscure. Their components are good, but sometimes I found myself digging into the source code only to understand a simple thing. Indy lacks on good documentation and good support.

The last thing that i came across was a big problem for me: I must detect when a client disconnects non gracefully (When the the client crashes or shutdown, for instance. Not telling the server that it will disconnect) and indy was not able to do that. If I want that, I will have to develop a algorithm like heartbeat, pooling or TCP keep-alive. I do not want to spend more time doing a, at least I think, component job. After a few study, I found out that this is not Indy's fault, but this is an issue of all blocking sockets components.

Now I am really thinking of changing the core of the Server to another good suite. I must admit I am tending to use a non-blocking socket. Based on that, I have some questions:

What do a benefit from changing from blocking to non-blocking sockets?
Will I be able to detect client disconnects (non gracefully)?
What component suite has the best product? By best product I mean: fast, good support, good tools and easy to implement.

I know this must be a subjective question, but I really want to hear that from you. My first question is the one I care most. I do not care if I have to pay 100, 500, 1000, 10000 dollars, but I want a complete solution. For now, I am thinking about Ip*works .

EDIT

I think some guys are not understand what I want. I don't want to create my own socket. I have been working with sockets for a long time and I am getting tired of it. Really.

And non-blocking sockets CAN detect client disconnects. That is a fact and it has good documentation all over the internet. A non-blocking socket checks the socket state for new incoming data all the time, and it makes possible to detect that the socket is not valid. This is not a heartbeat algorithm. A heartbeat algorithm is used on client side and it sends periodically packets (aka keep-alive) to the server to tells it is still alive.

EDIT

I am not make myself clear. Maybe because English is not my main language. I am not saying that it is possible to detect a dropped connection without trying to send or receiving data from a socket. What I am saying is that every non-blocking socket is able to do that because they constantly tries to read from the socket for new incoming data. Why is that so hard to understand? If you guys download and run ip*works demos, in special, the echoserver and echoclient ones (both use TCP) you can test by yourselves. I already tested it, and it works like I expected to do. Even if you use the old TCPSocketServer and TCPSocketClient in a non-blocking mode you will see what I meant.

"What do a benefit from changing from blocking to non-blocking sockets?
Will I be able to detect client disconnects (non gracefully)?"

Just my two cents to get the ball rolling on this question - I'm not a socket EXPERT, but I do have a good deal of experience with them. If I'm mistaken, I'm sure someone will correct me... :-)

I assume that since you're running a server using blocking sockets with 50 connections per second, you have a threading mechanism in place to handle client requests. If so, you don't really stand to gain anything from non-blocking sockets. On the contrary - you will have to change your server logic to be event driven- based on events fired in your main thread from the non-blocking sockets, or use constant polling to know what your sockets are up to.

Non-blocking sockets can't detect clients disconnecting without notification any more than blocking sockets can - they don't have telepathic powers... The nature of the TCP/IP 'conversation' between client and server is the same - blocking and non-blocking is only with respect to your application's interaction with the socket connection conducting the 'conversation'.

If you need to purge dead connections, you need to implement a heartbeat or timeout mechanism on your socket (I've never seen a modern socket implementation that didn't support timeouts).

What do a benefit from changing from blocking to non-blocking sockets?

Increased speed, availability, and throughput (from my experience). I had an IndySockets client that was getting about 15 requests per second and when I went directly to asynchronous sockets the throughput increased to about 90 requests per second (on the same machine). In a separate benchmark test on a server at a data-center with a 30 Mbit connection I was able to get more than 300 requests per second.

Will I be able to detect client disconnects (non gracefully)?

That's one thing I haven't had to try yet, since all of my code has been on the client side.

What component suite has the best product? By best product I mean: fast, good support, good tools and easy to implement.

You can build your own socket client in a couple of days and it can be very robust and fast... much faster than most of the stuff I've seen "off the shelf". Feel free to take a look at my asynchronous socket client: http://codesprout.blogspot.com/2011/04/asynchronous-http-client.html

Update:
(Per Mikey's comments)

I'm asking you for a generic, technical explanation of how NBS increase throughput as opposed to a properly designed BS server.

Let's take a high load server as an example: say your server is supposed to handle 1000 connections at any given time, with blocking sockets you would have to create 1000 threads and even if they're mostly idle, the CPU will still spend a lot of time context switching. As the number of clients increases you will have to increase the number of threads in order to keep up and the CPU will inevitably increase the context switching. For every connection you establish with a blocking socket, you will incur the overhead of spawning of a new thread and you eventually you will incur the overhead of cleaning up after the thread. Of course, the first thing that comes to mind is: why not use the ThreadPool, you can reuse the threads and reduce the overhead of creating/cleaning-up of threads.

Here is how this is handled on Windows (hence the .NET connection): sure you could, but the first thing you'll notice with the .NET ThreadPool is that it has two types of threads and it's not a coincidence: user threads and I/O completion port threads. Asynchronous sockets use the IO completion ports which "allows a single thread to perform simultaneous I/O operations on different handles, or even simultaneous read and write operations on the same handle."(1) The I/O completion port threads are specifically designed to handle I/O in a much more efficient way than you would ever be able to achieve if you used the user threads in ThreadPool, unless you wrote your own kernel-mode driver.

"The completion port uses some special voodoo to make sure only a specific number of threads can run at once — if one thread blocks in kernel-mode, it will automatically start up another one."(2)

There are other advantages also: "in addition to the nonblocking advantage of the overlapped socket I/O, the other advantage is better performance because you save a buffer copy between the TCP stack buffer and the user buffer for each I/O call." (3)

I am using Indy and Synapse TCP libraries with good results for some years now, and did not find any showstoppers in them. I use the libraries in threads - client and server side, stability and performance was not a problem. (Six thousand request and response messages per second and more with the server running on the same system are typical.)

Blocking sockets are very useful if the protocol is more advanced than a simple 'send a string / receive a string'. Non-blocking sockets cause a higher coupling of message protocol handlers with the socket read / write logic, so I quickly moved away from non-blocking code.

No library can overcome the limitations of the TCP/IP protocol regarding detection of connection loss. Only trying to read or send data can tell wether the connection is still present.

In Windows, there is a third option which is overlapped I/O. Non-blocking sockets are essential a model using Windows messages developed to avoid single-threaded GUI apps to become "blocked" while waiting for data. A modern application IMHO would be better designed using threads and overlapped I/O.

See for example http://support.microsoft.com/kb/181611

Aahhrrgghh - the myth of being able to always detect "dropped" connections. If you pull the power on a machine with a client connection then the server cannot tell, without sending data, that the connection is "dead". The is through the design of the TCP protocol. Don't take my word for it - read this article (Detection of Half-Open (Dropped) TCP/IP Socket Connections).

This article explains the main differences between blocking and non-blocking:

Introduction to Indy, by Chad Z. Hower

Pros of Blocking

Easy to program - Blocking is very easy to program. All user code can exist in one place, and in a sequential order.

Easy to port to Unix - Since Unix uses blocking sockets, portable code can be written easily. Indy uses this fact to achieve its single source solution.

Work well in threads - Since blocking sockets are sequential they are inherently encapsulated and therefore very easily used in threads.

Cons of Blocking

User Interface "Freeze" with clients - Blocking socket calls do not return until they have accomplished their task. When such calls are made in the main thread of an application, the application cannot process the user interface messages. This causes the User Interface to "freeze" because the update, repaint and other messages cannot be processed until the blocking socket calls return control to the applications message processing loop.

He also wrote:

Blocking is NOT Evil

Blocking sockets have been repeatedly attacked with out warrant. Contrary to popular belief, blocking sockets are not evil.

It is not is an issue of all blocking sockets components that they are unable to detect a client disconnect. There is no technical advantage on the side of non-blocking components in this area.