Question on non-blocking socket
On 2/15/23 16:44, Ben Engbers wrote:
Hi Op 15-02-2023 om 14:38 schreef Tomas Kalibera:
On 2/15/23 01:24, Ben Engbers wrote:
Hi, December 27, 2021 I started a thread asking for help troubleshooting non-blocking sockets.
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I have two questions. The first is where I can find R documentation on proper use of non-blocking sockets and on the proper use of the socketSelect function?
In addition to the demos I sent to you in that 2021 thread on R-pkg-devel, you could also have a look at how it is used in R itself, in the parallel package, in snowSOCK.R, to set up the snow cluster in parallel. Some hints may be also found in the blog post https://blog.r-project.org/2020/03/17/socket-connections-update/. But, in principle, R API is just a thin layer on top of what the OS provides, so general literature and tutorials on sockets should help, there should be even textbooks used at CS universities in networking classes.
Thanks for the suggestions!
Basically select() can tell you when data is ready (on input), when the socket interface is able to accept more data (on output) or when there is an incoming connection. In practice, you should not need any delays to be inserted in your program to make it work - if that is needed, it means that is an error in it (a race condition). If the program is polling (checking in a loop whether something has already happened, and then sleeping for a short while), the duration of the sleep may indeed influence latency, but should not affect correctness - if it does, there is an error.
In RBaseX I first calculate an MD5 hash that is send to the server and then I check the status byte that is returned by the server. writeBin(auth, private$conn) socketSelect(list(conn)) Accepted <- readBin(conn, what = "raw", n = 1) == 0 Without the second line, 'Accepted' is always FALSE. With this line it is TRUE. BaseX provides example API's in several languages. I've looked at several but indeed none uses any form of delay. All API's follow the same pattern, calculate a MD5, send it to the server and check the status byte. So the server is not likely to enforce a delay. So there is nothing left but to look for that racing condition ;-(
Without knowing more details, this looks ok. If you have a non-blocking connection, and the server produces a response based on the client request, the client has to take into account that it takes the server some time to produce the response. Right, the sockets are full duplex and so could be the communication protocol, but in this case it apparently isn't, it is request/response. Without the second line, there would be a race condition between the server sending a response and the client receiving it. With the second line, the client waits for the server before it starts receiving. In theory, one could be waiting for the response actively in a loop (polling), but socketSelect() is better. Both ways would resolve the race condition. Adding a single fixed-time wait, instead, would not remove the race condition, because one can never be sure that the server wouldn't take longer (apart from waiting too long most of the time). In the example you are waiting only for a single byte. But if the response may be longer, one needs to take into account in the client that not all bytes of the response may be available right away. One would keep receiving the data in a loop, as they become available (e.g. socketSelect() would tell), keep appending them to a buffer, and keep looking for when they are complete. Tomas
Ben