[R-pkg-devel] socketConnection, delay when reading from

[Duncan Murdoch]

On 27/11/2021 4:09 a.m., Ben Engbers wrote:
> 
> Hi,
> 
> I have been working on a R-client for the BaseX XML-database and version
> 0.9.2 is nearly finished (submitting version 0.9.0 was rejected by CRAN).
> Version 0.3 of RBaseX can be found here
> (https://cran.microsoft.com/web/packages/RBaseX/index.html).
> 
> The client-server protocol specifies that the communication between the
> client and the database is based on a socket. The code (below) shows how
> I create that socket.
> 
> Writing to the socket works perfect. Reading from the sockets (see
> second codeblock) also produces correct results. The problem however is
> that the timeout, as specified when initializing the socket, causes a 1
> second delay for every read-operation.
> I have experimented a lot with different settings and have been
> searching a lot on internet, but I can't find any method to get rid of
> that delay. (In C or C++ that should be easier but I have never before
> had any need to use those languages).
> The very first version of my client used a block-size of 1 when reading.
> That gave acceptable response times for small query-results but reading
> large responses from the database took very long time.
> 
> Do you have any suggestions on how to cope with this problem?

You are attempting to read 1024 bytes.  If the socket returns fewer, the 
code presumably assumes it should wait for more.  You should specify the 
number to read to be no larger than you'll receive.  That's why reading 
a byte at a time works well for small reads.

To handle larger ones, you need to know what size blocks you are going 
to receive with each read.  If that varies unpredictably, I doubt if 
there's much you can do other than reducing the timeout.

Duncan Murdoch

> 
> Ben Engbers
> 
> -----------------------------
>       CreateSocket = function(host, port = 1984L, username, password) {
>         tryCatch(
>           {conn <- private$conn <- socketConnection(
>             host = "localhost", port,
>             open = "w+b", server = FALSE, blocking = TRUE, encoding =
> "UTF-8", timeout = 1)
>           }, error = function(e) {
>             stop("Cannot open the connection")
>           }
>         )
> 
> -----------------------------
> 
> readBin_ <- function(conn) {
>     chars_read <- raw(0)
>     rd <- readBin(conn, what = "raw", 1024)
>     while(length(rd) == 1024) {
>       chars_read <- c(chars_read, rd)
>       rd <- readBin(conn, "raw", 1024)
>       }
>     if (length(rd) > 0) chars_read <- c(chars_read, rd)
>     return(chars_read)
> }
> 
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