Implements a caller/client for the req node of the req/rep protocol. Sends data to the rep node (executor/server) and returns an Aio, which can be called for the value when required.
Arguments
- context
a Context.
- data
an object (if send_mode = ‘raw’, a vector).
- send_mode
[default 'serial'] character value or integer equivalent - either ‘serial’ (1L) to send serialised R objects, or ‘raw’ (2L) to send atomic vectors of any type as a raw byte vector.
- recv_mode
[default 'serial'] character value or integer equivalent - one of ‘serial’ (1L), ‘character’ (2L), ‘complex’ (3L), ‘double’ (4L), ‘integer’ (5L), ‘logical’ (6L), ‘numeric’ (7L), ‘raw’ (8L), or ‘string’ (9L). The default ‘serial’ means a serialised R object; for the other modes, received bytes are converted into the respective mode. ‘string’ is a faster option for length one character vectors.
- timeout
[default NULL] integer value in milliseconds or NULL, which applies a socket-specific default, usually the same as no timeout.
- cv
(optional) a ‘conditionVariable’ to signal when the async receive is complete, or NULL. If any other value is supplied, this will cause the pipe connection to be dropped when the async receive is complete.
Details
Sending the request and receiving the result are both performed
async, hence the function will return immediately with a ‘recvAio’
object. Access the return value at $data.
This is designed so that the process on the server can run concurrently without blocking the client.
Optionally use call_aio on the ‘recvAio’ to call
(and wait for) the result.
If an error occured in the server process, a nul byte 00 will be
received. This allows an error to be easily distinguished from a NULL
return value. is_nul_byte can be used to test for a nul
byte.
It is recommended to use a new context for each request to ensure consistent state tracking. For safety, the context used for the request is closed when all references to the returned ‘recvAio’ are removed and the object is garbage collected.
Send Modes
The default mode ‘serial’ sends serialised R objects to ensure
perfect reproducibility within R. When receiving, the corresponding mode
‘serial’ should be used. Custom serialization and unserialization
functions for reference objects may be enabled by the function
next_config.
Mode ‘raw’ sends atomic vectors of any type as a raw byte vector, and must be used when interfacing with external applications or raw system sockets, where R serialization is not in use. When receiving, the mode corresponding to the vector sent should be used.
Signalling
By supplying a ‘conditionVariable’, when the receive is complete, the ‘conditionVariable’ is signalled by incrementing its value by 1. This happens asynchronously and independently of the R execution thread.
Examples
if (FALSE) { # \dontrun{
# works if req and rep are running in parallel in different processes
req <- socket("req", listen = "tcp://127.0.0.1:6546")
rep <- socket("rep", dial = "tcp://127.0.0.1:6546")
reply(.context(rep), execute = function(x) x + 1, timeout = 50)
aio <- request(.context(req), data = 2022)
aio$data
close(req)
close(rep)
# Signalling a condition variable
req <- socket("req", listen = "tcp://127.0.0.1:6546")
ctxq <- context(req)
cv <- cv()
aio <- request(ctxq, data = 2022, cv = cv)
until(cv, 10L)
close(req)
# The following should be run in another process
rep <- socket("rep", dial = "tcp://127.0.0.1:6546")
ctxp <- context(rep)
reply(ctxp, execute = function(x) x + 1)
close(rep)
} # }