Multiply a single band raster by a vector in parallel in R terra

I would like to perform the following vector*raster multiplication in parallel, since my vector is very long:

library(terra)

# Create a raster example

set.seed(123)

raster_example <- rast(nrow = 10, ncol = 10, vals = rnorm(100))

# Create a vector example
vector_ejemplo <- c(2, 3, 4, 5, 6, 7, 8, 9, 10, 11)

# Perform multiplication 
result_no_parallel <- vector_ejemplo * raster_example


print(result_no_parallel)

I tried with terra::app, but I am getting errors only.

Is there a way to do this?

Solution

Rast object cannot be serialized (have a look here and related links). But you can send names or paths to nodes, or you can wrap your raster to send it to nodes. Here is a toy example with your data. Note that this example has no time gain since the overheads of having multiple nodes are more significant than the speed gain. Since multiplication is a C++ function in terra optimized for speed and memory usage, I imagine your raster is enormous if you need to parallelize such a simple operation.

library(terra)
library(foreach)
library(doParallel)
# Create a raster example

set.seed(123)

raster_example <- rast(nrow = 10, ncol = 10, vals = rnorm(100))
path <-file.path(tempdir(),"ejemplo.tif")
writeRaster(raster_example,path)

# Create a vector example
vector_ejemplo <- c(2, 3, 4, 5, 6, 7, 8, 9, 10, 11)



f <- function(path,n) {
  x <- rast(path)
  y <- x*n
  wrap(y)
}

cl <- makeCluster(10)
registerDoParallel(cl)

result <- foreach(n=vector_ejemplo,.packages="terra")%dopar%{
  f(path,n)
}

stopCluster(cl)
x <- do.call(c,lapply(result, unwrap))

See ?wrap for more details on this beautiful function that makes parallelization with rast objects possible.