Evaluate An Expression On The Backend

This function can be used to evaluate an arbitrary base::expression() a backend created by start_backend().

Usage

evaluate(backend, expression)

Arguments

backend: An object of class Backend as returned by the start_backend() function.
expression: An unquoted expression to evaluate on the backend.

Value

This method returns the result of the expression evaluation. It throws an error if the value provided for the backend argument is not an instance of class Backend.

Details

This function is a convenience wrapper around the lower-lever API of parabar aimed at developers. More specifically, this function calls the evaluate method on the provided backend instance.

Examples

# Create an asynchronous backend.
backend <- start_backend(cores = 2, cluster_type = "psock", backend_type = "async")

# Check that the backend is active.
backend$active
#> [1] TRUE

# Check if there is anything on the backend.
peek(backend)
#> [[1]]
#> character(0)
#> 
#> [[2]]
#> character(0)
#> 

# Create a dummy variable.
name <- "parabar"

# Export the `name` variable in the current environment to the backend.
export(backend, "name", environment())

# Remove the dummy variable from the current environment.
rm(name)

# Check the backend to see that the variable has been exported.
peek(backend)
#> [[1]]
#> [1] "name"
#> 
#> [[2]]
#> [1] "name"
#> 

# Run an expression on the backend.
# Note that the symbols in the expression are resolved on the backend.
evaluate(backend, {
    # Print the name.
    print(paste0("Hello, ", name, "!"))
})
#> [[1]]
#> [1] "Hello, parabar!"
#> 
#> [[2]]
#> [1] "Hello, parabar!"
#> 

# Clear the backend.
clear(backend)

# Check that there is nothing on the backend.
peek(backend)
#> [[1]]
#> character(0)
#> 
#> [[2]]
#> character(0)
#> 

# Use a basic progress bar (i.e., see `parabar::Bar`).
configure_bar(type = "basic", style = 3)

# Run a task in parallel (i.e., approx. 1.25 seconds).
output <- par_sapply(backend, x = 1:10, fun = function(x) {
    # Sleep a bit.
    Sys.sleep(0.25)

    # Compute and return.
    return(x + 1)
})

# Print the output.
print(output)
#>  [1]  2  3  4  5  6  7  8  9 10 11

# Stop the backend.
stop_backend(backend)

# Check that the backend is not active.
backend$active
#> [1] FALSE