Inspect a Backend

This function can be used to check the names of the variables present on a backend created by start_backend().

Usage

peek(backend)

Arguments

backend

An object of class Backend as returned by the start_backend() function.

Value

The function returns a list of character vectors, where each list element corresponds to a node, and each element of the character vector is the name of a variable present on that node. 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 peek method on the provided backend instance.

See also

start_backend(), export(), evaluate(), clear(), configure_bar(), par_sapply(), par_lapply(), par_apply(), stop_backend(), and BackendService.

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