Adding Your First Feature

This guide describes how to add new functionality to arkouda. For each new feature, you need to add the client-side interface in Python and the server-side implementation in Chapel. To demonstrate this process, this guide walks through the process of implementing a custom function times2 that will multiply all elements of an array by 2.

Adding Python Functionality (Client Interface)

Python functions should follow the API of NumPy or Pandas, where possible. In general, functions should conform to the following:

  1. Be defined somewhere in the arkouda subdirectory, such as in arkouda/

  2. Have a complete docstring in (NumPy format)[]

  3. Check argument types and properties, raising exceptions if necessary

  4. Send a request message using generic_msg(request)

  5. Process the reply message

  6. Possibly create one or more pdarray objects

  7. Return any results


First, add the client-side implementation to construct the message to the server:

def times2(pda: pdarray) -> pdarray:
    Returns a pdarray with each entry double that of the input.

    pda : pdarray
        The array to double.

        The doubled array
    if isinstance(pda, pdarray):
        repMsg = generic_msg(cmd="times2", args={"arg1" : pda})
        return create_pdarray(repMsg)
        raise TypeError("times2 only supports pdarrays.")

Note that the function signature makes use of Python’s optional type checking features. You can learn more about Python type checking in (this documentation)[].

Second, add your function to the __all__ list near the top of the file.

__all__ = ["in1d", "concatenate", "union1d", "intersect1d", "setdiff1d", "setxor1d", "times2"]

Adding Functionality to the Arkouda Server

Your contribution must include all the machinery to process a command from the client, in addition to the logic of the computation. This is broken into function(s) that implement the actual operation, a function that processes the command message and calls the appropriate implementation, and code to register the message processing function with the message dispatch system. For the times2 example, we will add our functions to the ArraySetOps and ArraySetOpsMsg modules for the sake of simplicity.

When the client issues a command times2 arg1 to the arkouda server, this is what typically happens:

  1. The select block in ServerDaemon.chpl sees “times2”, looks it up in the commandMap, and calls the function responsible for processing the message: times2Msg.

  2. The times2Msg function is found via the ArraySetopsMsg module, which contains use ArraySetops and thus gets all symbols from the ArraySetops module where the implementation function times2() is defined.

  3. The times2Msg() function (in the ArraySetopsMsg module) parses and executes the command by

    1. Casting any scalar args

    2. Looking up pdarray (GenSymEntry) args in the symbol table with getGenericTypeArrayEntry

    3. Checking dtypes of arrays and branching to corresponding code

    4. Casting GenSymEntry objects to correct types with toSymEntry()

    5. Executing the operation, usually on the array data entry.a

    6. If necessary, creating new SymEntry and adding it to the symbol table with st.addEntry()

    7. Returning an appropriate message string

      1. If the return is an array, “created

      2. If the return is multiple arrays, one creation string per array, joined by “+”

      3. If the return is a scalar, “

      4. If any error occurred, then “Error: ” (see ServerErrorStrings.chpl for functions to generate common error strings)


First, define your message processing logic in src/ArraySetopsMsg.chpl in the following manner:

module ArraySetopsMsg {

    Parse, execute, and respond to a times2 message 
    :arg cmd: request command
    :type reqMsg: string 
    :arg msgArgs: request arguments
    :type msgArgs: borrowed MessageArgs
    :arg st: SymTab to act on
    :type st: borrowed SymTab 
    :returns: (MsgTuple) response message
    proc times2Msg(cmd: string, msgArgs: borrowed MessageArgs, st: borrowed SymTab): MsgTuple throws {
        var repMsg: string; // response message
        var vName = st.nextName(); // symbol table key for resulting array

        var gEnt: borrowed GenSymEntry = getGenericTypedArrayEntry(msgArgs.getValueOf("arg1"), st);

        select gEnt.dtype {
            when DType.Int64 {
                var e = toSymEntry(gEnt,int);

                var aV = times2(e.a);
                st.addEntry(vName, createSymEntry(aV));

                repMsg = "created " + st.attrib(vName);
                return new MsgTuple(repMsg, MsgType.NORMAL);
            // add additional when blocks for different data types...
            otherwise {
                var errorMsg = notImplementedError("times2",gEnt.dtype);
                return new MsgTuple(errorMsg, MsgType.ERROR);


Second, define your operation implementation in src/ArraySetops.chpl in the following manner:

module ArraySetops {

    // returns input array, doubled.
    proc times2(a: [] ?t) throws {
        var ret = a * 2; //scalar promotion
        return ret;


Finally, register your new function within the commandMap back in src/ArraySetopsMsg.chpl in the following manner:

module ArraySetopsMsg {

    use CommandMap;
    resisterFunction("times2", times2Msg, getModuleName());

Now, you should be able to rebuild and launch the server and use your new feature. We close with a client-side python script that uses the new feature.

import arkouda as ak
undoubled = ak.arange(0,10)
doubled = ak.times2(undoubled)