# Developing Codelets in C++¶

The goal of this tutorial is to develop a codelet in C++ that is effectively a machine that goes “ping”. For this tutorial, no external dependencies or special hardware is required.

Start by creating a new package by creating a folder in the isaac/packages directory with a command similar to the following:

bob@desktop:~/isaac/packages/$mkdir ping  For the rest of the tutorial whenever you are asked to create a new file place it directly into this folder. More complicated packages will have subfolders but for this tutorial things are kept simple. Every Isaac application is based on a JSON file. The JSON file describes the dependencies of the application, the node graph and the message flow, and contains custom configuration. Create a new JSON file called ping.app.json and specify its name as seen in the following snippet: { "name": "ping" }  Next, create a bazel build file for compiling and running the application. Bazel provides very good dependency management and excellent build speed for large projects, and bazel build files are very easy to write. Create a file BUILD with a new app target with name ping, as shown below: load("//engine/build:isaac.bzl", "isaac_app", "isaac_cc_module") isaac_app( name = "ping" )  Now you can build the application by running the following command in the ping directory. From now on when you are asked to execute a command, execute it in the ping directory. bob@desktop:~/isaac/packages/ping$ bazel build ping


The command may take some time as all external dependencies of the Isaac Robot Engine are downloaded and compiled. After a while the first build should have succeeded with output similar to the following:

bob@desktop:~/isaac/packages/ping$bazel build ping Starting local Bazel server and connecting to it... INFO: Analysed target //packages/ping:ping (54 packages loaded, 2821 targets configured). INFO: Found 1 target... Target //packages/ping:ping up-to-date: bazel-genfiles/packages/ping/run_ping bazel-bin/ping/packages/ping INFO: Elapsed time: 112.170s, Critical Path: 30.14s, Remote (0.00% of the time): [queue: 0.00%, setup: 0.00%, process: 0.00%] INFO: 691 processes: 662 linux-sandbox, 29 local. INFO: Build completed successfully, 1187 total actions  Next you can run your new application by executing the following command: bob@desktop:~/isaac/packages/ping$ bazel run ping


This will start the ping application and keep it running. You can stop a running application by pressing Ctrl+C in a console. This will gracefully shut down the application. You will notice that not much is happening, because we don’t have an application graph yet. Next we will create some nodes for the application.

## Creating a Node¶

An Isaac application consists of many nodes running in parallel. They can send each other messages or interact with each other using various other mechanisms provided by the Isaac Robot Engine. Nodes are light-weight and do not require their own processes, or even their own threads.

To customize the behavior of the ping node, we have to equip it with components. We will create our own component called Ping. Create a new file Ping.hpp in the ping directory, with the following contents:

#pragma once
#include "engine/alice/alice_codelet.hpp"
class Ping : public isaac::alice::Codelet {
public:
void start() override;
void tick() override;
void stop() override;
};
ISAAC_ALICE_REGISTER_CODELET(Ping);


Codelets provide three main functions which can be overloaded: start, tick and stop. When a node is started the start functions of all attached codelets are called first. For example, start is a good place to allocate resources. You can configure a codelet to tick periodically or each time a new message is received. Most of the functionality is then performed by the tick function.

At the end when a node stops the stop function is called. You should free all previously allocated resources in the stop function. Do not use constructors or destructors. You do not have access to any of the Isaac Robot Engine functionality such as configuration in the constructor.

Each custom codelet you create needs to be registered with Isaac Robot Engine. This is done at the end of the file using the ISAAC_ALICE_REGISTER_CODELET macro. In case your codelet is inside a namespace you have to provide the fully qualified type name, for example ISAAC_ALICE_REGISTER_CODELET(foo::bar::MyCodelet).

To add some functionality to the codelet, create a source file called Ping.cpp which contains this functionality:

#include "Ping.hpp"
void Ping::start() {}
void Ping::tick() {}
void Ping::stop() {}


Codelets can tick in multiple different ways, but for now use periodic ticking. This can be achieved by calling the tickPeriodically function in the codelet Ping::start function. Add the following code to the start function in Ping.cpp

void Ping::start() {
tickPeriodically();
}


To verify that something is in fact happening we print a message when the codelet ticks. The Isaac SDK includes utility functions for logging data. LOG_INFO can be used to print a message on the console. It follows the printf-style syntax. Add the tick function to Ping.cpp as shown below:

void Ping::tick() {
LOG_INFO("ping");
}


Add a module to BUILD as shown below:

isaac_app(
...
)

isaac_cc_module(
name = "ping_components",
srcs = ["Ping.cpp"],
hdrs = ["Ping.hpp"],
)


An Isaac module defines a shared library that encapsulates a set of codelets and can be used by different applications.

In order to use the Ping codelet in the application we first need to create a new node in the application JSON file:

{
"name": "ping",
"graph": {
"nodes": [
{
"name": "ping",
"components": []
}
],
"edges": []
}
}


Each node can contain multiple components which define its functionality. Add the Ping codelet to the node by adding a new section in the components array:

{
"name": "ping",
"graph": {
"nodes": [
{
"name": "ping",
"components": [
{
"name": "ping",
"type": "Ping"
}
]
}
],
"edges": []
}
}


An application graph normally has edges connecting different nodes, which determine the message passing sequence between different nodes. Because this application does not have any other nodes, leave the edges blank.

If you would try to run this application it would panic and show the error message Could not load component ‘Ping’. This happens because all components used in an applications must be added to the modules list. You need to do this both in the BUILD file and in the application JSON file:

load("//engine/build:isaac.bzl", "isaac_app", "isaac_cc_module")

isaac_app(
name = "ping",
modules = ["//packages/ping:ping_components"]
)

{
"name": "ping",
"modules": [
"ping:ping_components"
],
"graph": {
...
}
}


Note that the expression ping:ping_components referes to the module //package/ping:ping_components which we created previously.

If you would run the application now you would get a different panic message: Parameter ‘ping/ping/tick_period’ not found or wrong type. This message appears because we need to set the tick period of the Ping codelet in the configuration section. We will do this in the next section.

## Configuration¶

Most code requires various parameters for customizing behavior. For example, you might want to give the user of our ping machine the option to change the tick period. In the Isaac framework this can be achieved with configuration.

Let’s specify the tick period in the configuration section of the application JSON file so that we can finally run the application.

{
"name": "ping",
"modules": [
"ping:ping_components"
],
"graph": {
...
},
"config": {
"ping" : {
"ping" : {
"tick_period" : "1Hz"
}
}
}
}


Every configuration parameter is referenced by three elements: node name, component name and parameter name. In this case we are setting the parameter tick_period of the component ping in the node ping.

Now the application will run successfully and will print “ping” once a second. You should see an output similar to the snippet below. You can gracefully stop the application by pressing Ctrl+C.

bob@desktop:~/isaac/packages/ping$bazel run ping 2019-03-24 17:09:39.726 DEBUG engine/alice/backend/codelet_backend.cpp@61: Starting codelet 'ping/ping' ... 2019-03-24 17:09:39.726 DEBUG engine/alice/backend/codelet_backend.cpp@73: Starting codelet 'ping/ping' DONE 2019-03-24 17:09:39.726 DEBUG engine/alice/backend/codelet_backend.cpp@291: Starting job for codelet 'ping/ping' 2019-03-24 17:09:39.726 INFO packages/ping/Ping.cpp@8: ping 2019-03-24 17:09:40.727 INFO packages/ping/Ping.cpp@8: ping 2019-03-24 17:09:41.726 INFO packages/ping/Ping.cpp@8: ping  The tick_period parameter is automatically created for us, but we can also create our own parameters to customize the behavior of codelets. Add a parameter to your codelet as shown below: class Ping : public isaac::alice::Codelet { public: void start() override; void tick() override; void stop() override; ISAAC_PARAM(std::string, message, "Hello World!"); };  ISAAC_PARAM takes three arguments. First is the type of the parameter. Most often this is either double, int, bool, or std::string. The second argument is the name of our parameter. The name is used to access or specify the parameter. The third argument is the default value to use for this parameter. If there is no default value given and the parameter is not specified via a configuration file, the program asserts when the parameter is accessed. The ISAAC_PARAM macro creates an accessor called get_message and a bit more code to properly connect the parameter with the rest of the system. We can use the parameter now in the tick function instead of the hard-coded value: void tick() { LOG_INFO(get_message().c_str()); }  The next step is to add the configuration for the node. The config parameter uses node names, component names, and the parameter name to specify desired values. { "name": "ping", "modules": [ "ping:ping_components" ], "graph": { ... }, "config": { "ping" : { "ping" : { "message": "My own hello world!", "tick_period" : "1Hz" } } } }  That’s it! You now have an application that can periodically print a custom message. Run the application with the following command: bob@desktop:~/isaac/packages/ping$ bazel run ping


As expected, the codelet prints the message periodically on the command line.

## Sending Messages¶

The custom codelet Ping is happily ticking. In order for other nodes to react to the ping, the Ping codelet must send a message which other codelets can receive.

Publishing a message is easy. Use the ISAAC_PROTO_TX macro to specify that a codelet is publishing a message. Add it to Ping.hpp as shown below:

 #pragma once

#include "engine/alice/alice.hpp"
#include "messages/messages.hpp"

class Ping : public isaac::alice::Codelet {
public:
...

ISAAC_PARAM(std::string, message, "Hello World!");
ISAAC_PROTO_TX(PingProto, ping);
};

ISAAC_ALICE_REGISTER_CODELET(Ping);


The ISAAC_PROTO_TX macro takes two arguments. The first one specifies the message to publish. Here, use the PingProto message which comes as part of the Isaac message API. Access PingProto by including the corresponding header. The second argument specifies the name of the channel under which we want to publish the message.

Next, change the tick function to publish a message instead of printing to the console. The Isaac SDK currently supports cap’n’proto messages. Protos are a platform and language independent way of representing and serializing data. Creating a message is initiated by calling the initProto function on the accessor which the ISAAC_PROTO_TX macro created. This function returns a cap’n’proto builder object which can be used to write data directly to the proto.

The ProtoPing message has a field called message of type string, so in this instance we can use the setMessage function to write some text to the proto. After the proto is populated we can send the message by calling the publish function. This immediately sends the message to any connected receivers. Change the tick() function in Ping.cpp to the following:

...
void Ping::tick() {
// create and publish a ping message
auto proto = tx_ping().initProto();
proto.setMessage(get_message());
tx_ping().publish();
}
...


Lastly, upgrade the node (in the JSON file) to support message passing. Nodes in the Isaac SDK are by default light-weight objects requiring minimal setup of mandatory components. Not necessarily every node in your applications publish or receive messages. To enable message passing on a node we need to add a component called MessageLedger. This component handles incoming and outgoing messages and relays them to MessageLedger components in other nodes.

{
"name": "ping",
"graph": {
"nodes": [
{
"name": "ping",
"components": [
{
"name": "message_ledger",
"type": "isaac::alice::MessageLedger"
},
{
"name": "ping",
"type": "Ping"
}
]
}
],
"edges": []
},
"config": {
...
}


Build and run the application. It appears that nothing happens, because right now nothing is connected to your channel. While you are publishing a message, no one is there to receive it and react to it. You will fix that in the next section.

## Receiving Messages¶

You need a node which can receive the ping message and react to it in some way. For this purpose let us create a Pong codelet which gets triggered by the message sent by Ping. Create a new file Pong.hpp with the following contents:

#pragma once
#include "engine/alice/alice.hpp"
#include "messages/messages.hpp"

class Pong : public isaac::alice::Codelet {
public:
void start() override;
void tick() override;

// An incoming message channel on which we receive pings.
ISAAC_PROTO_RX(PingProto, trigger);

// Specifies how many times we print 'PONG' when we are triggered
ISAAC_PARAM(int, count, 3);
};

ISAAC_ALICE_REGISTER_CODELET(Pong);


The Pong codelets need to be added to the ping_components module in order to be compiled. Add it to the BUILD file as shown below:

isaac_cc_module(
name = "ping_components",
srcs = [
"Ping.cpp",
"Pong.cpp"
],
hdrs = [
"Ping.hpp",
"Pong.hpp"
],
)


In the application JSON, create a second node and attach the new Pong codelet to it. Connect the Ping and the Pong nodes via the edges:

{
"name": "ping",
"modules": [
"ping:ping_components"
],
"graph": {
"nodes": [
{
"name": "ping",
"components": [
{
"name": "message_ledger",
"type": "isaac::alice::MessageLedger"
},
{
"name": "ping",
"type": "Ping"
}
]
},
{
"name": "pong",
"components": [
{
"name": "message_ledger",
"type": "isaac::alice::MessageLedger"
},
{
"name": "pong",
"type": "Pong"
}
]
}
],
"edges": [
{
"source": "ping/ping/ping",
"target": "pong/pong/trigger"
}
]
},
"config": {
"ping" : {
"ping" : {
"message": "My own hello world!",
"tick_period" : "1Hz"
}
}
}
}


Edges are connecting receiving RX channels to transmitting TX channels. A transmitting channel can transmit data to multiple receivers. A receiving channel can also receive data from multiple transmitters, however this comes with caveats and is discouraged. Similar to parameters, channels are referenced by three elements: node name, component name and channel name. An edge can be created by adding it to the “edges” section in the application JSON file. Here source is the full name of the transmitting channel and target is the full name of the receiving channel.

The last remaining task is to set up the Pong codelet to do something when it receives the ping. Create a new file Pong.cpp. Call the tickOnMessage function in start to instruct the codelet to tick each time it receives a new message on that channel. In tick we add the functionality to print out “PONG!” as many number of times as defined by the “count” parameter in Pong’s header file:

#include "Pong.hpp"

#include <cstdio>

void Pong::start() {
tickOnMessage(rx_trigger());
}

void Pong::tick() {
// Parse the message we received
auto proto = rx_trigger().getProto();
const std::string message = proto.getMessage();

// Print the desired number of 'PONG!' to the console
const int num_beeps = get_count();
std::printf("%s:", message.c_str());
for (int i = 0; i < num_beeps; i++) {
std::printf(" PONG!");
}
if (num_beeps > 0) {
std::printf("\n");
}
}


By using tickOnMessage instead of tickPeriodically we instruct the codelet to only tick when a new message is received on the incoming data channel, in this case trigger. The tick function now only execute whenever you receive a new message. This is guaranteed by the Isaac Robot Engine.

Run the application. You should see how a “pong” is generated every time the Pong codelet receives a ping message from the Ping codelet. By changing the parameters in the configuration file you can change the interval at which a ping is created, change the message which is sent together with each ping, and print pong more or less often whenever a ping is received.

This is just quick start with a very simple application. A real-world application consists of dozens of nodes, each with multiple components and most with one or more codelets. Codelets receive multiple types of messages, call specialized libraries to solve hard computational problems, and publish their results again to be consumed by other nodes.