How to Write an OpenCog Service to SingularityNET


Before following this tutorial, make sure you’ve installed Docker

Run this tutorial from a bash terminal.

In this tutorial we’ll create an Opencog service and publish it in SingularityNET.

Step 1: Setting up with Docker

Setup and run a docker container.

$ git clone
$ docker build -t opencog_service_dev opencog-services/Docker/
$ docker run --name OPENCOG_SERVICE_DEV -ti opencog_service_dev /bin/bash

From this point we follow the turorial in the Docker container’s prompt.

The code in this repo is used to start and publish an Opencog service in SingularityNET. bin/server is the executable that actually listen for requests. bin/client can be used to send commands to the server locally (without blockchain).

Note that there’s only one Opencog service, which expects comands like this:

# ./bin/client sync Echo foo bar
foo bar

The service will issue the comand Echo passing 2 arguments: foo and bar. Thus, to implement a new Opencog service, we’ll actually need to implement a class which is linked to bin/server to provide the new command. So from now on we use the terminology “command” instead of “service” because we’ll actually implement a new “command” to the Opencog “service” which is already in place.

The sync keyword in the above command line is not relevant here. Take a look at this document for details.

Note: If you decided to run the server to actually test the client command above, make sure you kill it before moving on.

Step 2: Implementing “Hello World” commands

We’ll implement 2 versions of a “Hello world” command, one in C++ and another in Scheme. We need different names for the two commands so we’ll call them HelloWorld and HelloWorld2 respectively.

Step 2a: C++

For the C++ command, we need to implement a C++ class (with separated .h and .cc) in src/cpp-services. The class is supposed to have the exact name of the command in CamelCase notation. So the header file src/cpp-services/HelloWorld.h will look like this:

#include "../OpencogSNETService.h"
namespace opencogservices
class HelloWorld : public OpencogSNETService
    bool execute(std::string &output, const std::vector<std::string> &args);

The command class need to inherit from OpencogSNETService. The only required method is execute() but OpencogSNETService has other implemented helper methods explained here.

execute() expects a reference to std::string where its output is supposed to be written and a std::vector with the input arguments. In our case we expect no arguments.

So we only need to implement execute() in src/cpp-services/ In our case it may look like this:

#include "HelloWorld.h"
using namespace opencogservices;
using namespace std;
HelloWorld::HelloWorld() {}
HelloWorld::~HelloWorld() {}
bool HelloWorld::execute(string &output, const vector<std::string> &args)
    output.assign("Hello World");
    return false;

execute() is supposed to return true if and only if an error occurred so our HelloHorld implementation will just return false meaning that everything is OK.

Once the class is ready, edit src/ and add an #include statement and another else if clause in the method factory() according to your new command name.

Although OpencogSNETServiceFactory may compile if you use different names for the command and C++ class, you are supposed to use the exact same name.

To compile our new command just cd to src/ and:

OK we are ready to test our new C++ command.

Step 2b: Scheme

For the Scheme command, we need to implement a Scheme file in src/scm-services with the exact name of the command in CamelCase notation defining a funcion named execute which expects one single argument (a list with all the arguments to the function).

So our Scheme file src/scm-services/HelloWorld2.scm will look like this:

(define (execute args)
    "Hello World 2"

If the command expects arguments, they will be passed as a list to the function. You can make an arbritary number of valid Scheme calls in your file e.g. load new modules, define several global variables or helper functions etc. The only requisite is that you nned to define a function with the exact name of the command expecting a list of arguments as the only parameter.

Important: Anything the command send to stdout (e.g. by calling the function (display)) will be considered as part of the output.

So we’re done with the Scheme command.

Step 3: Testing Commands

We are ready to test our two new commands. cd to the root directory and start the server:

Now call our new commands:

# ./bin/client sync HelloWorld
Hello World
# ./bin/client sync HelloWorld2
Hello World 2

Step 4: Integrate into OpenCog Service

Once your commands are working properly, it’s time to make a PR to integrate then in the SingularityNET Opencog service. Make sure you read our contribution guidelines before going on.

Before issuing a PR, chek if your new commands implementation is compliant with some basic standards for tests and documentation. in the root directory run:

# ./scripts/
tests/HelloWorld is mising
tests/HelloWorld/testCases.txt is mising
tests/HelloWorld/baseline is mising
docs/ is mising

Note that we’re missing a lot of files. Basically we need to provide test cases for a regression test and a .md documentation for our new commands.

Step 4a: Integration Test

First, we’ll create the test cases for both commands.

# cd tests
# mkdir -p HelloWorld/baseline

Create a new file HelloWorld/testCases.txt like this:

{"test-cases": [
{"input": "", "output": "t.txt"}

testCases.txt lists all the test cases in a JSON array. Each entry is a hash with “input” and “output”. the first key is the input parameters of the test case and second one is the name of the file (which is expected to be in HelloWorld/baseline/) with the respective expected output.

We have only one test case so create HelloWorld/baseline/t.txt like this:

Execute integration tests:

Runing tests for Echo...
Runing tests for EchoScheme...
Runing tests for PatternMiner...
Runing tests for HelloWorld...
Runing tests for HelloWorld2...
Could execute tests for HelloWorld2

Still missing the test cases for HelloWorld2. Follow the same steps and run the regression tests again to make sure all the tests pass. NOTE: The Scheme commands append an extra \n at the end of the output. So your baseline output file need to have this empty line at the end.

Step 4b: Creating Documentation

Now we need to create a proper documentation for our new commands. Create docs/ and docs/ You are advised to use one of the documents of other commands as template for your own.

Use a MarkDown previewer of your choice. When you are done with the documentation, call:

This script will use your newly created documents to update the HTML user’s guide of SingularityNET Opencog Services. Those HTML files are in the repository as well so don’t forget to commit them in your PR.

You also need to update the main of the repository. Edit it and look for the secion “Opencog services to SingularityNET”. Add your newly created commands to the list with a simple description of their arguments and what is their expected output.

Step 5: Submitting PR

You are ready to submit your PR. Read our contribution guidelines before submiting it.

Additional helper methods in OpencogSNETService

The superclass inherited by your command has a couple of helper methods and variables.

    opencog::AtomSpace atomSpace;
    virtual void loadModules();
    bool loadAtomeseFile(std::string &errorMessage, const std::string &url);
    void evaluateScheme(const std::string &scmLine);
    void evaluateScheme(std::string &output, const std::string &scmLine);
    void setConfigurationParameters(const std::string jsonString);
  • loadModules(): Loads required SCM modules. Default implementation loads only (use-modules (opencog)) This method is called in OpencogSNETService constructor.
  • atomSpace: a variable containing the AtomSpace object used to execute any Opencog call.
  • loadAtomeseFile(): Fetches the contents of the passed URL (which is supposed to be a .scm file with an Atomese knowledge base) and load it in atomSpace. Any errors are reported in errorMessage.
  • evaluateScheme(): Evaluates the passed Scheme command (any Opencog’s scheme functions use the public atomSpace). The Scheme command output is discarded.
  • evaluateScheme(): Evaluates the passed Scheme command (any Opencog’s scheme functions use the public atomSpace). The Scheme command output is returned in output.
  • setConfigurationParameters(): Use the passed JSON hash to set Opencog’s configuration parameters (e.g. {"Max_thread_num": "8", "Pattern_Max_Gram": "3"})

This is a companion discussion topic for the original entry at