Exercise 3: Using Bedrock and composing with other services

In this exercise we will use Bedrock to deploy a daemon containing an instance of our phonebook provider. We will then implement a phonebook backend that uses Yokan, and organize the composition of the two within the same daemon. Everything in this exercise relies on the codebase from Exercise 2, however you don’t need to have completed Exercise 2 to do this exercise.

First, make sure that the Spack environment from Exercise 2 is activated (spack env status should show you in which environment you are).

From the build directory, re-run cmake as follows.


This time a libYP-bedrock-module.so is being built. This is the Bedrock module for our phonebook service, i.e. the library that tells Bedrock how to instanciate and use our phonebook provider.

To make sure Bedrock finds this library, execute the following command from the build directory.


examples/bedrock-config.json is an example of Bedrock configuration that spins up a phonebook provider with provider ID 42. This provider manages one phonebook of type “dummy”. You can try our this configuration using the bedrock program as follows.

bedrock na+sm -c ../examples/bedrock-config.json

You can copy the address printed by bedrock on the last line of its log, and in another terminal (don’t forget to activate your spack environment), run the following command.

bedrock-query na+sm -a <copied-address> -p

You will see the current configuration of the service, including a phonebook provider that manages a phonebook. Bedrock has completed the input configuration with a lot of information about Mercury, Argobots, etc. These information can be very useful to communicate to Mochi developers when you try to find out what’s wrong with your service.

We will now add Yokan in our service. To add Yokan as dependency to our spack environment, run the following command.

spack add mochi-yokan+bedrock
spack install

This will install Yokan.


spack add will add the dependency to your active environment, but you should also make sure to list this new dependency in the list of specs in spack.yaml at the root of your project so that it is taken into account next time you want to build a new environment from it.

Edit CMakeLists.txt at the root of your project to add find_package(yokan REQUIRED) (e.g. after the call to find_package(thallium REQUIRED)).


When developing your own service, don’t forget to also edit the src/*.cmake.in and src/*.pc.in files to add relevant dependencies there. Those are the files used by cmake and pkg-config respectively to search for dependencies when people are using your code.

Edit src/CMakeLists.txt to add yokan-client as a dependency for the YP-server library (i.e. find the call to target_link_libraries for YP-server and add yokan-client in the list of public dependencies).

From the build directory, re-run cmake .. to make it find Yokan.

Open examples/bedrock-config.json and add the Yokan library in the libraries section.

"yokan": "libyokan-bedrock-module.so"

In this file as well, we will instanciate a Yokan provider with a Yokan database. In the providers section, before the phonebook provider, add the following provider definition.

  "type": "yokan",
  "name": "my-yokan-provider",
  "provider_id": 123,
  "config": {
    "databases": [
        "type": "map",
        "name": "my-db"


It is important that the Yokan provider be defined before the YP provider because the former will be a dependency of the latter.

If you re-run bedrock with this new configuration then call bedrock-query, you should be able to confirm that your Bedrock daemon is now running two providers: one YP provider and one Yokan provider. Of course, these two don’t know about each other, they simply share the resources of the same process. We will now introduce a dependency between YP and Yokan.

Edit src/BedrockModule.cpp and find the getProviderDependencies member function at the end. Change static const std::vector<bedrock::Dependency> no_dependency; into a variable that lists en actual dependency, i.e.:

static const std::vector<bedrock::Dependency> dependencies =
    {{"yokan_ph", "yokan", BEDROCK_REQUIRED}};

The first field, "yokan_ph", is the name by which YP will reference this dependency. "yokan" is the type of dependency. BEDROCK_REQUIRED indicates that this dependency is required.

If you rebuild your code now and re-run the Bedrock configuration, it will display an error message:

[critical] Missing dependency yokan_ph in configuration

So let’s fix that by going again into examples/bedrock-config.json, and add the following in the field in the definition of our YP provider.

"dependencies": {
  "yokan_ph": "yokan:123@local"

You can also use "my-yokan-provider" instead of "yokan:123". Now Bedrock should restart accepting your configuration.

Edit src/BedrockModule.cpp once again. This time we will look at the registerProvider function at the beginning of the file. Use the args variable to find the dependency to Yokan as follows.

auto it = args.dependencies.find("yokan_ph");
yk_provider_handle_t yokan_ph =

You will need to include yokan/provider-handle.h to get the definition of yk_provider_handle_t.

You have now retrieved a Yokan provider handle pointing to your Yokan provider, in the context of registering a YP provider.

Bonus: continuing to wire YP with Yokan

At this point, you have learned what this exercise aimed for you to learn, namely how to write a Bedrock module, a Bedrock configuration with multiple providers, and how to manage dependencies in Bedrock configurations and Bedrock modules.

This bonus section invites you to complete the dependency injection of Yokan into YP. It is a lot more complicated and a lot less guided, hence feel free to stop there and ignore this bonus section.

First, you will need to instanciate a yk::Provider in your tests (tests/AdminTest.cpp and tests/PhonebookTest.cpp) and make sure it manages at least one database by passing it an appropriate configuration string. Here we are doing manually what Bedrock would normally do from a JSON configuration file.

In src/ProviderImpl.hpp add a const tl::provider_handle& yokan_ph argument to the constructor. Add a corresponding m_yokan_ph field to the class and assign the provided constructor argument to it.

In include/YP/Provier.hpp, add a const tl::provider_handle& yokan_ph argument to the two constructors. In src/Provider.cpp change the signature of the constructor accordingly as well as the call to the underlying ProviderImpl constructor.

You can convert this yokan_ph into a thallium provider handle as follows before passing it to the Provider constructor.

tl::provider_handle ph{
    args.engine, yokan_ph->addr, yokan_ph->provider_id, false};

You have successfully injected a Yokan dependency into the YP provider!

The rest of this exercise will be less directed. The goal is now to pass this provider handle down to the dummy phonebook so that it can use Yokan as an implementation of a key/value store instead of relying on an unordered_map. You should now be familiar enough with the code to make the necessary changes bellow without too much guidance. Keep the API of Yokan open in a web browser for reference. Yokan also has a C++ API here.

To be able to pass the Yokan provider handle down to a backend (e.g. a dummy phonebook), you will need to change the signature of the functions that create and open a phonebook (the createPhonebook and openPhonebook in include/YP/Backend.hpp, as well as the type of std::function stored in create_fn and open_fn maps, and the signatures of the lambdas in the __PhonebookBackendRegistration class down the file).

This then implies changing src/dummy/DummyBackend.cpp and src/dummy/DummyBackend.hpp accordingly.

You will need to tell your dummy phonebook backend which database to use. Yokan databases can be identified by a name, so you may want to look for the name of this database in the configuration passed to the phonebook (std::string db_name = config["db_name"].get<std::string>();), using the engine to create a yokan::Client (yokan::Client yk_client{engine.get_margo_instance_()};) and use this yokan::Client::findDatabaseByName() with the appropriate arguments. (tl::provider_handle::provider_id() and tl::provider_handle::get_addr() can be useful to obtain the provider handle’s provider ID and hg_addr_t respectively).

yokan::Client::findDatabaseByName returns a yokan::Database instance that you can store in the DummyBackend class.

In the insert and lookup functions of the dummy phonebook, you may now use the put and get methods of this yokan::Database instance to put and get phone numbers.

In practice, you could copy the dummy backend implementation into a new type of backend that specifically uses Yokan. Don’t hesitate to implement multiple backends for your service, with different dependencies or different strategies for solving the same problem.