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.

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 to add find_package(yokan REQUIRED) (e.g. after the call to find_package(PkgConfig 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"

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/bedrock-module.c and find the struct bedrock_module definition at the end. Its provider_dependencies field is where we will be able to introduce this dependency on Yokan. Before the declaration of the bedrock_module structure, add the following declaration:

static struct bedrock_dependency provider_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.

You can now assign the field in the bedrock_module structure.

.provider_dependencies = provider_dependencies

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.

In include/YP/YP-server.h, include the yokan/provider-handle.h header and add a yokan_provider_handle_t yokan_ph field in the YP_provider_args structure.

Edit src/bedrock-module.c once again. This time we will look at the YP_register_provider function at the beginning of the file. Use bedrock_args_get_dependency(args, "yokan_ph", 0); to retrieve a pointer to a Yokan provider handle (yoken_provider_handle_t), which you can now use to set the corresponding field in the YP_args structure. You have successfully injected a Yokan dependency into the 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.

The goal is now to pass this Yokan 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 the phonebook.h implementation. 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.

First, you will need to register a Yokan provider in your tests (tests/test-admin.cpp and tests/test-client.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.

To add and keep a reference to the Yokan provider handle in the YP_provider structure (in src/provider.h), you will need to copy the provided yokan_provider_handle_t in YP_provider_register, and free this copy in YP_finalize_provider (in src/provider.c). yokan_provider_handle_t is a public structure with no yokan_provider_handle_ref_incr function. You will have to manually copy its fields, and call margo_addr_ref_incr on the hg_addr_t field to increase the reference count of the address (and call margo_addr_free on it in YP_finalize_provider).

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 (in include/YP/YP-backend.h).

This then implies changing their implementation (in src/dummy/dummy-backend.c).

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 implement a way to look for the name of this database in the configuration passed to the phonebook (in the dummy_create_phonebook and dummy_open_phonebook functions in src/dummy/dummy-backend.c).

Once a backend knows the name of the database it should use, you can use yk_database_find_by_name to look for its ID, then yk_database_handle_create to create a handle for the database (don’t forget to call yk_database_handle_release when you no longer need it, e.g. when closing/destroying the dummy backend).

In the insert and lookup functions of the dummy phonebook, you may now use yk_put and yk_get 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.