I'm trying to learn and use Fuchsia for fun, and a pretty basic concept is keeping me from progressing.
I thought that, as a learning experience, I could write a simple HTTP client that prints the content of some random URL to the log. Really nothing fancy.
As I understand, using the network (in my case I'd like to utilize fuchsia.net.http.Loader) is a capability, which has to be granted to a running component. Makes sense, that's pretty much the core of the OS.
I also understand that the initiating component, the one that runs my component, needs to grant this capability to my component. That's fair.
What I don't understand, and I'd very much appreciate any additional information (pretty please!) is how I can grant this to my component?
Specifically all demos and examples I saw had a custom client & server under a realm, which talked to each other. That's a good practice, but it doesn't bring in any capability that's built in.
What am I missing? Thanks in advance!
I'm trying to learn and use Fuchsia for fun, and a pretty basic concept is keeping me from progressing.
Thanks for your interest in Fuchsia! First of all, if you haven't already gone through Fuchsia Fundamentals I would strongly suggest that as a starting point for many of the foundational concepts.
Specifically all demos and examples I saw had a custom client & server under a realm, which talked to each other. That's a good practice, but it doesn't bring in any capability that's built in.
This is primarily because there's isn't necessarily a concept of any set of components or capabilities being "built in" to the system. The capabilities available to components in the system are entirely dependent on the rest of the components in a particular product build and how they are organized (this is called the component topology).
I thought that, as a learning experience, I could write a simple HTTP client that prints the content of some random URL to the log. Really nothing fancy.
The answer has a few sharp edges to it at the moment, as Fuchsia is a rapidly evolving open source project. Hopefully some of the details below will help you move forward.
Determine the capability routes
So you'll have to do a bit of work to figure out where the capability you need is provided and routed. In fact, one of the components exercises shows you how to do this for the fuchsia.net.http.Loader capability. Knowing where a capability is offered/used allows you to determine where your component would need to be instantiated to obtain the necessary capability.
You might also find some of the content in the Connect components developer guide useful in accessing the capability.
Run the component
Knowing where a capability is routed allows you to determine how to run your component. The most straightforward way of instantiating a component in the topology is to do so dynamically using ffx component. However, this requires a collection somewhere on the system with the capabilities you need. The ffx-laboratory realm where most examples are run has a very limited set of capabilities that does not include fuchsia.net.http.Loader.
You'll likely need to add your component statically to the topology using a core realm shard so that the necessary routes can be declared explicitly between the components that offer fuchsia.net.http.Loader and your component. With the component included statically in your product build, you can execute it using ffx component commands.
For more details on component execution, check out the Run components developer guide as well.
Run a CLI binary
Since this is a learning exercise, another option is to build your code as a binary that runs within the context of a component that already has the capabilities you need vs. creating and running an entirely new component. This is commonly used for CLI tools. With the ffx component explore command you can run your code as a binary inside the existing component that provides the HTTP capability you are looking for using the --tools argument, without the need to work through all the capability routing pieces described above.
For more details on ffx component explore, see Explore components.