Resource bindings

Some bindings are like libraries for programs and are not doing anything by themselves but are needing some extra elements like a configuration.

A typical example is the MODBUS binding. That binding requires a configuration file describing connected hardwares.

Because bindings and configurations are coming from differents sources and are developed by different teams, there is a need for guidelines on how to assemble them.

The redpesk framework gives flexibility on how to implement such assembly. This document presents the recommended solution(s) to solve the problem.

Basic usage of a resource bindings

The resource bindings are bindings that are exported to third parties. It is in the framework since 2018. To achieve it the manifest file must include the provided-binding declaration, as on the example:

provided-binding:
   - name: extra
     value: lib/extra-binding.so
file-properties:
   - name: lib/extra-binding.so
     value: public

On that example, the binding lib/extra-binding.so (a shared library) is exported under the name extra. Note well that the provided binding must be exported publicly using the file-properties section.

Such provided bindings can be imported by applications by requiring it in the manifest. This is achieved by adding required-binding to the target that requires it, as on the below example:

targets:
  - target: main
    content:
      src: .rpconfig/manifest.yml
      type: application/vnd.agl.service
    required-binding:
      - name: extra
        value: extern

On that example, the service imports the binding exported as extra.

Using a resource binding with a config

When a service imports a binding and when that binding expect a configuration specific to the service, the service must provide a configuration.

This is done in two steps:

• tell through manifest file to use a config file
• in that config file, tell the binding what is its configuration

Since version 12.2.4, the framework allows to tell what configuration files are to required to be loaded. This is done using required-config at target level, as on the below example:

targets:
  - target: main
    content:
      src: .rpconfig/manifest.yml
      type: application/vnd.agl.service
    required-binding:
      - name: extra
        value: extern
    required-config:
      - etc/config-extra-common.yml
      - etc/config-extra-extra.yml

On that example, the service requires to load 2 configurations.

When more that one configuration is given, the configuration are merged together to form the final configuration.

Such given configuration are equivalent to giving options on the binder command lines. Then for setting the configuration of a binding it is the afb-binder’s option --set. Here is the manual extract of this option:

   -s, --set VALUE
       Set parameters values for APIs.  The  value  can  be  of  the  form
       [API]/[KEY]:JSON or {"API":{"KEY":JSON},...}

So as explained in the manual, the configuration file should look as below:

{
   "set": {
      "API": {
         ...
      }
   }
}

Here caution has to be taken because two kinds of bindings exist:

• when the binding dynamically creates APIs without using afb_binding_t static description and when the API name depends of the configuration, in that case, the API name is the basename of the binding library must be used (example: if the binding is lib/hello.so then the API is hello.so).

• In other cases, this is the usual API name;