Extensions.md

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# Apache MiNiFi Extensions Guide

To enable all extensions for your platform, you may use -DENABLE_ALL=TRUE OR select the option to "Enable all Extensions" in the bootstrap script. [ReadMe](https://github.com/apache/nifi-minifi-cpp/#bootstrapping)

# Extension internals
Extensions are dynamic libraries loaded at runtime by the agent.

## C extensions
You can build shared libraries using the API defined in `minifi-c.h`
For the shared library to be considered a valid extension, it must export a global symbol with the name `MinifiApiVersion`
with its value equal to the uint32_t constant `MINIFI_API_VERSION` from `minifi-c.h`.

### Resource Lifetime

Unless otherwise specified, the following lifetime rules apply to all functions called by the agent (e.g., `MinifiInitExtension`, `MinifiProcessorCallbacks::onTrigger`, or other callbacks):

* Arguments: The lifetime of any resource provided as a function argument is limited to the duration of that function call.

* Created Resources: The lifetime of resources created within these functions (e.g., a handle returned by `MinifiProcessSessionGet` inside `MinifiProcessorCallbacks::onTrigger`)
is limited to the scope of the innermost callback.
(the return value of `MinifiRegisterExtension` is only valid during the execution of `MinifiInitExtension`).

Because of these scoping rules, all processor and controller service registrations must occur within the `MinifiInitExtension` call.
One possible example of this is:

```C++
extern "C" const uint32_t MinifiApiVersion = MINIFI_API_VERSION;

extern "C" void MinifiInitExtension(MinifiExtensionContext* extension_context) {
  MinifiExtensionDefinition extension_definition{
    .name = minifi::api::utils::toStringView(MAKESTRING(EXTENSION_NAME)),
    .version = minifi::api::utils::toStringView(MAKESTRING(EXTENSION_VERSION)),
    .deinit = nullptr,
    .user_data = nullptr
  };
  auto* extension = MinifiRegisterExtension(extension_context, &extension_definition);
  minifi::api::core::useProcessorClassDescription<minifi::extensions::llamacpp::processors::RunLlamaCppInference>([&] (const MinifiProcessorClassDefinition& description) {
    MinifiRegisterProcessor(extension, &description);
  });
}
```

## C++ extensions
You can utilize the C++ api, linking to `minifi-api` and possibly using the helpers in `extension-framework`.
No compatibilities are guaranteed beyond what extensions are built together with the agent at the same time.

An extension makes its capabilities (classes) available to the system through registrars. Registration must happen in source files, not headers.

```C++
// register user-facing classes as
REGISTER_RESOURCE(InvokeHTTP, Processor);
// or
REGISTER_RESOURCE(SSLContextService, ControllerService);

// register internal resources as
REGISTER_RESOURCE(HTTPClient, InternalResource);
// or
REGISTER_RESOURCE(RESTSender, DescriptionOnly);
```

Some extensions (e.g. `OpenCVExtension`) require initialization before use.
You need to define an `MinifiInitCppExtension` function of type `MinifiExtension*(MinifiExtensionContext*)` to be called.

```C++
extern "C" void MinifiInitCppExtension(MinifiExtensionContext* /*extension_context*/) {
  const auto success = org::apache::nifi::minifi::utils::Environment::setEnvironmentVariable("OPENCV_FFMPEG_CAPTURE_OPTIONS", "rtsp_transport;udp", false /*overwrite*/);
  if (!success) {
    return nullptr;
  }
  MinifiExtensionDefinition extension_definition{
    .name = minifi::utils::toStringView(MAKESTRING(MODULE_NAME)),
    .version = minifi::utils::toStringView(minifi::AgentBuild::VERSION),
    .deinit = nullptr,
    .user_data = nullptr,
  };
  minifi::utils::MinifiRegisterCppExtension(extension, &extension_definition);
}
```

# Loading extensions

The agent will look for the `nifi.extension.path` property in the `minifi.properties` file to determine what extensions to load.

The property expects a comma separated list of paths.
The paths support wildcards, specifically, a standalone `**` matches any number of nested directories, the `*` matches any number of characters in a single segment and `?` matches one single character in a segment.
Relative paths are relative to the agent executable.
```
// This matches all files in the 'extensions' directory next to the directory the executable is in.
nifi.extension.path=../extensions/*
```

### Exlusion
If you want to exclude some extensions from being loaded, without having to specify the rest, you can do so by prefixing the pattern with `!`.
```
// This loads all extensions but the azure extension. (the exact name differs by platform: dylib, dll, so)
nifi.extension.path=../extensions/*,!../extensions/libminifi-azure.so
```

You could even exclude some subdirectory and then re-include specific extensions/subdirectories in that.
```
// The last pattern that matches an extension will determine if that extension is loaded or not.
nifi.extension.path=../extensions/**,!../extensions/private/*,../extension/private/my-cool-extension.dll
```