Docs: Add section about plugins

Resolve #3

README.md

@@ -157,3 +157,10 @@ After the `find_package(FairMQ)` call the following CMake variables are defined:
    4. [File output](docs/Logging.md#54-file-output)
    5. [Custom sinks](docs/Logging.md#55-custom-sinks)
 6. [Examples](docs/Examples.md#6-examples)
+7. [Plugins](docs/Plugins.md#7-plugins)
+   1. [Usage](docs/Plugins.md#71-usage)
+   2. [Development](docs/Plugins.md#72-development)
+   3. [Provided Plugins](docs/Plugins.md#73-provided-plugins)
+       1. [DDS](docs/Plugins.md#731-dds)
+       2. [PMIx](docs/Plugins.md#732-pmix)
+

docs/Plugins.md

@@ -0,0 +1,93 @@
+← [Back](../README.md)
+
+# 7. Plugins
+
+FairMQ devices can be integrated with external configuration and control systems through its plugin system. FairMQ plugins are special dynamic libraries that can be loaded at runtime. Plugins have access to the Plugin API which includes the capability to control/monitor the device [state machine](Device.md#13-state-machine) and change/monitor configuration properties.
+
+A simple plugin may add the feature to read configuration from a certain desired file format once at the start of a device. A more complex plugin may create a long-running thread that integrates a network client to an external API of a central experiment control system.
+
+Because plugins are loaded dynamically, they can be developed in separate repositories/projects and also have their own set of runtime dependency that are not needed to be known at compile-time of the FairMQ device.
+
+## 7.1 Usage
+
+To load a plugin pass the `-P <name>[,<name>]` (or long `--plugin`) command line option. Multiple plugins can be loaded at the same time. The load order is as specified at the command line. This determines the order in which the plugins are instantiated (ctor call order) and in which order they are notified, should they subscribe to any notifications.
+
+When passing `-h/--help` on the command line one can find more detailed information:
+
+```
+Plugin Manager:
+  -S [ --plugin-search-path ] arg       List of plugin search paths.
+                                        * Override default search path, e.g.
+                                          -S /home/user/lib /lib
+                                        * Append(>) or prepend(<) to default
+                                        search path, e.g.
+                                          -S >/lib </home/user/lib
+                                        * If you mix the overriding and
+                                        appending/prepending syntaxes, the
+                                        overriding paths act as default search
+                                        path, e.g.
+                                          -S /usr/lib >/lib </home/user/lib
+                                        /usr/local/lib results in
+                                        /home/user/lib,/usr/local/lib,/usr/lib/
+                                        ,/lib
+                                        If nothing is found, the default
+                                        dynamic library lookup is performed,
+                                        see man ld.so(8) for details.
+  -P [ --plugin ] arg                   List of plugin names to load in
+                                        order,e.g. if the file is called
+                                        'libFairMQPlugin_example.so', just list
+                                        'example' or 'd:example' here.To load a
+                                        prelinked plugin, list 'p:example'
+                                        here.
+```
+
+## 7.2 Development
+
+To develop a custom FairMQ plugin, one simply needs to inherit from the `fair::mq::Plugin` base class (`#include <fairmq/Plugin.h>`) and call the `REGISTER_FAIRMQ_PLUGIN` macro. It is possible to introduce new command line option together with a plugin.
+
+The Plugin API includes:
+  * `Take/Steal/ReleaseDeviceControl()`/`GetCurrent/ChangeDeviceState()`/`SubscribeTo/UnsubscribeFromDeviceStateChange()` APIs enable controlling the device state machine. Only one plugin is authorized to control at the same time. Which one is determined by which plugin calls `TakeDeviceControl()` first.
+  * `Set/GetProperty()`/`GetPropertyKeys()`/`SubscribeTo/UnsubscribeFromPropertyChange()` APIs enable configuration of device properties.
+See [`<fairmq/Plugin.h>`](/fairmq/Plugin.h) for the full API.
+
+A more complete example which may serve as a start including example CMake code can be found here: [FairRootGroup/FairMQPlugin_example](https://github.com/FairRootGroup/FairMQPlugin_example).
+
+## 7.3 Provided Plugins
+
+### 7.3.1 DDS
+
+When launching a FairMQ topology via [DDS](http://dds.gsi.de/) the DDS plugin enables FairMQ devices to interact with DDS' custom command and property subsystems - enable the plugin by passing `-P dds` on the command line.
+
+Via the property subsystem a FairMQ topology may exchange channel connection data (essentially to do service discovery) needed to connect/bind all FairMQ channels appropriately. DDS is highly optimized for this use case. See [examples/dds](examples/dds/README.md) for more details.
+
+Via the custom command subsystem a FairMQ device can receive a number of commands. FairMQ provides a convenient command line tool `fairmq-dds-command-ui` that allows interactive or scripted control of a running FairMQ topology managed via DDS. If one develops directly against the custom command DDS API, the following table lists all the commands the DDS plugin currently understands:
+
+| Custom Command | Response | Error | Description |
+| --- | --- | --- | --- |
+| `check-state` | `<ID>: <STATE> (pid: <PID>)` | n/a | Query current device state, see state machine for possible states |
+| `dump-config` | `(<ID>: <PKEY> -> <PVALUE>\n)+` | n/a | Query current device config (list property key/value pairs) |
+| `INIT DEVICE` | `<ID>: queued <CMD> transition` | `<ID>: could not queue <CMD> transition` | Initiate state transition |
+| `BIND` | `<ID>: queued <CMD> transition` | `<ID>: could not queue <CMD> transition` | Initiate state transition |
+| `CONNECT` | `<ID>: queued <CMD> transition` | `<ID>: could not queue <CMD> transition` | Initiate state transition |
+| `INIT TASK` | `<ID>: queued <CMD> transition` | `<ID>: could not queue <CMD> transition` | Initiate state transition |
+| `RUN` | `<ID>: queued <CMD> transition` | `<ID>: could not queue <CMD> transition` | Initiate state transition |
+| `STOP` | `<ID>: queued <CMD> transition` | `<ID>: could not queue <CMD> transition` | Initiate state transition |
+| `RESET TASK` | `<ID>: queued <CMD> transition` | `<ID>: could not queue <CMD> transition` | Initiate state transition |
+| `RESET DEVICE` | `<ID>: queued <CMD> transition` | `<ID>: could not queue <CMD> transition` | Initiate state transition |
+| `END` | `<ID>: queued <CMD> transition` | `<ID>: could not queue <CMD> transition` | Initiate state transition |
+| `subscribe-to-heartbeats` | `heartbeat-subscription: <ID>,OK` | n/a | Subscribe to heartbeats |
+| on heartbeat subscription | `heartbeat: <ID>,<PID>` | n/a | Heartbeat every 100ms |
+| `unsubscribe-from-heartbeats` | `heartbeat-unsubscription: <ID>,OK` | n/a | Unsubscribe from heartbeats |
+| `subscribe-to-state-changes` | `state-changes-subscription: <ID>,OK` | n/a | Subscribe to state changes |
+| on state changes subscription | `state-change: <ID>,<STATE>` | n/a | State change notification |
+| `unsubscribe-from-state-changes` | `state-changes-unsubscription: <ID>,OK` | n/a | Unsubscribe from state changes |
+
+If unknown commands are received the plugin will print a warning.
+
+### 7.3.2 PMIx
+
+The [PMIx](https://pmix.org/) plugin enables launching a FairMQ topology with any PMIx capable launcher, e.g. the [Open Run-Time Environment (ORTE) of OpenMPI](https://www.open-mpi.org/doc/v4.0/man1/mpirun.1.php) or the [Slurm workload manager](https://slurm.schedmd.com/srun.html). This plugin is not (yet) very mature and serves as a proof of concept at the moment.
+
+TODO example usage
+
+← [Back](../README.md)