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\chapter{Integration with \texorpdfstring{\gls{ITASK}}{iTask}}%
\label{chp:integration_with_itask}
\begin{chapterabstract}
	This chapter shows the integration of \gls{MTASK} with \gls{ITASK} by showing:
	\begin{itemize}
		\item an architectural overview \gls{MTASK} applications;
		\item on the interface for connecting devices;
		\item the interface for lifting \gls{MTASK} tasks to \gls{ITASK} tasks;
		\item a interface for lowering \gls{ITASK} \glspl{SDS} to \gls{MTASK} \glspl{SDS};
		\item and finishes with non-trivial home automation example application using all integration mechanisms;
	\end{itemize}
\end{chapterabstract}

The \gls{MTASK} language is a multi-view \gls{DSL}, there are multiple interpretations possible for a single \gls{MTASK} term.
Using the byte code compiler (\cleaninline{BCInterpret}) \gls{DSL} interpretation, \gls{MTASK} tasks are fully integrated in \gls{ITASK}.
They are executed as if they are regular \gls{ITASK} tasks and they can access \glspl{SDS} from \gls{ITASK}.
Devices in the \gls{MTASK} system contain a domain-specific \gls{OS} and are little \gls{TOP} engines in their own respect, being able to execute tasks.

\Cref{fig:mtask_integration} shows the architectural layout of a typical \gls{IOT} system created with \gls{ITASK} and \gls{MTASK}.
The entire system is written as a single \gls{CLEAN} specification where multiple tasks are executed at the same time.
Tasks can access \glspl{SDS} according to many-to-many communication and multiple clients can work on the same task.
The diagram contains three labelled arrows that denote the integration functions between \gls{ITASK} and \gls{MTASK}.
Devices are integrated into the system using the \cleaninline{withDevice} function (see \cref{sec:withdevice}).
Using \cleaninline{liftmTask}, \gls{MTASK} tasks are lifted to a device (see \cref{sec:liftmtask}).
\glspl{SDS} from \gls{ITASK} are lowered to the \gls{MTASK} device using \cleaninline{lowerSds} (see \cref{sec:liftsds}).

\begin{figure}
	\centering
	\includestandalone{mtask_integration}
\caption{An architectural view of an \imtask{} applications.}%
	\label{fig:mtask_integration}
\end{figure}

\section{Connecting edge devices}\label{sec:withdevice}
Edge devices in an \gls{MTASK} application are always coordinated by a server.
This means that they wait for a server to connect to them and send work.
The heavy lifting of connecting an \gls{MTASK} device to an \gls{ITASK} server is done with the \cleaninline{withDevice} function.
This function is given a communication specification and a function producing an \gls{ITASK} task that does something with an \gls{MTASK} device, e.g.\ lift tasks.
By using \gls{HOAS} like this, setting up and tearing down the connection to the device is fully controlled.

In the implementation of the function, all communication with a device happens through a so-called \emph{channels} \gls{SDS}.
The channels contain three fields, a queue of messages that are received, a queue of messages to send and a stop flag.
Every communication method that implements the \cleaninline{channelSync} class can provide the communication with an \gls{MTASK} device.
As of now, serial port communication, direct \gls{TCP} communication and \gls{MQTT} over \gls{TCP} are supported as communication providers (see \cref{lst:connection_types}).
Internally, the \cleaninline{withDevice} task sets up the communication, exchanges specifications with the device, executes the inner task while handling errors, and finally cleans up after closing.
\Cref{lst:mtask_device} shows the types and interface to connecting devices.

\begin{lstClean}[label={lst:mtask_device},caption={Device communication interface in \gls{MTASK}.}]
:: MTDevice //abstract
:: Channels :== ([MTMessageFro], [MTMessageTo], Bool)

class channelSync a :: a (Shared sds Channels) -> Task () | RWShared sds

withDevice :: a (MTDevice -> Task b)
	-> Task b | iTask b & channelSync, iTask a
\end{lstClean}

\subsection{Implementation}
\Cref{lst:pseudo_withdevice} shows a pseudocode implementation of the \cleaninline{withDevice} function.
The \cleaninline{MTDevice} abstract type is internally represented as three \gls{ITASK} \gls{SDS} that contain all the current information about the tasks.
The first \gls{SDS} is the information about the \gls{RTS} of the device, i.e.\ metadata on the tasks that are executing, the hardware specification and capabilities, and a list of fresh task identifiers.
The second \gls{SDS} is a map storing downstream \gls{SDS} updates.
When a lowered \gls{SDS} is updated on the device, a message is sent to the server.
This message is initially queued in the map to allow for asynchronous handling of multiple updates.
Finally, the \cleaninline{MTDevices} type contains the communication channels.

The \cleaninline{withDevice} task itself first constructs the \glspl{SDS} using the \gls{ITASK} function \cleaninline{withShared} to create anonymous local \glspl{SDS}.
Then, it performs the following four tasks in parallel to monitor the edge device.
\begin{enumerate}
	\item It synchronises the channels using the \cleaninline{channelSync} overloaded function.
		Errors that occur here are converted to the proper \gls{MTASK} exception.
	\item Watches the channels for the shutdown flag.
		If the connection is lost with the device unexpectedly, an \gls{MTASK} exception is thrown.
	\item Watches the channels for messages and processes them accordingly by changing the device information \gls{SDS} or adding the lowered \gls{SDS} updates to the corresponding \gls{SDS} update queue.
	\item Sends a request for a specification. Once the specification is received, the device task is run.
		The task value of this device task is then used as the task value of the \cleaninline{withDevice} task.
\end{enumerate}

\begin{lstClean}[caption={Pseudocode for the \texttt{widthDevice} function},label={lst:pseudo_withdevice}]
withDevice spec deviceTask =
	withShared newMap \sdsupdates->
	withShared ([], [MTTSpecRequest], False) \channels->
	withShared default \dev->parallel
		[ channelSync spec channels
		, watchForShutdown channels
		, watchChannelMessages dev channels
		, waitForSpecification
			>>| deviceTask
			>>* [ifStable: issueShutdown]
		]
\end{lstClean}

If at any stage an unrecoverable device error occurs, an \gls{ITASK} exception is thrown on the \cleaninline{withDevice} task.
This exception can be caught in order to devise fail-safe mechanisms.
For example, if a device fails, the task can be sent to another device as can be seen in \cref{lst:failover}.
This function executes an \gls{MTASK} task on a pool of devices connected through \gls{TCP}.
If a device error occurs during execution, the next device in the pool is tried until the pool is exhausted
If another error occurs, it is rethrown for a parent task to catch.

\begin{lstClean}[caption={An \gls{MTASK} failover combinator.},label={lst:failover}]
failover :: [TCPSettings] (Main (MTask BCInterpret a)) -> Task a
failover []     _     = throw "Exhausted device pool"
failover [d:ds] mtask = try (withDevice d (liftmTask mtask)) except
where except MTEUnexpectedDisconnect = failover ds mtask
	  except _                       = throw e
\end{lstClean}

\section{Lifting \texorpdfstring{\gls{MTASK}}{mTask} tasks}\label{sec:liftmtask}
Once the connection with the device is established, \gls{MTASK} tasks are lifted to \gls{MTASK} tasks using the \cleaninline{liftmTask} family of functions (see \cref{lst:liftmtask}).
Given an \gls{MTASK} task in the \cleaninline{BCInterpret} view and a device obtained from \cleaninline{withDevice}, an \gls{ITASK} task is returned.
This \gls{ITASK} task proxies the \gls{MTASK} task that is executed on the microcontroller.
Hence, when for example observing the task value, the actual task value from the microcontroller is observed.

\begin{lstClean}[label={lst:liftmtask},caption={The interface for lifting \gls{MTASK} tasks to \gls{ITASK} tasks.}]
liftmTask :: (Main (MTask BCInterpret a)) MTDevice -> Task a | iTask a
\end{lstClean}

\subsection{Implementation}
\Cref{lst:liftmTask_pseudo} shows the pseudocode for the \cleaninline{liftmTask} implementation
The first argument is the task and the second argument is the device which is just an \gls{ADT} containing the \glspl{SDS} referring to the device information, the \gls{SDS} update queue, and the channels.
First a fresh identifier for the task is generated using the device state.
With this identifier, the cleanup hook can be installed.
This is done to assure the task is removed from the edge device if the \gls{ITASK} task coordinating it is destroyed.
Tasks in \gls{ITASK} are destroyed when for example it is executed in parallel with another task and the parallel combinator terminates or when the condition to step holds in a sequential task combination.
Then the \gls{MTASK} compiler is invoked, its only argument besides the task is a function doing something with the results of the compilation, i.e.\ the lowered \glspl{SDS} and the messages containing the compiled and serialised task.
With the result of the compilation, the task can be executed.
First the messages are put in the channels, sending them to the device.
Then, in parallel:
\begin{enumerate*}
	\item the value is watched by looking in the device state \gls{SDS}, this task also determines the task value of the whole task
	\item the downstream \glspl{SDS} are monitored, i.e.\ the \cleaninline{sdsupdates} \gls{SDS} is monitored and updates from the device are applied to the associated \gls{ITASK} \gls{SDS}
	\item the upstroam \glspl{SDS} are monitored by spawning tasks that watch these \glspl{SDS}, if one is updated, the novel value is sent to the edge device.
\end{enumerate*}

\begin{lstClean}[label={lst:liftmTask_pseudo},caption={Pseudocode implementation for \texttt{liftmTask}.}]
liftmTask task (MTDevice dev sdsupdates channels)
	= freshTaskId dev
	>>= \tid->withCleanupHook (sendmessage [MTTTaskDel tid] channels) (
		compile task \mrefs msgs->
			sendMessage msgs channels
			>>| waitForReturnAndValue tid dev
			-|| watchSharesDownstream mrefs tid sdsupdates
			-|| watchSharesUpstream mrefs channels tid)
\end{lstClean}

Sending the complete byte code to the device is not always a suitable option.
For example, when the device is connected through an unstable or slow connection, sending the entire byte code may induce lots of delay.
To mitigate this problem, \gls{MTASK} tasks can be preloaded on a device.
Preloading means that the task is compiled and integrated into the device firmware.
On receiving a \cleaninline{TaskPrep}, a hashed value of the task to be sent is included.
The device then checks the preloaded task registry and uses the local version if the hash matches.
Of course this only works for tasks that are not tailor made for the current work specification and not depend on run time information.
The interface for task preloading can be found in \cref{lst:preload}.
Given an \gls{MTASK} task, a header file is created that is placed in the source code directory of the \gls{RTS} to include it in the firmware.

\begin{lstClean}[label={lst:preload},caption={Preloading tasks in \gls{MTASK}.}]
preloadTask :: (Main (MTask BCInterpret u)) -> Task String
\end{lstClean}

\section{Lowering \texorpdfstring{\gls{ITASK}}{iTask} \texorpdfstring{\glsxtrlongpl{SDS}}{shared data sources}}\label{sec:liftsds}
Lowering \gls{ITASK} \glspl{SDS} to \gls{MTASK} \glspl{SDS} is something that mostly happens at the compiler level using the \cleaninline{lowerSds} function (see \cref{lst:mtask_itasksds}).
\Glspl{SDS} in \gls{MTASK} must always have an initial value.
For regular \gls{SDS} this value is given in the source code, for lowered \gls{ITASK} \glspl{SDS} this value is obtained by reading the values once just before sending the task to the edge device.
On the device itself, there is just one difference between lowered \glspl{SDS} and regular \glspl{SDS}: after changing \pgls{SDS}, a message is sent to the server containing this new value.
The \cleaninline{withDevice} task (see \cref{sec:withdevice}) receives and processes this message by writing to the \gls{ITASK} \gls{SDS}.
Tasks watching this \gls{SDS} get notified then through the normal notification mechanism of \gls{ITASK}.

\begin{lstClean}[label={lst:mtask_itasksds},caption={Lowered \gls{ITASK} \glspl{SDS} in \gls{MTASK}.}]
class lowerSds v where
	lowerSds :: ((v (Sds t)) -> In (Shared sds t) (Main (MTask v u)))
		-> Main (MTask v u) | RWShared sds
\end{lstClean}

As an example, \cref{lst:mtask_liftsds_ex} shows a light switch function producing an \imtask{} task when given a device handle.
First an \gls{ITASK} \gls{SDS} of the type boolean is created.
This boolean represents the state of the light.
The \gls{MTASK} task uses this \gls{SDS} to turn on or off the light.
The \gls{ITASK} task that runs in parallel allows interactive updating of this state.

\begin{lstClean}[label={lst:mtask_liftsds_ex},caption={Interactive light switch program.}]
lightswitch :: MTDevice -> Task Bool
lightswitch dev  =
	withShared False \sh->
		    liftmTask (mtask sh) dev
		-|| updateSharedInformation [] sh
		<<@ Hint "Light switch"
where
	mtask :: (Shared sds Bool) -> Main (MTask v Bool)
		| mtask, lowerSds v & RWShared sds
	mtask sh =
		declarePin D13 PMOutput \d13->
		lowerSds \ls=sh
		In fun \f=(\st->
			     getSds ls
			>>*. [IfValue ((!=.)st) (\v->writeD d13 v)]
			>>|. f (Not st))
		In {main=f true}
\end{lstClean}

\subsection{Implementation}
The compilation of the code and the serialisation of the data throws away all typing information.
\Glspl{SDS} are stored in the compiler state as a map from identifiers to either an initial value or an \cleaninline{MTLens}.
The \cleaninline{MTLens} is a type synonym for a \gls{SDS} that represents the typeless serialised value of the underlying \gls{SDS}.
This is done so that the \cleaninline{withDevice} task can write the received \gls{SDS} updates to the according \gls{SDS} independently.
The \gls{ITASK} notification mechanism then takes care of the rest.
Such \pgls{SDS} is created by using the \cleaninline{mapReadWriteError} which, given a pair of read and write functions with error handling, produces \pgls{SDS} with the lens embedded.
The read function transforms converts the typed value to a typeless serialised value.
The write function will, given the new serialised value and the old typed value, produce a new typed value.
It tries to decode the serialised value, if that succeeds, it is written to the underlying \gls{SDS}, an error is thrown otherwise.
\Cref{lst:mtask_itasksds_lens} provides the implementation for this:

% VimTeX: SynIgnore on
\begin{lstClean}[label={lst:mtask_itasksds_lens},caption={Lens applied to lowered \gls{ITASK} \glspl{SDS} in \gls{MTASK}.}]
lens :: (Shared sds a) -> MTLens | type a & RWShared sds
lens sds = mapReadWriteError
	( \r->   Ok (fromString (toByteCode{|*|} r)
	, \w r-> ?Just <$> iTasksDecode (toString w)
	) ?None sds
\end{lstClean}
% VimTeX: SynIgnore off

\Cref{lst:mtask_itasksds_lift} shows the code for the implementation of \cleaninline{lowerSds} that uses the \cleaninline{lens} function shown earlier.
First, the \gls{SDS} to be lowered is extracted from the expression by bootstrapping the fixed point with a dummy value.
This is safe because the expression on the right-hand side of the \cleaninline{In} is never evaluated.
Then, using \cleaninline{addSdsIfNotExist}, the identifier for this particular \gls{SDS} is either retrieved from the compiler state or generated freshly.
This identifier is then used to provide a reference to the \cleaninline{def} definition to evaluate the main expression.

% VimTeX: SynIgnore on
\begin{lstClean}[label={lst:mtask_itasksds_lift},caption={Lens applied to lowered \gls{ITASK} \glspl{SDS} in \gls{MTASK}.}]
instance lowerSds BCInterpret where
	lowerSds def = {main =
			let (t In _) = def (abort "lowerSds: expression too strict")
			in addSdsIfNotExist (Right $ lens t)
				>>= \sdsi->let (_ In e) = def (pure (Sds sdsi)) in e.main
		}\end{lstClean}
% VimTeX: SynIgnore off

\section{Home automation}
\todo[inline]{Staat dit hier goed of moet dit naar een andere sectie?}
This section presents a interactive home automation program (\Cref{lst:example_home_automation}) to illustrate \gls{MTASK}'s integration with \gls{ITASK}.
It consists of a web interface for the user to control which tasks may be executed on either of two connected devices: an \gls{ARDUINO} UNO, connected via a serial port; and an ESP8266 based prototyping board called NodeMCU, connected via \gls{TCP} over \gls{WIFI}.

\Crefrange{lst:example:spec1}{lst:example:spec2} show the specification for the devices.
The UNO is connected via serial using the unix filepath \path{/dev/ttyACM0} and the default serial port settings.
The NodeMCU is connected via \gls{WIFI} and hence the \cleaninline{TCPSettings} record is used.
Both types have \cleaninline{channelSync} instances.

The code consists of an \gls{ITASK} part and several \gls{MTASK} parts.
\Crefrange{lst:example:task1}{lst:example:task2} containing the \gls{ITASK} task that coordinates the \gls{IOT} application.
It first connects the devices (\crefrange{lst:example:conn1}{lst:example:conn2}) followed by launching a \cleaninline{parallel} task, visualized as a tabbed window, and a shutdown button to terminate the program (\crefrange{lst:example:par1}{lst:example:par2}).
This parallel task is the controller of the tasks that run on the edge devices.
It contains one task that allows adding new tasks (using \cleaninline{appendTask}) and all other tasks in the process list will be \gls{MTASK} tasks once they are added by the user.
The controller task, \cleaninline{chooseTask} as shown in \crefrange{lst:example:ct1}{lst:example:ct2}, allows the user to pick a task, sending it to the specified device.
Tasks are picked by index from the \cleaninline{tasks} list (\crefrange{lst:example:tasks1}{lst:example:tasks2}) using \cleaninline{enterChoice}.
The interface that is generated for this can be seen in \cref{fig:example_screenshots1}.
After selecting the task, a device is selected (see \cref{fig:example_screenshots2,lst:example:selectdev}).
When both a task and a device is selected, an \gls{ITASK} task is added to the process list using \cleaninline{appendTask}.
Using the helper function \cleaninline{mkTask}, the actual task is selected from the \cleaninline{tasks} list and executed by providing the device argument.
For example, when selecting the \cleaninline{temperature} task, the current temperature is shown to the user (\cref{fig:example_screenshots3}).
This task just sends a simple temperature monitoring task to the device using \cleaninline{liftmTask} and provides a view on its task value using the \cleaninline{>\&>}\footnotemark{} \gls{ITASK} combinator.
\footnotetext{\cleaninline{(>\&>) infixl 1 :: (Task a) ((SDSLens () (? a) ()) -> Task b) -> Task b \| iTask a \& iTask b}}
The light switch task at \crefrange{lst:example:ls1}{lst:example:ls2} is a task that has bidirectional interaction using the definition of \cleaninline{lightswitch} shown in \cref{lst:mtask_liftsds_ex}.
Using \cleaninline{liftsds}, the status of the light switch is synchronised with the user.
The task on the edge device continuously monitors the value of the lowered \gls{SDS}.
If it is different from the current state, the new value is written to the digital \gls{GPIO} pin 13 and the monitoring function is recursively called.

\begin{figure}[ht]
	\centering
	\begin{subfigure}[b]{.3\linewidth}
		\includegraphics[width=\linewidth]{home_auto1}
		\caption{Select task.}%
		\label{fig:example_screenshots1}
	\end{subfigure}
	\begin{subfigure}[b]{.3\linewidth}
		\includegraphics[width=\linewidth]{home_auto2}
		\caption{Select device.}%
		\label{fig:example_screenshots2}
	\end{subfigure}
	\begin{subfigure}[b]{.3\linewidth}
		\includegraphics[width=\linewidth]{home_auto3}
		\caption{View result.}%
		\label{fig:example_screenshots3}
	\end{subfigure}
	\caption{Screenshots of the home automation example program in action.}%
	\label{fig:example_screenshots}
\end{figure}

\begin{figure}
	\cleaninputlisting[firstline=12,lastline=50,numbers=left,belowskip=0pt]{lst/example.icl}
	\begin{lstClean}[numbers=left,firstnumber=40,aboveskip=0pt,caption={An example of a home automation program.},label={lst:example_home_automation}]
	, ...][+\label{lst:example:tasks2}+]\end{lstClean}
\end{figure}

\section{Conclusion}
When \gls{IOT} edge devices run the \gls{MTASK} \gls{RTS}, they become little \gls{TOP} engines of their own.
Using just three \gls{ITASK} functions, \gls{MTASK} devices are integrated in \gls{ITASK} seamlessly.
Devices, using any supported type of connection, are integrated in \gls{ITASK} using the \cleaninline{withDevice} function.
Once connected, \gls{MTASK} tasks are sent to the device for execution using \cleaninline{liftmTask}, lifting them to full-fledged \gls{ITASK} tasks.
To lower the bandwidth, tasks can also be preloaded.
Furthermore, the \gls{MTASK} tasks interact with \gls{ITASK} \glspl{SDS} using the \cleaninline{lowerSds} construct.
All of this together allows programming all layers of an \gls{IOT} system from a single source and in a single paradigm.
All details regarding interoperation are automatically taken care of.

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