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\chapter{Integration of mTask and iTask}%
\label{chp:integration_with_itask}
\begin{chapterabstract}
	This chapter shows the integration of \gls{MTASK} and \gls{ITASK} by showing:
	\begin{itemize}
		\item an architectural overview of \gls{MTASK} applications;
		\item 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 a non-trivial home automation example application using all integration mechanisms;
	\end{itemize}
\end{chapterabstract}

The \gls{MTASK} system is a \gls{TOP} \gls{DSL} for edge devices.
It is a multi-view \gls{DSL}, there are multiple interpretations possible for a single \gls{MTASK} term.
The main interpretation of \gls{MTASK} terms is the byte code compiler, \cleaninline{:: BCInterpret a}.
When using this interpretation and a few integration functions, \gls{MTASK} tasks are fully integrated in \gls{ITASK}.
They execute as regular \gls{ITASK} tasks and they can access \glspl{SDS} from \gls{ITASK}.
Devices in the \gls{MTASK} system are set up with a domain-specific \gls{OS} and become 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 connected to 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 overview of an \imtask{} application.}%
	\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 take initiative, set up a connection, and send the work.
The heavy lifting of connecting an \gls{MTASK} device to an \gls{ITASK} server is done with the \cleaninline{withDevice} \gls{ITASK} function.
This function has two parameters, a communication specification, and a function using a device handle.
The device handle is required to interact with \gls{MTASK} devices, e.g.\ lift tasks.
By using \gls{HOAS} like this, setting up and tearing down the connection to the device is fully controlled.

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.
At the time of writing, serial port, direct \gls{TCP}, and \gls{MQTT} over \gls{TCP} are supported communication methods (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 for 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 in order to properly handly multiple updates asychronously.
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}.
Then, it performs the following four tasks in parallel to monitor the edge device.
\begin{enumerate}
	\item The channels are synchronised using the overloaded \cleaninline{channelSync} function.
		Errors that occur here are converted to the proper \gls{MTASK} or \gls{ITASK} exception.
	\item The shutdown flag of the channels is watched.
		If the connection is lost with the device unexpectedly, an \gls{MTASK} exception is thrown.
	\item The received messages in the channels are watched and processed.
		Depending on the type of message, either the device information \gls{SDS} is updated, or the \gls{SDS} update is added to the lowered \gls{SDS} updates \gls{SDS}.
	\item A request for a specification is sent.
		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{withDevice} function in \gls{MTASK}.},label={lst:pseudo_withdevice}]
withDevice spec deviceTask =
	withShared default \dev->parallel
	withShared newMap \sdsupdates->
	withShared ([], [MTTSpecRequest], False) \channels->
		[ channelSync spec channels
		, watchForShutdown channels
		, watchChannelMessages dev channels
		, waitForSpecification
			>>| deviceTask (MTDevice dev sdsupdates channels)
			>>* [ifStable: issueShutdown]
		]
\end{lstClean}

If at any stage an unrecoverable device error occurs, an \gls{ITASK} exception is thrown in 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 type of 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 mTask tasks}\label{sec:liftmtask}
Once the connection with the device is established, \gls{MTASK} tasks are lifted to \gls{ITASK} tasks using the \cleaninline{liftmTask} function (see \cref{lst:liftmtask}).
Given an \gls{MTASK} task in the \cleaninline{BCInterpret} view and a device handle 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.
So, when another task observes 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 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 upstream \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 induces a lot 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 preloaded 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 should be placed in the source code directory of the \gls{RTS} before building to include it in the firmware.

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

\section{Lowering iTask shared data sources}\label{sec:liftsds}
Lowering \gls{ITASK} \glspl{SDS} to \gls{MTASK} \glspl{SDS} is something that mostly happens at the \gls{DSL} level using the \cleaninline{lowerSds} function (see \cref{lst:mtask_itasksds}).
Lowering \pgls{SDS} proxies the \gls{ITASK} \gls{SDS} for use in \gls{MTASK}.
\Glspl{SDS} in \gls{MTASK} 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, there is just one difference between lowered \glspl{SDS} and regular \glspl{SDS}: after changing a lowered \gls{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}.
\Cref{lst:imp_sds} shows the implementation of this type class for the byte code compiler.

\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 in \gls{MTASK}.}]
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 (\v->v !=. st) (\v->writeD d13 v)]
			>>|. f (Not st))
		In {main=f true}
\end{lstClean}

\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.
The following section contains an elaborate example using all integration functions that has deliberately been placed after the conclusion so that the code listing and description are on facing pages.

\begin{figure}[p]
	\begin{fullpage}
%	\begin{leftfullpage}
		\vspace{\headsep}
\section{Home automation}
This section presents an interactive home automation program (\cref{lst:example_home_automation}) to illustrate the integration of the \gls{MTASK} language and the \gls{ITASK} system.
It consists of a web interface for the user to control which tasks are executed on either one 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} contains the \gls{ITASK} task that coordinates the \gls{IOT} application.
First the devices are connected (\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, and 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 is 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 are 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 it 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{>\&>} \gls{ITASK} combinator.
This combinator allows the observation of the left-hand side task's value through \pgls{SDS}.
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{lowerSds}, the status of the light switch is synchronised with the user.
Finally, a task that calculates the factorial of a user-provided number is shown in the list.

		\vspace{4ex}
		\begin{center}
			\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{center}
	%\end{leftfullpage}
	\end{fullpage}
\end{figure}

\begin{figure}[p]
	\begin{fullpage}
		\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{fullpage}
\end{figure}

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