top/int.tex

   1 \documentclass[../thesis.tex]{subfiles}
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   3 \input{subfilepreamble}
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   5 \begin{document}
   6 \input{subfileprefix}
   7
   8 \chapter{Integration with \texorpdfstring{\gls{ITASK}}{iTask}}%
   9 \label{chp:integration_with_itask}
  10 \begin{chapterabstract}
  11         This chapter shows the integration of \gls{MTASK} with \gls{ITASK} by showing:
  12         \begin{itemize}
  13                 \item an architectural overview \gls{MTASK} applications;
  14                 \item on the interface for connecting devices;
  15                 \item the interface for lifting \gls{MTASK} tasks to \gls{ITASK} tasks;
  16                 \item and interface for lifting \gls{ITASK} \glspl{SDS} to \gls{MTASK} \glspl{SDS}.
  17         \end{itemize}
  18 \end{chapterabstract}
  19
  20 The \gls{MTASK} language is a multi-view \gls{DSL}, i.e.\ there are multiple interpretations possible for a single \gls{MTASK} term.
  21 Using the byte code compiler (\cleaninline{BCInterpret}) \gls{DSL} interpretation, \gls{MTASK} tasks can be fully integrated in \gls{ITASK}.
  22 They are executed as if they are regular \gls{ITASK} tasks and they communicate may access \glspl{SDS} from \gls{ITASK} as well.
  23 \Gls{MTASK} devices contain a domain-specific \gls{OS} and are little \gls{TOP} engines in their own respect, being able to execute tasks.
  24 \Cref{fig:mtask_integration} shows the architectural layout of a typical \gls{IOT} system created with \gls{ITASK} and \gls{MTASK}.
  25 The entire system is written as a single \gls{CLEAN} specification where multiple tasks are executed at the same time.
  26 Tasks can access \glspl{SDS} according to many-to-many communication and multiple clients can work on the same task.
  27 Devices are integrated into the system using the \cleaninline{withDevice} function (see \cref{sec:withdevice}).
  28 Using \cleaninline{liftmTask}, \gls{MTASK} tasks are lifted to a device (see \cref{sec:liftmtask}).
  29 \Gls{ITASK} \glspl{SDS} are lifted to the \gls{MTASK} device using \cleaninline{liftsds} (see \cref{sec:liftmtask}).
  30
  31 \begin{figure}[ht]
  32         \centering
  33         \includestandalone{mtask_integration}
  34         \caption{\Gls{MTASK}'s integration with \gls{ITASK}.}%
  35         \label{fig:mtask_integration}
  36 \end{figure}
  37
  38 \section{Connecting edge devices}\label{sec:withdevice}
  39 When interpreted by the byte code compiler view, an \gls{MTASK} task produces a compiler.
  40 This compiler is exceuted at run time so that the resulting byte code can be sent to an edge device.
  41 All communication with this device happens through a so-called \emph{channels} \gls{SDS}.
  42 The channels contain three fields, a queue of messages that are received, a queue of messages to send and a stop flag.
  43 Every communication method that implements the \cleaninline{channelSync} class can provide the communication with an \gls{MTASK} device.
  44 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}).
  45 The \cleaninline{withDevice} function transforms such a communication provider and a task that does something with this device to an \gls{ITASK} task.
  46 Internally, the task sets up the communication, exchanges specifications with the device, executes the inner task while handling errors, and finally cleans up after closing.
  47 \Cref{lst:mtask_device} shows the types and interface to connecting devices.
  48
  49 \begin{lstClean}[label={lst:mtask_device},caption={Device communication interface in \gls{MTASK}.}]
  50 :: MTDevice //abstract
  51 :: Channels :== ([MTMessageFro], [MTMessageTo], Bool)
  52
  53 class channelSync a :: a (Shared sds Channels) -> Task () | RWShared sds
  54
  55 withDevice :: a (MTDevice -> Task b)
  56         -> Task b | iTask b & channelSync, iTask a
  57 \end{lstClean}
  58
  59 \subsection{Implementation}
  60 The \cleaninline{MTDevice} abstract type is internally represented as three \gls{ITASK} \gls{SDS} that contain all the current information about the tasks.
  61 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.
  62 The second \gls{SDS} is a map storing downstream \gls{SDS} updates.
  63 When a lifted \gls{SDS} is updated on the device, a message is sent to the server.
  64 This message is initially queued in the map to allow for asynchronous handling of multiple updates.
  65 Finally, the \cleaninline{MTDevices} type contains the communication channels.
  66
  67 The \cleaninline{withDevice} task itself first constructs the \glspl{SDS} using the \gls{ITASK} function \cleaninline{withShared} to create anonymous local \glspl{SDS}.
  68 Then, it performs the following four tasks in parallel to monitor the edge device.
  69 \begin{enumerate}
  70         \item It synchronises the channels using the \cleaninline{channelSync} overloaded function.
  71                 Errors that occur here are converted to the proper \gls{MTASK} exception.
  72         \item Watches the channels for the shutdown flag.
  73                 If the connection is lost with the device unexpectedly, an \gls{MTASK} exception is thrown.
  74         \item Watches the channels for messages and processes them accordingly by changing the device information \gls{SDS} or adding the lifted \gls{SDS} updates to the corresponding \gls{SDS} update queue.
  75         \item Sends a request for a specification. Once the specification is received, the device task is run.
  76                 The task value of this device task is then used as the task value of the \cleaninline{withDevice} task.
  77 \end{enumerate}
  78
  79 If at any stage an unrecoverable device error occurs, an \gls{ITASK} exception is thrown on the \cleaninline{withDevice} task.
  80 This exception can be caught in order to device some kind of fail-safe mechanism.
  81 For example, when a device fails, the tasks can be sent to another device.
  82 \todo[inline]{Example of failsafe?}
  83
  84 %withDevice spec deviceTask
  85 %       withShared newMap \sdsupdates->
  86 %       withShared ([], [MTTSpecRequest], False) \channels->
  87 %       withShared default \dev->parallel
  88 %               [ channelSync spec // unexpected disconnect
  89 %               , watchForShutdown // unexpected disconnect
  90 %               , watchChannels    // process messages
  91 %               , waitForSpecification
  92 %                       >>| deviceTask
  93 %                       >>* [ifStable: issueShutdown]
  94 %               ]
  95
  96 \section{Lifting \texorpdfstring{\gls{MTASK}}{mTask} tasks}\label{sec:liftmtask}
  97 Once the connection with the device is established, \gls{MTASK} tasks can be lifted to \gls{MTASK} tasks using the \cleaninline{liftmTask} family of functions (see \cref{lst:liftmtask}).
  98 Given an \gls{MTASK} task in the \cleaninline{BCInterpret} view and a device obtained from \cleaninline{withDevice}, an \gls{ITASK} task is returned.
  99 This \gls{ITASK} task tethers the \gls{MTASK} task that is executed on the microcontroller.
 100 Hence, when for example observing the task value, the actual task value from the microcontroller is observed.
 101
 102 \begin{lstClean}[label={lst:liftmtask},caption={The interface for lifting \gls{MTASK} tasks to \gls{ITASK} tasks.}]
 103 liftmTask :: (Main (MTask BCInterpret u)) MTDevice -> Task u | iTask u
 104 \end{lstClean}
 105
 106 Under the hood, \cleaninline{liftmTask}:
 107 \begin{itemize}
 108         \item Generates a fresh task identifier for the device.
 109         \item Compiles the task and fetches the values for the tethered \glspl{SDS}.
 110         \item Sends the task to the device
 111         \item Watches, in parallel: the tethered \glspl{SDS} in \gls{ITASK}, if they are updated, a message is sent to the device; the \gls{SDS} update queue, if there is a downstream update, the \gls{ITASK} \gls{SDS} it references is updated as well; and the task value.
 112 \end{itemize}
 113
 114 The task value of the \cleaninline{liftmTask} task is the task value of the task on the edge device.
 115
 116 \section{Lifting \texorpdfstring{\gls{ITASK}}{iTask} \texorpdfstring{\glsxtrlongpl{SDS}}{shared data sources}}\label{sec:liftsds}
 117 Lifting \gls{ITASK} \glspl{SDS} to \gls{MTASK} \glspl{SDS} is something that mostly happens at the compiler level using the \cleaninline{liftsds} function (see \cref{lst:mtask_itasksds}).
 118 \Glspl{SDS} in \gls{MTASK} must always have an initial value.
 119 For regular \gls{SDS} this value is given in the source code, for lifted \gls{ITASK} \glspl{SDS} this value is obtained by reading the values once just before sending the task to the edge device.
 120 On the device itself, there is just one difference between lifted \glspl{SDS} and regular \glspl{SDS}: after changing \pgls{SDS}, a message is sent to the server containing this new value.
 121 The \cleaninline{withDevice} task (see \cref{sec:withdevice}) receives and processes this message by writing to the \gls{ITASK} \gls{SDS}.
 122 Tasks watching this \gls{SDS} get notified then through the normal notification mechanism of \gls{ITASK}.
 123
 124 \begin{lstClean}[label={lst:mtask_itasksds},caption={Lifted \gls{ITASK} \glspl{SDS} in \gls{MTASK}.}]
 125 class liftsds v where
 126         liftsds :: ((v (Sds t)) -> In (Shared sds t) (Main (MTask v u)))
 127                 -> Main (MTask v u) | RWShared sds
 128 \end{lstClean}
 129
 130 The compilation of the code and the serialisation of the data throws away all typing information.
 131 \Glspl{SDS} are stored in the compiler state as a map from identifiers to either an initial value or an \cleaninline{MTLens}.
 132 The \cleaninline{MTLens} is a type synonym for a \gls{SDS} that represents the typeless serialised value of the underlying \gls{SDS}.
 133 This is done so that the \cleaninline{withDevice} task can write the received \gls{SDS} updates to the according \gls{SDS} independently.
 134 \Gls{ITASK}'s notification mechanism then takes care of the rest.
 135 Such a \gls{SDS} is created by using the \cleaninline{mapReadWriteError} which, given a pair of read and write functions with error handling, produces a \gls{SDS} with the lens embedded.
 136 The read function transforms, the function that converts a typed value to a typeless serialised value, just applies the serialisation.
 137 The write function, the function that, given the new serialised value and the old typed value, produces a new typed value.
 138 It tries to decode the serialised value, if that succeeds, it is written to the underlying \gls{SDS}, an error is thrown otherwise.
 139 \Cref{lst:mtask_itasksds_lens} provides the implementation for this:
 140
 141 % VimTeX: SynIgnore on
 142 \begin{lstClean}[label={lst:mtask_itasksds_lens},caption={Lens applied to lifted \gls{ITASK} \glspl{SDS} in \gls{MTASK}.}]
 143 lens :: (Shared sds a) -> MTLens | type a & RWShared sds
 144 lens sds = mapReadWriteError
 145         ( \r->Ok (fromString (toByteCode{|*|} r)
 146         , \w r-> ?Just <$> iTasksDecode (toString w)
 147         ) ?None sds
 148 \end{lstClean}
 149 % VimTeX: SynIgnore off
 150
 151 \Cref{lst:mtask_itasksds_lift} shows the code for the implementation of \cleaninline{liftsds} that uses the \cleaninline{lens} function shown earlier.
 152 First, the \gls{SDS} to be lifted is extracted from the expression by bootstrapping the fixed point with a dummy value.
 153 This is safe because the expression on the right-hand side of the \cleaninline{In} is never used.
 154 Then, using \cleaninline{addSdsIfNotExist}, the identifier for this particular \gls{SDS} is either retrieved from the compiler state or generated freshly.
 155 This identifier is then used to provide a reference to the \cleaninline{def} definition to evaluate the main expression.
 156
 157 % VimTeX: SynIgnore on
 158 \begin{lstClean}[label={lst:mtask_itasksds_lift},caption={Lens applied to lifted \gls{ITASK} \glspl{SDS} in \gls{MTASK}.}]
 159         liftsds def = {main =
 160                         let (t In _) = def (abort "liftsds: expression too strict")
 161                         in addSdsIfNotExist (Right $ lens t)
 162                                 >>= \sdsi->let (_ In e) = def (pure (Sds sdsi)) in e.main
 163                 }
 164         where
 165 \end{lstClean}
 166 % VimTeX: SynIgnore off
 167
 168 \section{Conclusion}
 169
 170
 171 \input{subfilepostamble}
 172 \end{document}