Communicating with Devices Prasun Dewan Department of Computer Science University of North Carolina [email protected]

Communicating with Devices

Prasun Dewan

Department of Computer Science University of North Carolina

[email protected]

2

Steps to UPnP Networking

0 Control point and device get addresses1 Control point finds interesting device2* Control point learns about device capabilities3 Control point invokes actions on device4 Control point listens to state changes of device5 Control point controls device and/or views device status

using HTML UI

0 Addressing0 Addressing

1 Discovery1 Discovery

2* Description2* Description

5 Presentation5 Presentation4 Eventing4 Eventing3 Control3 Control

3

Example Service

<?xml version="1.0" ?> <scpd xmlns="urn:schemas-upnp-org:service-1-0"> <specVersion> <major>1</major> <minor>0</minor> </specVersion>

4

Example Action List

<actionList> <action> <name>PowerOn</name> </action> <action> <name>SetChannel</name>

<argumentList> <argument>

<name>Channel</name> <relatedStateVariable>Channel</relatedStateVariable> <direction>in</direction>

</argument> </argumentList>

</action> ….

<actionList>

5

Example Variables<serviceStateTable> <stateVariable sendEvents="yes"> <name>Power</name> <dataType>Boolean</dataType> <defaultValue>0</defaultValue> </stateVariable> <stateVariable sendEvents="yes"> <name>Channel</name> <dataType>i4</dataType> <allowedValueRange> <minimum>1</minimum> <maximum>100</maximum> <step>1</step> </allowedValueRange> <defaultValue>1</defaultValue> </stateVariable> …..</serviceStateTable>

6

Example Server CodeObject processMessages (Message message) {

switch (message.kind) case GET_VAR_REQUEST:

switch ((GetVarMessage) message).varName) case “Channel”: message.result = getChannel();

… case ACTION_REQUEST:

actionMessage = (ActionMessage) message.switch (actionMessage.actionName) case “setChannel”: setChannel(parseString(actionMessage.params[0])) …

}int channel;int getChannel () { return curentChannel};void setChannel (int newChannel) { channel = newChannel; UPnP.announce(“Channel”, toString(getChannel());}...

7

High Programming Cost Device developer must:

define the XML-based external representation of the device

translate the client operation-invocation and variable-access requests to local operation-invocations

announce variable change notifications Exporting functionality requires more code than

implementing it in example Intel device not counting UI code

Cost proportional to complexity of type. Reason for not allowing complex types?

8

High Maintenance Cost Information repeated multiple times

operation implementation code action list state variable table message processing code event announcement code

All repetitions must be changed tedious error-prone

Channel -> Current Channel requires 9 changes!

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Changed Action List<actionList> <action> <name>PowerOn</name> </action> <action> <name>SetCurrentChannel</name> <argumentList> <argument>

<name>CurrentChannel</name> <relatedStateVariable>CurrentChannel</relatedStateVariable>

<direction>in</direction> </argument> </argumentList>

</action> ….

<actionList>

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Changed State Variable Table<serviceStateTable> <stateVariable sendEvents="yes"> <name>Power</name> <dataType>Boolean</dataType> <defaultValue>0</defaultValue> </stateVariable> <stateVariable sendEvents="yes"> <name>CurrentChannel</name> <dataType>i4</dataType> <allowedValueRange> <minimum>1</minimum> <maximum>100</maximum> <step>1</step> </allowedValueRange> <defaultValue>1</defaultValue> </stateVariable> …..</serviceStateTable>

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Changed Server CodeObject processMessages (Message message) {

switch (message.kind) case GET_VAR_REQUEST: switch ((GetVarMessage) message).varName) case “CurrentChannel”: message.result = getCurrentChannel();

… case ACTION_REQUEST: actionMessage = (ActionMessage) message. switch (actionMessage.actionName)

case “setCurrentChannel”: setCurrentChannel(parseString(actionMessage.params[0]))

…}int channel;int getCurrentChannel () {return channel};void setCurrentChannel (int newChannel) { channel = newChannel; UPnP.announce(“CurrentChannel”, toString(getCurrentChannel());}...

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Ideal Developer Effort

int channel;int getChannel () { return channel};void setChannel (int newChannel) { channel = newChannel;}...

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No Programmer-Defined Types XML defines 24 data types

ui1, ui2, ui4, i1, i2, i4, int r4, r8, number, fixed.14.4, float char, string date, dateTime, dateTime.tz, time, time.tz boolean bin.base64, bin.hex uri uuid

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No Programmer-Defined Types Only supports predefined types

state variables action parameters

No way to model dynamic composites

Dynamic programmer-defined collection

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Generating Action List

Generator

External DescriptionDevice Code

Actions

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<action list><action> <name>SetCurrentChannel</name> <argumentList> <argument>

<name>NewChannel</name> <relatedStateVariable>

CurrentChannel </relatedStateVariable>

<direction>in</direction> </argument> </argumentList> </action>….</actionList>

Generating Action Descriptions

Mainly syntactic issue Static approach

precompiler Dynamic approach

reflection vcr.getClass().getMethods() method.getName() method.getParameterTypes()

Done in ICrafter (Stanford) How to get related state

variable?

int channel;int getChannel () { return channel};void setChannel (int newChannel) { channel = newChannel;}

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Why variables? Clients may view device as

state manager Because their user has this

view. VCR user interface

May compose state aggregating sensors

More abstract to associate events with variable changes rather than method invocations Channel changed

vs.SetChannel called Similarly access and

concurrency control state specific

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Generating External Representation

Generator

External Description

Actions

Variables

Problem in general unsolvable

• Need help from programmer

Device class

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Generating Variable DescriptionsBasic idea Object state stored in internal vars. Make them public.

Drawbacks Violates encapsulation

Changing internal representation changes clients

Not all state may be stored How to get

Related state vars Min, max range

How to generate events public var could be changed

directly

public int channel;int getChannel () { return channel};void setChannel (int newChannel) { channel = newChannel;}…

<serviceStateTable> <stateVariable sendEvents="yes"> <name>Channel</name> <dataType>i4</dataType> <allowedValueRange> <minimum>1</minimum> <maximum>100</maximum> <step>1</step> </allowedValueRange> <defaultValue>1</defaultValue> </stateVariable> …..</serviceStateTable>

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Interface-based extraction Abstract data type helps by exposing

logical structure using standard system-defined interface (TreeNode) Approach used in Swing table

widget Equivalent to writing external

description extra programming effort extra maintenance effort no type checking (everything

TreeNode) Cannot generate types of

variables

must use common interfaces and associated object files

public Vector getVarNames();public TreeNode getVar(String fieldName);public void setVar(String fieldName, val TreeNode)

public class VCR implements TreeNode{ int channel; int getChannel () { return channel}; void setChannel (int newChannel) { channel = newChannel; }…}

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int channel;int getChannel () { return channel};void setChannel (int newChannel) { channel = newChannel;}…

Pattern-based Approach Can use relationships

between operation names and signatures to derive external variables

(properties) channel : int

related state variable setChannel changes

channel Relationships defined by

Java beans

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Simple Properties in Java

Defined by a pair of ‘getter’/’setter’ methods:String getTitle();

void setTitle(String newTitle)

define a title property of type String.

In general,<PropertyType> get<PropertyName>()void set<PropertyName>(<PropertyType>)

define a simple property named <PropertyName> of type <PropertyType> with read/write semantics.

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Role of JavaBeans Properties

‘Properties are discrete named attributes of a Java Bean that can affect its appearance, or its behavior.’ typical properties:

font color size

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Properties as Record FieldsMain idea: consider properties as

elements of the logical structure of the object.

record fieldsProblems Forces conventions

Channel() instead of getChannel()?

Legacy code Min, max range?



25

Property Specification

Approach: separate property specification and

interpretation Mechanism

introduce a pattern language for property specification

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Property Extractor

Pattern specification

Property Definitions

Device Operations

Pattern specifications

Implementation must conform to specified pattern

User-defined, but less work and no duplication

Approach: separate property

specification and interpretation

Mechanism introduce a pattern

language for property specification

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Examples Specification 1

String getTitle();


define a title property of type String. Specification 2

String title();



Free Variables Unified

Literals

Grammar? Methods as

sentences Property name

and types not known

Must be extracted Unification

Methods as facts Property names

and types as unified variables

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Property Specification Language

Canonical representation of method signatures:<return_type> method_name(arg1_type, …,argN_type)

An example specification: simple properties1. Method pattern specifications: getter = <.GetType.> get.PropName.() setter = <void> set.PropName.(.SetType.) pattern variables: GetType, PropName, SetType example matches:getter: String getTitle() {GetType == “String”,PropName == “Title”}

setter: void setTitle(String) {SetType == “String”, PropName == “Title”}

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Java Bean Specification with Constraints

property type = simple access methods = getter, settergetter = <.GetType.> get.PropName.()

setter = <void> set.PropName.(.SetType.) constraints getter.PropName == setter.PropName getter.GetType == setter.SetType name = getter.PropNameend

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Examples Specification 1

String getTitle();


define a title property of type String. Specification 2

String title();



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Alternative Bean Specification

property type = simple access methods = getter, setter

getter = <.GetType.>.PropName.()

setter = <void> set.PropName.(.SetType.) constraints

getter.PropName == setter.PropNamegetter.GetType == setter.SetType

name = getter.PropNameend

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Properties as Record Fields Allows

Channel() instead of getChannel()

Legacy code Issues

Min, max range?



33

Min, Max Extensions

Additional Constraints

Propertytype = simple

access methods = min, max

min = <int> getMin.PropName.()

max = <int> getMax.PropName.()

step = <int> getStep.PropName.() … constraints getter.PropName==min.PropName== max.PropNameend

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Alternative Min, Max Extensions

Constraints on variable declarations

property type = simple

access fields = min, max, stepmin = <final> <int> MIN_.PropName.max = <final> <int> MAX_.PropName.step = <final> <int> STEP_.PropName.

constraints min.PropName==max.PropName== step.PropName

== getter.PropNameend

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No Programmer-Defined Types Only supports predefined types

state variables action parameters

No way to model dynamic composites

Dynamic programmer-defined collection

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Two issues

Representation of programmer-defined types in XML Seems to be syntactic issue

Generating external representation Static (Record) structures addressed

JavaBeans defines field properties Language allows convention definitions

Dynamic structures?

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Indexed Properties in Java

An indexed property isa simple property of an array type:

Section[] getSection()

void setSection( Section[] section)

with two additional methods:

Section getSection( int index)

void setSection( Section s, int index) Emulates array semantics in procedural languages

38

Understand Java Vector

class VCR { void power(); … int getCurrentChannel() ; void setCurrentChannel(int newVal) ; Vector getSettings(); void setSettings(Vector newVal)}

Cannot support programmer-defined dynamic list

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Example Programmer-defined List class VCR { void power(); … int getCurrentChannel() ; void setCurrentChannel(int newVal) ; int size(); Setting elementAt(int index); void removeElement(Setting newVal); void addElement (Setting newVal); void setElementAt(Setting newVal, int index); }

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Sequence propertiesinsert = <void> insert.Prop.at( int, .InsType.)remove = <void> remove.Prop.at( int)lookup = <.GetType.> .Prop.At( int)set = <void> set.Prop.At( int, .SetType.)count = <int> size()property type = sequence access methods = insert, remove, lookup, count, set constraints insert.Prop == remove.Prop == lookup.Prop insert.InsType == lookup.SetType == lookup.GetType name = insert.Prop element type = insert.InsType

public int size();public int settingAt(int index); public void setSettingAt(int index, Setting newVal);public void insertSettingAt(int index, Setting newVal);

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Programmer-Defined Types in External Representation

Concrete Constructors Pascal enumeration Subrange Hashtable Array Sequence (variable size list) Record

Not abstract types Appliance object is abstract type Must deconstruct object so that variables are

known, that is, must map it to concrete type

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Property Extractor

Pattern specification

Property Definitions

Device Operations

Pattern specifications

Programmer must conform to specified pattern

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Conforming to Patterns Makes code understandable to humans Also makes code understandable to system tools

Bean builder Property Extractor

Unlike Bean framework, the patterns chosen by users

System adapts to application interfaces rather than vice versa

More flexible than the alternative of system-defined interfaces Pattern specification without pattern variables is

simply an interface

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public int size();public int elementAt(int index); public void setElementAt(int index, Object newVal);public void insertElementAt(int index, Object newVal);public void insertElementAt(int index, Object newVal);

insert = <void> insertElementat( int, Object)

remove = <void> removeElementAt( int)lookup = Object elementAt( int)set = <void> setElementAt( int, Object)count = <int> size()property type = sequence access methods = insert, remove,

lookup, count, set name = vector element type = Object

Sequence interface

Fixes method names

Fixes element type to Object not Setting

no type

checking

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Pattern Variables Allowing Multiple Methods Names/Element Types

insert = <void> insert.Prop.at( int, .InsType.)remove = <void> remove.Prop.at( int)lookup = <.GetType.> .Prop.At( int)set = <void> set.Prop.At( int, .SetType.)count = <int> size()property type = sequence access methods = insert, remove, lookup, count, set constraints insert.Prop == remove.Prop == lookup.Prop insert.InsType == remove.SetType == lookup.GetType name = insert.Prop element type = insert.InsType

public int size();public int elementAt(int index); public void setElementAt(int index, Object newVal);public void insertElementAt(int index, Object newVal);public void removeElementAt(int index, Object newVal)

public int size();public int settingAt(int index); public void setSettingAt(int index, Setting newVal);public void insertSettingAt(int index, Setting newVal);public void removeElementAt(int index);

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Alternative Sequence properties with same access methods

insert = <void> insert.Prop.at( int, .InsType.)remove = <void> remove.Prop.at( int)index = <.GetType.> .Prop.At( int)set = <void> set.Prop.At( int, .SetType.)count = <int> count()property type = sequence access methods = insert, remove, index, count, set constraints insert.Prop == remove.Prop == index.Prop insert.InsType == remove.SetType == lookup.GetType name = insert.Prop element type = insert.InsType

public int count();public int settingAt(int index); public void setSettingAt(int index, Setting newVal);public void insertSettingAt(int index, Setting newVal);public void removeElementAt(int index);

47

Alternative Sequence properties with different access methods

insert = <void> insert.Prop.at( int, .InsType.)remove = <void> remove.Prop.at( int)iterate = Enumeration elements()set = <void> set.Prop.At( int, .SetType.)property type = sequence access methods = insert, remove, iterate, set constraints insert.Prop == remove.Prop == lookup.Prop insert.InsType == remove.SetType == lookup.GetType name = insert.Prop element type = insert.InsType

public Enumeration elements(); public void setSettingAt(int index, Setting newVal);public void insertSettingAt(int index, Setting newVal);public void removeElementAt(int index);

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Reducing High Programming Cost Device developer must:

define the XML-based external representation of the device system does it automatically, mapping operations to

external representation translate the client operation-invocation and

variable-access requests to local operation-invocations based on its knowledge of the mapping it can

translate automatically announce variable change notifications

system does successive object diffs to announce events automatically

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Device ObjectDiffer

Subscription Manager

Generating Events

Subscription Manager Separate from device Allows clients to register

interests in device Forwards announcements

to them Differ assumes subscription

manager exists Takes successive snapshots

When? How?

Client

register interest

forward event

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When Snapshots Taken? Client requests

user refreshed wants the differences rather than complete values

Some client has taken an action on device informs some global object about it other clients can now be notified

Timer-based differ polls when upto date information not crucial

Device requests Differ.announce() vs. UPnP.announce(“Channel”, toString(getChannel());

More maintainable

51

How Snapshots Compared?

Operations diff(s1, s2) for each property p of s1 and s2

diffs += diff (s1.p1, s2.p2)

diff(o1,o2) returns a sequence of operations that when applied to o1 take it to o2.

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How properties found?

Property extraction based on pattern specifications. But pattern specifications defined and interpreted by

user. matchedProperty.getMethod(“getter”).invoke (params) “getter” is user-defined

Also multiple access methods possible per property type indexing and iteration for sequences

Need a standard device-independent interface for each property constructor

property type = sequence access methods = getter, setter

...

53

Device Independent Interface Unix file operations for all devices

do not model structured objects UPnP

standard operation to read any variable no standard operation to write variable

client must determine device-specific write operation via relatedStateVariable clause

no programmer-defined types Need standard operations for each property

say indexing for sequence how mapped to device-specific methods?

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Mapping Approaches Global table/registry

Standard OS approach may not have one to one

mapping from device-independent to device-dependent indexing to iteration

Mapping per device? Additional effort on device

programmer Per-pattern mapping

done by property handlers apply to multiple device

classes

Device

Mapper

Device-Indep

Device-Dep

read() get ()

write() set ()

DeviceProperty Handlers

Device

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Property Handlers Specified in pattern

definition Implement device-

indep interface

property type = sequence access methods = elements, ... ... handlers lookup = IteratorToIndexing ...

interface Indexing { Object elementAt(Object target, MatchedProperty mp, int index;}

class IteratorToIndexing implements Indexing { ... Object elementAt(Object target, MatchedProperty mp, int index { Enumeration elements = mp.getMethod( ”elements")).invoke(target, null); for (i = 0; i < index; i++)

elements.nextElement(); return elements.nextElement(); }}

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Three Levels of Indirection Level 0

Client calls operation directly device.elementAt(index) bound to specific names

Level 1 Client calls pattern-specific access methods

mp.getMethod(“index”).invoke(device, {index}) lookup may be bound to elementAt() or settingAt()

Level 2 Client calls property handler mp.getClass(“lookup”).newInstance().invoke ( device, mp, {index}) Independent of access methods

indexing iteration

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Three Levels of Indirection

device1.elementAt (index) device2.settingAt(index)device3.iterate

()

mp.getAccessMethod(“index”). invoke(device, {index})

mp.getAccessMethod(“iterate”). invoke(device, {})

mp.getPropertyHandler (“lookup”). invoke(device, mp, {index})

58

Late Binding vs. Programmability/Efficiency

The later the binding the more levels of indirection the more awkward the programming the less efficient the program

Choose the minimum amount of indirection for the lateness desired

Device-independent interface meant really for system functions such as diff may be useful to clients

59

Vassil’s thesis Reduces programming cost

No cost of exporting functionality to distributed clients Like RPC, but accommodates late binding as UPnP

does dynamic service discovery

Reduces maintenance cost No duplication of information as it is all automatically

generated. Supports predefined and programmer-defined types

Makes reduction of programming and maintenance costs harder

60

Channel -> Current Channel requires 9 changes! getChannel -> getCurrentChannel setChannel -> setCurrentChannel

Reducing High Prog/Maintenance Cost

Information manually repeated multiple times operation implementation code action list state variable table message processing code event announcement code

2

Generated automatically

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Device Operations

Access Controller

Event Generator

External Description Generator

Property Extractor

Subscription Manager

Inter-Process Communication

Reflection

Differ

Access Specification

Device Handler

Device-Independent Interface

Property Specification

Architecture

Documents

Communicating with Devices Prasun Dewan Department of Computer Science University of North Carolina [email protected]