Comet - an Erlang-to-COM Port

1
Comet - an Erlang-to-COM Port

• Comet is a port and a gen_server module that
  enables Erlang programs to deploy COM components
• Comet is under development, an early version is
  part of OTP release 7B

2
COM

• Common Object Model
• A standard for component development from
  Microsoft
• Windows-only (although a third-party version
  exist on Solaris)
• Rival to CORBA on the Windows platform
• All Microsoft programs use COM
• Support for distribution, DCOM and COM

3
COM Model

• Classes presents interfaces
• Interfaces are a bunch of related functions or
  methods
• No data are exposed, only interfaces
• Properties of objects accessible through
  access-functions

4
COM Model continued

• IDL describes classes and interfaces
• IDL compiles into a type-library that can be
  browsed with a tool
• Two ways to use a class
• Dispatch - a special interface for interpreted
  languages
• Virtual Interface - faster, for compiled
  languages
• Comet can use both (although dispatch is safer)

5
COM Memory Handling

• Reference-counting
• Language support in Visual Basic, Java (and C)
• Erlang programs must (currently) explicitly free
  interfaces

6
Erlang Ports

• A way to call external code from Erlang
• Implemented as a linked in driver (DLL) or a port
  program
• Comet offers both
• port driver is considerably faster
• port program is safer, if the COM server crashes,
  it wont bring the emulator down
• A gen_server module interfaces to the port
  program or driver
• The Comet port driver and program are
  multithreaded

7
Comet as a Port Driver

• An application calling a COM Object
• Comet as a driver

Erlang process
ErlComDrv.dll
some_application
erl_com
thread
IWhatever
8
Comet as a Port Program

• An application calling a COM Object
• Comet as a port program

Erlang process
ErlComDrv.exe
some_application
erl_com
thread
IWhatever
9
Calling COM from Erlang

• All calls through the gen_server module erl_com
• erl_com provides methods for calling
• erl_com has functions for
• creating objects
• fetching interfaces
• releasing interfaces and objects
• retrieving type information of objects and types
• creating threads for calling COM objects
  asynchronously

10
A Simple Example

• An interface that implements some utilities
• Calling it from Erlang

interface ISomeUtilities IDispatch
id(1) HRESULT DaysBetween(in DATE date1,
in DATE date2, out,
retval double daysBetween) id(2)
HRESULT ReplaceAll(in BSTR inStr, in BSTR
keyStr, in BSTR newStr,
out, retval BSTR outStr) ...
... S It was a dark and stormy night..., I
erl_comcreate_dispatch(T, class id for
SomeUtilities), S2 erl_cominvoke(I,
ReplaceAll, S, stormy, still), D
erl_cominvoke(I, DaysBetween, vt_date,
2000, 1, 1, 0, 0, 0,
vt_date, erlangnow()) erl_comr
elease(I), ...
11
Mapping COM Types to Erlang

• COM uses a small set of types
• Comet mapps Erlang types to COM types through the
  use of tuples
• Basic types are converted properly integers,
  floats, strings and booleans
• Other types are prefixed in a tuple, e.g.
  vt_date 1999, 12, 12,
• Constants in COM are enumerations
• Strings currently 8-bits in Comet
• Complex types as structures, are currently not
  supported

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Invoke (dispatch interface)

• The invoke method in the dispatch interface is
  used to late-bind to interfaces
• Comet provides the methods invoke, property_put
  and property_get
• Errors returned as com_error, Code
• Can have named parameters (not support in Comet
  yet)

... Obj erl_comcreate_object(T, class
id), I erl_comquery_interface(Obj, a
dispatch interface id), Value
erl_cominvoke(I, Method, parameters), erl_com
property_put(I, Property, parameters,
Value2), Value3 erl_comproperty_get(I,
AnotherProperty), ...
13
Calling a Virtual Interface

• A virtual interface is an array of function
  pointers
• Virtual Method Table used for C objects
• Called in Comet using assembler glue
• Every parameter, including return value, must be
  explicitly typed
• Address of virtual function must be specified
• Only practical when code is generated
• Wrong parameters cause Comet to crash

... Outstr erl_comcom_call(I_, 36, vt_str,
InStr, vt_str, out), ...
14
Browser Example

• The Internet Explorer browser presents COM
  interfaces
• Example creating an Internet Explorer and
  navigating to a URL
• 1 opens a Comet process and a thread in it
• 2 creates an object, retrieves its default
  interface
• 3 invokes the methods navigate and the visible

15
Excel Example

• Excel is also accessible through COM
• Easiest way is to start with a Visual
  Basic-program
• The Excel macro recorder can generate these
• Example adding a graph

Visual Basic Charts.Add ActiveChart.ChartType
xlPieExploded Erlang Charts erl_compackage_int
erface(E, erl_comproperty_get(E,
"Charts")), erl_cominvoke(Charts, "Add"), C
erl_compackage_interface(E, erl_comproperty_get(
E, "ActiveChart")), erl_comproperty_put(C,
"ChartType", ?XlPieExploded),
16
Generating Glue Code

• Can be used for both virtual- and
  dispatch-interfaces
• Type libraries, compiled from COM IDL, describes
  COM classes and interfaces
• Comet reads information from Type Libraries
• Erlang modules are generated with glue code
• Each interface generates a module
• Each enum (set of constants) generates a module
  and a header-file with macros

17
Excel Example with Generated Code

• (Code is generated from the Excel type-library)

Visual Basic Charts.Add ActiveChart.ChartType
xlPieExploded ActiveChart.SetSourceData _
SourceSheets("Sheet1").Range("B2C4"), _
PlotByxlColumns ActiveChart.Location
WherexlLocationAsObject, Name"Sheet1" Erlang
chartsadd(xc_Applicationcharts(E)), ActiveChart
xc_ApplicationactiveChart(E), chartchartType(Ac
tiveChart, ?XlPieExploded), R sheetsrange(xc_App
lication.sheets(E, Sheet1), B2C4), chartsetS
ourceData(ActiveChart, R, ?xlColumns), chartlocat
ion(ActiveChart, ?xlLocationAsObject, "Sheet1"),
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Problems

• Combining an object-oriented approach with
  Erlangs semi-functional
• Handling state
• Memory management
• Type conversions between Erlang and other system
• Asynchronous operations
• Performance considerations
• Robustness

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Future Improvements

• Feedback needed
• Improvements considered
• Full Unicode support
• Calling Erlang from COM
• Event Sinks
• Erlang COM Servers
• COM Distribution
• Complex types
• Other APIs on other platforms
• Combining COMs ref-counting with Erlangs GC

20
References

• Comet documentation from OTP
• Don Box Essential COM (Addison Wesley)
• Box, Brown, Ewald and Sells Effective COM
  (Addison Wesley)
• Oberg Understanding Programming COM (Prentice
  Hall)
• Jason Pritchard COM and Corba Side by Side
  (Addison Wesley)