Extensible and Dynamic Topic Types for DDS

Extensible and Dynamic
Topic Types for DDS
MARS – Jacksonville, FL – March 2010
Presenter: Rick Warren, RTI
Submitting POCs:
 Angelo Corsaro, PrismTech: angelo.corsaro@prismtech.com
 Gerardo Pardo, RTI: gerardo.pardo@rti.com
 Robert Poth, PrismTech: robert.poth@prismtech.com
 Rick Warren, RTI: rick.warren@rti.com
Supporting POCs:
 Virginie Watine, Thales: virginie.watine@thalesgroup.com
document number: 2010-03-02

Structure of this Presentation
Overview of specification and process
Specification contents
 Very detailed discussion last time
 This presentation focuses on what’s changed
AB feedback
Copyright © 2010 RTI - All rights Reserved 2

DDS Types: Assessment and Challenges
Good
Unmatched type safety
=> fewer defects
Unique type awareness
=> faster integration
Unique type awareness
=> higher performance
Bad
No standard API to
define / access types
at runtime
Hard to change types
without rebuilding,
redeploying
Hard to upgrade
systems one
subsystem at a time
Vendor-Specific
Solutions
OMG-Standard
Solutions

Where We Are
RFP issued: Jun. 2008
Initial submission: Nov 2008
Revised submission extended: Feb. 2009
Revised submission extended: Aug. 2009
Revised submission extended: Nov. 2009
Revised submission deadline: Feb. 2010
 Submitters back on the same page
 Responded to comments from 2 (now 3) AB reviewers
 Discussed with Task Force in Long Beach meeting

Design Principles
Don’t break anybody.
 Many people depend on DDS
 Don’t modify existing APIs
 Don’t modify existing compliance points
 Many people depend on RTPS/DDSI 2
 Don’t modify existing protocols or formats
 Don’t modify existing compliance points
Keep it fast.
 New capabilities must permit efficient implementations.
 Don’t use them? Don’t pay for them.
Keep it orthogonal. Type compatibility doesn’t depend on:
 Types’ definition language(s)
 Data representation(s)
 Programming language(s)
 QoS configuration

Overview
Type System
Type
Representation
Language
Binding
Data
Representation

Overview: Example
Type
Representation
Language
Binding
Data
Representation
IDL:
Foo.idl
struct Foo {
string name;
long ssn;
};
IDL to Language Mapping:
Foo.h
Foo.c
FooTypeSupport.c
struct Foo {
char *name;
int ssn;
};
Foo f = {"hello", 2};
IDL to CDR:
00000006
68656C6C
6F000000
00000002

Conformance Changes
Feedback on previous submission: conformance
criteria, relationships to other specs unclear
New submission more precise, better organized:
 Conformance (part II, section 3):
2 conformance levels correspond to DDS, RTPS
 Programming language conformance:
Type system, type representations, language bindings,
DDS Minimum Profile
 Network interoperability conformance:
Type system, data representations, RTPS
 Relations to other specs (part III, section 8)
 Extensions to DDS:
 “Programming language conformance” sections constitute
new optional conformance level (“profile”)
 No changes to existing DDS profiles
 Extensions to IDL:
 Normative to impl.’s of this spec only;
no change to CORBA conformance

Type System
Extensible, Compatible
 6.5.1: (a) Type System as UML meta-model.
(b) Type substitutability rules.
 6.5.2: Type extensibility, mutability rules
 6.5.3: Type compatibility in presence of change
 6.5.17: Allow type to evolve without breaking interoperability
 6.5.5: Support keys. How does extensibility apply?
More Expressive
 6.5.4: Map data type
 6.5.6, 6.5.19: Sparse data: object contains only subset of fields
defined by its type

Type System Overview
class Type System
NamedElement
Type
+ nested: boolean {readOnly}
PrimitiveType
«enumeration»
TypeKind
Collection
Namespace
Aggregation
Namespace
Enumeration
NamedElement
Namespace
Module
Namespace
BitSet
Alias
ConstructedType
«enumeration»
ExtensibilityKind
FINAL_EXTENSIBILITY {readOnly}
EXTENSIBLE_EXTENSIBILITY {readOnly}
MUTABLE_EXTENSIBILITY {readOnly}
+/container
0..*
{frozen}
+/containedType
*
{addOnly}
+extensibility_kind
1
{frozen}
+element_type
1
{frozen}
+kind
1
{frozen}
+base_type
1

Type System Overview
Entirely familiar from IDL:
 Primitives
 Strings, narrow and wide
 Arrays and sequences
 Aliases (typedefs)
 Unions
 Modules
As in IDL, but extended:
 Structures—including single inheritance
(for substitutability: 6.5.1)
 Enumerations—specify bit width and constant values
New relative to IDL:
 Maps—like std::map or java.util.Map;
required by RFP (6.5.4), fundamental OO collection
 Annotations—for extensibility (6.5.2, 6.5.3)
module Foo {
struct Bar {
long baz;
};
typedef Bar Quux;
}

Type System Changes
Clearer, more-flexible type extensibility kinds
1. Previous kind (Boolean): “extensible”=true:
 Add/remove/reorder type members
 Renamed to “mutable”
2. Previous kind (Boolean): “extensible”=false:
 Add to end, but don’t add elsewhere, remove, or reorder
 Renamed to “extensible”
 Now allow removing from end as well as adding to end
3. New kind “final”:
 Don’t allow any modifications
 Explicit annotations identify all levels in IDL
 Un-annotated types presumed “extensible” (2)
 Can be overridden by IDL compiler option
Several concrete examples added

Type Extensibility Example
@Extensibility(
EXTENSIBLE)
struct Coord2D {
long x;
long y;
};
@Extensibility(
EXTENSIBLE)
struct Coord3D {
long x;
long y;
long z;
};
Assignable

Type Extensibility Example
@Extensibility(
FINAL)
struct Coord2D {
long x;
long y;
};
@Extensibility(
FINAL)
struct Coord3D {
long x;
long y;
long z;
};
Not Assignable

Type Representations
 IDL
 XML, XSD
 TypeObject
Static Definition
 6.5.11: Programming-language-
independent type serialization format(s)
 6.5.9: Represent types in IDL and XML
 6.5.7: Support IDL subset used by DDS 1.2
Dynamic Definition
 6.5.12: Type discovery

IDL Type Representation Changes
Added optional pragmas to delimit IDL segments
using new extended syntax
#pragma dds_xtopics begin [<version_number>]
// IDL definitions
#pragma dds_xtopics end
 Document conformance requirements for human readers
 Help non-conforming compilers recognize problems
sooner, give more meaningful errors
 No particular errors or warnings mandated

Use struct syntax for inheritance, not valuetype
struct MyStruct : MyBaseStruct { … };
 Type system doesn’t include all valuetype features, so
syntax was confusing
 Unclear whether valuetype could extend struct

Cleaner user-defined annotation syntax
 Previously used methods, like Java annotations:
@Annotation local interface MyAnnotation {
long my_annotation_member();
};
 Now uses attributes:
@Annotation local interface MyAnnotation {
attribute long my_annotation_member;
};
 Still uses local interface (alternative: struct)
 Emphasizes that no runtime state exists
 Allows future support for multiple inheritance, if desired

XML Type Representations (No Change)
XML Type Representation
 Terse XML syntax designed ground-up for this Type
System
 Easy to read, plays nicely with auto-completing editors
XSD Type Representation
 Based on existing IDL-to-WSDL mapping specification
 Extended for new Type System concepts from this spec

TypeObject Changes
Type Representation suitable for DDS discovery
Renamed from “TypeCode”
 Name similarity to CORBA TypeCode deemed A Bad
Thing: this spec doesn’t depend on that one
 .Net also has “TypeCode,” something else entirely…
Improved string sharing to make representations
much more compact

Data Representations
 Extended CDR
 XML

CDR Data Representation (No Change)
Requirements:
 Continue supporting existing (compact) CDR used by user-defined
types (6.5.16)
 Continue supporting existing (extensible, parameterized) CDR
used by discovery types (6.5.16)
 Provide extensibility to user-defined types (6.5.2, 6.5.3, 6.5.17)—
all Data Representations should have equivalent expressiveness
Solution: Make existing extensible, parameterized CDR
available to all
 Same rules for every type, built-in or user-defined
 100% compatible with existing types and serialization
Parameterized CDR: structs & unions marked “mutable”
 Each member == parameter
 Member ID == parameter ID
Compact CDR: everything else

XML Data Representation (No Change)
Optional Requirement (6.6.1):
Dynamic data access based on XML or JSON
Given: XSD Type Representation
Data object == XML document
Validated by XSD Representation of its type
SAX, DOM-based access follow naturally
New RTPS encapsulation ID lets you use XML
on the network
struct OriginalLandData {
long x;
long y;
};
<OriginalLandData>
<x>5</x>
<y>42</y>;
</OriginalLandData>

Language Bindings
 Plain Language Binding
 Dynamic Language Binding

Plain Language Binding (No Change)
Defined by existing IDL-to-language mappings:
1. Start with IDL Representation of a type
2. Apply mapping to desired programming language
Mappings expanded to cover new concepts
 e.g. maps, annotations, optional members
 For C, C++, Java as required by RFP (6.5.8)
// IDL
struct
OrigLandData {
long x;
long y;
};
// Java
public class
OrigLandData {
public int x;
public int y;
}

Dynamic Language Binding (No Change)
Classes
 DynamicTypeFactory: Creates new types (6.5.20)
 DynamicType: Introspect type definitions (6.5.13)
 Informed by CORBA TypeCode API
 DynamicData: Introspect objects (6.5.18)
 Informed by CORBA DynAny, JMS MapMessage,
TIBCO Rendezvous self-describing messages
 DynamicTypeSupport: Registers types with DDS (6.5.20)
 DynamicDataWriter: Writes objects of any (single) type,
each represented by a DynamicData object (6.5.21)
 DynamicDataReader: Reads objects of any (single) type,
each represented by a DynamicData object (6.5.21)

Use by DDS
 Data Representation Compatibility
 Type Compatibility
 Built-in Types

Data Representation Compatibility
Reader and writer must agree
on which Data Representation to use
 6.5.14: Encapsulation specification, negotiation
 6.5.15: Is encapsulation request-offer?
 6.6.2: Optional: Custom encapsulation API
Largely unchanged from previous submission
Solution: DataRepresentationQosPolicy
 List of representation IDs (standard = XCDR, XML)
 Applies to both readers and writers
 Included in corresponding built-in topic data types
 Request-Offer semantics
 Writer offers single representation it will use
 Reader requests a list of representations
 Compatible iff writer’s representation is in reader’s list
 Can describe proposed representations, future additions, and
vendor extensions

Type Compatibility
If I write type “Foo” and you read type “Bar”, can
we communicate?
New submission clearer, more flexible
based on feedback on previous submission
3 parts:
1. Physical: type assignability
2. Intentional: type signature
3. Enforcement: QoS policy governs how above are applied
(NEW in this submission)

Type Compatibility Example: What I Want
struct Coord2D
{
long x;
long y;
};
struct Coord3D :
Coord2D
{
long z;
};
Read Write
Write Read

1. Type Assignability
Structural compatibility:
Can sample serialized according to one type be
deserialized according to another?
 Reminder: Rules designed to require no writer-specific
runtime checks at deserialization time
Intrinsic property of types’ definitions
 Governed in part by extensibility kind
 No action required by application programmer

2. Type Signature
Do I intend the types to be compatible?
Lists of equivalent types, base types provided at
registration time
Clarification to previous submission: both reader
and writer declarations taken into account
New since previous submission: lists can contain
patterns, not just literal names

3. Type Consistency Enforcement Policy
QoS policy on Topic, DataReader, DataWriter
governs how (1) and (2) take effect
 RxO: reader and writer must have the same setting
 EXACT_TYPE:  Types must also be equal
(as if extensibility were “final”)
 Registered names must be equal
 Like DDS of today but with extra safety
 Assumed during endpoint matching if remote policy
is unspecified
 EXACT_NAME:  Assignability assumed without inspection
 Registered names must be equal
 This is what DDS guarantees today.
 DECLARED:  Types must be assignable
 Type signatures must be compatible
 Default for conformant implementations
 ASSIGNABLE:  Types must be assignable
 Type names, signatures ignored: talk to any
compatible type

Type Compatibility Example: How to Get It
1. Is Coord2D assignable from Coord3D? Yes.
2. Type Signature:
Coord3DTypeSupport.register_type(
my_participant,
"Coord3D : Coord2D")
3. Type Consistency Enforcement: DECLARED
(default)
struct Coord2D
{
long x;
long y;
};
struct Coord3D :
Coord2D
{
long z;
};

Built-in Types (No Change)
Goal: Improve out-of-box experience for new
users, those with simple needs
Background: Usability gap vs. competition
 Other middlewares allow sending textual or binary data
without explicit type definition, registration
Response: Provide simple types built in
 Pre-defined, pre-registered
 String: single string payload
 Bytes: octet sequence payload
 KeyedString: string payload + string key
 KeyedBytes: octet sequence payload + string key

Summary

AB Feedback
Incorporates feedback from 3 AB reviewers:
Define IDL pragma to delimit extensions: Done
Clarify relationships to other specs: Done
Provide more detail about modules and
namespaces: Done
Make annotation type definition syntax more
natural: Done
Standardize VerbatimText behavior for C, C++,
Java languages: Done
Assorted typos, wording changes, organizational
suggestions: Done

Summary
The proposal:
Satisfies all mandatory and optional requirements and
supports identified real-world use cases
 Without sacrificing backwards compatibility
 Without sacrificing performance
Is easily extensible w/ new type, data representations to
handle additional use cases
Only small changes since previous submission
Incorporates all AB feedback
We will be asking for a vote to recommend adoption.

Q & A

Extensible and Dynamic Topic Types for DDS

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