Publish and Subscribe The Information Bus®— An Architecture for Extensible Distributed Systems...

Publish and SubscribeThe Information Bus®—An Architecture for Extensible Distributed Systems

Oki, Pfluegl, Siegel, Skeen. 1993.

Matching Events in a Content-based Subscription System

Aguilera, Strom, Sturman, Astley, Chandra. 1999.

Dan Sandler | COMP 520 | October 7, 2004

Distributed Systems in the Real World

So far: Tools for building distributed systems

Focused on certain problems Redundancy Distribution Marshalling and communication

Less attention paid to others Discoverable systems Maintainable, upgradeable systems

Generative Programming in Linda

Review: Linda Typed data organized

into tuples Stored indefinitely in

global “tuple space” Tuples requested by

partial specification Anonymous

communicationTUPLE SPACE

Problems in Tuple Space

Open Issues Unbounded storage

requirements of tuple space

Tuple contents weak on flexibility, metadata, discoverability

General tuple-searching can be complex, slow

TUPLE CLUTTER

Take-aways from Linda

The content itself connects senders to receivers

Participants have no other formal relationship

Let’s explore this model further

Publish and Subscribe

Recall Linda’s simple in/out operators

If there is an in() pending when a matching out() is invoked, the scenario resembles what we now call Publish and Subscribe

The Information Bus is such a system

Producer out(<…>)

Consumerin(<…>)

The Information Bus

Goal: develop real-time, “24/7” systems Circuit fabrication Securities trading systems

Specific requirements derived from these situations Continuous operation Legacy systems integration “Dynamic system evolution”

Evolution is hard

Capacity for change must be planned from the beginning

Systems may need to “evolve” in many ways New kinds of data New applications (services, clients) Fault recovery and scalability can be considered

evolution

Remember: Evolution must occur without interruption of service

Architecture of the Information Bus

Clients may publish data objects under a specific subject

Clients may subscribe to one or more subjects to receive data

Note: The bus broadcasts all published data to all participating hosts

A snapshot of the Information Bus

Subject: …Data: <object>

THE INFORMATION BUS

SUBSCRIBERPUBLISHER

UNINTERESTED




Properties of the Information Bus

P1. Minimal core semantics Recall the “end to end argument” – complexity at a

low level is usually either insufficient…or overkill

Two styles of communication: Remote method invocation Publish/subscribe

Two kinds of objects: Data (things sent on the bus) Services and Clients (things that use the bus)

Properties of the Information Bus (cont.)

P2. Self-describing objects We might call this “introspection” today

Given an object, we can ask at run-time for object type, property types and values, method signatures, etc.

All participants and data play by these rules

Effect: loose coupling and run-time discovery


P3. “Dynamic classing” A fancy way of expressing the ability of the

system implementation to be changed at run-time

Without interruption of the system: New classes can be defined New code can be introduced

This is clearly necessary for evolvability


P4. Anonymous communication The hallmark of publish-and-subscribe

Data objects are sent and received based on content alone Details of the participants are irrelevant In this system, the content which controls

subscription is a “subject” string No other part of the data is involved in delivering the

object to subscribers Subjects typically organized with hierarchy

(cf. Usenet groups: rice.owlnews.comp520)

Other features of the Information Bus

What else is going on in the bus?

Object discovery

Point-to-point remote method invocation

Legacy data conversion

Discovery protocol

Discovering participants in a given subject A, B, D all subscribed to “Little Green Apples”

A B C D

Subject: apples.little.greenData: Who’s there?

Subject: apples.little.greenData: I’m here, my name is “B”

Subject: apples.little.greenData: I’m here, my name is “D”

THE INFORMATION BUS

RMI brokering

Finding a participant to invoke methods Like the discovery protocol

A B C D

Subject: apples.little.greenData: I want to make a method call.

Subject: apples.little.greenData: Sure, my address is “2”

Subject: apples.little.greenData: Sure, my address is “4”

THE INFORMATION BUS

1 2 3 4

Adapters


THE INFORMATION BUS

Adapters convert data from legacy systems to pub/sub messages

Other clients don’t know that there’s a legacy system involved

Dynamic System Evolution

New clients can be brought on-line at any time Subscribe to current subjects Publish objects of conventional type Publish objects of novel type and implementation Create new subjects for subscription

Existing subscriptions unaffected

Problems solved by the Information Bus

System is available, evolvable Maintenance may be performed on-line New services and clients can be rolled out

incrementally, without downtime

Is subject-based subscription a limitation? Simple subject easier to test than arbitrary tuple

signatures

Let’s look closer at this “matching problem”

Matching Events in a Content-based Subscription System

Scenario: The content-based pub/sub system

Like the Information Bus: subscriptions based on content, rather than a membership list

A participant has (potentially) many subscriptions

A participant receives (potentially) many “publications”

The Matching Problem

Each participant must test each event to see which subscriptions it matches

Attribute-based subscription model Each event may have multiple attributes, some or

all of which may be tested Example subscriptions:

Fruit=“apple”; Size=“little”; Color=“green”

Fruit=“apple”; Size=*; Color=“red” Fruit=*; Size=“little”; Color=*

* == “don’t care” (match anything)

The Matching Problem

Trivially, this problem is linear in the number of subscriptions

By adding multiple attributes, it’s now linear in the number of attributes too

Can we do better than the naïve matching implementation?

The Exact Attribute Problem

Consider a special case of this problem

Each attribute is to be matched exactly (Alternatives: substring match, lexicographic

comparison, etc.)

General algorithm

Pre-process all subscriptions into a “matching tree” Like a decision tree of attribute tests Goal: If multiple subscriptions have the same

attribute requirements, only test that attribute once for all subscriptions

Similar problem: matching multiple strings in text consider each char of each string an attribute

Naïve Matching

Subscriptions: SUB1: apples.little.green SUB2: apples.*.yellow SUB3: bananas.little.green

Algorithm Search each subscription

separately

For each event, For each subscription, For each attribute,

Test against event

Naïve algorithm

1

2

3

SUB1

apples?

little?

green?

1

2

3

SUB2

apples?

*

yellow?

1

2

3

SUB3

bananas?

little?

green?

Matching Tree

Subscriptions: SUB1: apples.little.green SUB2: apples.*.yellow SUB3: bananas.little.green

Algorithm: Search all subscriptions

together For each event, Recursive tree search For each attribute (node)

Test against event Follow all matching edges

Leaf nodes = matches

Matching tree algorithm

2

3

SUB1

little?

green?

1

3

SUB2

apples?

*

yellow?

2

3

SUB3

little?

green?

bananas?

Complexity of the matching tree

Why is this better?

By inspection, the matching tree tends to have fewer tests than the trivial implementation

Fewer nodes, that is, assuming there’s some overlap in attribute values among your subscriptions

Still linear in number of subscriptions, however

Complexity of the matching tree (cont.)

Deeper insight

For the exact-matching problem, the number of branches you can follow is at most 2 i.e. some event’s attri = “X”; you can only follow “X”

and *

It gets better, however If there are no * subscriptions for attri, you will follow 0

or 1 branches Intuition: more like a traditional search tree


Time complexity shown to be O(N1-λ) (The expected complexity for random events) λ related to number of non-* edges in the

matched path; can be as high as ½ Intuition: the more exact tests there are, the

fewer branches you will follow

Other complexity characteristics Space complexity: linear Pre-computation: linear


Simulation with random data

SpaceTime

# of subscriptions

“com

ple

xit

y”

Optimizations

Collapse multiple “don’t care” edges into a single edge Rationale: Many subscriptions “don’t care” about

most attributes of data (60% speedup in simulation)

Pre-compute “successor nodes” Short-circuit parts of the matching tree in special

situations

Successor Node Optimization

Subscriptions: SUB1: *.little.green SUB2: *.*.yellow SUB3: bananas.little.green

What’s going on? Annotate nodes with

links to other nodes you know will also match at that point

Example: if we match bananas.little, we know *.little and *.* will also match for sure

Matching tree algorithm

2

3

SUB1

little?

green?

1

3

SUB2

*

*

yellow?

2

3

SUB3

little?

green?

bananas?

Summary and Discussion

Publish/subscribe: participants connected only by exchanged data Flexible, loose connections — an evolvable system No Linda-like storage

(but you could implement a storage service in a pub/sub system)

So what about the matching problem? It only exists in broadcast pub/sub

Each participant sees each event Question: Is this realistic?

Trend: multicast instead of broadcast Subscription lists — more administration, but potentially

better publication performance P2P?

Publish and Subscribe The Information Bus®— An Architecture for Extensible Distributed Systems...

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