Orchestrating Services with shortlived changes: the StPowla Approach

1
Orchestrating Services with short-lived changes
the StPowla Approach

• Stephen Gorton
• Joint work with Stephan Reiff-Marganiec, Carlo
  Montangero and Laura Semini

2
Overview

• The meaning of it all??
• Business Processes and Workflows
• A simple supplier model
• Model Refinement
• Model Reconfiguration
• StPowla Overview
• How SOA fits in
• Workflows
• Policies
• Something more interesting
• Some open issues
• Summary

3
The meaning of it all?

• Orchestration?
• Composition?
• Business Process?
• Business Process Management?
• Business Process Modelling?
• Workflows?

4
Business Processes and Workflows

• A set of scheduled abstract activities that
  contribute towards the satisfaction of a business
  goal.
• BPs can be specified through natural language,
  graphical notation, process languages or formal
  languages (e.g. process algebra or logic).
• Workflows are process templates, they have
  instances or cases.
• Analogous to OO classes and instances.
• Notations include UML Activity Diagrams and BPMN.
• Process languages include BPEL and YAWL.
• Formal languages include StAC and -calculus

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A Simple Supplier Model

• Supplier receives a valid order from a registered
  customer
• Supplier packs the order
• Supplier ships the order to the customer
• Supplier invoices the customer
• A simple workflow model

Receive order
Ship
Pack
Invoice
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Varying the Model 1 Refinement
Receive order
Ship
Pack
Invoice

• Shipping is done through a contractor.
• We add the policy
• message transfer should include 128-bit
  encryption
• Only to the Ship task
• No visible change to the workflow, but tasks
  specifications become more specific, i.e.
  refinement.

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Varying the Model 2 Reconfiguration
Receive order
Ship
Pack
Invoice

• The general process is fine, but
• What if the order was in excess of Euro 250k?
• The supplier might prefer to get some security
  for the order and request a deposit.
• We add a policy that states
• if the order is above Euro 250, then get a
  deposit before shipping.

Request Deposit
Receive order
Ship
Pack
Invoice
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Case Study VoIP Pre-Delivery Process
Policy No test for small customers and orders of
small entity
Policy Avoid legal assistance for small orders
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The Problem

• Refinement
• Our needs change frequently, but we dont
  necessarily want permanent changes.
• In fact, sometimes we want changes for a short
  time only.
• Reconfiguration
• Only in some cases do we want to make a change.
• This change can change itself!
• Enter StPowla

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Overview of StPowla

• StPowla the Service-Targeted, Policy-Oriented
  WorkfLow Approach
• Ingredients
• Workflows
• Present the core business model
• Policies
• Present system variability
• Service Oriented Architecture
• Implement the requirements of the end user

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StPowla and SOA

• Services are platform independent, hetereogenous
  software components, available through the
  Internet and are
• Based on open standards
• composable
• optionally self-describing and discoverable
• Services map to tasks as follows

data
control
data
task
service
error
side effect
compensate
data
error
control
data
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StPowla Workflows

• Workflows define the core process
• In a graphical notation
• Suitable for end users (e.g. business analysts)
• Actually the workflow specification doesnt
  matter that much
• The key thing is that a common set of breakpoints
  can be found

random choice
decision point
non-synchronising merge
sequence
parallel split
synchronising merge
strict preference
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Workflow Breakpoints

• A couple of slides ago
• Plus 0a Workflow entry, 0b Workflow Success and
  0c Workflow failure

2a Service invocation/entry
1a Task entry
data
control
data
task
service
error
1b Task exit (success)
side effect
compensate
data
error
control
data
1c Task failure
2c Service failure
2b Service completion
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StPowla Policies

• We use the Appel policy description language
• Specifies Event-Condition-Action (ECA) rules
• Or goals (just As)
• Needs some domain specific information to address
  all conditional values (e.g. ontology)

• Appel has
• A natural language expression
• An XML syntax for system processing
• Formal semantics via mapping to
• Good for checking for the presence of conflicts

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StPowla Default Policies

• Policies in natural English syntax look like
  this
• Every task has a default policy

Functional requirements specification of the task
SLA dimensions
Invocation parameters
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Something a little more interesting

• How exactly can we define reconfigurations
  exactly?
• Consider this system overview
• I briefly introduce two complimentary methods

This is where reconfigurations must be specified
This part has no knowledge of policy updates
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A Process Calculi Approach

• For system behaviour specification
• Describes precisely to the programmer what should
  happen
• Can be analysed easily and verified
• We need constructs such as
• Skip
• Done
• Fail
• Sequence
• Parallel
• Choice
• (Non-deterministic) Choice
• Compensation
• Scoping
• Restriction
• Insertion
• Deletion
• Communication

Note the last three constructs are missing!
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A Graph Transformation Approach

• For determining what we need to express in a
  policy to present the transformation rule
• The type of change
• The location of the change
• We do not need to express the if part, this is
  done in the policy

Q
P
R
A
B
reconfigured by rule to
Q
P
R
A
C
B
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Some Open Issues

• Overarching constraints are not easy!
• How can we calculate the total cost of the
  process if we dont know what services are going
  to be used?
• But cant we use a policy?
• Yes, but that still affects the rest of the
  process! (e.g. insertions)
• When is it better to override the default policy
  and when is it better to add a new policy?
• The compensation scoping problem
• If Q fails, what do we compensate and where to?

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Summary

• Business processes are there to satisfy a
  business goal
• But the requirements may change, especially over
  time
• And those changes may change between cases!
• StPowla is being developed to address refinement
  and reconfiguration of service-targeted
  workflows, from the point of the end user.
• It consists of a graphical workflow specification
• A set of policies
• Service Oriented Architecture
• Formal specification of short-lived
  reconfigurations?
• With process calculi
• And graph transformations.

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Thank you for listening

• Any Questions?