2. Analysis. 2.1 Resource Planning. Resource Planning. 1st Question. Example. BPM & WfM. Lecture 9 III. Workflow Theory & Analysis

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University of Paderborn Software Engineering Group E. Kindler

BPM & WfM

Lecture 9

III. Workflow Theory & Analysis

IV. WfM & Transactions

SS 2006, L09 2 BPM & WfM: III. Wf Theory & Analysis / IV. Transactions

2. Analysis

Consistency and plausibility

Structural analysis: “Awkward nets” Unused / redundant data or resources Unnecessary orderings

Consistency among different aspects Compliance with business rules ...

Performance Resource planning Performance analysis

2.1 Resource Planning

Problem

:

Given a business process model.

How many resources / agents are necessary

for completing a certain number of workflows

per time unit?

Resource Planning

Questions

:

What information do we need to answer this

question (in addition to what we have in the

model already)?

How do we calculate the necessary

resources?

1st Question

Number of processes to be performed per

time unit

Average processing time for each activity

Probabilities for each choice

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2nd Question (Idea)

! ! 6 + 2 = 8 B 3 + 3 = 6 A ! " "# # ! " SS 2006, L09 8 BPM & WfM: III. Wf Theory & Analysis / IV. Transactions

2nd Question

For each activity, calculate the number of

processed instances per time unit

For each activity, multiply this number with

the average processing time

Sum up the results for the different resources;

this is the number of resources we need (in

an very ideal case)

Remark

Everyday experience as well as basic results from queuing theory tell us that we can never reach 100% utilization of the resources!

Therefore, the number of resources must be higher than the calculated resources

(for a detailed analysis, more information is needed performance analysis).

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Remaining Questions

Where do the average processing time and

the probabilities come from?

How do we calculate the number of

processed instances for each activity?

Remaining Questions

Where do the average processing time and

the probabilities come from?

Experience Measurements Audit trail

Remaining Questions

How do we calculate the number of

processed instances for each activity?

Inductively over the structure of the workflow net (built from workflow constructs)

For each construct, we give a rule to calculate the number of processed instances for each activity

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Calculation for each Construct

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Calculation for each Construct

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Remarks

The presented method works for workflows built from workflow constructs only. But, there is a more general theory that works for arbitrary workflow nets (basically the same theory as for performance analysis).

The same method can be used to calculate the average costs of a process.

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2.2 Performance Analysis

The resource calculation gives the necessary resources for the ideal case (100% utilization) only.

waiting times go to infinity

ignores dependencies among different workflows ...

A more detailed performance analysis can give more accurate results (considering all workflows in a system): average completion times, average waiting times, utilization rate etc.

Disclaimer

Here, we cannot go into the details of the mathematics of performance analysis

Operations research Markov chains Queuing theory Stochastic Petri nets

We restrict ourselves to discuss the additional information necessary for doing (more accurate) performance analysis and to the results

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Necessary information

Distribution of interarrival timefor externally triggered activities (in particular start activities)

Type of distribution

(markovian, non-markovian, ...)

mean interarrival time and standard deviation

Distribution of processing timefor all activities Probabilities for selection of particularchoices (this depends on the underlying stochastic model)

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Results

Dependent on the underlying stochastic model and the used distributions, analytic techniques or simulation can give us the following results:

Average completion time for each process (time for finishing the process)

Average processing time for each process (time effectively processing activities of the process) Average waiting timefor each process

(completion time – processing time)

Utilizationof resources ...

Remark

As with resource planning, performance analysis can be also used for analysing costs.

IV. Transactions

Motivation & Basic Concepts

Atomic spheres

Compensation spheres

...

12 , 34+ SS 2006, L09 22 BPM & WfM: III. Wf Theory & Analysis / IV. Transactions

1. Motivation and Basic Concepts

Motivation

Transactions and ACID principle

Problems with long-lived transactions

Advanced transaction models

Motivation

Sometimes,

workflows are

defined as

long-lived transactions

This is not completely true, but there is much

truth in it.

Here will discuss this definition and introduce

some transaction concepts to workflows.

-University of Paderborn Software Engineering Group E. Kindler

Transaction: Definition

A transaction is a set of related actions that

virtually appear to be executed

instantaneously,

i.e. they are executed according to the

ACID

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ACID principle

Atomicity: Either all of the actions are (appear to be) executed or no action is executed (all or nothing). Consistency: The activities are (appear to be)

executed only when they results in a consistent state.

Isolation: The activities (appear to be) executed without the interference of activities of other transactions

Durability: The changes of a successfully executed transaction survive system failures.

Examples of transaction

Sale:

transfer goods and transfer money Booking a trip:

book a hotel, book a flight, and rent a car Money transfer:

debit one account and credit the other account

BPs and the ACID principle

It would be nice to apply the ACID principle to

business processes.

business processes are long-lived

transactions

But, the price is too high!

The price of the ACID principle

The longer a process runs the higher the probability that some of its activities fail

This applies in particular when traditional transaction protocols are used (locking protocols are prone to deadlocks).

Then, the ACID principle implies that the complete process fails.

The effort put into the process is completely lost – years of work (and millions of Euros) would be lost.

The price of the ACID principle

Moreover,

performance would be extremely poor.

some activities must have an externally visible effect (letters, payments, ...) before other activities are executed;

then, the ACID principle guarantees that all remaining activities will be successfully executed in the future.

Nobody can give such a guarantee.

Advanced Transaction Models

There are some concepts for solving some of the above problems:

Nested transactions: A transaction consists of sub-transactions, which are independent from each other

Compensations: Each action within a transaction has an associated compensation action, which “undoes” the effect of the action.

Reliable messaging systems: Each successfully sent message is guaranteed to be successfully delivered exactly once.

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Workflows

are

advanced

long-lived

transactions

- 5 ' . SS 2006, L09 32 BPM & WfM: III. Wf Theory & Analysis / IV. Transactions

2. Atomic Spheres

Basic idea

:

An atomic sphere is a set of activities within a

workflow, that are executed atomically (i.e. all

or nothing).

no consistency no isolation no durability $ 3+ ' 5 2 5 SS 2006, L09 33 BPM & WfM: III. Wf Theory & Analysis / IV. Transactions

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Example: Book a Trip

All activities of an atomic sphereare executed under the control of a transaction

(the corresponding activities must be transactional) If one activity of the atomic sphere aborts(fails), all activities are aborted

If all enabled activities were successful, all activities are committed.

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Example: Book a Trip

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Discussion

If we cannot contact the customer, this activity can be aborted; accordingly, all activities of the atomic sphere are aborted.

Atomic spheres help to concentrate on the normal behaviour of a process

The transaction monitors deals with the exceptions (details depend on the underlying transaction monitors).

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Extensions

Retries: Each atomic sphere is assigned an upper limit for the number of retries.

Final fail: When an atomic sphere finally fails, one of the following actions is taken:

Notifysome agent (e.g. process administrator)

Skipatomic sphere, i.e. treat all activities of the sphere as dead activities and do dead path elimination

University of Paderborn Software Engineering Group E. Kindler 6 ' 6 6

Extensions:

77 8 ," 9 , SS 2006, L09 39 BPM & WfM: III. Wf Theory & Analysis / IV. Transactions

Problems

In principle, atomic spheres have the same problem as classical transaction!

But, atomic spheres can be used to restrict the problem to small parts of a workflow!

For increasing performance, atomic spheres should run fast.

To this end and for technical reasons, there are some syntactical restrictions for atomic spheres:

Restrictions

Single entry point

:

1 1

Restrictions: Single Entry

Also okay:

:

Restrictions: Single Entry

Not okay:

:

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Restrictions: Single Entry

Not okay:

1 1 -, ' 5 SS 2006, L09 44 BPM & WfM: III. Wf Theory & Analysis / IV. Transactions

Restrictions

Atomic spheres must not overlap!

Restrictions

The

applications

of the activities of an

atomic sphere must be

transactional

(i.e. it must be able to execute them under

transaction control)

This is a technical restriction only; but serious

one.

Desirable

Activities within the atomic sphere should be

automatic

(manual activities are slow)