Work Flows

Done BySendhil AnnamalaiSanghamitra Das

Nandini T. GopinathSanjeev U.N.

Sunil B Varghese

What is an workflow

It is an activity involving the coordinated execution of multiple tasks performed by different processing entities.

A

B

C

C D1

2

3

4

5

Our Goal

The user should be able to specify certain constraints in the workflow.

Our layer should be able to enforce those constraints.

Types of constraints Dependency (inter and intra workflow) And Or Mutually Exclusive (on ID, on Component,

on Workflows)

Middleware : Event Channel

Push driven Real time event channel. Uses object oriented techniques (Java). The event channel defines supplier and

consumer participants.

For our project we are using the event channel to pass the events between two components of a workflow.

Push Consumer Push Consumer Push Consumer

Consumer Proxy Consumer Proxy Consumer Proxy

Supplier Proxy Supplier Proxy Supplier Proxy

Push Supplier Push Supplier Push Supplier

Dispatcher Consumer Admin

Supplier Admin

WORKING OF THE EVENT CHANNEL

Some assumptions

Reliability. Each component works on a single ID at a time. Mutual exclusion sets are disjoint AND receiver gets the same ID from both sides. send(event) is the last statement of execution in

a component.(i.e. No execution on the same ID occurs after a send has been done).

In writing the XML file all rules and constraints are followed.

Architecture

Event Channel

TAM

- Constraint Data Structure

-Reference to Stubs

B

A

C

D E

Stub A

Stub B

Stub C

Stub D

Stub E

Stub A

Functions TAM Reads the XML file with the constraints. <wf-definition>

<wf-names><workflow> <workflow-name> A </workflow-name>

<wf-components> <wf-component> C1</wf-component> <wf-component> C2</wf-component>

</wf-components> </wf-names>

</wf-definition>

<wf-specification> <wf-dependency> <wf-name> A </wf-name>

<dep> <dep-name> dep1 </dep-name> <dep-def> <dep-comp> C1 <dep-comp>

<dep-event> E1 </dep-event> <dep-result>C2 </dep-result>

</dep-def> </dep> </wf-dependency> </wf-specification>

Creates a data structure which maintains the constraints

Work Flow 1

Work Flow 2

Component A Component DComponent CComponent B

Component C Component B

Workflows and Components hold the condition objects

DEP A,1 AND B,C

Component E

OR C,B

Mutual Exclusion

Mutual Ex. (B,C)

Mutual Ex.

(C,B)

A|

B|

C|

D|

C|

B|

E|

Creates the stubs.

There is a stub associated with each component. Each stub holds a supplier (If its corresponding

component sends events) Each stub holds a consumer (If its corresponding

component receives events) Each component interacts only with the stubs to

send/receive events. Each stub informs the TAM each time it has

published an event Each stub checks with the TAM if it can call the

receive of the component.

Component A

Stub AComponent A

Stub A

send(event)receive(event)

Event Channel

Inform TAM

push(event)

Check with TAM

push(event)

Component and Stub Interaction

How TAM takes care of constraints

TAM has a constraint object for each type of constraint which can be specified in the constraint file.

When a component calls the send method of its stub,it first informs TAM that it is going to send the event,

then the stub calls the push method of its supplier. When a stub receives an event from the event

channel, it checks with TAM to see if it can deliver it to its component.

‘Dependency’

When the stub of component B gets event 2, it will inform TAM about the receipt of the event.

TAM will return a true to the stub to go ahead with the delivery (if there are no other constraints).

If any other constraint exists like mutual exclusion, TAM returns a false and the event header is stored in the dependency object.

On receiving false the stub will buffer the event in its queue till further notification from TAM.

Component A

Component B2

‘Or’ constraint

Component A

1

2

3

The component in the TAM data structure holds the OR object.

1. TAM will receive a request from the Stub of Component A when it has received Event 1 whether it can deliver the event to the component.

2. TAM will reply with a true (if there is no other constraint). 3. TAM will again receive a request from Stub A when it has

received event 2. It returns a true (if there is no other constraint) to the Stub.

TAMStub A

A

Event Channel

1

Request to Deliver

True

2

‘OR’ constraint (contd..)

1 2

‘And’ constraint

Component A

1

2

3

The component in the TAM data structure holds the AND object.

1. TAM will receive a request from the Stub of Component A when it has received Event 1 whether it can deliver the event to the component.

2. TAM will reply with a false and will set a field in the constraint object indicating the receipt of event 1. The stub buffers the event on receiving a false.

3. TAM will again receive a request from Stub A when it has received event 2. It returns a false to the Stub, the event gets buffered in the stub.

4. TAM will now send an indication to Stub A to deliver both event 1 and event 2 to its component.

TAMStub A

A

Event Channel

1

Request to Deliver

Deliver 1 and 2

Set event in constraint

object

2

‘And’ constraint (contd..)

1 & 2

Mutual Exclusion

Mutual exclusion objects are held by components or workflows1. The mutual exclusion object holds the set of

components/workflows which may not execute simultaneously.2. When the receiver stub requests TAM permission to forward an

event to the external receiver component, if the TAM finds a mutual exclusion object with the associated internal receiver component, it checks if any of the members of the exclusion set is active.

3. If so, it forwards the components to the mutual exclusion object of the executing member.

4. When the executing member finishes, the list of components blocked are returned to the TAM.

5. TAM allows the first receiver stub within the list returned to execute and stores the rest of the blocked components in its associated internal receiver component.

Mutual Exclusion within a workflow (between components)

Each component maintains a mutual exclusion object with the same exclusion set

The component with the mutual exclusion that is currently executing maintains a list of blocked component names.

In the case of mutual exclusion between components based on ID, TAM checks, the ID being processed and the status of the other components in the mutual exclusion set.

TAMStub A

A

Event Channel

1Request to Deliver

Forward B

Check for active Mutual

Exclusion Components

Mutual Exclusion (contd..)

Stub B

B

Return B

Execute Deliver 1

Mutual Exclusion at workflow level on a ID

In this case, a workflow has processing on an ID only at one of its components.

Two workflows can also be mutually exclusive with each other

Advantages of TAM

Workflow and component level constraints are taken care of in the TAM layer.

Components can be coded independently without having to worry about decision making.

Uses an XML file to specify constraints (parsers already available)

Workflow goes ahead even if some components fail.

Disadvantages of TAM

Decision making is centralized. Synchronized manager. Can’t change the data-structure

dynamically.

A scenario

Component

A

Component

B

Component

C

Component

D

Component

E

1

2

3

4

5

- Component D has an and condition. Event 5 can go ahead only if it has received event 3 and 4

-The workflow will also have a constraint of mutual exclusion on an ID.

The xml file to specify this workflow

<wf-definition>

<wf-names><workflow>

<workflow-name>WF1</workflow-name> <wf-components> <wf-component> Component A</wf-component> <wf-component> Component B</wf-component> <wf-component> Component C</wf-component> <wf-component> Component D</wf-component> <wf-component> Component E</wf-component>

</wf-components><workflow>

</wf-names> </wf-definition>

The xml file

<wf-specification><wf-dependency> <workflow-name>WF1</workflow-name><dep> <dep-name> DEP1 </dep-name> <dep-defn>

<dep-comp> Component A <dep-comp> <dep-event> Event1 </dep-event>

<dep-result> Component B</dep-result> </dep-defn> </dep> <dep>

<dep-name> DEP2 </dep-name> <dep-defn>

<dep-comp> Component A <dep-comp> <dep-event> Event2 </dep-event>

<dep-result> Component C</dep-result> </dep-defn> </dep>

<and-cond> <cond-param> <cond-comp> Component B</cond-comp> <cond-event> Event3 </cond-event> </cond-param>

<cond-param> <cond-comp> Component C</cond-comp> <cond-event> Event4 </cond-event> </cond-param>

<cond-result> Component D</cond-result>

</and-cond>

The xml file

The data structure which TAM makes to store all these constraints

Work Flow 1

ComponentA

ComponentD

ComponentC

ComponentB

ComponentE

AND B,C

3,4

DEP A, 1 DEP A, 2 DEP D,5

MutualExclusionID

Dummy

Logical representation of the data structure.

Work Flow 1

Component

A Component

C

Component

B Component

D

Component

E

Holds an AND object

Holds a DEPENDANCY

object

Holds a DEPENDANCY

object

Holds a DEPENDANCY

object

Holds a MutualExclusionID

object

Dummy

The stubs created.

Component

A

Component

B

Component

C

Component

D

Component

E

1

2

3

4

5

Stub A

Stub B

Stub C

Stub D Stub E

Holds a supplier which will publish event types 1 and 2

Holds a Consumer for the event which the Dummy sends.

Holds a supplier which will publish event type 3.

Holds a consumer which will subscribe to event type 1

Holds a supplier will publish event type 4.Holds a consumer which will subscribe to event type 2

Holds a supplier will publish event type 3.Holds a consumer which will subscribe to event type 1

Holds a consumer which will subscribe to event type 5

Dummy

Dummy

Send

Publish

Recieve

Recieve

Questions ?