Using BPMN for ETL Conceptual Modelling: A Case Study

Bruno Oliveira

, Óscar Oliveira

and Orlando Belo

CIICESI, School of Management and Technology, Porto Polytechnic, Rua do Curral, Felgueiras, Portugal

ALGORITMI R&D Centre, University of Minho, Braga, Portugal

Keywords: Data Warehousing, ETL, Conceptual Modelling, BPMN.

Abstract: One of the most important parts of a Data Warehousing System is the Extract-Transform-Load (ETL)

component. It is responsible for extracting, transforming, conciliating, and loading data for supporting

decision-making requirements. Usually, due to the complexity of managing heterogeneous data, this

component is responsible for consuming most of the resources required for implementing a Data Warehousing

System, representing a critical component that compromises the adequacy of the system. Despite their

importance, the ETL development method is essentially ad-hoc, which does not always follow or embodies

the best practices. With the emergence of Big Data and associated tools, script-based ETL became, even more,

a common approach. In the last years, BPMN – Business Process Model and Notation – have been proposed

and used to support ETL conceptual models. Still, as an expressive language, it provides different approaches

for representing the same requirements. In this paper, we explore the use of BPMN for ETL conceptual

modelling, analyzing existing approaches, and proposing a set of guidelines for using this notation in a more

consistent way.

1 INTRODUCTION

Data Warehousing System (DWS) implementations

face some challenges due to the ever-growing need

for data. These challenges are most visible in typical

Extract, Transform and Load (ETL) tasks, which are

usually implemented in one of the most critical

components of a DWS.

The implementation of any analytical system is

strongly influenced by the correspondent populating

system's quality and adequacy (Kabiri & Chiadmi,

2013). Analyzing data with poor quality can provide

wrong insights that can have disastrous results in

business activities. GUI-based tools are a common

approach for ETL development, providing visual

constructs for defining workflows and pallets of

predefined tasks that are typically used in ETL

processes. They provide a certain standardization

degree since most of the available tasks encapsulate

best practices associated with specific application

scenarios. With the rise of Big Data, several code-

https://orcid.org/0000-0001-9138-9143

https://orcid.org/0000-0003-3807-7292

https://orcid.org/0000-0003-2157-8891

https://www.bpmn.org/

based tools emerged for taking advantage of specific

models and architectures. Despite their potential for

data integration, these tools make these procedures

very specific, requiring specialized knowledge to deal

with their design, implementation, and maintenance.

In the last years, Business Process Modelling and

Notation

(BPMN) (Aagesen & Krogstie, 2015)), is

being used for modelling (or representing) ETL

processes at higher levels of abstraction, allowing for

the development team to focus on the most critical

workflow aspects. The choice of BPMN for ETL

modelling is mainly due to its simplicity in

representing and modelling business processes,

coupled with its expressiveness.

In this paper, we explore the use of BPMN for

ETL conceptual modelling, analyzing some of the

existing approaches, and proposing a set of guidelines

to use this notation in a more standardized way. In the

remaining sections, we present some work related to

ETL modelling (Section 2), describe the

fundamentals of BPMN, focusing on ETL

development (Section 3), and present and discuss

Oliveira, B., Oliveira, Ó. and Belo, O.

Using BPMN for ETL Conceptual Modelling: A Case Study.

DOI: 10.5220/0010575702670274

In Proceedings of the 10th International Conference on Data Science, Technology and Applications (DATA 2021), pages 267-274

ISBN: 978-989-758-521-0

267

how BPMN can be applied in ETL application

scenarios (Section 4). Finally, in Section 5, we

present conclusions and future work directions.

2 RELATED WORK

Considering ETL processes as a critical component

for DW implementation, the definition of a detailed

model will be beneficial throughout the entire process

development, installation, and validation, providing

to its architects and engineers a handy guide. The

ETL development is still firmly based on physical

characteristics used to support its execution, meaning

that the processes are developed considering specific

tools and languages constrained or framed by

architectural characteristics. While most of these

tools are very powerful for ETL execution, they do

not provide the necessary resources for documenting

and representing integration processes at a higher and

common abstraction model. With these problems in

mind, several authors made recognized and relevant

efforts to provide a methodology for ETL conceptual

modelling.

In (Vassiliadis et al., 2002) a new set of elements

is proposed for expressing ETL natural features,

while in (Simitsis & Vassiliadis, 2003) a

methodology for representing attribute mapping

between data sources and the DW schemas is

proposed. Another approach was proposed in

(Trujillo & Luj, 2003) with the goal to extend the

Unified Model Language

(UML) for minimizing

notations and methodology learning curve's efforts

and costs. (Dupor & Jovanovi, 2014) proposed a

method and notation focused on a simple visual

overview to simplify processes representation, and

(Biswas et al., 2017) an approach for exploring

requirement and activity diagrams of the Systems

Modelling Language (SysML). The conceptual

representation in SysML can also be transformed into

XML Metadata Interchange (XMI) format, allowing

its programable interpretation. More recently, in

(Biswas et al., 2019), the authors extended their work,

presenting how the SysML model's validation can be

automated. More recently, (Raj et al., 2020) presented

a conceptual way for modelling data pipelines. This

work covered an extended scope considering the use

of ETL/ELT transformations, independently of the

DW environment, several applications, and different

data types (e.g., continuous or batch). Some of the

approaches presented focus on specific notations,

which can cause an extra effort to the ETL

http://www.uml.org/

development team to learn the specifics of notation

and posteriorly to communicate to non-technical

users.

The adaptation of existing ETL notations can

reduce some of the referred problems since some of

them are already widely used and commonly

supported by a large diversity of modelling tools. In

(Akkaoui et al., 2009), it is stated that the ETL

process can be considered as a particular type of

business process, which can facilitate communication

with more technical and non-technical staff. Since

BPMN is a widely used notation for business process

modelling and execution, it is not rare to see it used

for helping in other scenarios.

A Model-Driven Development based vendor-

independent BPMN metamodel and automatic code

generation for any vendor-specific platform was

proposed (El Akkaoui et al., 2011). One year later, the

same authors addressed in (El Akkaoui et al., 2012)

two architectural layers related to the specification of

ETL processes using BPMN, namely: process

orchestration, which can be accomplished by several

BPMN elements, like events and flow control

gateways, and data process operations, which are

related to some specific operations that allow for the

manipulation of data among several data sources

coordinated by control process elements. Meanwhile,

other approaches emerged, such as a Pattern-Oriented

Approach proposed by (Oliveira & Belo, 2015). In

this proposal, patterns represent some of the most

commonly used ETL procedures, such as change data

capture, Slowly-Changing Dimensions, or Data

Quality Enhancement, among others. These authors

used a Collaboration Diagram to represent

independent components' interaction (patterns),

providing a first approach to a multi-layer ETL

system using BPMN, showing in a simpler way how

ETL patterns can be used for supporting ETL

conceptual modelling with BPMN.

Figure 1: ETL process modelled using BPMN.

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268

Figure 2: High-level ETL representation using BPMN.

3 BPMN FOR ETL

Understanding the data generated from business

processes and integrating them into the Data

Warehouse (DW) is very important to guarantee DW

conformity and identify potential data quality

problems. Using a common language such as the

BPMN can be valuable since it enhances

communication between business analysts, technical

developers, and business people (Akkaoui et al.,

2009).

BPMN provides three types of diagrams:

process/collaboration, choreography, and

conversation. The process/collaboration diagram

models the process flow using several BPMN

elements. It is also possible to represent one process

(process diagram) or the collaboration between two

or more processes with their exchanged messages

(collaboration diagram). The choreography diagram

allows for modelling the data exchanged between

partners. The main difference to collaboration

diagrams is that with choreography diagrams data

exchange is modelled as an activity. The conversation

diagram represents the involved partners (from

different domains) and their relationships.

The BPMN model presented in Figure 1 is based

on the Akkaoui et al. (2009) proposal, and can be

understood even with basic knowledge about process

modelling and ETL. The process begins with a start

event (represented an unfilled circle). The rounded

rectangles represent an activity. BPMN tasks such as

"Load Cities.txt" describe atomic tasks, while tasks

such as "Fill TempGeo.State" describe compound

tasks that can be detailed using another BPMN

diagram. The BPMN sub-processes also include a

Loop marker, i.e., an execution control mechanism to

repeat the ETL task's executions until a specific

condition evaluates False. The connecting arrows

between BPMN activities are used for describing the

sequence flow. The process contains a BPMN

gateway (the diamond shape). In this case, a Splitting

and Joining Parallel Gateway is used, meaning that

several parallel paths will be initiated and

synchronized before proceeding to the next step. The

gateway is "blocking" the process sequence flow until

the data from each data source (identified by BPMN

tasks) completes. Additionally, the process depicted

includes two intermediate events: Error Boundary

Event that interrupts the associated task, executing

the "Reject to bad data" activity; and a Compensation

Boundary Event that references a specific

compensation activity ("Load to ready data"). BPMN

artifacts and annotations are also used to describe

attribute values and additional information. It is

Using BPMN for ETL Conceptual Modelling: A Case Study

269

Figure 3: BPMN conceptual model with patterns representation.

possible to add informal expressions to activities and

gateways to define properties and conditions that

enrich semantics. Keywords such as: "Input",

"Output", "Parameters", or "Comments" can be used

for improving process readability. Noteworthy that

several BPMN elements defined in (Akkaoui et al.,

2009) are not presented in Figure 1.

4 THE CASE STUDY

To provide an overview and foster the discussion on

how BPMN can be used in more complex scenarios,

the Wide World Importers

(WWI) Microsoft ETL

example was selected. The WWI is a wholesale

novelty goods importer and distributor operating

from the San Francisco bay area. The DW is

composed of six "modules" referring to events

generated by specific business processes, which

results in several star schemas. For this work, we

selected the Sale schema ETL processes. This schema

integrates a "Sale" fact table representing invoiced

sales to customers, and the "Date", "City",

"Employee", "Customer", and "Stock Item"

dimensions. The ETL processes are implemented

using Microsoft Integration Services workflows.

An ETL conceptual model can be used as an

abstract view, contributing with metadata that can be

added to enrich the the ETL system. It is noteworthy

that an over-specification of conceptual models

(mainly considering the ETL context) can

compromise the process interpretation, turning

BPMN diagrams complex to read and understand

(Griethuysen, 2009).

https://docs.microsoft.com/en-us/sql/samples/wide-

world-importers-what-is?view=sql-server-ver15

Figure 2 illustrates an ETL conceptual model

representing the processes and execution order. This

case represents an abstract view of the processes

responsible for loading data to each DW dimension

and correspondent fact tables. This is one of the most

abstract representations that can be done over the

implemented process. This model followed some

design approaches:

 Pools are not represented – Pools are especially

interesting for modelling collaboration,

representing several partners' interactions.

 Data Annotations are reduced – They could be

included in every activity to improve process

readability; however, since each BPMN

subprocess is self-explanatory, this detail was

omitted.

 Data artifacts were not used – Considering each

BPMN subprocess represents a coarser grain

component (potentially with several data

repositories involved), the number of data

artifacts can result in over-specification.

 Only sub-processes were used – Only high-level

components are represented.

 A timer start event was used – indicating the

process will execute with a given periodicity. A

data annotation was used to inform this ETL

process will execute daily.

Since dimensions need to be populated before the fact

tables (due to the existence of some referential

integrity constraints), they are grouped between

BPMN Parallel Gateways. The conceptual

representations presented till now are focused on the

control flow between the core ETL components.

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Figure 4: Elementary ETL process level.

In this case, the process is driven, considering the DW

tables that will be populated. This abstract

representation describes the main concepts and

process logic without referring to technical aspects.

Further detailed models can be created, providing a

hierarchical decomposition of the ETL process to

develop.

Using BPMN, processes can be modeled using

multiple abstraction levels. The main idea is to use

specific elements to add or remove detail to the

process. Thus, process understandability can be

adapted by hiding irrelevant information to a

particular development stage or even to a specific

user profile. BPMN sub-processes can be useful for

creating different abstraction levels and for

simplifying model representation (Reijers &

Mendling, 2010). Thus, a top-down modelling

approach can be followed for describing different

modelling perspectives. To illustrate the discussed

hierarchical decomposition, Figure 3 presents a

detailed view over the "Load Customer Dimension"

(Figure 2). This process is responsible for:

1) extracting data from each of the identified tables

needed for customer data load;

2) joining data according to the needs of the target

dimension;

3) storing the data in a data staging area (DSA)

table.

At this point, both BPMN sub-processes and atomic

tasks can be used. If a specific task is further

decomposed at a conceptual level, a sub-process

should be used; otherwise, a BPMN task should be

used. This conceptual layer is based on the Pattern-

Using BPMN for ETL Conceptual Modelling: A Case Study

271

Oriented Approach (Oliveira & Belo, 2012). In the

diagram presented in Figure 3, three patterns are used:

 Change Data Capture (CDC) is used to detect

data changes in data sources.

 Surrogate Key Generation (SKG) is used to

generate a surrogate key for each new row

coming from data sources.

 Slowly Changing Dimensions (SCD) (type 2) is

used in the DW design for handling customer's

data history.

In Figure 3, we can see CDC and SCD patterns

represented as sub-processes, meaning that they will be

decomposed in the next BPMN conceptual layer. At

this abstraction layer, patterns are used for identifying

common ETL procedures to simplify the development.

Additionally, the ETL development complexity starts

to emerge, providing a high-level but descriptive view

of the efforts required to accomplish each task. This

ETL representation also includes BPMN Datastores to

visually represent the repositories involved in each

task, with the association arrow's directionality

indicating if they are used as input or output. Coupled

with each pattern, BPMN Data Annotations with a

specific data structure are used. The pre-condition

keyword indicates any requirements that can be used to

validate the execution of a particular task, input

indicates the data repositories in which data will be

extracted (prefixed as: database name(dot)schema

name(dot)table name), a description (as the name

suggest a textual description), the output indicating the

data repositories in which data will be loaded, and

finally, the post-condition block indicating the

conditions/rules that should be guaranteed to qualify

the pattern output as success/unsuccess.

Figure 4 presents a more granular view over a

"#CDC# Extract Customer Data" sub-process. A

collaboration diagram is used for modelling processes

framed within different "partners". Partners were

associated with each source data object, and DSA used

to support ETL execution. The "Staging Area" BPMN

pool represents all atomic tasks needed to support the

execution of the CDC procedure. In this scenario,

lineage keys are used to track ETL execution. The

"Lineage" table generates a new key for each record,

identifying the table that is being loaded with data, the

process starts and ends time, state

(successful/unsuccessful), and source system cut-off

time (that it is used to control the amount of data that is

transferred using the cut-off date in the past). The task

"Update Lineage Table" is used to store a new record

indicating the load process date registered for customer

dimension (this table will be updated when the process

finishes). Next, the "Clean Staging Table" truncates the

staging table used to store the extracted customer data

in the DSA (in each moment, this table only stores the

records handled for each DW load). The

"GetLastETLCutoffTime" task represents the

procedure responsible for getting the cut-off time

generated before starting the dimension and fact tables

loading process. Next, the subsequent populating

process is focused on the source system that holds all

the operational data needed to populate the Customer

dimension (and for that reason, it is represented in the

"Source" pool).

Message flows are exchanged between the

represented pools (the one representing source data

ETL activities and another representing the staging

area ETL activities). Each BPMN lane inside the

"Source" pool describes the process scoped within the

handled source table. The parallel gateway indicates

the tasks for loading data from each source table are

independent (they can be executed in parallel if the

physical architecture allows). For each source table,

BPMN sub-processes are used to describe the

extraction data processes from source data. A sub-

process is used for process representation since it can

be detailed in a more specific diagram. Two tables are

used: one referring to the current data and the other

one referring to the archived/historical data. A CDC

based on audit columns is used to identify all the

changes made since the last ETL execution (using the

lineage table) and the ETL cut-off time. Next, the

"Identify changes for each record" sub-process

represents the procedure responsible for building data

records from the modified BuyingGroups,

Customers, and Categories (joining each of the

respective tables from the source system). With all

changes identified, the "Set record valid Date" task

works out the expiration date by using the "Valid

From". The "Insert into Customer Staging" task is

used for representing the procedure responsible for

storing new/changed data into the customer staging

table. Finally, the lineage table is updated through the

"Update Lineage Table" task. Considering the example

from Figure 4, several decisions were made for this

abstraction layer, namely:

 Use of BPMN collaboration diagram: since the

two data repositories (source repository and

DSA) are represented as separate

entities/partners, the collaboration diagrams

represent synchronized interactions supported by

exchanging messages between two or more

processes. Process diagrams can also be used. At

this conceptual level, the "Identify changes for

each record" and "Set record valid Date" sub-

processes can be drilled down, with their internal

tasks being modelled using a Process diagram.

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Collaboration diagrams help to document the co-

operation of several ETL components. As stated

in (Akkaoui et al., 2009), pools and lanes allow

ETL processes to be organized according to

several strategies, such as technical architecture,

user profile, or business entities.

 Sub-processes are used for activities described

with more detail in other diagrams (mainly for

simplifying diagrams representation).

 Datastores are still used at this conceptual level

for representing the repositories involved and the

communication direction with the respective

activity.

 Data Annotations use the same format described

before. Description, Pre-conditions, Input,

Output, and Post-conditions keywords can be

used to provide a semi-structured way to

document BPMN activities.

 The loop marker is used in some sub-processes

("Identify changes for each record" and "Set

record valid date") to indicate that the grain used

for describing the associated process is related to

each data instance/record.

 Pools are included but are optional. If pools are

not crucial for understanding the process, they

should not be included in the diagram.

 Intermediate events are not included in this

diagram. They can identify scenarios for handling

exceptions in ETL processes.

Table 1 presents an overview of the three different

abstraction layers. The first column identifies the level

at which ETL processes can be represented. The

Process level allows for an overview of the ETL

system's main processes directly related to each one of

the involved objects that should be populated. It can

represent only the dependencies between dimensions

and fact table population processes and describe sub-

processes related to each data object. For example, it

can be used to identify the need to apply an SCD

technique or to identify the constraints applied to the

use of bridge tables. This Process model can be used as

a top-down mechanism to develop other layers

progressively.

The Pattern level represents a set of predefined sub-

processes associated with a specific procedure

typically used in ETL development. The design team

can provide additional configurable components,

forming a pattern palette that must be included in the

project documentation. It provides a more

straightforward way for describing the main ETL

components without specifying how such procedures

will be implemented. For example, this level can

identify the need to apply a specific CDC or SCD

mechanism without detailing how it will be handled.

At this level, activities are documented for identifying

inputs, outputs, and potential error handling

approaches in a high-level view. Data objects are also

identified, revealing in more detail the complexity of

the ETL system and the specific techniques used (for

example, for CDC or SCD).

Finally, the Task level mainly represents BPMN

tasks describing the algorithm for implementing the

patterns identified in the Pattern level. The

representation tasks such as joins, projections or

selections can be used at a logical level to describe how

each pne of the tasks will be implemented.

Table 1: Summary of BPMN abstraction layer for ETL

conceptual modelling.

Abstraction

level

Purpose

Main BPMN

artifacts

Process

Representing system

abstraction and

providing process

dependencies

desc

iption.

Subprocesses are

used to represent

data flows for DW

populating

processes.

Pattern

Representing the macro

activities presented in

the ETL system

regarding extraction,

quality and load

tecnhiques.

Subprocesses

represent common

ETL procedures/sub

systems.

Task

Represent the

elementary level for

ETL representation

using (mainly) atomic

tasks. Processes are

represented in a row-

-row processing.

Task are a

predominant

modelling artifact

5 CONCLUSIONS

This paper presented a BPMN conceptual modelling

approach for modelling ETL processes using three

different abstraction layers. Due to BPMN

expressiveness, which can be very useful for ETL

representation, ETL conceptual models can

significantly vary since people have different ways of

thinking, which promote different ways of

representing the same ETL process. In the last years,

the experience we acquired shows us that ETL

modelling based on BPMN is focused on language

semantics rather than a methodology that can be used

to represent the ETL development at different stages.

This sometimes results in BPMN processes mixing

different detail levels, which sometimes difficult

process interpretation and understanding. For that

reason, the approach proposed represents ETL

conceptual modelling in different layers, each one

Using BPMN for ETL Conceptual Modelling: A Case Study

273

representing a different process detail, providing to

the ETL development team specific tools for

communicating at different ETL development phases.

Each layer represents a new detail level applied to a

specific construct described in the previous layer.

This contributes to a more agile development

approach since models can enrich system

requirements incrementally. To show how this

technique can be used, we explored a specific sub-

process from an ETL scenario. We show how the

ETL specificities can be represented at a conceptual

level for conceptualizing ETL processes in an

effective way.

As future work, we want to provide a complete

system specification for the WWI case study, explore

different ETL (and ELT) application scenarios,

discussing how this modelling approach can be

adapted for different types of data integration

projects. Additionally, we want also to explore

BPMN choreography and conversation diagrams for

ETL representation, which may help model systems

that rely on a service-oriented architecture.

ACKNOWLEDGEMENTS

This work has been supported by national funds

through FCT - Fundação para a Ciência e Tecnologia

through project UIDB/04728/2020.

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