Event Sourcing in Python - Event-oriented Analysis and Design with Applications

348 Pages·2020·2.997 MB·English

by  John Bywater

A Pattern Language for Event
Sourcing in Python
Event-oriented analysis and design with applications.
By John Bywater
Copyright © 2020 John Bywater
All rights reserved. No part of this book may be reproduced or transmitted
in any form or by any means, electronic or mechanical, including
photocopying, recording, or by any information storage and retrieval
system, without permission in writing from the publisher.
Contents
Preface
INTRODUCTION
Prologue
Whitehead’s Scheme
Domains and Domain Models
Overview of the Patterns
PART 1 DOMAIN MODEL
Chapter 1 Domain Event
Chapter 2 Aggregate
Chapter 3 Mapper
Chapter 4 Recorder
Chapter 5 Event Store
Intermission 1 Notifications
PART 2 APPLICATION
Chapter 6 Notification Log
Chapter 7 Snapshot
Chapter 8 Repository
Chapter 9 Application
Chapter 10 Remote Log
Intermission 2 Tracking
PART 3 SYSTEM
Chapter 11 Log Reader
Chapter 12 Policy
Chapter 13 Process
Chapter 14 System
Chapter 15 Runner
APPLICATIONS
Bank Accounts
Cargo Shipping
Epilogue
Index
Preface
I began writing the main parts of this book (in Parts 1-3) after giving a talk
to the Domain-Driven Design London meetup group in November 2019
about event sourced building blocks for Domain-Driven Design, based on
my experience of event sourcing in Python. During this talk, I presented a
set of patterns for event-sourced Domain-Driven Design based on the
design and capabilities of the Python eventsourcing library. I identified
fifteen main patterns and the dependencies of each pattern on the others.
The directed graph of patterns revealed a linear ordering that would allow
the patterns to be discussed one-by-one without the need to refer to patterns
which had not already been introduced.
The main chapters of this book follow the structure of that talk, with each of
the main chapters describing one of the patterns. These fifteen patterns fell
naturally into three groups of five. These three groups were taken as the
three main parts of the talk, and have become the main parts of this book.
These chapters present the material with more care and with more detail
than I was able to provide in the talk. A map showing the graph of
dependencies and the linear ordering is included in the final introductory
chapter (Overview of the Patterns).
The first two chapters of the book (Prologue, and Whitehead’s Scheme)
were written following some earlier considerations about the general
applicability of event sourcing. The material for these two chapters was
developed during 2018 and 2019, and was gradually distilled and refined
whilst preparing for presentations given at software conferences in 2018
and 2019.
In the Prologue, I discuss Christopher Alexander’s pattern language scheme
and its relationship to the modern process philosophy of Alfred North
Whitehead. I make a respectful but slightly critical appraisal of how pattern
language was taken up in software development, identifying the conceptual
distance of classical object-oriented analysis and design from modern
process philosophy, and the apparent overlooking of the event-oriented
character of Christopher Alexander’s scheme. This conceptual gap left
room for the unanticipated turn to event-oriented analysis and design, as
expressed by event sourcing, event storming, and event modelling. I argue
that the event-oriented approach is generally applicable, on the grounds that
Whitehead’s scheme is generally applicable. This argument goes against
classical object-oriented analysis and design which views the world as built
up of instances of substance-quality categories with events having a
marginal status, and of the early view of event sourcing that it is only
applicable in limited circumstances.
I had initially accepted those views, and was merely seeking how to identify
the limits of an event-oriented approach, so that I would be able to tell when
event sourcing is not appropriate in any given domain. The surprising result
of my search was: to discover Whitehead’s scheme; to realise that
Alexander’s scheme was a creative application of Whitehead’s scheme; that
there are no domains that are not fundamentally comprised of events; and
that the limits of the applicability of event sourcing is determined by the
skills of the software developers rather than the nature of the domain they
are desiring to support.
In order to provide a logical and coherent conceptual foundation on which
to construct a general event-oriented analysis and design approach for
software development, a general introduction to modern process philosophy
is provided as a second introductory chapter.
An introduction to the notions ‘domain’, ‘domain model’, and ‘domain
event’ is presented in the third introductory chapter (Domains and Domain
Models). The notions are explained in basic terms, and put on an event-
oriented foundation by refering to Whitehead’s scheme. This material has
been separated from the patterns in the main parts of the book, in order to
keep the patterns as concise as possible.
Connections are made with person-centred psychology, which is described
in more detail in the final chapter of the book (Epilogue).
Acknowledgements
I am very grateful to Kacper Gunia and the other organisers of the DDD
London meetup for inviting me to give the talk about event sourcing in
Python. I was greatly encouraged by Kacper’s generous remark, “You’ve
come up with a particularly nice narrative and way of structuring the
content.”
I am similarly grateful for the support of the organisers of the software
architecture and Python conferences at which I have spoken about these
topics. I would like to express deep gratitude to Robert Smallshire and
Mathias Verraes for involving me in the DDD EU conference in 2019,
which gave me an opportunity to collect my thoughts about “events in
software, process and reality” into a moderately communicable form.
I am especially grateful to Michael Mehaffy, Rebecca Wirfs-Brock, Eric
Evans, Ward Cunningham, Jim Coplien, Vaughn Vernon, Grady Booch, and
David Heath for detailed discussions during 2019 and 2020, for their
interest in these topics, and for the encouragement these discussion have
given me to write this book.
I have also greatly appreciated discussions I have been having with various
academic philosophers, in particular Prof James R Williams and Dr Peter
Sjöstedt-H. Their guidance and conversation about the work of Alfred
North Whitehead helped to reassure me in my understanding of
Whitehead’s work.
I would also like to thank the many companies that have asked me to work
with them professionally over the past five years to create event-sourced
applications and systems. Without these practical encounters, the Python
eventsourcing library would likely not have come into existence, and my
sustained interest in these topic would not have been possible.
Last but not least, I would also like to thank the users and supporters of the
Python eventsourcing library for their friendliness, expressions of gratitude,
contributions, good will, pleasantness, and encouragement.
INTRODUCTION