Python Domain Driven Design (DDD) example project
Disclaimer: this is a work in progress project, stay tuned for updates.
The goal of this project
AUCTION APPLICATION
The goal is to implement an automatic bidding system using DDD tactical patterns, described here: https://www.ebay.co.uk/pages/help/buy/bidding-overview.html
Domain
Online Auctions domain was selected for the purpose of this project, which is loosely based on Ebay bidding system.
The main reason for selecting this domain is general familiarity and limited complexity - many people understand how internet bidding work and the whole concept is not difficult to understand (or at least it's much simpler that healthcare or banking domains). On the other hand it's not simply a CRUD system - there are some business rules that must be implemented in the system.
Domain description
This is a summary of the information we got from a domain expert about how the auction system should work.
Selling
When you want to sell an item, the first step in getting your item in front of buyers is creating a Listing. For now we only focus on selling through auctions (https://www.ebay.com/help/selling/listings/auction-format?id=4110), but in the future we may consider selling with "Buy It Now" (https://www.ebay.com/help/selling/listings/selling-buy-now?id=4109) or in some other ways. When presenting an Listing for sale (which we call publishing in the Catalog), seller must provide a Listing duration and initial price. Also it is also possible to schedule a Listing publication (https://www.ebay.com/help/selling/listings/selecting-listing-duration?id=4652).
You can cancel your listing when bidding takes place, but only under certain circumstances: if time left in listing < 12 hrs you can cancel your listing only if no bids were places (but we might change it in the future).
If you are a new seller (you never sold an item before), you can list only one Item in the Catalog at a time.
Buying
When Listing is selled through auction Bidding takes place. As a Buyer, you can place a bid, which must be greated than the current price + 1 USD and which sets the the highest price you are willing to pay for an item. System will let you know if someone outbids you and you can decide if you want to increase your maximum limit. Sometimes you can can be automatically outbidded (if some other buyer sets his maximum limit higher that yours) - see https://www.ebay.com/help/buying/bidding/automatic-bidding?id=4014.
After a fixed time since the bidding was started a bidding ends and the Winner is announced.
Payments
At this point, payments are out of scope for this project.
Users
Each user can be a Seller or a Buyer (or both). User priveledges can be elevated to a Staff member or Administrator.
Event storming
Event storming technique was used to discover the business domain and the most important business processes.
Listing draft management
Publishing Listing to Catalog
Bidding process
Context Map
Lising, Bidding and Payment bounded contexts were identified as a result of event storming. Relationship between these bounded contexts is presented in the following context map.
Since Payment context will be provided by a 3rd party payments provider (via REST API), the downstream context (Bidding) must conform to whatever the upstream provides.