This repository is a place for experimenting with a concept of process managers (in CQRS context).

What is a process manager?

Process Manager is an entity capable of processing domain events and issuing commands in reaction to them. It can keep its state persisted. It should not implement any complex domain rules. It can't do anything on its own - it calls other aggregates to complete parts of the process it manages.

Sources:

• https://medium.com/@drozzy/long-running-processes-event-sourcing-cqrs-c87fbb2ca644
• https://www.youtube.com/watch?v=WvjTCmeGlGA
• https://www.enterpriseintegrationpatterns.com/patterns/messaging/ProcessManager.html

Examples

Voiding

In this example I'm trying to implement documents voiding process which involves two remote services communication.

• There is a service called Billing which is responsible for issuing various billing documents (invoices, receipts etc.).
• There is a service called Reports which is responsible for aggregating documents issued by Billing in form of a report. A report is generated for every customer separately.

When Billing issues a document, it emits a domain event DocumentIssued. Reports service listens to those events and builds a report based on them.

Problem arises when someone decides that a particular document should be voided. A rule which cannot be broken is that a document cannot be voided if a report in which the document is included has already been sent to a customer (published).

A user issues a VoidDocument command to Billing service which emits DocumentVoidingRequested, what triggers the process. There can be no more than 1 voiding process running for a single document at one time.

Process prerequisites

1. DocumentIssued event from Billing

Process flow

1. VoidDocument command to Billing
2. DocumentVoidingRequested event from Billing
3. MarkDocumentAsVoided command to Reports
4. MarkingDocumentAsVoidedSucceeded/Failed event from Reports (in case report wasn't sent yet/was sent already)
5. Abort/CompleteDocumentVoiding command to Billing
6. (optional) AcknowledgeProcessFailure command to process manager

A process has its identifier which is passed along with commands and events related to it as a correlation id.

What's important - the process manager creates process instances what makes them first class citizens - they're very similar to any different aggregate. They keep their own state and make the process visible and inspectable. In this particular example I made use of that and provided a command which allows to acknowledge a process failure (making it disappear from the view and allowing triggering new process for a particular document).

State machine describing the process

 +------------------+      +-------------------------------+          +----------------------+
 | VoidingRequested | +--> | MarkingDocumentAsVoidedFailed | +------> | Failure Acknowledged |
 +------------------+      +-------------------------------+          +----------------------+
         |
         +                 +----------------------------------+          +----------------+
         +---------------> | MarkingDocumentAsVoidedSucceeded | +------> | DocumentVoided |
                           +----------------------------------+          +----------------+

It's possible to play with the system using a simplistic HTML interface. To run a server:

$ cd 1_voiding && go run .

In order to make the process last longer, so you could observe the progress, run the server as following:

$ SLOW_DOWN=1 cd 1_voiding && go run .

The interface will be available under http://localhost:8080.