Temporal POC

Goals

Created a small POC for implementing Temporal. My goals were:

• to keep low level abstractions(the way temporal stuff is executed) and higher level abstractions(business logic) separated as much as possible
• to allow for proper unit testing, with mocking certain parts
• to keep the Temporal surface as small as possible to prevent a vendor-lockin

This POC simulates the simplified version of upgrading an existing reservation, this upgrade consists of:

1. get the current room name for this specific reservation from a different microservice(mocked)
2. create an email message using this reservation and the fetched room name
3. calling the email microservice(mocked) to send this created e-mail to the user with the upgrade link

There are 2 non-deterministic pieces of logic involved with accompanied specific retry behavior, they are captured in Temporal activities:

1. getting the room name via http request
2. sending the e-mail via http request

Running the POC

To run this project

1. open a terminal and navigate into the TemporalServer directory and run docker compose up
2. run the worker/worker main function
3. run the start/main main function

Design choices

Chain of Responsibility Pattern

The Chain of Responsibility Design Pattern is chosen as this creates an abstraction between the workflow and the pieces of logic getting executed, and a logical separation between Temporal and non-Temporal code. This allows for cleaner code and better testability as well as a limited Temporal specific code surface. It adheres to the Single Responsibility Principle and the Open Closed Principle costing a limited amount of overhead via using Go's embedding.

Mocking the Workflow

In the EmailWorkflow.go the UpgradeEmailWorkflowV3 function is kept at a minimum, it sets up the handler and the receiver struct and passes it along to a handler. This is designed this way due to 3 limitations imposed by Temporal.

1. All input to a workflow must be serializable
2. The workflow.Context can not be mocked
3. Everything not inside an activity inside a workflow must be deterministic

These limitations would mean having all logic inside the workflow that logic not bound to a Temporal activity can not be mocked in a clean way. This is because there is no way to inject or access a mock handler or a mock handler factory into or from the workflow, as this makes a workflow non-deterministic, thus limiting the workflow unit test to actual used handlers.

Not being able to mock the workflow.Context results in not being able to test the handleUpgrade directly, because the activities wrapped in handlers do require to be called with a context otherwise they panic. Therefore, keeping the workflow as minimal as possible is required, otherwise all this logic must be duplicated inside the unit tests because we create a mock workflow which gets called by the Temporal testing environment passing in a workflow.Context.

Alternative Design

One alternative would be to create a handler factory which acts upon a serializable configuration object, which gets passed to the workflow. I believe this solution would introduce a lot of overhead for every workflow ergo this solution was discarded.

Archimate Diagram POC