Other Things We Want to Optimize for
For example, a feasibility concern for the chat app may be memory availability on the device that the chat app runs on.
A conversation can have an arbitrarily large number of messages. If we load all the messages into memory without pagination when the app first mounts, we will quickly run out of memory and the app will crash.
Practicality
Choosing to pursue a more technically complex and robust solution at a higher engineering cost is not always appropriate for a business that needs to be agile to test hypotheses and iterate quickly.
A social media platform like OkCupid operates in a very competitive landscape so there’s an urgency to launch quickly and iterate on experimental features in response to new market insights. A flexible architectural design that is easier and faster to implement but has more technical debt is often the right choice for OkCupid.
Another practicality constraint is the tolerance for risk which depends on the business case for the feature. For example, if the KPI is measured in the number of new user onboarding and conversion of these users to paid users, the business cost of shipping a broken onboarding flow or broken table-stakes feature like matching and chatting can be very high. In this case, it is better to trade off velocity for higher code quality.
Maintainability is a goal for any software engineering project. We want to make sure that the solution we implement is consistent with the existing patterns in the codebase and is easy to maintain.
A more technically complex solution may be more robust but it also introduces more risk of bugs and crashes. A more technically complex solution may also be more expensive to maintain and scale.
The recurring cost can often be higher than the upfront cost because the upfront cost is amortized over the lifetime of the product.
Engineering effort is required not only to implement the solution but also to maintain and evolve the solution over time. The more complex the solution, the more effort is required to maintain and evolve the solution.
One way to offset the upfront cost is to use a third-party solution. But this comes with the risk of vendor lock-in and the cost of integration.
To lower recurring cost, many codebases use industry standards and have style guides to discourage deviations from existing patterns because inconsistencies in codebases add maintenance cost.
Another way to lower maintenance costs is to avoid duplication of code. Duplication is generally considered tech debt it makes the codebase hard to evolve (you have to make the same changes in multiple places), which could introduce inconsistency in the codebase and bugs.
However, duplication is not always bad! Sometimes it makes things simpler. As complexity can also be a source of maintainability concern. Sometimes it could be a good tradeoff to introduce a little bit of duplication for a lot of simplicity.
Solution Approach
To make the chat app offline-first, we need to find a way to manage a collection of ordered data (messages between two users) which can be added to the collection by the server or by the user. Server-added messages come from an API request or a WebSocket event. Client-added messages come from the user sending a message.
We can think of the offline-first chat app as a collaborative editing tool – two users are collaborating on the same conversation thread. The conversation thread is a collection of messages ordered by the time they were created. A collaborating session in the context of a chat app is when all the participants of the chat are online and actively contributing to the conversation.