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Integrate esigning notification

Hi. And welcome to the system design interview channel. Today we design a notification service. Let's start with the problem statement. In the world of web services there are many scenarios when messages need to be sent in a reaction to some event. For example, when credit card transaction amount exceeded a limit and card holder needs to be notified. Or service monitoring system encountered a large number of faults produced by API and on-call engineer needs to be notified. In more general terms, let's say there is a component called Publisher which produces messages that need to be delivered to a group of other components, called Subscribers. We could have setup a synchronous communication between Publisher and Subscribers, when Publisher calls each Subscriber in some order and waits for the response. But this introduces many different challenges: hard to scale such system when number of subscribers and messages grow and hard to extend such solution to support different types of subscribers. Instead, we can introduce a new system that can register an arbitrary large number of publishers and subscribers and coordinates message delivery between them. The problem statement is ambiguous, as usual. To make it less vague we need to start asking an interviewer clarifying questions. We need to scope both functional and non-functional requirements. Just on the side note, when we talk about functional requirements, we want to define system behavior, or more specifically APIs - a set of operations the system will support. When we talk about non-functional requirements, we basically mean such system qualities as scalability, maintainability, testability and others. The core APIs our notification service need to support are: create a topic, publish a message to a topic and subscribe to a topic to receive published messages. Topic represents a named resource to which messages are sent. You can think of it as a bucket that stores messages from a publisher and all subscribers receive a copy of a message from the bucket. As for non-functional requirements, we want our service to be scalable and support a big number of topics, publishers and subscribers; highly available and survive hardware failures and network partitions; fast, so that messages are delivered to subscribers as soon as possible; and durable, so that messages are not lost and delivered to each subscriber at least once. The interviewer may ask to focus on other requirements as well. For example, security or operational cost. And our usual recommendation is to be proactive and clarify requirements upfront. Let’s move on and outline a high-level architecture. All requests coming from our clients will go through a load balancer first. This will ensure requests are equally distributed among requests processing servers. And the component that does this initial request processing is a FrontEnd service. We will use a database to store information about topics and subscriptions. We will hide the database behind another miscroservice, Metadata service. There are several reasons for this decision. First, separation of concerns, a design principle that teaches us to...