Add more to what-is-kafka.md

A few appearance tweaks. Mostly for headings.
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Sage Vaillancourt 2022-09-08 00:30:56 -04:00
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Initially built by LinkedIn, Apache Kafka is a "distributed event store" and "stream processing" platform. Initially built by LinkedIn, Apache Kafka is a "distributed event store" and "stream processing" platform.
But what does that mean exactly? But what does that mean exactly?
## Basics of Kafka
The core principles of Kafka are very simple. There are three key components: producers, consumers, and the kafka cluster.
### Producers
Producers are responsible for creating new messages for your Kafka system. Every time an event occurs that the system
should know about, a producer will generate a message describing this event.
For example, a bookstore may have an application that _produces_ a message every time someone buys a new book:
```json
{
"action": "purchase",
"title": "The Metamorphosis",
"price": 9.99,
"buyerId": "84694fc7",
"storeNum": "FL-231",
}
```
The rest of the system decides what exactly to _do_ with this new information. Those parts of the system are called
**Consumers**.
### Consumers
Whenever a message is produced in the Kafka system, a consumer can act in response to that new information.
Returning to the previous example: this particular bookstore may have a service that informs authors and publishers
every time one of their books has been purchased. This service may work as a consumer. Then, every time a book is
purchased, the service _consumes_ the new message, and alerts the creators however it sees fit.
One of the main benefits of this structure is an inherent loose coupling. Producers do not care how consumers work. If a
consumer wants to send a text, rent a billboard, or order an ice cream, the producer simply is not interested. This
allows each part of the system to focus on its own piece of the domain, each staying smaller and more self-contained.
### Kafka Cluster
The Kafka cluster, in short is what facilitates the connection between producers and consumers. An indefinite number of
producers and consumers can connect to the cluster, and the cluster itself can be powered by many servers.
The cluster decides exactly which messages need to go where, and a key part of this process is its use of **Topics**.
#### Topics
"Topics" are Kafka's way of dividing up messages into different categories. A simple topic for our bookstore might be
`book_purchases`, or `coffee_orders` (for the requisite café inside). So, when a book is sold, our producer would push a
new message into the `book_purchases` topic. And of course, when someone orders a fancy latte or something hot and
bitter, a message is instead put into the `coffee_orders` topic.
Importantly, consumers do not blindly receive every message produced in the system. Instead, they subscribe to
individual topics. Our author-notification service might subscribe just to the `book_purchases` topic, not being
very interested in messages from `coffee_orders`.
Thus, every time the Kafka cluster receives a newly-produced message with a given topic, it only forwards it along to
consumers that have intentionally subscribed.
## TODO:
## Distributed Event Store ## Distributed Event Store
## Stream Processing Essentially
## Stream Processing