Understanding Inter-component communication in Distributed Systems

1. Introduction to Distributed Systems

Distributed systems are systems that are composed of multiple independent components that communicate with each other over a network. These components can be on the same physical machine or on different machines. The primary goal of distributed systems is to provide a way for different components to work together to achieve a common goal.

Let’s understand this with a simple real-world example: A distributed system is like a team of chefs in a restaurant, each handling a specific task — preparing, cooking, or plating. Though they work at different stations, they constantly communicate to deliver a single meal. Similarly, components in a distributed system work together over a network to achieve a common goal.

2. Communication Models in Distributed Systems

In distributed systems, components communicate with each other using different communication models. These models define how components interact with each other and exchange information. The two main communication models in distributed systems are:

1. Synchronous Communication — In synchronous communication, components communicate in a blocking manner, where the sender waits for the receiver to respond before continuing. Implementing this model is easy, but it may result in performance issues if the receiver is slow or unresponsive. Example protocols that use synchronous communication include HTTP and RPC.
2. Asynchronous Communication — In asynchronous communication, components communicate in a non-blocking manner, where the sender does not wait for the receiver to respond. Implementing this model is more challenging, but it enhances performance by enabling components to multitask while awaiting a response. Example protocols that use asynchronous communication include message queues and publish-subscribe systems.

3. Communication Mechanisms in Distributed Systems

Different communication mechanisms are utilized to facilitate interaction among components in distributed systems. These mechanisms define how components send and receive messages and how they handle communication failures. Some common communication mechanisms in distributed systems are:

1. Remote Procedure Calls (RPC) — RPC is a communication mechanism that allows a component to invoke a procedure or method on a remote component as if it were a local call. This mechanism abstracts the details of network communication and makes it easier to build distributed systems.

Let’s understand how RPC gets used in a real-world with an example: with Apache Spark, have you ever wondered how the spark driver knows the state of the executors? Well, that is done using RPC. Executor periodically sends a heartbeat to the driver with task metrics and its state using RPC. When the driver receives the heartbeat, it internally updates the state, allowing the Task scheduler to schedule tasks based on resources.

2. RESTful APIs — RESTful APIs are communication mechanisms that use HTTP methods (GET, POST, PUT, DELETE) to interact with resources on a server. This mechanism is widely used in web applications and allows components to communicate over the internet.

To illustrate the use of RESTful APIs for state communication, consider a social media application. When a user creates a new status, the front-end client sends a POST request to the server’s API containing the status content. The server then processes the request and updates the status in the database. The front-end client, in this scenario, can be a mobile app, web browser, or any other client on a separate machine from the server.

3. Message Queues?—?Message queues are communication mechanisms that allow components to send and receive messages asynchronously. A queue allows components to publish messages, which can then be consumed by other components. This mechanism decouples the sender and receiver, allowing them to operate independently.

To grasp how it is used in real life, consider an e-commerce application scenario. When a user makes an order, the front-end client sends a message to a message queue containing the order information. The order processing may involve various components, including payment processing, inventory management, and shipping, before we sent the confirmation. Here, each of these components will consume the message from the queue, perform its individual task and drop back the message in the queue. If all the components successfully process the order, the confirmation is sent to the client as the order progresses in the processing pipeline.

4. Publish-Subscribe Systems?—?Publish-subscribe systems are communication mechanisms that allow components to publish messages to topics or channels, and other components can subscribe to these topics to receive messages. This mechanism enables one-to-many communication and is commonly used in event-driven architectures.

Let’s understand its real-world usage: In case of a stock trading application, when a stock price changes, the stock exchange publishes a message to a topic with the updated price. Subscribers, such as traders or algorithms, can subscribe to this topic to receive real-time updates on the stock price. This enables traders to make informed decisions based on the latest market information.

4. Choosing the Right Communication Model and Mechanism

When designing a distributed system, it is important to choose the right communication model and mechanism based on the requirements of the system. Here are some factors to consider when selecting a communication model and mechanism:

1. Latency Requirements?—?What are the latency requirements of the system? Is this a real-time system that requires low latency communication or a batch processing system that can tolerate higher latency?
2. Scalability?—?How does the communication model scale with the number of components? As the system grows, will the communication model be able to handle the increased load?
3. Complexity?—?How complex is the communication model to implement and maintain? Is ease of implementation and maintenance important for the system?
4. Reliability?—?How reliable is the communication mechanism? Can it handle failures and ensure message delivery?

Below is the baseline comparison of the communication mechanisms based on the above factors:

5. Conclusion

Inter-component communication is a critical aspect of distributed systems that enables components to work together to achieve a common goal. By understanding the different communication models and mechanisms available, developers can design systems that meet the requirements of their applications. Choosing the right communication model and mechanism is essential for building scalable, reliable, and performant distributed systems.