Why is service dependency defined in Docker Compose?

Service dependency in Docker Compose is primarily defined to ensure that services start in a specific order. In a multi-container application, certain services may rely on others to be fully operational before they can correctly initialize. For example, a web application service might depend on a database service being up and running before it can successfully connect to it.

By explicitly declaring service dependencies, Docker Compose can manage the orchestration of starting those services in the required sequence. This avoids potential issues where, for instance, an application tries to connect to a database that is not yet available, leading to initialization errors.

While there are other aspects of service management, such as scaling or managing network communication, those do not specifically relate to the concept of ensuring a certain start order, which is crucial for maintaining the stability and reliability of the application as a whole.