Which protocol is commonly used for communication between services in a service-oriented architecture?

The commonly used protocol for communication between services in a service-oriented architecture (SOA) is HTTP/HTTPS. This is primarily due to its widespread adoption and compatibility with web services. HTTP (Hypertext Transfer Protocol) and its secure variant HTTPS (HTTP Secure) enable communication over the internet and are well-suited for RESTful APIs and web-based services, which are integral components of SOA.

HTTP/HTTPS allows for stateless communication between services, thereby simplifying the exchange of data and invoking services from remote systems. It supports various data formats such as JSON and XML, making it versatile for service interactions. This flexibility makes it a preferred choice in modern architectures, where services often need to interact seamlessly across diverse platforms and environments.

Other protocols like FTP (File Transfer Protocol) are designed for transferring files, which may not align with the typical needs of service communication in an SOA. SMTP (Simple Mail Transfer Protocol) is focused on email transmission, while SSH (Secure Shell) is primarily used for secure access to networked devices. None of these protocols possess the same broad applicability and utility as HTTP/HTTPS in the context of service-oriented architectures.