To make API keys more secure during transmission, it's essential to use HTTPS (SSL/TLS) to encrypt the communication. Transmitting keys in plain text, via email, or in public chat channels can expose them to potential attackers. HTTPS provides secure end-to-end encryption, making it significantly more difficult for malicious parties to intercept and misuse API keys.

GraphQL subscriptions are used to get real-time updates when an event or data change occurs on the server. This can include events like new data being added, data updates, or deletions. GraphQL subscriptions enable clients to receive real-time updates without the need for continuous polling.

Designing a RESTful API for a banking application requires a strong focus on security and reliability. Using HTTPS, implementing robust authentication, authorization, and encryption mechanisms are essential to protect sensitive financial data. Minimizing error handling and returning detailed error messages can pose security risks. Storing sensitive data in plain text is a security vulnerability.

A characteristic feature of gRPC is that it supports multiple programming languages, making it suitable for building cross-language and cross-platform applications. It uses Protocol Buffers (protobufs) for data serialization and supports asynchronous communication, which can improve application performance.

Web APIs act as a bridge between "Clients" and allow them to communicate and exchange data. Clients can be web applications, mobile apps, or other software that make requests to servers, which provide data and services. This interaction allows for the exchange of information and functionality between different systems, making it a fundamental aspect of modern web development.

To ensure the reliability of an API, it's important to implement Monitoring Tools that can detect and alert on potential issues. Monitoring tools track the performance and health of an API in real-time, helping to identify and respond to issues such as downtime, slow responses, or errors that can affect the end-users.

OpenID Connect returns an ID Token, which contains claims about the authentication of an end-user by an Authorization Server. This ID Token is a JSON Web Token (JWT) and includes information about the authenticated user, such as their user ID and authentication timestamp. It is a critical component of user authentication in OpenID Connect.

In Express, implementing "Middleware" can help in securing APIs from common vulnerabilities like Cross-Site Scripting (XSS) and Cross-Site Request Forgery (CSRF). Middleware functions can intercept requests and responses, allowing you to perform security checks and other operations before or after handling the request.

gRPC (gRPC Remote Procedure Call) improves communication between microservices by providing a high-performance framework for remote procedure calls (RPC). It uses a binary serialization format and supports multiple programming languages, making it efficient and language-agnostic. This enhances communication speed and reduces latency in microservices architectures.

To handle a large number of simultaneous requests efficiently, you should consider using HTTP/2. HTTP/2 allows for multiplexing, which enables multiple requests and responses to be processed in parallel over a single connection. This reduces latency and can significantly improve the performance of your Web API.