3-Tier Auto-scalable Web Application Architecture

"A web application or web app is any application software that runs in a web browser or is created in a browser-supported programming language (such as the combination of JavaScript, HTML and CSS) and relies on a common web browser to render the application. ... Applications are usually broken into logical chunks called "tiers", where every tier is assigned a role. ...
For more complex applications, a 3-tier solution may fall short, and it may be beneficial to use an n-tiered approach, where the greatest benefit is breaking the business logic, which resides on the application tier, into a more fine-grained model. Another benefit may be adding an integration tier that separates the data tier from the rest of tiers by providing an easy-to-use interface to access the data. For example, the client data would be accessed by calling a "list_ clients()" function instead of making an SQL query directly against the client table on the database. This allows the underlying database to be replaced without making any change to the other tiers." [Web application. Wikipedia]
The AWS architecture diagram example "3-Tier Auto-scalable Web Application Architecture" was created using the ConceptDraw PRO diagramming and vector drawing software extended with the AWS Architecture Diagrams solution from the Computer and Networks area of ConceptDraw Solution Park.

This Azure cloud architecture pattern diagram template was created on the base of figure in the article "Valet Key Pattern" from the Microsoft Developer Network (MSDN) website.
"Valet Key Pattern.
Use a token or key that provides clients with restricted direct access to a specific resource or service in order to offload data transfer operations from the application code. This pattern is particularly useful in applications that use cloud-hosted storage systems or queues, and can minimize cost and maximize scalability and performance. ...
Client programs and web browsers often need to read and write files or data streams to and from an application’s storage. ...
Data stores have the capability to handle upload and download of data directly, without requiring the application to perform any processing to move this data, but this typically requires the client to have access to the security credentials for the store.
... applications must be able to securely control access to data in a granular way, but still reduce the load on the server by setting up this connection and then allowing the client to communicate directly with the data store to perform the required read or write operations. ...
To resolve the problem of controlling access to a data store where the store itself cannot manage authentication and authorization of clients, one typical solution is to restrict access to the data store’s public connection and provide the client with a key or token that the data store itself can validate.
This key or token is usually referred to as a valet key. It provides time-limited access to specific resources and allows only predefined operations such as reading and writing to storage or queues, or uploading and downloading in a web browser. Applications can create and issue valet keys to client devices and web browsers quickly and easily, allowing clients to perform the required operations without requiring the application to directly handle the data transfer. This removes the processing overhead, and the consequent impact on performance and scalability, from the application and the server." [msdn.microsoft.com/ ru-RU/ library/ dn568102.aspx]
The Azure cloud system architecture diagram template "Valet key pattern" for the ConceptDraw PRO diagramming and vector drawing software is included in the Azure Architecture solutin from the Computer and Networks area of ConceptDraw Solution Park.