Design Patterns Used at Different Software Layers

Client Layer

Pattern	Description
Client Side Load Balancing	Distributes requests from the client directly to the available service instances.
State Pattern	Allows an object to alter its behaviour when its internal state changes.
Composite UI Pattern	Composes responses from multiple microservices to render the complete UI.

Load Balancer Layer

Pattern	Description
Geographical Distribution	Routes traffic based on the geographical location of the client.
Health Checks	Periodically checks the health of the servers and routes traffic only to healthy ones.
Affinity Based Routing	Routes the user's request to the same server for maintaining session persistence.
Least Connections	Routes traffic to the server with the fewest active connections.

API Gateway Layer

Pattern	Description
Backend for Frontend (BFF)	Tailors API responses to the needs of individual client types.
Circuit Breaker	Detects failures and prevents applications from trying to perform actions that are doomed to fail.
Retry Pattern:	Helps to handle transient failures when it tries to connect to a remote service or network resource.
Request Collapsing	Collapses multiple requests for the same operation into a single request

Web Server Layer

Pattern	Description
Page Cache Pattern	Stores the output of expensive operations and reuse it to avoid duplicated work.
Compression Pattern	Reduces the size of the response to improve load times.
Lazy Loading	Defers initialization of an object until the point at which it is needed.
Content Negotiation Pattern	The server generates different versions of a resource and serves the one matching the client’s criteria.

Appplication Server Layer

Pattern	Description
Saga Pattern	Manages long-running transactions and deals with failures and compensating transactions.
CQRS (Command Query Responsibility Segregation)	Separates read and write operations to improve performance and scalability.
Proxy Pattern	Provides a surrogate or placeholder for another object to control access to it.
Chain of Responsibility	Passes the request along a chain of handlers.

Caching Layer

Pattern	Description
Sidecar Caching	Deploy a dedicated cache alongside each microservice to provide isolated and scalable caching functionality.
Cache Chaining	Arrange multiple cache layers hierarchically to handle different granularity or lifetime, querying each layer sequentially on a cache miss.
Time-to-Live (TTL) Caching	Assigns a predefined lifespan to each cache entry, removing or refreshing the entry once its lifespan expires.

Content Delivery Network Layer

Pattern	Description
Prefetching	Anticipates user actions and loads resources ahead of time.
Parallel Requesting	Makes multiple requests in parallel to improve load times.
Edge Computing	Processes data closer to the location where it is needed.
Domain Sharding	Splits resources across multiple domains to increase parallel downloads.
Adaptive Image	Delivery: Delivers images tailored to the device and user context.

Database Layer

Pattern	Description
Sharding Pattern	Distributes data across multiple databases to improve scalability.
Replication Pattern	Keeps copies of data in multiple locations for availability and reliability.
Read-Replica Pattern	Uses read replicas to offload read operations from the primary database instance.
Query Object Pattern	An object that represents a database query.