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‎USE_CASES.md‎

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# ElixirScope Infrastructure Protection: Use Cases
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The `ElixirScope.Foundation.Infrastructure` module, particularly its `execute_protected/3` function, provides a unified and powerful way to apply essential resiliency patterns (connection pooling, rate limiting, and circuit breaking) to interactions with various resources. This document outlines key use cases where this architecture is ideal.
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## Core Principle
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This architecture excels when your application needs to interact with resources that are:
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* **External or Remote:** Residing outside your application's direct control.
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* **Shared:** Used by multiple parts of your application or multiple instances.
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* **Potentially Unreliable or Slow:** Subject to network latency, outages, or performance degradation.
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* **Rate-Limited or Quota-Bound:** Imposing restrictions on usage frequency or volume.
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* **Resource-Intensive to Connect To:** Where establishing a new connection is costly.
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The `execute_protected/3` function simplifies applying these patterns in a consistent, configurable, and observable manner.
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---
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## 1. Consuming Third-Party HTTP APIs
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This is a primary use case for combining all three protection patterns.
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* **Scenario:** Your application integrates with external services like payment gateways (Stripe, PayPal), data enrichment services (Clearbit), social media APIs (Twitter, Facebook), weather APIs, mapping services, etc.
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* **Protections Applied:**
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* **Connection Pooling (`connection_pool: :http_client_pool`):**
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* **Benefit:** Reuses HTTP connections (e.g., via `ConnectionManager` managing `Finch` or `Mint` workers), reducing latency from TLS handshakes and TCP connection setup for frequent calls. Improves overall throughput.
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* **Rate Limiter (`rate_limiter: {:api_calls_per_user, "user_id_123"}`):**
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* **Benefit:** Prevents your application from exceeding the API provider's usage quotas (e.g., requests per second/minute per API key). Avoids 429 (Too Many Requests) errors, throttling, or temporary bans. Can be configured per API, per user, or globally.
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* **Circuit Breaker (`circuit_breaker: :api_fuse`):**
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* **Benefit:** If the third-party API becomes slow, unresponsive, or returns a high rate of errors, the circuit opens. Subsequent requests fail fast without hitting the faulty API, preventing your application's resources (e.g., request handling processes) from being tied up. This improves your application's stability and user experience, and gives the external API time to recover.
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* **Ideal for:**
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* Applications making frequent calls to multiple external REST or GraphQL APIs.
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* Systems relying on external services for core functionality.
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* Integrations where external service reliability can vary.
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---
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## 2. Inter-Microservice Communication
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When building a distributed system with multiple ElixirScope-powered (or other) microservices.
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* **Scenario:** Service A needs to call Service B to fulfill a request. Service B might be under heavy load, temporarily unavailable, or have its own dependencies.
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* **Protections Applied:**
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* **Connection Pooling:**
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* **Benefit:** Efficiently manages and reuses connections (e.g., HTTP or gRPC) between your internal microservices, especially important for high-volume internal traffic.
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* **Rate Limiter:**
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* **Benefit:** Protects downstream services (Service B) from being overwhelmed by upstream services (Service A), especially during traffic spikes or if one service is significantly more resource-intensive. Helps prevent cascading overload.
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* **Circuit Breaker:**
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* **Benefit:** If Service B becomes unhealthy or unresponsive, Service A's circuit breaker for calls to Service B will open. This allows Service A to fail fast, potentially return a cached response, or trigger a fallback mechanism, rather than waiting for timeouts. It isolates failures and improves the overall resilience of the distributed system.
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* **Ideal for:**
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* Microservice architectures where services frequently call each other.
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* Systems where the failure of one service could impact others.
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* Ensuring fair resource usage among internal services.
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---
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## 3. Advanced Database Interactions
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While Ecto provides robust connection pooling, there are scenarios where additional layers of protection around database calls are beneficial.
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* **Scenario:**
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* Specific queries are known to be extremely resource-intensive on the database.
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* A multi-tenant application where one tenant's heavy database usage could impact others.
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* The database is a managed service with strict query-per-second (QPS) or connection limits.
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* The database link is occasionally unstable.
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* **Protections Applied:**
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* **Connection Pooling:** Typically handled by Ecto (`DBConnection` and `poolboy`/`sbroker`). `ElixirScope.Foundation.Infrastructure.ConnectionManager` could be used for non-Ecto databases or highly specialized pooling needs.
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* **Rate Limiter:**
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* **Benefit:** Limit the execution frequency of very expensive queries. Enforce fair usage policies in multi-tenant systems by rate-limiting database operations per tenant ID. Adhere to QPS limits imposed by managed database services.
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* **Circuit Breaker:**
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* **Benefit:** If the database server is overloaded, undergoing maintenance, or experiencing network issues, the circuit breaker can prevent the application from continuously hammering it. This protects the application from long wait times and allows the database to recover.
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* **Ideal for:**
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* Applications with high database load or specific performance-critical queries.
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* Multi-tenant systems sharing database resources.
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* Systems using managed databases with operational quotas.
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---
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## 4. Interacting with Caching Systems (e.g., Redis, Memcached)
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Protecting interactions with external or shared caching tiers.
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* **Scenario:** Your application relies heavily on an external cache (like Redis) for performance. The cache server itself could become a bottleneck or point of failure.
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* **Protections Applied:**
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* **Connection Pooling:**
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* **Benefit:** Efficiently manage and reuse connections to the cache server, reducing overhead for frequent cache lookups or writes.
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* **Rate Limiter:**
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* **Benefit:** Less common for caches, but could be useful if certain cache commands are particularly expensive, if the cache server has its own command rate limits, or to prevent cache stampedes for specific keys under certain conditions.
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* **Circuit Breaker:**
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* **Benefit:** If the cache server becomes slow, unresponsive, or starts erroring, the circuit breaker can open. This allows the application to gracefully degrade by, for example, fetching data directly from the primary data source (e.g., database) or serving slightly stale data, instead of failing or blocking requests due to cache issues.
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* **Ideal for:**
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* Applications with high-throughput caching requirements.
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* Systems where cache availability is critical but can fluctuate.
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---
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## 5. Publishing Messages to Message Queues (e.g., RabbitMQ, Kafka)
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Ensuring reliable and controlled message production.
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* **Scenario:** Your application publishes events or commands to a message broker like RabbitMQ or Kafka. The broker might be temporarily unavailable or overloaded.
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* **Protections Applied:**
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* **Connection Pooling:**
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* **Benefit:** Manage connections and channels to the message broker efficiently, especially important if publishing many messages.
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* **Rate Limiter:**
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* **Benefit:** Control the rate of message publishing to avoid overwhelming the broker or downstream consumers. Useful for smoothing out bursts of messages or adhering to broker-imposed throughput limits.
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* **Circuit Breaker:**
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* **Benefit:** If the message broker is unavailable, or if publishing messages (and receiving acknowledgments) consistently fails, the circuit breaker can open. This allows the application to temporarily stop publishing attempts, potentially buffer messages locally, trigger an alert, or implement other fallback strategies.
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* **Ideal for:**
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* Systems that publish a high volume of messages.
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* Event-driven architectures where message broker stability is crucial.
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* Applications needing to control the flow of data into asynchronous processing pipelines.
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---
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## 6. Guarding Resource-Intensive Internal Operations
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While primarily for external resources, the same patterns can protect critical, shared, and costly internal components.
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* **Scenario:** An application has an internal GenServer, a pool of workers, or a set of functions that perform very resource-intensive tasks (e.g., complex computations, large file processing, local AI model inference) and could become a bottleneck if overused.
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* **Protections Applied:**
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* **Connection Pooling (Conceptual):** If these are GenServer-based workers, `ConnectionManager` could manage a pool of them.
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* **Rate Limiter:**
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* **Benefit:** Limits how frequently these costly internal operations can be invoked by different parts of the system or by different users/tenants, ensuring fair use and preventing self-inflicted denial of service.
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* **Circuit Breaker:**
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* **Benefit:** If the underlying resource for this internal operation (e.g., a specific hardware accelerator, a large in-memory data structure being processed, a third-party library that can crash) starts failing or becomes excessively slow, the circuit breaker can prevent further attempts, allowing the component to recover or for an alternative to be used.
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* **Ideal for:**
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* Protecting shared, computationally expensive internal services from overload.
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* Ensuring stability when interacting with internal components that have finite capacity or can enter error states.
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---
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## 7. Systems Requiring High Observability and Dynamic Resilience Control
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The unified nature of `execute_protected/3`, combined with `configure_protection/2` and ElixirScope's integrated telemetry, allows for easy monitoring and dynamic adjustment of resilience strategies.
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* **Scenario:** An application operates in an environment where load patterns and external service health can change dynamically. Operators need to observe the behavior of protected calls and adjust resilience parameters (e.g., rate limits, circuit breaker thresholds) without code deployments.
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* **Protections Applied:** All three, as configured per `protection_key`.
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* **Benefits:**
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* **Centralized Monitoring:** Telemetry emitted by `execute_protected/3` provides a holistic view of how the combined protections are performing for a specific external call (e.g., "external_api_call").
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* **Dynamic Configuration:** `configure_protection/2` allows runtime adjustments to rate limits, circuit breaker settings (failure thresholds, recovery times), and even potentially pool configurations for a given `protection_key`. This enables adaptive resilience.
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* **Ideal for:**
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* Production systems requiring robust monitoring of external interactions.
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* Environments where operational conditions necessitate dynamic tuning of resilience strategies (e.g., tightening rate limits during an incident, temporarily relaxing circuit breaker thresholds during a planned maintenance window of an external service).
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* Teams that want to standardize and easily manage resilience patterns across their application.
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---
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By using `ElixirScope.Foundation.Infrastructure.execute_protected/3`, developers can easily implement a defense-in-depth strategy for any critical interaction point, improving the overall stability, performance, and predictability of their Elixir applications.

‎docs/foundation/API_FULL.md‎

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- **GitHub Repository**: [https://github.com/elixir-scope/elixir_scope](https://github.com/elixir-scope/elixir_scope)
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- **Issue Tracker**: [https://github.com/elixir-scope/elixir_scope/issues](https://github.com/elixir-scope/elixir_scope/issues)
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- **Documentation**: [https://hexdocs.pm/elixir_scope](https://hexdocs.pm/elixir_scope)
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- **Elixir Forum**: [https://elixirforum.com/c/elixir-scope](https://elixirforum.com/c/elixir-scope)
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- **Discord Channel**: Join the `#elixir-scope` channel on the Elixir Discord server.
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- **Elixir Forum**: [https://elixirforum.com/c/elixir-scope](https://elixirforum.com/c/elixir-scope)

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