Chapter 6 - Distributed Caching
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What is the primary benefit of application caching?

  • Increased data storage capacity
  • Simplified database architecture
  • Automatic data updates
  • Reduced latencies and increased responsiveness (correct)

Which policy is commonly used by caches to evict entries?

  • Least recently used (correct)
  • First in, first out
  • Least frequently used
  • Most frequently used

What happens when a cache miss occurs?

  • The request is satisfied through the cache
  • The request is satisfied by querying databases (correct)
  • The entry is immediately evicted
  • The cache data is automatically refreshed

What is a unique feature of read-through, write-through, and write-behind caches compared to the cache-aside pattern?

<p>They involve automatic interaction with backend storage systems. (A)</p> Signup and view all the answers

Why is it important to monitor cache usage in production?

<p>To maintain the hit and miss rates as expected (C)</p> Signup and view all the answers

Which cache technology supports application-specific handlers for performing database actions?

<p>NCache (A)</p> Signup and view all the answers

What can negate the benefits of caching when items are updated frequently?

<p>The cost of cache misses (C)</p> Signup and view all the answers

What is the main advantage of the cache-aside strategy?

<p>It is resilient to cache failures. (D)</p> Signup and view all the answers

Which caching pattern allows bypassing the data storage systems when results are in the cache?

<p>Cache-aside (D)</p> Signup and view all the answers

How do web caches primarily enhance the responsiveness of websites?

<p>By intercepting requests and returning cached resources instead of contacting the service. (A)</p> Signup and view all the answers

What is the ideal cache usage pattern regarding reads and updates?

<p>Much more reads than updates (D)</p> Signup and view all the answers

What is a characteristic of dedicated database caches mentioned in the content?

<p>They require cache access to align with the database model. (A)</p> Signup and view all the answers

What utilization is typically provided by caching systems such as memcached?

<p>Statistics including hit and miss counts (D)</p> Signup and view all the answers

What is one implication of web caches being physically closer to clients?

<p>They reduce the latency of requests made to services. (A)</p> Signup and view all the answers

What feature of caching systems like Redis and Memcached simplifies their scalability?

<p>Their simple distributed hash table model. (A)</p> Signup and view all the answers

Which of the following statements about caching strategies is FALSE?

<p>Read-through caches do not need to interact with backend storage. (A)</p> Signup and view all the answers

What do memcached and Redis have in common?

<p>They serve as distributed in-memory hash tables. (B)</p> Signup and view all the answers

What is a cache hit?

<p>When the data requested is found in the cache. (D)</p> Signup and view all the answers

What happens during a cache miss?

<p>Query results are written to the cache for future requests. (A)</p> Signup and view all the answers

What type of data is typically stored in application-level caches?

<p>Dynamic web pages and user session data. (C)</p> Signup and view all the answers

Why is calculating lift wait times considered expensive?

<p>It aggregates and processes tens of thousands of records simultaneously. (D)</p> Signup and view all the answers

How long are lift wait time results deemed valid before recalculation?

<p>Five minutes (D)</p> Signup and view all the answers

What is a potential use for a distributed cache in application services?

<p>To cache the results of API calls for faster response times. (A)</p> Signup and view all the answers

How does a cache allocate objects to individual cache servers?

<p>Using a hash function on the object key. (C)</p> Signup and view all the answers

What is the primary benefit of using caching in scalable applications?

<p>It allows information to be served multiple times without re-creation costs. (A)</p> Signup and view all the answers

What does a high cache hit rate indicate in a scalable system?

<p>Most requests are fulfilled using cached data. (C)</p> Signup and view all the answers

How does HTTP caching affect request loads on downstream services?

<p>It can significantly reduce request loads if used wisely. (B)</p> Signup and view all the answers

What must an application do when implementing distributed caching?

<p>Check for cached values upon receiving client requests. (B)</p> Signup and view all the answers

What function do cache directives in HTTP headers provide?

<p>They dictate what can be cached and how long it should remain cached. (B)</p> Signup and view all the answers

What is the role of CDN in the context of caching?

<p>They facilitate fast content delivery for geographically dispersed users. (C)</p> Signup and view all the answers

Which of the following is NOT a benefit of caching in applications?

<p>Increased data security. (B)</p> Signup and view all the answers

What is a primary resource for learning about caching strategies?

<p>Gerardus Blokdyk's <em>Memcached</em>, 3rd ed. (B)</p> Signup and view all the answers

What is the primary function of edge caches?

<p>To cache frequently accessed data close to clients. (D)</p> Signup and view all the answers

Which statement accurately describes web browser caches?

<p>They are also known as private caches. (A)</p> Signup and view all the answers

What is the role of HTTP caching directives?

<p>To control how caching should be utilized for resources. (A)</p> Signup and view all the answers

Where are CDN edge caches typically located?

<p>In various strategic geographical locations globally. (C)</p> Signup and view all the answers

What happens when a client sends a GET request?

<p>It can be intercepted by multiple caches along the request path. (D)</p> Signup and view all the answers

What type of content do caches typically store?

<p>Results of GET requests. (D)</p> Signup and view all the answers

What is the significance of the value 'Expires' in an HTTP response?

<p>It specifies when the resource is no longer fresh. (A)</p> Signup and view all the answers

How many locations did Akamai have as a CDN provider?

<p>2,000 locations. (C)</p> Signup and view all the answers

What happens when a request arrives and the latest wait times are available in the cache?

<p>The results are immediately returned to the client. (B)</p> Signup and view all the answers

What key is used to access the cached lift wait times?

<p>&quot;liftwaittimes:&quot; concatenated with the resort identifier (B)</p> Signup and view all the answers

What does the time to live (TTL) value signify in the cache operations?

<p>The duration after which the cache key-value pair is removed. (C)</p> Signup and view all the answers

How does the service handle the situation when the lift wait times are not found in the cache?

<p>It calls a downstream service for calculations. (B)</p> Signup and view all the answers

What is the primary benefit of using caching for lift wait time calculations?

<p>It reduces the need for expensive calculations for subsequent requests. (C)</p> Signup and view all the answers

What occurs when the cache value is deemed stale after its TTL expires?

<p>A cache miss happens, triggering a calculation of new values. (D)</p> Signup and view all the answers

What would happen if cached items are not flushed periodically?

<p>The cache may fill up and reject further requests. (D)</p> Signup and view all the answers

When the service successfully retrieves lift wait times from the cache, how long is the typical response time in a fast network environment?

<p>Approximately 1 millisecond (B)</p> Signup and view all the answers

Flashcards

Cache

A mechanism that stores data temporarily for faster retrieval, avoiding repetitive calculations.

Time to Live (TTL)

The time a cached item remains valid before being removed.

Cache Hit

Accessing data from the cache, avoiding expensive calculations.

Cache Miss

When the requested data is not present in the cache.

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Cache Key

The unique identifier used to access data in a cache.

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Cache Eviction

The process of removing expired or outdated data from the cache.

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Cache Optimization

Avoiding redundant calculations by using cached data instead of performing expensive operations.

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Stale Data

Providing stale or outdated information to the client.

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Application-level Caching

A type of caching that stores data in memory for quick retrieval, reducing database query time.

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Memcached

A distributed in-memory key-value store, known for its speed and scalability. Widely used for caching data that's frequently requested.

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Redis

A similar in-memory cache like Memcached, but offers more data structures and features. Widely used for caching, session management, and real-time analytics.

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Cache Expiration

The time after which cached data is considered invalid and needs to be refreshed. This helps ensure data freshness.

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LiftWaitService

A service that uses application-level caching to estimate wait times at a ski resort, using data about skier traffic patterns.

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Cache Eviction Policy

A strategy used by caches to decide which items to remove when the cache is full. Examples include LRU (Least Recently Used) and LFU (Least Frequently Used).

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Cache Hit Rate

The percentage of requests that can be served directly from the cache. A higher hit rate indicates a more efficient cache.

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Cache Miss Rate

The percentage of requests that cannot be served from the cache and require a database or service query.

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Reduced Latencies

Reducing the time it takes for users to receive a response from an application, making it feel faster and more responsive.

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Increased Throughput

The ability of an application to handle a high volume of requests efficiently, often achieved through optimization techniques like caching.

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Cache-Aside Pattern

A caching pattern where the application code checks the cache first, only accessing the database if the data is not found. This is the most common caching pattern.

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Cache Invalidation

The process of removing outdated or stale cache entries to ensure that the cache contains the most current data.

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Read-Heavy Cache Design

The design principle of aiming for more reads than updates in a cache, as updates can lead to expensive cache misses.

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Read-Through Caching

A caching technique where the application always uses the cache for reads and writes. The cache manages interactions with the backend storage, making application logic simpler.

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Write-Through Caching

A caching technique where write operations are performed in both the cache and the backend storage simultaneously, ensuring consistency.

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Write-Behind Caching

A caching technique where write operations are initially performed only in the cache, with asynchronous updates to the backend storage later.

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Cache Resilience

The ability to continue operating even when the cache is unavailable. This typically involves handling all requests as cache misses, potentially impacting performance.

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Scalability

The ease of expanding a system's capacity to handle more requests. This usually involves adding more servers or instances to the system.

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Web Cache

A temporary storage that holds copies of web resources (like web pages or images) for a specified time.

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Web Caching

An approach where a web cache intercepts client requests, providing the cached content if available. This reduces the load on the target service and improves request latency.

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Edge Cache

A type of web cache situated closer to users, usually in various geographic locations, to reduce latency for content delivery, especially for global users.

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Content Delivery Network (CDN)

A network of servers designed to deliver content quickly and efficiently to users around the world.

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Private Cache

A private cache used by individual users, like a web browser's cache, to store recently accessed content.

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Shared Cache

A cache used by multiple users within an organization, often managed by a dedicated server, to improve efficiency.

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Web Proxy Cache

A web cache that intercepts client requests and attempts to serve them from cached copies. If the content is not in the cache, it will forward the request to the origin server.

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HTTP Caching Directives

HTTP headers included in responses from servers to indicate how client applications (and caches) should handle the cached content.

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Cache-Control

A directive used in HTTP headers to control caching behavior. It specifies how a resource should be cached, for how long, and by whom.

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Proxy Cache

A type of caching where data is stored in memory on a server that acts as an intermediary between clients and backend servers.

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Distributed Cache

A storage mechanism that allows applications to store and retrieve data quickly, often used for caching.

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HTTP Caching

The ability to utilize existing caching infrastructure on the internet to speed up content delivery.

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Cache Directives

Instructions within HTTP headers that tell a browser or cache how to manage cached content.

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Study Notes

Distributed Caching

  • Caches exist in various places within applications, including hardware (CPUs) and software (database engines). They speed up access to data by storing frequently accessed information.
  • Distributed caching is important for scalable systems. It increases system capacity because it avoids recalculating results for each request.
  • Two types of caching are application caching and web caching.
  • Application caching uses distributed caches to store and retrieve computation results, improving response time in applications. An example is a news site where comments are cached.
  • Web caching leverages the mechanisms in the HTTP protocol for caching information, using the Internet's infrastructure.
  • Caching reduces load on databases and services by allowing frequent access to information and reducing computation costs for complex queries.
  • Caching requires extra resources (and therefore cost). However, it is often less expensive than upgrading servers. Approximately 3% of infrastructure resources are typically used for caching.
  • Caching uses dedicated engines like memcached and Redis (in-memory hash tables).
  • Caching stores items based on keys, and those items are placed on the servers using a hash function.
  • Caching schemes utilize hit rates and miss rates for efficiency and performance.

Application level Caching

  • Application-level caching stores query results/computation results improving responsiveness.
  • Caching improves retrieval for frequently accessed data, such as storing user data and session information.
  • Information is stored as a single store to application services.
  • In a busy winter resort example, mobile application users obtain wait times for lifts. Caches the information and reduce wait times for the user.

Web Caching

  • Web caches (Content Delivery Networks - CDNs) store copies of resources.
  • Caches intercept requests and retrieve information and resources.
  • Caching is efficient for large or frequent requests.
  • They place caches geographically, close to clients' to reduce response time.
  • Web caching architecture includes various levels, such as client browsers' caches, organizational caches, and internet service providers (ISPs) caches.
  • Edge caches (part of CDNs) are positioned in strategic locations worldwide to improve efficiency and performance. Akamai is one example of a major CDN.

Cache Strategies

  • Read-through: All requests pass through the cache. If not found, the data is retrieved, stored in the cache, and returned.
  • Write-through: When an update occurs, the cache and the database are updated simultaneously.
  • Write-behind: First update is to the cache. Then, the database is updated later.
  • Cache-aside: The application first looks in the cache. If not found, it retrieves the data from the database, stores it in the cache, then returns the result.

Cache Directives

  • Cache-Control: A directive that determines how resources are cached. Common values include public (allows caching by anything) and max-age=X (setting a time limit for keeping content in the cache).
  • Expires and Last-Modified: Values used to calculate how long a cached resource is considered fresh.

Etag

  • An opaque value that web caches use to check if a resource in the cache is still valid.
  • This value (like a version number) is updated when the resource's content changes.

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Distributed Caching Chapter 6

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This quiz covers the fundamentals of distributed caching, including its importance in scalable systems and the two main types: application and web caching. It also explores how caching improves system performance by reducing database load and computation costs. Test your knowledge on these key concepts in distributed caching.

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