Caching in Application Servers
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Questions and Answers

Which cache eviction strategy removes items that are used least often?

  • Least Frequently Used (LFU) (correct)
  • Least Recently Used (LRU)
  • Most Recently Used (MRU)
  • Random Replacement (RR)

    • Most Recently Used (MRU) eviction strategy keeps the most recently accessed items in cache.

    False (B)

    What is the main characteristic of the Last In First Out (LIFO) caching method?

    It evicts the block accessed most recently first.

    The _____ Replacement (RR) strategy discards a randomly selected candidate item to make space.

    <p>Random</p> Signup and view all the answers

    Stale-while-revalidate is a method where outdated content is immediately served to the user while a background update occurs.

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

    Match the following caching strategies with their descriptions:

    <p>LIFO = Evicts the most recently accessed block first LRU = Discards the least recently used items first MRU = Discards the most recently used items first LFU = Removes items based on usage frequency</p> Signup and view all the answers

    In a read-through cache, the cache is responsible for retrieving data from the ______ when a cache miss occurs.

    <p>underlying data store</p> Signup and view all the answers

    Which of the following is NOT a benefit of using stale-while-revalidate?

    <p>Always serving the most up-to-date content (D)</p> Signup and view all the answers

    What happens when a request is made for content using the stale-while-revalidate technique?

    <p>The cached version of the content is served to the user, and a background request is made to the origin server for the latest version.</p> Signup and view all the answers

    Match the caching strategy to its description:

    <p>Read-through cache = Cache itself retrieves data from the data store during a cache miss. Stale-while-revalidate = Serves stale content while updating the cache in the background.</p> Signup and view all the answers

    What happens when the latest version of content becomes available in the stale-while-revalidate method?

    <p>The cached version is updated with the latest version. (A)</p> Signup and view all the answers

    Read-through caching is a method primarily used in web browsers.

    <p>False (B)</p> Signup and view all the answers

    Describe the primary aim of using stale-while-revalidate.

    <p>To ensure content is delivered quickly to the user, even if the cached version is slightly outdated.</p> Signup and view all the answers

    What is the primary purpose of a Content Delivery Network (CDN)?

    <p>To serve large amounts of static media (B)</p> Signup and view all the answers

    A CDN serves content directly from the back-end servers at all times.

    <p>False (B)</p> Signup and view all the answers

    What is one way to prepare for a future transition to a CDN?

    <p>Serve static media off a separate subdomain</p> Signup and view all the answers

    If a CDN doesn't have the requested file locally, it will query the _____ servers.

    <p>back-end</p> Signup and view all the answers

    What lightweight HTTP server is suggested for serving static media?

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

    CDNs can help reduce the load on back-end servers.

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

    What action does a CDN take when it has a piece of content available?

    <p>Serve the content to the requesting user</p> Signup and view all the answers

    Match the following actions in a CDN setup with their descriptions:

    <p>Check local cache = The CDN first looks for the requested file Query back-end servers = If not found locally, the CDN requests the file from the server Cache the file = Save the received file for future requests Serve the user = Deliver the requested file to the end-user</p> Signup and view all the answers

    What must happen to cache data when the database is modified?

    <p>It should be invalidated. (A)</p> Signup and view all the answers

    Cache invalidation is not necessary if the database is updated frequently.

    <p>False (B)</p> Signup and view all the answers

    What is the primary benefit of using a write-through cache?

    <p>Data consistency between cache and storage.</p> Signup and view all the answers

    The main purpose of cache invalidation is to maintain _____ between the cache and the database.

    <p>consistency</p> Signup and view all the answers

    Match the following cache invalidation schemes with their descriptions:

    <p>Write-through cache = Data is written to cache and database simultaneously Write-back cache = Data is written to cache and updated later in the database Cache expiration = Data is invalidated after a predetermined time Manual invalidation = Data is invalidated based on specific operations or triggers</p> Signup and view all the answers

    Which scenario describes a potential problem without cache invalidation?

    <p>Inconsistent application behavior. (C)</p> Signup and view all the answers

    Data stored in the cache is generally more reliable than data stored in the database.

    <p>False (B)</p> Signup and view all the answers

    What is a possible consequence of a crash or power failure when using a write-through cache?

    <p>No data loss.</p> Signup and view all the answers

    What happens during a cache miss?

    <p>Data is retrieved from the data store and the cache is updated. (C)</p> Signup and view all the answers

    The read-aside cache strategy simplifies application code because it automatically updates the cache.

    <p>False (B)</p> Signup and view all the answers

    What is the primary benefit of writing to permanent storage after specified intervals?

    <p>High throughput for write-intensive applications (C)</p> Signup and view all the answers

    Why might an application choose to implement a read-aside cache strategy?

    <p>To gain better control over the caching process and optimize cache usage based on specific data access patterns.</p> Signup and view all the answers

    In the FIFO cache eviction policy, the cache evicts the first block __________.

    <p>accessed</p> Signup and view all the answers

    Writing data directly to permanent storage eliminates the risk of data loss in the case of a crash.

    <p>False (B)</p> Signup and view all the answers

    What characteristic does a read-aside cache provide to an application?

    <p>Manual control over when to update the cache (B)</p> Signup and view all the answers

    What happens when a purge request is received?

    <p>The cached content for a specific object or URL is removed.</p> Signup and view all the answers

    The two common cache invalidation methods are ___ and ___.

    <p>Purge, Refresh</p> Signup and view all the answers

    Cache eviction policies have no impact on application performance.

    <p>False (B)</p> Signup and view all the answers

    What must an application do when a cache miss occurs?

    <p>Retrieve the data from the data store and update the cache.</p> Signup and view all the answers

    What does the refresh method do in cache invalidation?

    <p>Fetches content from the origin server and updates the cache (A)</p> Signup and view all the answers

    Match the cache eviction policies with their definitions:

    <p>FIFO = Evicts the oldest accessed block regardless of access frequency LRU = Evicts the least recently used block LFU = Evicts the least frequently used block Random = Evicts a random block from the cache</p> Signup and view all the answers

    The refresh method will use the existing cached content instead of going back to the origin server.

    <p>False (B)</p> Signup and view all the answers

    Match the following cache invalidation methods with their descriptions:

    <p>Purge = Removes cached content for a specific object or URL Refresh = Updates cached content with the latest from the origin server</p> Signup and view all the answers

    What risk is associated with using cache for write-intensive applications?

    <p>Data loss in case of a crash or adverse event.</p> Signup and view all the answers

    Study Notes

    Caching

    • Load balancing scales servers horizontally, but caching optimizes existing resources and makes demanding product requirements feasible.
    • Caching leverages the principle of locality of reference—recently requested data is likely to be requested again.
    • Caches are integrated into various computing components: hardware, OS, web browsers, web apps, etc.
    • Caches function like short-term memory: they have limited space, are often faster than the original source, and store recently accessed data.
    • Caches exist at different architectural levels, typically near the front end to quickly return data without affecting downstream processes.

    Application Server Cache

    • Placing a cache on the request layer node allows local storage of response data.
    • Upon request, the node returns locally cached data, if available, otherwise fetches it from the disk.
    • Caches can reside both in memory (fast) and on the node's local disk (faster than network storage).
    • Scaling to multiple nodes allows each node to have its own cache, but a load balancer distributing requests across different nodes increases cache misses.

    Content Delivery Network (CDN)

    • CDNs are caches designed for static media-heavy websites.
    • A CDN request initially seeks the requested content locally.
    • If the content isn't available locally, the CDN queries backend servers, caches the content locally, and serves the content to the user.
    • CDNs can be a separate subdomain (e.g., static.yourservice.com) integrated using a lightweight server like Nginx, and DNS can be switched later.

    Cache Invalidation

    • Maintaining cache coherence with the source of truth (e.g., database) requires cache invalidation.
    • If data in the database changes, the cached data should be invalidated to prevent inconsistencies.

    Three Main Invalidation Schemes

    • Write-through: Simultaneously writes data to the cache and the database; ensures data consistency but has higher latency due to double writes.
    • Write-around: Directly writes data to the database, bypassing the cache, reducing latency, but read requests for recently written data will create a cache miss, resulting in slower response.
    • Write-back (write-behind): Writes data only to the cache; immediately confirms to the client; writes to the database occurs periodically or under specific conditions. This potentially loses data during failures and has lower latency.

    Cache Invalidation Methods

    • Purge: Removes cached content for specific objects, URLs, or sets of URLs, typically used when content is updated or changed. A purge request immediately removes the content from the cache, and serves subsequent requests from the origin server.
    • Refresh: Updates the cache with the latest version of the data even if cached content is available.
    • Ban: Invalidates cached content based on criteria like URL or headers, immediately removing matching data from the cache. Subsequent requests are served from the origin server.
    • Time-to-live (TTL): Content in the cache expires after a specified time; the cache checks expiry and re-fetches data if it is expired.
    • Stale-while-revalidate (SWI): Serves stale cached content while asynchronously fetching updated data from the origin server; the update results in an updated cache.

    Cache Read Strategies

    • Read-through: The cache is responsible for retrieving data from the data store on a cache miss. The application requests data from the cache first; on a cache miss, the cache retrieves the data, updates the cache, and returns the data to the application. Simplifies application logic and ensures data consistency.
    • Read-aside: The application is responsible for retrieving data from the underlying data store on cache misses. The application first checks the cache: if found, uses cached data; otherwise, retrieves the data, updates the cache, and uses that data. Provides more control over caching but adds complexity to the application code.

    Cache Eviction Policies

    • First-In, First-Out (FIFO): Evicts the oldest entry in the cache.
    • Last-In, First-Out (LIFO): Evicts the most recently added entry.
    • Least Recently Used (LRU): Evicts the least recently used entry.
    • Most Recently Used (MRU): Evicts the most recently used entry.
    • Least Frequently Used (LFU): Evicts the least frequently used entry.
    • Random Replacement (RR): Randomly selects an entry to evict.

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    Description

    This quiz covers the fundamentals of caching in application servers, including its integration with various computing components and its role in optimizing resource use. Understand how caches function like short-term memory and how they improve data retrieval efficiency. Get ready to test your knowledge on caching strategies and their architectural significance!

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