Chapter 6 - Distributed Caching

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Questions and Answers

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?

They involve automatic interaction with backend storage systems. Signup and view all the answers

Why is it important to monitor cache usage in production?

To maintain the hit and miss rates as expected Signup and view all the answers

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

NCache Signup and view all the answers

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

The cost of cache misses Signup and view all the answers

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

It is resilient to cache failures. Signup and view all the answers

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

Cache-aside Signup and view all the answers

How do web caches primarily enhance the responsiveness of websites?

By intercepting requests and returning cached resources instead of contacting the service. Signup and view all the answers

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

Much more reads than updates Signup and view all the answers

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

They require cache access to align with the database model. Signup and view all the answers

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

Statistics including hit and miss counts Signup and view all the answers

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

They reduce the latency of requests made to services. Signup and view all the answers

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

Their simple distributed hash table model. Signup and view all the answers

Which of the following statements about caching strategies is FALSE?

Read-through caches do not need to interact with backend storage. Signup and view all the answers

What do memcached and Redis have in common?

They serve as distributed in-memory hash tables. Signup and view all the answers

What is a cache hit?

When the data requested is found in the cache. Signup and view all the answers

What happens during a cache miss?

Query results are written to the cache for future requests. Signup and view all the answers

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

Dynamic web pages and user session data. Signup and view all the answers

Why is calculating lift wait times considered expensive?

It aggregates and processes tens of thousands of records simultaneously. Signup and view all the answers

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

Five minutes Signup and view all the answers

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

To cache the results of API calls for faster response times. Signup and view all the answers

How does a cache allocate objects to individual cache servers?

Using a hash function on the object key. Signup and view all the answers

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

It allows information to be served multiple times without re-creation costs. Signup and view all the answers

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

Most requests are fulfilled using cached data. Signup and view all the answers

How does HTTP caching affect request loads on downstream services?

It can significantly reduce request loads if used wisely. Signup and view all the answers

What must an application do when implementing distributed caching?

Check for cached values upon receiving client requests. Signup and view all the answers

What function do cache directives in HTTP headers provide?

They dictate what can be cached and how long it should remain cached. Signup and view all the answers

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

They facilitate fast content delivery for geographically dispersed users. Signup and view all the answers

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

Increased data security. Signup and view all the answers

What is a primary resource for learning about caching strategies?

Gerardus Blokdyk's Memcached, 3rd ed. Signup and view all the answers

What is the primary function of edge caches?

To cache frequently accessed data close to clients. Signup and view all the answers

Which statement accurately describes web browser caches?

They are also known as private caches. Signup and view all the answers

What is the role of HTTP caching directives?

To control how caching should be utilized for resources. Signup and view all the answers

Where are CDN edge caches typically located?

In various strategic geographical locations globally. Signup and view all the answers

What happens when a client sends a GET request?

It can be intercepted by multiple caches along the request path. Signup and view all the answers

What type of content do caches typically store?

Results of GET requests. Signup and view all the answers

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

It specifies when the resource is no longer fresh. Signup and view all the answers

How many locations did Akamai have as a CDN provider?

2,000 locations. Signup and view all the answers

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

The results are immediately returned to the client. Signup and view all the answers

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

"liftwaittimes:" concatenated with the resort identifier Signup and view all the answers

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

The duration after which the cache key-value pair is removed. Signup and view all the answers

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

It calls a downstream service for calculations. Signup and view all the answers

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

It reduces the need for expensive calculations for subsequent requests. Signup and view all the answers

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

A cache miss happens, triggering a calculation of new values. Signup and view all the answers

What would happen if cached items are not flushed periodically?

The cache may fill up and reject further requests. 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?

Approximately 1 millisecond Signup and view all the answers

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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Description

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.