What is the difference between a session and a cache?

Sessions: Session data is stored on the server, usually in memory or in a session store. Session data is private to each user and cannot be accessed by other users. Sessions are commonly used to store user-specific information such as login credentials, shopping cart contents, user preferences, and temporary data needed during the user's session. Caches: Caches are used to store frequently accessed or expensive-to-compute data to improve application performance. Cache data can be accessed and modified throughout the application on any page. Caches are commonly used to store data that is shared by multiple users or clients, such as frequently accessed database queries, API responses, or static content.

Session: - When two systems (App layer and Backend or two backend systems server to server) communicate with each other, there is an authentication and authorisation involved to ensure the identity of the caller is ascertained and only the services that can be used by the caller be authorised for the caller's use. Hence, after authentication, typically a session is established and for the session various context are retained at the caller's end and the services' end to have conversation (context sensitive communication). Like when you go to a doctor you have a conversation versus introduce yourself for every single statement. - Cache is the context data stored either at the caller end or service end to retain context for this conversation

A session is like a grocery shopping trip, you first start with a empty shopping cart, then fill it up with the groceries and food you need, then finally after check out your cart is cleared again

Imagine that you are going to see someone at Google HQ, upon reaching the security desk, a person will enter your information in their computer and during check-in (sign-in), you will be given the Visitor Pass, this visitor pass will allow you to hang freely in Google campus and is your session for the day, what areas you have access to, how many areas you have access to are all determined by this session. Once you checkout (logout), the session is terminated. The information that was entered in the computer by the security will serve as a Cache so when you visit Google later, security will have your information to serve you faster. The word cache is generally used when you introduce an additional layer of storage for better performance.

Sessions: typically have a limited lifespan and may expire when a user logs out or their session becomes inactive. They are often designed to be short-term, user-specific storage mechanisms. Caches: are generally designed to persist data for a longer duration and can be set with specific expiration policies. They are not tied to user sessions and are often used for longer-term data optimization and performance improvements.

1. Caches improve application performance by reducing the need to fetch data from the primary source. 2. Caches can exist at different form : Browser cache, web server cache, Application cache, CDN cache. 3. Cache policies determine which data to store and when to refresh it. 4. Caches optimize performance, reduce server load, minimize bandwidth consumption and enhance user experience. 5. Caches can be implemented using various data structures, such as hash tables, linked lists or arrays.

Sessions are used to track individual users and their activities on a website. This can be used for things like keeping track of what items a user has added to their shopping cart, or remembering what pages they have visited. Sessions are typically stored on the user's browser, and they expire after a period of inactivity. Caches are used to store frequently accessed data, such as images, CSS files, and JavaScript files. This can improve the performance of a website or app by reducing the number of times that data needs to be downloaded from the server. Caches can be stored on the user's browser, or they can be stored on the server.

Notice that Memcached does not persist data to disk, only in memory. On the other hand, Redis supports storing data to disk. This means that if Redis was restarted, we could retain the stored data; this is not true for Memcached.

It might be fun for the readers to know that "Caching" is known as one of the hardest problems to solve right in the tech world for its sneaky nature. Also, worth talking about different level at which caching happens - starting from database query, to view rendering, to CDN, in browser caching and how they work together to create a cohesive system.

A cache is a mechanism for reducing the cost of a response to a request. This can be computation cost (complex cpu bound algorithm), time cost (need to access slow medium), or actual cost (external web service). Each part pf any stack can implement caching. Cache data is always disposable (you repay the cost to regenerate). The most difficult aspect of a good cache is knowing when to destroy it so that it does not become stale. Typical invalidation strategies include time based (eg cache rendered html for a few minutes), condition based (auto invalidate when X happens, eg an entity is created), or manual (cache indefinitely until someone invalidates). Each strategy can be a good fit, but you need to understand the data source to decide.

One of the key characteristics of a cache is that its data can be thrown away and recomputed whenever you like. If you throw away data stored in a user session, that user might lose their progress and have to log in again. But if you lose some cached data, you just have to spend a little extra time and resources to recompute that data. It could be misleading to say that caches last longer than sessions. The lifetime of a cache entry is totally up to you, and should be chosen based on how long it takes to recompute the data and how often it will be used. Depending on the use case, cache durations can be measured in anything from milliseconds to days.

Sessions have transactional data, helping stitch together data from different transactions initiated by a single user for a continuous short period of time. Logged in user data, shopping cart, selections made in booking until payment, are all maintained in a session. Cache can have meta data or master data, that can be used by multiple users for a longer period of time, used across multiple users. Depending on the data, duration can be determined. Example - all currency exchange rates downloaded and stored in cache monthly. Or page rendering times reduced for high volume traffic pages by ensuring page content is cached, esp in the geographical regions where there is a sudden influx of traffic on a web page due to a sale.

Sessions and caches differ in a number of ways, including: a) Purpose: Sessions are used to track a user's activity on a website, while caches are used to store frequently accessed data. b) Scope: Sessions are typically scoped to a single user, while caches can be shared by multiple users. c) Lifetime: Sessions typically end when the user closes their browser, while caches can live for a longer period of time. d) Storage mechanism: Session data is typically stored on the server, while cached data can be stored on the client side or on the server side.

Our project's scope highlighted the differences between sessions and caches. While sessions were user-specific, keeping track of individual user data, caches were data-centric, improving overall system performance. The sessions ensured personalized user interactions, whereas the caches, shared across multiple users, enhanced the application's efficiency and response time.

Some use cases for Sessions - Sessions are user specific and a way to store temporarily the activity in the service i.e for UI it can store enabled options for this user like see UI V2, for APIs that are limited or restricted one might use a session to keep the time to live of the user calls. Cache - this is data centric, multi user , the server can keep the most frequently accessed resources for a given time, to save access to primary source like dbs, object storage, in general is to improve performance but there is always a trade off - caller might see outdated data if not refreshed - if cached is too big , it could lead to increase memory usage (memory is cheap these days , but depends on the context)

Both caches and sessions offers significant advantages : 1. Enhance User Experience 2. Optimize Application Performance 3. Reduce Bandwidth Consumption 4. Improve Resource Utilization 5.Resilience in backend service disruptions

Sessions and caches offer a number of advantages, including: a) Improved performance: Sessions and caches can improve the performance of websites and applications by reducing the number of requests that need to be made to the server. b) Reduced bandwidth usage: Sessions and caches can reduce bandwidth usage by storing data on the client side. c) Enhanced user experience: Sessions and caches can enhance the user experience by providing a more personalized and responsive experience.

Sessions maintain stateful information about individual users during their interactions with a website. They store user-specific data, such as login credentials or shopping cart items, enabling personalized experiences and secure transactions. Caches, on the other hand, temporarily store frequently accessed data, such as images, scripts, or database queries, to reduce server load and improve page loading speed. By retrieving data from the cache websites become more responsive Web developers strategically employ sessions and caches to create seamless user experiences and optimize website performance. Sessions personalize interactions and maintain continuity across multiple pages, while caches accelerate page loading and reduce server strain

Session and Cache offer several advantages: Sessions can be helpful in long conversation flow such as e-commerce application where a user might add a item to a cart and continue shopping, might decide to come back and remove the item, session can be used to track user activity over a period of time. It should be noted that session synchronization should be implemented when the application is accessed over multiple devices such as Web, Mobile and Tablets. Session invalidation is essential when user logs off to prevent information leakage to other users. Cache can be every effective in retrieving frequently accessed data. In the case of an e-commerce application, the list of products displayed for a specific search criteria can be cached.

Advantages of Sessions: -User Persistence: Sessions maintain user state and identity throughout their visit. -Personalization: You can store user-specific data and preferences. -Security: Sessions help authenticate and authorize users securely. -State Management: Ideal for tracking shopping carts, login status, and more. Advantages of Caches: -Performance: Caches boost speed by storing frequently accessed data. -Reduced Load: Lessens the load on databases and servers, improving efficiency. -Scalability: Enables horizontal scaling by reducing backend data fetches. -Offline Access: Caches provide data availability even when the original source is temporarily unreachable. Each has its own unique advantages depending on the use case.

Sessions and caches also have some disadvantages, including: a) Security risks: Sessions and caches can introduce security risks, such as session hijacking and cache poisoning. b) Complexity: Sessions and caches can add complexity to the development and maintenance of websites and applications. c) Cost: Sessions and caches can increase the cost of hosting and maintaining websites and applications.

The disadvantages of sessions include increased server load and potential for data loss when sessions expire or when the server restarts. The disadvantages of caches is that there is a risk of serving state data, meaning that the information stored in the cache may be outdated or it may no longer be accurate when the underlying data is changed.

Be wary of security issues that can be introduced by caching. If you store some data in a cache shared by multiple users, you need to be certain that the cached data doesn't include anything that can vary between different users. Caching is an easy way to accidentally expose one user's data or preferences so they can be seen by other users.

Sessions introduced server-side complexity, requiring a robust management system to ensure security. Caches, while improving performance, introduced challenges in data consistency, requiring a well-defined cache invalidation strategy to ensure data accuracy and relevancy, which was a crucial aspect to consider in the solution's architecture.

Cache Problem: - Old & new data. If data is actively written & read by many user. It makes the data which should be same, the data is read differently - There is no data-lock mechanism. Data can be double consumed, meanwhile data is intended for 1 time consumed. - Data flush policy. When is the data should be written to disk? (if data is written to disk, I/O write busy waiting must be your concern) - Full of Memory Policy or error policy. The policy define your cache fault. - Cache key management: Wrong cache key, data is null Session Problem: - Can be misused. Everyone who has session can do everything. It means security problem. - Where is session stored? Big session data can leads longer time to process. - Multi server problem.

A cache is a performance optimisation. The application should still be able to function if you remove the cache completely; it'll just be slower. Sessions provide actual behaviour, and you can't remove them without changing or breaking the way your app works. Caches and sessions aren't something where you need to pick one or the other, and it's normal to use both. There's nothing stopping you from adding a cache as part of an individual user session, if you have data that's accessed frequently during the session and is specific to a single user.

To choose between sessions and caches, consider the nature of your data, its size and volatility, user identities and interactions, application architecture, and associated trade-offs. Sessions are best for user-specific, frequently changing data, while caches excel with common, less volatile data that boosts application performance and scalability. The choice should align with your web application's specific requirements and goals.

I'm not sure the comparison of sessions and caches is specifically useful unless you are new to both terms. A cache is a temporary storage that should/could be transparent to an application (used primarily for performance reasons) whereas a session is an application level concept to facilitate user-related state during otherwise stateless communication (typically implemented as a combination of client and server side storage). The only related concept here is storage of state and the temporary nature of that storage. The use cases are completely different and doesn't seem to warrant comparison.

Baseline: - If data is big, use cache - If data is small & user-scoped, use session. - If data is shared between user, use cache. Additional - If data can be pre-heated to optimize execution time, use cache. - Your session will be mainly simple table. - Your session data can have cache key to optimize the process/execution time.

It is very easy that you introduce a cache within a session without knowing this. So ask yourself for each entry in your session whether it is something that you can recreate if you loose it, if so, make it clear that this is cached data. This is important in case the code detects error conditions, where throwing away cached data might be a way out of this error condition.

For building a robust and secure session management system to meet your application's requirements and protect user data and privacy you should consider the following : Session Expiry and Inactivity Session Timeout Handling Session Revocation Session Hijacking Prevention Cross-Site Request Forgery (CSRF) Protection Data Serialization and Storage Load Balancing Session Testing Logging and Monitoring Cross-Origin Resource Sharing (CORS) Fallback Mechanisms

In addition to the fundamental concepts of sessions and caches, it's essential to address their interactions and trade-offs. Balancing data consistency with performance can be a tricky challenge. I recall a project where we used caching to boost response times significantly. However, when dealing with sensitive financial data, we had to carefully manage cache expiration and refresh to maintain accuracy. Striking the right balance between real-time data and performance gains is a critical consideration in such scenarios. It's crucial to evaluate the specific requirements of your application and ensure that your caching strategy aligns with those needs.