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Building Scalable Web Architecture and Distributed Systems


There are two common forms of global caches depicted in the diagrams. In Figure 10, when a cached response is not found in the cache, the cache itself becomes responsible for retrieving the missing piece of data from the underlying store. In Figure 11 it is the responsibility of request nodes to retrieve any data that is not found in the cache.

Figure 10: Global cache where cache is responsible for retrieval......

Figure 11: Global cache where request nodes are responsible for retrieval

The majority of applications leveraging global caches tend to use the first type, where the cache itself manages eviction and fetching data to prevent a flood of requests for the same data from the clients. However, there are some cases where the second implementation makes more sense. For example, if the cache is being used for very large files, a low cache hit percentage would cause the cache buffer to become overwhelmed with cache misses; in this situation it helps to have a large percentage of the total data set (or hot data set) in the cache. Another example is an architecture where the files stored in the cache are static and shouldn't be evicted. (This could be because of application requirements around that data latency — certain pieces of data might need to be very fast for large data sets — where the application logic understands the eviction strategy or hot spots better than the cache.)

Distributed Cache

In a distributed cache (Figure 12), each of its nodes own part of the cached data, so if a refrigerator acts as a cache to the grocery store, a distributed cache is like putting your food in several locations — your fridge, cupboards, and lunch box — convenient locations for retrieving snacks from, without a trip to the store. Typically the cache is divided up using a consistent hashing function, such that if a request node is looking for a certain piece of data it can quickly know where to look within the distributed cache to determine if that data is available. In this case, each node has a small piece of the cache, and will then send a request to another node for the data before going to the origin. Therefore, one of the advantages of a distributed cache is the increased cache space that can be had just by adding nodes to the request pool.

A disadvantage of distributed caching is remedying a missing node. Some distributed caches get around this by storing multiple copies of the data on different nodes; however, you can imagine how this logic can get complicated quickly, especially when you add or remove nodes from the request layer. Although even if a node disappears and part of the cache is lost, the requests will just pull from the origin — so it isn't necessarily catastrophic!

Figure 12: Distributed cache

The great thing about caches is that they usually make things much faster (implemented correctly, of course!) The methodology you choose just allows you to make it faster for even more requests. However, all this caching comes at the cost of having to maintain additional storage space, typically in the form of expensive memory; nothing is free. Caches are wonderful for making things generally faster, and moreover provide system functionality under high load conditions when otherwise there would be complete service degradation.

One example of a popular open source cache is Memcached (which can work both as a local cache and distributed cache); however, there are many other options (including many language- or framework-specific options).

Memcached is used in many large Web sites, and even though it can be very powerful, it is simply an in-memory key value store, optimized for arbitrary data storage and fast lookups (O(1)).

Facebook uses several different types of caching to obtain their site performance (see "Facebook caching and performance"). They use $GLOBALS and APC caching at the language level (provided in PHP at the cost of a function call) which helps make intermediate function calls and results much faster. (Most languages have these types of libraries to improve Web page performance and they should almost always be used.) Facebook then use a global cache that is distributed across many servers (see "Scaling memcached at Facebook"), such that one function call accessing the cache could make many requests in parallel for data stored on different Memcached servers. This allows them to get much higher performance and throughput for their user profile data, and have one central place to update data (which is important, since cache invalidation and maintaining consistency can be challenging when you are running thousands of servers).

Now let's talk about what to do when the data isn't in the cache…

Proxies

At a basic level, a proxy server is an intermediate piece of hardware/software that receives requests from clients and relays them to the backend origin servers. Typically, proxies are used to filter requests, log requests, or sometimes transform requests (by adding/removing headers, encrypting/decrypting, or compression).

Figure 13: Proxy server

Proxies are also immensely helpful when coordinating requests from multiple servers, providing opportunities to optimize request traffic from a system-wide perspective. One way to use a proxy to speed up data access is to collapse the same (or similar) requests together into one request, and then return the single result to the requesting clients. This is known as collapsed forwarding.

Imagine there is a request for the same data (let's call it littleB) across several nodes, and that piece of data is not in the cache. If that request is routed thought the proxy, then all of those requests can be collapsed into one, which means we only have to read littleB off disk once. (See Figure 14.) There is some cost associated with this design, since each request can have slightly higher latency, and some requests may be slightly delayed to be grouped with similar ones. But it will improve performance in high load situations, particularly when that same data is requested over and over. This is similar to a cache, but instead of storing the data/document like a cache, it is optimizing the requests or calls for those documents and acting as a proxy for those clients.

In a LAN proxy, for example, the clients do not need their own IPs to connect to the Internet, and the LAN will collapse calls from the clients for the same content. It is easy to get confused here though, since many proxies are also caches (as it is a very logical place to put a cache), but not all caches act as proxies.

Figure 14: Using a proxy server to collapse requests

Another great way to use the proxy is to not just collapse requests for the same data, but also to collapse requests for data that is spatially close together in the origin store (consecutively on disk). Employing such a strategy maximizes data locality for the requests, which can result in decreased request latency. For example, let's say a bunch of nodes request parts of B: partB1, partB2, etc. We can set up our proxy to recognize the spatial locality of the individual requests, collapsing them into a single request and returning only bigB, greatly minimizing the reads from the data origin. (See Figure 15.) This can make a really big difference in request time when you are randomly accessing across TBs of data! Proxies are especially helpful under high load situations, or when you have limited caching, since they can essentially batch several requests into one.

Figure 15: Using a proxy to collapse requests for data that is spatially close together

It is worth noting that you can use proxies and caches together, but generally it is best to put the cache in front of the proxy, for the same reason that it is best to let the faster runners start first in a crowded marathon race. This is because the cache is serving data from memory, it is very fast, and it doesn't mind multiple requests for the same result. But if the cache was located on the other side of the proxy server, then there would be additional latency with every request before the cache, and this could hinder performance.

If you are looking at adding a proxy to your systems, there are many options to consider; Squid and Varnish have both been road tested and are widely used in many production Web sites. These proxy solutions offer many optimizations to make the most of client-server communication. Installing one of these as a reverse proxy (explained in the load balancers section) at the Web server layer can improve Web server performance considerably, reducing the amount of work required to handle incoming client requests.


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