At least in the articles on Plenty of Fish and Slashdot it was mentioned that one can achieve higher performance by creating read-only and write-only databases where possible. I have read the comments and tried unsuccessfully to find more information on the net about this. I still do not understand the concept. Can someone explain it in more detail, as well as recommend resources for further investigation? (Are there books written specifically about this technique?) I think it is a very important issue, because databases are oftentimes the bottleneck.
The primero recommendation for speeding up a website is almost always to add cache and more cache. And after that add a little more cache just in case. Memcached is almost always given as the recommended cache to use. What we don't often hear is how to effectively use a cache in our own products. MySQL hosted two excellent webinars (referenced below) on the subject of how to deploy and use memcached. The star of the show, other than MySQL of course, is Farhan Mashraqi of Fotolog. You may recall we did an earlier article on Fotolog in Secrets to Fotolog's Scaling Success, which was one of my personal favorites. Fotolog, as they themselves point out, is probably the largest site nobody has ever heard of, pulling in more page views than even Flickr. Fotolog has 51 instances of memcached on 21 servers with 175G in use and 254G available. As a large successful photo-blogging site they have very demanding performance and scaling requirements. To meet those requirements they've developed a sophisticated approach to using memcached that others can learn from and emulate. We'll cover some of the highlightable strategies from the webinar down below the fold.
What is Memcached?The first part of the first webinar gives a good overview of memcached. Just in case the rock you've been hiding under recently disintegrated, you may not have heard about memcached (as if). Memached is: A high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load. Memcached essentially creates an in-memory shard on top of a pool of servers from which an application can easily get and set up to 1MB of unstructured data. Memcached is two hash tables, one from the client to the server and another one inside the server. The magic is that none of the memcached servers need know about each other. To scale up you just add more servers and the key hashing algorithm makes it all work out right. Memcached is not redundant, has no failover, and has no authentication. It's simple server for storing and getting data, the complex bits must be implemented by applications. The rest of the first webinar is Farhan explaining in wonderful detail how they use memcached at Fotolog. The beginning of the second seminar covers much of the same ground as the first. In the last half of the second seminar Farhan gives a number of excellent code examples showing how memcached is installed, used, and managed. If you've never used memcached before there's a lot of good stuff presented.
Memcached and MySQL Go Better TogetherThere's a little embrace and extend in the webinar as MySQL cluster is presented several times as doing much the same job as memcached, but more reliably. However, the recommended approach for using memcached and MySQL is:
Cache everything that is slow to query, fetch, or calculate.This is Fotolog's rule of deciding what to cache. What is considered slow depends on your requirements. But when something becomes slow it's a candidate for caching.
Fotolog's Caching TypologyFotolog has come up with an interesting typology of their different caching strategies:
Non-Deterministic CacheThis is the typical way of using memcached. I think the non-determinism comes in because an application can't depend on data being in the cache. The data may have been evicted because a slab is full or because the data simply hasn't been added yet. Other Fotolog strategies are deterministic, which means the application can assume the data is always present.
State CacheKeeps the current state of an application in cache.
Usage StepsIt's a form of non-deterministic caching so the same steps apply.
Deterministic CacheKeep all data for particular database tables in the cache. An application always assumes that the data they need is in the cache (deterministic), applications never go to the database for data. Applications don't have to check memcached before accessing the data. The data will exist in the cache because the database is essentially loaded into the cache and is always kept in sync.
Proactive CachingData magically shows up in the cache. As updates happen to the database the cache is populated based on the database change. Since the cache is updated as the database is updated the chances of data being in cache are high. It's non-deterministic caching with a twist.
Usage StepsThere are typically three implementation approaches:
File System CachingNFS has significant overhead when used with a large number of servers. Fotolog originally stored XML files on a SAN and exposed them using NFS. They saw contention on these files so they put files in memcached. Big performance improvements were seen and it kept NFS mounts open for other requests. Smaller media can also be stored in the cache.
Partial Page CachingCache directly displayable page elements. The other caching strategies cache data used to create pages, but some things are still compute intensive and require a lot of work. So instead of just caching objects, prepare and cache entire page elements for reuse. For page elements that are accessed many times per second this can be a big win. For example: calculating top users in a region, popular photo list, and featured photo list. Especially when using sharding it can take some time to calculate these lists, so caching the resulting page elements makes a lot of sense.
Application Based ReplicationWrite data to the cache through your own API. The API hides implementation details like:
MiscellaneousThere were a few suggestions on using memcached that didn't fit in any other section, so they're gathered here for posterity:
Final ThoughtsFotolog has obviously put a great deal of thought and effort into creating sophisticated scaling strategies using memcached and MySQL. What I'm struck with is the enormous amount of effort that goes into syncing rows and objects back and forth between the cache and the database. Shouldn't it be easier? What role is the database playing when the application makes such constant use of the object cache? Wouldn't more memory make the disk based storage unnecessary?
Ehcache is a pure Java cache with the following features: fast, simple, small foot print, minimal dependencies, provides memory and disk stores for scalability into gigabytes, scalable to hundreds of caches is a pluggable cache for Hibernate, tuned for high concurrent load on large multi-cpu servers, provides LRU, LFU and FIFO cache eviction policies, and is production tested. Ehcache is used by LinkedIn to cache member profiles. The user guide says it's possible to get at 2.5 times system speedup for persistent Object Relational Caching, a 1000 times system speedup for Web Page Caching, and a 1.6 times system speedup Web Page Fragment Caching. From the website: Introduction Ehcache is a cache library. Before getting into ehcache, it is worth stepping back and thinking about caching generally. About Caches Wiktionary defines a cache as A store of things that will be required in future, and can be retrieved rapidly . That is the nub of it. In computer science terms, a cache is a collection of temporary data which either duplicates data located elsewhere or is the result of a computation. Once in the cache, the data can be repeatedly accessed inexpensively. Why caching works Locality of Reference While ehcache concerns itself with Java objects, caching is used throughout computing, from CPU caches to the DNS system. Why? Because many computer systems exhibit locality of reference . Data that is near other data or has just been used is more likely to be used again. The Long Tail Chris Anderson, of Wired Magazine, coined the term The Long Tail to refer to Ecommerce systems. The idea that a small number of items may make up the bulk of sales, a small number of blogs might get the most hits and so on. While there is a small list of popular items, there is a long tail of less popular ones. The Long Tail The Long Tail is itself a vernacular term for a Power Law probability distribution. They don't just appear in ecommerce, but throughout nature. One form of a Power Law distribution is the Pareto distribution, commonly know as the 80:20 rule. This phenomenon is useful for caching. If 20% of objects are used 80% of the time and a way can be found to reduce the cost of obtaining that 20%, then the system performance will improve. Will an Application Benefit from Caching? The short answer is that it often does, due to the effects noted above. The medium answer is that it often depends on whether it is CPU bound or I/O bound. If an application is I/O bound then then the time taken to complete a computation depends principally on the rate at which data can be obtained. If it is CPU bound, then the time taken principally depends on the speed of the CPU and main memory. While the focus for caching is on improving performance, it it also worth realizing that it reduces load. The time it takes something to complete is usually related to the expense of it. So, caching often reduces load on scarce resources. Speeding up CPU bound Applications CPU bound applications are often sped up by: * improving algorithm performance * parallelizing the computations across multiple CPUs (SMP) or multiple machines (Clusters). * upgrading the CPU speed. The role of caching, if there is one, is to temporarily store computations that may be reused again. An example from ehcache would be large web pages that have a high rendering cost. Another caching of authentication status, where authentication requires cryptographic transforms. Speeding up I/O bound Applications Many applications are I/O bound, either by disk or network operations. In the case of databases they can be limited by both. There is no Moore's law for hard disks. A 10,000 RPM disk was fast 10 years ago and is still fast. Hard disks are speeding up by using their own caching of blocks into memory. Network operations can be bound by a number of factors: * time to set up and tear down connections * latency, or the minimum round trip time * throughput limits * marshalling and unmarhshalling overhead The caching of data can often help a lot with I/O bound applications. Some examples of ehcache uses are: * Data Access Object caching for Hibernate * Web page caching, for pages generated from databases. Increased Application Scalability The flip side of increased performance is increased scalability. Say you have a database which can do 100 expensive queries per second. After that it backs up and if connections are added to it it slowly dies. In this case, caching may be able to reduce the workload required. If caching can cause 90 of that 100 to be cache hits and not even get to the database, then the database can scale 10 times higher than otherwise. How much will an application speed up with Caching? The short answer The short answer is that it depends on a multitude of factors being: * how many times a cached piece of data can and is reused by the application * the proportion of the response time that is alleviated by caching In applications that are I/O bound, which is most business applications, most of the response time is getting data from a database. Therefore the speed up mostly depends on how much reuse a piece of data gets. In a system where each piece of data is used just once, it is zero. In a system where data is reused a lot, the speed up is large. The long answer, unfortunately, is complicated and mathematical. It is considered next.
Update: A very nice JavaWorld podcast interview with Google engineer Max Ross on Hibernate Shards. Max defines Hibernate Shards (horizontal partitioning), how it works (pretty well), virtual shards (don't ask), what they need to do in the future (query, replication, operational tools), and how it relates to Google AppEngine (not much). To scale you are supposed to partition your data. Sounds good, but how do you do it? When you actually sit down to work out all the details it’s not that easy. Hibernate Shards to the rescue! Hibernate shards is: an extension to the core Hibernate product that adds facilities for horizontal partitioning. If you know the core Hibernate API you know the shards API. No learning curve at all. Here is what a few members of the core group had to say about the Hibernate Shards open source project. Although there are some limitations, from the sound of it they are doing useful stuff in the right way and it’s very much worth looking at, especially if you use Hibernate or some other ORM layer.
What is Hibernate Shards?
Schema Design for Shards
The Sharding Code’s Relationship to Hibernate
Pluggable Strategies Determine How Data Are Split Across Shards
This is an unusually well written and useful paper. It talks in detail about experiences implementing a complex project, something we don't see very often. They shockingly even admit that creating a working implementation of Paxos was more difficult than just translating the pseudo code. Imagine that, programmers aren't merely typists! I particularly like the explanation of the Paxos algorithm and why anyone would care about it, working with disk corruption, using leases to support simultaneous reads, using epoch numbers to indicate a new master election, using snapshots to prevent unbounded logs, using MultiOp to implement database transactions, how they tested the system, and their openness with the various problems they had. A lot to learn here. From the paper: We describe our experience building a fault-tolerant data-base using the Paxos consensus algorithm. Despite the existing literature in the field, building such a database proved to be non-trivial. We describe selected algorithmic and engineering problems encountered, and the solutions we found for them. Our measurements indicate that we have built a competitive system. Introduction It is well known that fault-tolerance on commodity hardware can be achieved through replication [17, 18]. A common approach is to use a consensus algorithm  to ensure that all replicas are mutually consistent [8, 14, 17]. By repeatedly applying such an algorithm on a sequence of input values, it is possible to build an identical log of values on each replica. If the values are operations on some data structure, application of the same log on all replicas may be used to arrive at mutually consistent data structures on all replicas. For instance, if the log contains a sequence of database operations, and if the same sequence of operations is applied to the (local) database on each replica, eventually all replicas will end up with the same database content (provided that they all started with the same initial database state). This general approach can be used to implement a wide variety of fault-tolerant primitives, of which a fault-tolerant database is just an example. As a result, the consensus problem has been studied extensively over the past two decades. There are several well-known consensus algorithms that operate within a multitude of settings and which tolerate a variety of failures. The Paxos consensus algorithm  has been discussed in the theoretical  and applied community [10, 11, 12] for over a decade. We used the Paxos algorithm (“Paxos”) as the base for a framework that implements a fault-tolerant log. We then relied on that framework to build a fault-tolerant database. Despite the existing literature on the subject, building a production system turned out to be a non-trivial task for a variety of reasons: While Paxos can be described with a page of pseudo-code, our complete implementation contains several thousand lines of C++ code. The blow-up is not due simply to the fact that we used C++ instead of pseudo notation, nor because our code style may have been verbose. Converting the algorithm into a practical, production-ready system involved implementing many features and optimizations – some published in the literature and some not. • The fault-tolerant algorithms community is accustomed to proving short algorithms (one page of pseudo code) correct. This approach does not scale to a system with thousands of lines of code. To gain confidence in the “correctness” of a real system, different methods had to be used. • Fault-tolerant algorithms tolerate a limited set of carefully selected faults. However, the real world exposes software to a wide variety of failure modes, including errors in the algorithm, bugs in its implementation, and operator error. We had to engineer the software and design operational procedures to robustly handle this wider set of failure modes. • A real system is rarely specified precisely. Even worse, the specification may change during the im- plementation phase. Consequently, an implementation should be malleable. Finally, a system might “fail” due to a misunderstanding that occurred during its specification phase. This paper discusses a selection of the algorithmic and engineering challenges we encountered in moving Paxos from theory to practice. This exercise took more R&D efforts than a straightforward translation of pseudo-code to C++ might suggest. The rest of this paper is organized as follows. The next two sections expand on the motivation for this project and describe the general environment into which our system was built. We then provide a quick refresher on Paxos. We divide our experiences into three categories and discuss each in turn: algorithmic gaps in the literature, software engineering challenges, and unexpected failures. We conclude with measurements of our system, and some broader observations on the state of the art in our field.
Several months ago I attended a Joyent presentation where the spokesman hinted that Joyent had the chops to support a one billion page per month Facebook Ruby on Rails application. Even under a few seconds of merciless grilling he would not give up the name of the application. Now we have the big reveal: it was LinkedIn's Bumper Sticker app. For those not currently sticking things on bumps, Bumper Sticker is quite surprisingly a viral media sharing application that allows users to express their individuality by sticking small virtual stickers on Facebook profiles. At the time I was quite curious how Joyent's cloud approach could be leveraged for this kind of app. Now that they've released a few details, we get to find out.
* Joyent is a scale on demand cloud. Allows customers to have a dynamic data center instead of being stuck using their own rigid infrastructure.
* There's an API if you need one. The service is unmanaged, you get root on all your boxes.
* They consider their infrastructure to be better and more open than Amazon. You get access to a high end load balancer and the capabilities of OpenSolaris (Dtrace, Zones, lower request processing overhead, sub 10 second reboot times).
* Joyent's primary scalability principle is to organize apps around silos built from their powerful Accelerator blocks: put applications on different servers based on the quality of service you want to give them. For example, put static content on their own servers so the static content is always served fast and reliably. This allows you to prioritize based on what's important to you. You could, for example, prioritize the virality of your application by putting the Invite Friends functionality on their own servers, thus assuring the growth of your application through your viral functionality possibly at the expense of less important functionality.
* Has three data centers in the US and are opening a fourth, none in Europe.
* Considers their secret sauce to be their highly sophisticated administration system which allows a few people to easily manage a large infrastructure.
* Has a peering relationship with Facebook. That means there are direct high-speed fiber links between Joyent’s data center in Emeryville and Facebook’s data center in San Francisco.
Update: InfoQ links to a few excellent Eucalyptus updates: Velocity Conference Video by Rich Wolski and a Visualization.com interview Rich Wolski on Eucalyptus: Open Source Cloud Computing. Eucalyptus is generating some excitement on the Cloud Computing group as a potential vendor neutral EC2 compatible cloud platform. Two reasons why Eucalyptus is potentially important: private clouds and cloud portability: Private clouds. Let's say you want a cloud like infrastructure for architectural purposes but you want it to run on your own hardware in your own secure environment. How would you do this today? Hm.... Cloud portability. With the number of cloud offerings increasing how can you maintain some level of vendor neutrality among this "swarm" of different options? Portability is a key capability for cloud customers as the only real power customers have is in where they take their business and the only way you can change suppliers is if there's a ready market of fungible services. And the only way their can be a market is if there's a high degree of standardization. What should you standardize on? The options are usually to form a great committee and take many years to spec out something that doesn't exist, nobody will build, and will never really work. Or have each application create a high enough layer interface that portability is potentially difficult, but possible. Or you can take a popular existing API, make it the general API, and everyone else is accommodated using an adapter layer and the necessary special glue to take advantage of value add features for each cloud. With great foresight Eucalyptus has chosen to create a cloud platform based on Amazon's EC2. As this is the most successful cloud platform it makes a lot of sense to use it as a model. We see something similar with the attempts to port Google AppEngine to EC2 thus making GAE a standard framework for web apps. So developers would see GAE on top of EC2. A lot of code would be portable between clouds using this approach. Even better would be to add ideas in from RightScale, 3Tera, and Mosso to get a higher level view of the cloud, but that's getting ahead of the game. Just what is Eucalyptus? From their website: Overview ¶ Elastic Computing, Utility Computing, and Cloud Computing are (possibly synonymous) terms referring to a popular SLA-based computing paradigm that allows users to "rent" Internet-accessible computing capacity on a for-fee basis. While a number of commercial enterprises currently offer Elastic/Utility/Cloud hosting services and several proprietary software systems exist for deploying and maintaining a computing Cloud, standards-based open-source systems have been few and far between. EUCALYPTUS -- Elastic Utility Computing Architecture for Linking Your Programs To Useful Systems -- is an open-source software infrastructure for implementing Elastic/Utility/Cloud computing using computing clusters and/or workstation farms. The current interface to EUCALYPTUS is interface-compatible with Amazon.com's EC2 (arguably the most commercially successful Cloud computing service), but the infrastructure is designed to be modified and extended so that multiple client-side interfaces can be supported. In addition, EUCALYPTUS is implemented using commonly-available Linux tools and basic web service technology making it easy to install and maintain. Overall, the goal of the EUCALYPTUS project is to foster community research and development of Elastic/Utility/Cloud service implementation technologies, resource allocation strategies, service level agreement (SLA) mechanisms and policies, and usage models. The current release is version 1.0 and it includes the following features: * Interface compatibility with EC2 * Simple installation and deployment using Rocks cluster-management tools * Simple set of extensible cloud allocation policies * Overlay functionality requiring no modification to the target Linux environment * Basic "Cloud Administrator" tools for system management and user accounting * The ability to configure multiple clusters, each with private internal network addresses, into a single Cloud. The initial version of EUCALYPTUS requires Xen to be installed on all nodes that can be allocated, but no modifications to the "dom0" installation or to the hypervisor itself. For more discussion see:
A report from the CloudCamp conference on cloud computing, held in London in July 2008.
Given S3's recent failure (Cloud Status tells the tale) Kevin Burton makes the excellent suggestion of fronting S3 with a caching proxy server. A caching proxy server can reply to service requests without contacting the specified server, by retrieving content saved from a previous request, made by the same client or even other clients. This is called caching. Caching proxies keep local copies of frequently requested resources. In normal operation when an asset (a user's avatar, for example) is requested the cache is tried first. If the asset is found in the cache then it's returned. If the asset is not in the cache it's retrieved from S3 (or wherever) and cached. So when S3 goes down it's likely you can ride out the down time by serving assets out of the cache. This strategy only works when using S3 as a CDN. If you are using S3 for its "real" purpose, as a storage service, then a caching proxy can't help you... Amazon doesn't used S3 as a CDN either Amazon Not Building Out AWS To Compete With CDNs. They use Limelight Networks. Some proxy options are: Squid, Nginx, Varnish. Planaroo shares how a small startup responds to an S3 outage (summarized):
Robert Scoble in an often poignant FriendFeed thread commiserating PodTech's unfortunate end, shared what he learned about creating a successful startup. Here's a summary of a Robert's rules and why Machiavelli just may agree with them: