An Enterprise System should be Highly Scalable, Always Available, Easily Manageable, Fast Performer, Auto-Healer and Capable of Super Fast Searching and Intelligent Analysis through Machine Learning.

Scalability means the system does not downgrade even when load increases multiple times.
For example, when number of users of an ecommerce system increases heavily further leading to sudden increase in transactions per user, the system should still keep performing with same SLA and handle the traffic smoothly ensuring that business is not down.
One of the design choice for large-scale system is to scale out horizontally. Create one DB per Entity (User/ Item/ Product) and store only Key-Value in that table. Replicate the DB in various machines and implement a scheme for data-sharding.
For example, in the Modulo(Entity_Pky, 100) approch, User Bob with id 1000 will be located in machine #10 . So if number of users increases suddenly, new User DB will be created in new machine and the new user info will be inserted automatically.
There are other data-sharding policies like Range-based partitions, Lookup based approach, Read-Most-Write-Least Model.
SimpleDB can be effectively used to manage lookup-orieneted entity info.
This is an out-of-the-box offering from Amazon AWS as a hosted and managed solution administered by Amazon itself.
SimpleDB does the heavy-lifting of multi-value data fetching for a key, batch operations, consistent reads.
Its better to delegate the tradional ‘data management operations’ like ‘Relations, Transactions, Locks, Sequences, Constraints’ to Application Layer as SimpleDB is meant for handling simple things !!   Such a typical example is eBay DAL layer.
Actually the System Architect should choose the Entities which are meant for mere lookup (like User, Item, Manufacturer, Order) and mark them as best candidates for SimpleDB items.
Those entities will be then guarnteed to be highly available. Off course this pattern is not suitable for financial transactions in banking domain and rule-based complex events where every fetch query triggers   sub-query-based rules or procedures.
Discussing further details on implementing SimpleDB is out-of-scope here.
If we want to handle complex business transactions while updating Entity Info and manage intricate relatiuonships then we should take resort to Cassandra or mongoDB.
But we should remember that one needs to take care of Scalability in Application layer while adopting NoSQL Dbs.
 If the data are hierarchical in nature – we can leverage a Graph Database (MonogoDB) which is as opposed to conventional sql-db is a de-normalized graph processor.
Availability : Load-balancing and Clustering are standard practices for making applications available.
Normally DNS Resolver routes to a pool of servers for a requested application and Load-balancer picks up one server.
Machines should be load-balanced in such a way the moving the user from one machine to another machine can be achieved easily without shutting down the system.
Fast Searching :
NoSql is a must for fast lookup of trillions of  Business Entity data and persisting time-critical entities by locking data storage row for a  negligible amount of time (contrary to traditional RDBMS), yet be able to write-through / broadcast write events to sub0system grids (Search Grid / Analytics Machine / Events Collector etc.)
Cassandra does a very neat job for that matter.
“The sparse two-dimensional “super-column family” architecture allows for rich data model representations (and better performance) beyond just a simple key-value look up…..
Some of the most attractive features of Cassandra are its uniquely flexible consistency and replication models. Applications can determine at call level what consistency level to use for reads and writes (single, quorum or all replicas).
This, combined with customizable replication factor, and special support to determine which cluster nodes to designate as replicas, makes it particularly well suited for cross-datacenter and cross-regional deployments. In effect, a single global Cassandra cluster can simultaneously service applications and asynchronously replicate data across multiple geographic locations….”
Its worth paying for the learning curve and operational compexities in exchange of the ‘scalability, availability and performance advantages of the NoSQL persistence model’….
In traditional DB, what happens if one of the node containing one of the table in a join condition is down !
Simple – now the whole application that depends on that join condition is unavailable !
Well there is no join condition in NoSql !!
Cassandra is best for cross-regional deployments and scaling with no single points of failure.
Next question – does the NoSql gurantees data-consistency the same way Relational DB vouches for read-after-write consistency at the cost of Blocking the read untill Transaction is finished !
Well NoSql follows CAP theorem not ACID principle !
So if you think ‘immediate consistency’ need to be ensured for super-critical tasks like Bidding / Buying, better await till data committed before reading the latest data !
In certain cases of ‘Eventual Consistency’ like searching data where we expect fast response we may not await for latest data to availble but rather instantly display the pre-computed search result !!
Also there is no concurrency bottlenecks ! Replicas are mostly Multi-master Highly Available !
NoSql ensures Asynchronous Lock-based Reconciliation as opposed to Synchronous Lock-based Reconciliation bby SQL.
This means more work but saving time !! Using Message -Multicast do write-behind to replicated database / grid without taouching master db.
Then synchronize with master DB after a specific time period. This saves lot of time spent in Index Arrangement and Sequential Updation.
Say you want to search – ‘Blue Sony mp3 player’ ! NoSql will hold the lock for a row only for couple of milliseconds as opposed to locking the entire table by RDBMS !
NoSql will just lookup the id of next entity (manufatures) against the id of the main entity (product) and move the mesage to the next table !!
At the same time, it looks up the product requested by the next user !!  No Joins , Just Intelligent Routing !!  Handling millions of requests concurrently has never looked so esy before !!!
Which is for What ?
Memcahed : Static Key-Value Pair
Neo4j  : Network Graph Store
Hbase  : Row-Orieneted Sparse Column Store
MongoDB : (Key, Document) Storage.
Fast Data Analysis
Apache Hbase – best suited for Data Analytics, Data Mining, Enterprise Search and other Data Query Services.
If the main business is to mine PeraBytes of data and perform parallel range-queries and then combine the results through batch Map-Reduce (say Enterprise Search for Video Content), then one  of the defacto choice is Apache Hbase configured with haddop Jobs, using HDFS as a shared storage platform.
Hbase comes with availability trade-off i.e. the persistent domain entities may not immediately available in the Search Result. The reason for this is huge Hadoop Map-Reduce jobs are performed parallely in offline mode so that main source of data is not hogged by Hadoop Jobs and is highly Scalable.
It should be noted that Hadoop is not meant for searching in real-time. Its actually an offline batch-processing system well-suited for BI analytics, data aggregation, normalization in parallel.
Hadoop provides a framework which will automatically take care of interProcess co-ordination, distributed counters, failure detection, automatic restart of failed process, co-ordinated, sorting and much more.
There is different stages like mapper, combiner, partitions, reducer etc, instead of we writing from the scratch, the framework takes care of it.
Erlang / Scala also provide out-of-the box parallel processing.
Its important to know that there are different tools based on hadoop that serve different purposes :
Hive : SQL query for MR
Pig : Scripting for MR
HBase : Low-latency, Big Table like database on hadoop
Oozie : Workflo Mgmt on Hadoop
Sqoop – RDBMS import / export
Zookeeper : Fast, distributed, coordination system
Hue : advanced web env. for Hadoop and custom applications
Here come the ultimate offering from Spring – http://www.springsource.org/spring-data
Apache has a solution for combining lucene with hadoop for blazing-fast document search.
Fast Cache :
Implementing a cold cache with minimum memory footprint (MySQL native memory, Memcached, Terracota Ehcahce) is absolutely important.
Application Server should not remeber the state of the Entity rather should just persist in DB. The metadata should be stored in Cold Cache.  The persistent pojo objects should never be cached in memory.
If queries are mostly read-only, very less updates – the mySQL native memory scores high ! MySQL InnoDB storage engine’s dictionary cache provides blazing fast data acceess. On demand, data blocks are read from disk and stored in memory using LRU chains or other advanced mechanisms(like MySQL’s Midpoint Insertion Strategy) .
In order to lookup the value for a key, Memcached clients issue multiple parallel requests to memcached servers where keys are sharded across those servers.
For a frequently changing data set , Memcahed is surely better than DB Cache
Probably the fastes cache on planet is – is Ehcache using Terracotta BigMemory http://blog.terracottatech.com/2010/09/bigmemory_explained_a_bit.html
Here directly the native byte-buffer of the OS is used bypassing main-memory.
A write-optimized data store. Something that aggregates the writes in RAM, and writes out generational updates. Take a look at Google’s BigTable paper for a good description of this strategy.
Everything Asynchronous
All communications in every layer should be asynchronous to reduce the latency.
There are 2 types of latency.
1. user latency – how fast user gets back the control on web site
2. execution latency – how fast the execution takes place in backend
UI behavior should be completely Ajax and send the main events to queue and schedule for batch updates in offline mode. There is absoulutely no room for a wait_state i.e. all response should be immediate and non-blocking.
Common Flow :
— Submit a job/task to a thread and return to the user immediately.
— The thread should perform the operation in background (it may communicate to LRU cache optimized for concurrent access / Graph Structure / Iry-IIry master-slave replicatioed env / Map-Reduce based sturctures)
— Once done with the opertaion it should return the result thru a CallbackHandler and client will get notification.
There are multiple patterns for Asynchronous Communications.
(1) Store all jobs in Event Queue. Select a queue based on a contract/ algo depending on type of task. Then use multiple event brokers/ listeners to handle the jobs from queue (thrid party ESB like Tibco / apache fuse/ apache camel / …) can be used.
(2) Message Multicast – say user enters a new item in system. do not update the iry db immediately. rather send messages to pollers / searchers – that there is a ‘New’ event. Return to user immedialtely. Now the updater thread will update Iry db. Then searcher will behind the scene query Iry db / data source to find what has been just added and it will add it to its search grid !
(3) Batch Processing (schedule offline procesing). Identify which type of job requests can be scheduled for offline processing and do not need immediate attention !
Prallel Threading
Use Executor Service to effectively manage pool of threads – 
 Remember  a simple set of worker threads ‘without the manager’ – can simply lead to 
 (i) Resource Thrashing (each thread is expensive – execution call stack / context switching)
 (ii) Request Overload ..  if all requests are provided threads for execution
 (iii) Thread Leakage : sive of threadpool diminishes due to uncaught errors but requests are not served !
So there should be a proper manager to allocate threads either FixedThreadPool / ScheduledThreadPool / SingleThreadPool / QueuedThreadPool with proper RejectionPolicy.
This manager should also place the completed result in a non-blocking queue from where result can be taken off asynchrounously.
Thread should not wait to acquire locks – leverage advanced processor optimizations to use LATCH concept to lock/unlock threads at the same time. Locks should be acquired / released in any order.
Fork/Join and CountDownLatch are powerful concepts for running threads parallely.
You can download and try out the open-source Thrift which is a C++ Fwk for multi-threading and asynchronous processing of job
Auto-Healing and Self-Recovery :
Systems should be falut-tolernt and should know how to degrade gracefully in the scenario of unprecented load.
The JMX agents and other Robot Apps should continously monitor the system and gather intelligence to take the best desicion.
A high-degree of automation is requireed for easily recovering from failures and managing the system smoothly.
Normally automation is driven though Centrallized Logging System and Self-organization Artificial Intelligence. BI tools are used to analyze user experience and provide Best Match through continous inhouse experimentations.
Scalable systems are mangeable if new partitions can be added, DB instance can be horizantally scaled out and new application servers turned on without affecting users of the system.
Case Study :
 Twitter :
Twitter Example :
user SAM tweets –
> store tweet > iterate social graph
> split chunks into parallel jobs > prepend packet into its memcahced blob / (some other cache) – if not present in cache – talk to db / hadoop
user RAM who follows SAM – sees sam’s tweet –
> read mysql blob from memcache (or other cache) > deserialze integers > sort > slice > probablistic truncation (fast but may not be all consistent).
Facebook Example :
Alex friend of bob – logs in 
> Web-tier Calls a C++ based Service (thrift)
> Thrift has the user-id of Bob and calls Aggregator to find all friends ids of Alex
> Aggregator in parallel calls the multi-feed leaves….  (each leaf node is one user for which their is one DB .. one DB .. has a key-value pair (uid, user-data) … no traditional sql (this is like noSql graph db)
> feed result says  [Bob, Sam, Paul] – these  are all alexe’s friends …. returns those ids  (multi-feed) – by calling all DB servers in parallel .. finds the Indexes ..
> Aggregator says ..now got  ids of 40 most interesting stories ….   It .. ranks them … based on certain criteria …..
> For each Id, .. get the metadata (timestamp, user name, comment..) from memcached (in ur cache it could be any other cache) – parallel query on multi-core Fedorra
> Now web tier renders the data.
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