Let us take a closer look at the event application framework we mentioned yesterday. Several designs compete for this space. We enumerate some below: 

First – we mention the design of MSMQ with its dead letter and poison letter queues.  This design includes support for retries by the queue processor, non-invasive approach which lets the clients send and forget, ability to create an audit log, support for transactional as well as non-transactional messages, support for distributed transactions, support for clusters instead of standalone single machine deployments and ability to improve concurrency control. 

This design is somewhat different from that of System.Messaging. The system.messaging library transparently exposes the underlying Message queuing windows APIs . For example, it provides GetPublicQueues method that enumerates the public message queues. It takes the message queue criteria as a parameter. This criterion can be specified with parameters such as category and label. It can also take machine name or cluster name, created and modified times as filter parameters. The GetPublicQueuesEnumerator is available to provide an enumerator to iterate over the results.  

Second – we mention the design of ZeroMQ, RabbitMQ and others of this family with their flexibility for several publishers and consumers topology.  These are superfast guaranteed delivery message bus but they are mostly for standalone deployments and not as cloud services.  

In this regard, Amazon Simple Queue Services offers almost unparalleled fully managed message queuing service that makes it easy to integrate with microservices and serverless applications. SQS also makes it simple and cost-effective to intermediate between these producers and consumers. 

Third – we mention the highly performant Erlang-based messaging framework. Whatsapp is a messaging architecture. It offers free texting in a SMS world. There are no ads, no gimmicks, no games and no feeds. They have hundreds of nodes, thousands of cores, hundreds of terabytes of RAM, serve billions of smartphones. Their server infrastructure is based on Erlang/FreeBSD.  The Erlang is a programming language that is used to build massively scalable soft real-time systems with requirements on high availability.  With this programming language they send out about 70 million messages per second.  

This architecture is famous for “it just works” because only a phone number is used as the address to send messages to and messages can be voice, video, images or text.   It also features voice message, group chats and read-receipts.  

The backend consists of Erlang, Jabber, FreeBSD, Yaws web server, PHP, BEAM, and custom XMPP. The Frontend consists of seven client platforms for the different mobile devices and SQLite. Every user is an actor. Messages flow between actors when they are online or they are stored offline. When the actor comes online, he pulls the messages sent to him.  User and actor differ in that the actor notion or proxy exists only in the WhatsApp backend framework. Message sent from user goes to actor inbox which then sends the message to the other actor who then repeatedly sends to the user behind the actor. When that user comes online, the messages appear.  The Mnesia backend DB stores the actors and their inboxes. The message is encrypted with a key as a tag. They use tools for system activity monitoring on all their server operating systems as well as BEAM Processor hardware perf counters and dtrace. BEAM is the erlang VM that is used to process the messages.  

The highlight of this architecture is its performance-oriented design. Even similar feature from Facebook also written in erlang could not match the widespread adoption of this messenger.  They have proven that Erlang and FreeBSD have unmatched SMP scalability.  

With these three different forms of design, we can now utilize cloud services or build a limited scale light-weight customized solution for the requirements we mentioned. 

#codingexercise
Yesterday we were discussing ways to climb up the staircase with jumps of 1, 2 or 3 steps.
We can generalize this for climbing up staircase with jumps of  1 to N-1 steps
The number of ways to climb up the staircase with N steps by taking a leap of 1 to N steps at a time is simply the sum of binomial coefficients. We see this by cumulating that there are n-1Choose1 ways to take one stop jump and n-1Choose2 ways to take two jumps and so on.
static int GetCount(int n)
 {
            if (n == 0)
                return 1;
            return (1 << (n - 1));
 }

• The binomial coefficient for increasing degrees form the Pascals triangle.