Post on 06-Dec-2014
description
NServiceBus: building a distributed system based on a messaging infrastructure
Mauro ServientiCTO @ Mastreeno, LTD
mauro.servienti@mastreeno.com@mauroservienti
//milestone.topics.it//github.com/mauroservienti
NServiceBus trainer/supportRavenDB trainer
Microsoft MVP – Visual C#
Resources
• Slides on SlideShare: //slideshare.net/mauroservienti
• Samples on GitHub: //github.com/mauroservienti– Repository: Conferences– Branches:• Full-Stack-Sample• 2014/Italian-Developers-Meet-up
TENETSArticles of faith…
None of the following is true
• Network is reliable;• Latency is near to zero or irrelevant;• Bandwidth is unlimited;• Network is secure;• Topology doesn’t change;• Transport cost is irrelevant;• Network is homogeneous;
DEFINITIONSLet’s get in touch…
Consistency
The rate of agreement of observers looking at a system at a given point in time.
The more the observers agree on what they see the more the system is consistent.
Coupling
The rate of dependency among parts of a system.
The more changing a portion of the system impacts on other portions of the system the
more the system is coupled.
Temporal Coupling
It’s a special form of coupling.
The more non-availability of a portion of the system impacts on other parts the more the
system is temporally coupled.
Scalability
the ability of a system to handle a growing amount of work in a capable manner
Scalability is generally difficult to define and in any particular case it is necessary to
define the specific requirements for scalability
The more we scale the more we cannot rely on
consistency
“ACD/C”Scaling can be achieved understanding that we need to choose and accept consequences of our decisions, our pillars should be:
- Asynchronous;- Cached;- Distributed;- And not Consistent;
CONSINSTENCY?Are you kidding me :-)
A strange world :-)
• A new order comes in;• The whole company is informed that a new order
we’ll be processed and we need to:– Understand if items are in stock;– Understand if we need to produce/buy something;
• At the same time production is trying to understand how to schedule the new order but is waiting for the warehouse that is currently used by the sales department to understand if the order can be shipped within the next week;
DEADLOCK
The real world…
• The obvious and only consequence of trying to scale a monolith is the collapse of the entire system;
• The real world:– Does not know at all what transactions are
(especially distributed);– Has a really low, if not null, coupling among parts;– Has no temporal coupling at all;
Transaction boundaries
• We cannot any more rely on transactions to guarantee consistency, e.g.: 1. Update the shopping chart;2. Checkout;3. Create the order;4. Create the shipment request at FedEx;
• “Simply” 1, 2, 3 and 4 can live in different systems on different machines with different databases;– And given our tenets we now have a problem– And a solution... :-)
EVENTUALCONSISTENCY
The rate of the agreement
• Will be low or really low;• Every communication must bring with itself its
version (or timestamp) in order to be able to sort stuff;
• Parts of the system are now free to move independently:– They can evolve due to the low coupling;– Be available or not, depending on their needs,
because there is no temporal coupling;
MESSAGESWe have async and distributed parts…but…how they talk to each other?
asynchronous
We cannot rely on RPC calls
the other part is not guaranteed to be there when we need it.
asynchronous
QueueSender Receiver
Now
Some time in the future
distributed
We need an atomic piece of information
we cannot rely on ordering
we cannot rely on receiving the information at the same place (is distributed);
distributed
QueueSender Receiver
C
Receiver
Receiver
BA
B
A
Brok
en!
C
non-coupled
We need the message as small as possible
The more the exchanged vocabulary is large the more coupling we have
Changing the vocabulary is hard, think twice about it
NServiceBusPlease welcome:
Concepts
• Message– An atomic piece of information that has a semantical meaning in
the business;• Component
– Something that can handle a message;• Service
– A set of components grouped by context;• Endpoint
– A set of services grouped by:• SLA(s);• Infrastructure concerns;• Etc..;
Concepts #2
• Command: A message that semantically identifies something to be done (imperative):– "CreateNewUser";
• Event: A message that semantically identifies something happened and immutable (past):– "NewUserCreated";
• Subscription: The notion that an endpoint is interested in an event;
DEMO
Transport
• Transport(s): The technology used to connect systems and transport the message:– MSMQ– RabbitMQ– SQL Server– Azure ServiceBus & Azure Queues
• Serialization: the way messages are"serialized" in order to be transported on the choosen transport;– it is transparent to the transport;
Advanced Concepts
• Saga: An orchestrator for a long running workflow, with the ability to store the saga state across requests and handling concurrency;
• Timeout: The way a Saga can take autonomous decisions;
• Retries: First level and Second level retry engine to handle transient failures;
• Error & Audit: error and auditing management
SCALE OUT & HIGH AVAILABILITYIn an eventual consistent world
Mail & Mail Servers
When we send an email message:• Our relation with the mail server is consistent?
Yes;• Cross-servers relation is consistent? Yes;• Relationship between the last server and the
recipient is consistent? Yes;
The entire system is consistent? No
But we have some guarantees:• Every single hop/node/BC is consistent;• If something along the way fails we will have,
with the same logic, an information back that our request is failed or succeeded;
• Do we need distributed transactions? No :-)• The message is fully enough to guarantee
consistency, in the long run.
DEMO
Publish/Subscribe
• Request/Response is generally considered an anti-pattern;
• Events are the easiest way to drive the world:– SomethingHappened;• DoSomethingToMoveOn;
• Lots of possible listener and lots of possible publishers:– CorrelationID
DEMO
QUESTIONS?We are all set :-)