Benefits of an ESB

Source application / service has not to be changed

Business events that happened are published by the source application to the bus.
Typically there is an application interested in this event, so it will consume it as this kind of processing would have been done by a point-to-point communication.
→ If business requirements changes and enterprise landscape grow more and more applications might potentially be interested in that kind of events. So the producing application and already existing consumers do not have to be changed at all.

Easy integration through a DSL

Additional systems that are interested in that event just subscribe to the topic or if there is a dispatcher application in place, that consumes special kind of messages laying on the bus, will then dispatch requests to the corresponding systems.
Integration scenarios are typically configured / implemented with the help of a DSL leading to more maintainability and faster realization.

Further benefits

• Heterogeneous environments get interoperable
• Service virutalization aka location transparency
• Service versioning
• Transaction-based message processing

Message routing over an ESB

How is the routing done over the ESB / bus?

Routing configuration can be available on the client or on the bus and can be changed during runtime.

Certain bus products allow you to configure routing of messaging by using a DSL. 

Here is an example:

from("invoices").choise().when(header("invoiceType").isEqualTo("clearing")

.to("clearingQueue")

.otherwise("costcenterQueue")

This shows a content-based routing. Depending on a meta data the message from one queue is routed to one of other 2 queues.

Further example with Camel:

For routing purpose we take a quick at Camel.

Camel is an integration platform, a framework with the target to provide EIP based components.

Is Camel itself already an ESB? As there is no standard definition, the answer is yes and no.

But the core functionality it brings is definitely  a routing and transformation engine.

Routes are defined in Camel with XML configuration or a DSL like above.

The messages are collected at the endpoints and processed through the defined routes.

A route itself contains of flow and integration logic. 

A message always has a producer and a consumer - inside the Camel context (runtime system of Camel)

there are processors that processes the message further by filtering, enriching, routing, transforming etc.

The component inside Camel that manages these processing that happens between the service provider and consumer

is called MEC (Message Exchange Container). This component has further information about the unique message ID, exception information, etc.

The routes are defined or to be more explicit are added to the Camel context.

This runtime system brings all the defined components in Camel together.

A route definition is a plain Java class that needs to extends RouteBuilder and needs to implement the configure-method.

In here the route must start with from(…) and end with to(…).

Like in the above snippet all the processing logic happens besides these two points.

What is the difference between an ESB, SOA and EAI?

ESB - Enterprise Service Bus

An ESB is an integration platform in that applications that want to communicate with each other have to integrate. It also defines a backbone of your enterprise landscape so that applications and services can easily communicate.

SOA - Service Oriented Architecture

A SOA describes an architecture style in that software resources of an enterprise get accessible and discoverable on network as dedicated and defined services.

EAI - Enterprise Application Integration

An EAI is driven by business needs to achieve a certain business objective by connecting applications inside an enterprise and external partner systems. So it is a concept for integrating business functions with the help of a dedicated IT infrastructure along the value change. As they are provided by different applications and platforms by nature EAI deals with data and business process integration.

So what is the difference?

To make it short: 

SOA is an architecture style based on services, EAI is a concept about connecting applications and services to new valuable services. Whereas the ESB is a concrete method for establishing an integration platform for inter-application communication.

Both SOA and EAI need in their concrete transformation components and the ESB plays an very important role for the realization.

ESB: Drawbacks and risks

Depending of the choice of the broker architecture:

Very often a central Hub is chosen as the broker architecture. That means the messages on the bus all go through the hub.

- Scalability

-- less scalable as a distributed broker architecture

- Maintenance

++ easy to monitor

++ easy understanding 

-- maintenance work on hub gets crucial

- Stability

-- single point of failure

Service locator

The service locator is responsible for locating / identifying the service of an application, that needs to be called to process a certain message.

Every service provider has to register the service at the service registry. Service consumer ask for the service and retrieve the endpoint for starting to consume the service.

The danger of a centralized service locator is similarly to the central Hub. If the service locator is to a certain moment not available no messages are processed as no services could be identified.

It is important that there are multiple instances of the service registry and that the configuration information is synchronized between them.

Messaging

Leads to decoupling between the communication partners - the messages are guaranteed delivered by buffering, saving and forwarding. 

But there are situation where asynchronous communication leads to just more overhead and complexity swallowing the advantages.

Batch processing

The bus should not be misused for batch processing as throughput may much less as direct batch processing.

Mass data processing should still be accomplished by corresponding batch applications that might publish their result or special cases to the bus.

Business logic

A big danger is the possible development that business logic leaks into the bus - leading to maintainability and scalability problems.

Business logic should not be handled directly in the bus. 

Commands

The bus should not be thought of an assembly line for placing orders. 

Much better is to place occurred business events on the bus and let the connected applications / services consume those events.

ESB: Typical tasks

- Transports
Transportation of the messages
- Messaging
Allowing asynchronous and synchronous messaging.
- Security
Application that wants access to the bus must authenticate and must get authorization for the requested operation.
- Datatransformations
A message of data model A is transformed into data model B without losing content.
- Service locator
On receive of a message the bus identifies / locates the service of an application to be called.
- Interceptors
After processing messages at certain stages processing can be intercepted by configured service calls - e.g. allowing realization of cross-cutting concerns.
- Protocol bindings
The bus allows forwarding requests from one endpoint to another one. Not changing the message at all or enriching or transforming it.
- Service model

Sources that will  / should be connected to the bus:
- Web services
- Queues
- Portals
- File / FTP / ...
- BPEL

Systems that would / should be connected to the bus:
- business applications
- mobile devices
- partners
- browsers
- rich clients

ESB concept / bus

1) Explanation
The ESB (Enterprise Service Bus) is the holy grail of applications integration.
It is actually an integration platform used to allow applications to integrate more easily into an enterprise landscape.
The ESB defines the infrastructure, the foundation, in which the different applications and services of an enterprise landscape will integrate. It stands for a specific architecture style enforcing a communication bus to be used for inter-application or inter-services communication. Depending on the structuring of the enterprise landscape, that means service oriented or application based, the bus is used for service integration in comparison to "only" connecting different applications over a common communication bus.

2) Initial situation
Having an enterprise landscape with many applications that need to communicate with each other,
lead to many point-to-point communications with all its disadvantages.
That are implementing retry mechanism in each application that interacts with another application.
Dealing with the complexity of timeouts(connection and read timeout) - finding the right timeout window, distinguishing between operations that can and that shouldn't be repeated.

3) Benefits
Business events that happened are published by the source application to  the bus.
Typically there is an application interested in this event, so it will consume it as this kind of processing would have been  done by a point-to-point communication.
But now comes the benefits of an enterprise bus into play. If business requirements changes and enterprise landscape grow more and more applications might potentially be interested in that kind of events. So the producing application does not have to be changed at all.
Additional systems that are interested in that event just subscribe to the topic or if there is a dispatcher application in place, that consumes special kind of messages laying on the bus, will then dispatch requests to the corresponding systems.
Integration scenarios are typically configured / implemented with the help of a DSL leading to more maintainability and faster realization.

ESB allows:
- Heterogeneous environments get interoperable

- Service virutalization aka as location transparency
The service consumer and the service provider is decoupled by the ESB.
With service virtualization the service consumer has not be reconfigured if the service provider is changed regarding
   endpoint information.

- Service versioning
With message transformation the message of an interface that is not support anymore is translated into a message of
  new interface version.
Benefitting from again the fact that the service consumer is decoupled from the service provider.
The additional "layer" can be used to allow a technical mapping between an old interface and the new one.
Allowing business adaption independent of interface availabilities.

- Transaction-based message processing
The message is taken from queue and during the processing of the defined workflow involving maybe different services
   and service calls, the whole transaction is only committed if the process arrived the last service that is a database
   adapter. The whole process is committed as the flow reaches the db adapter.
So the ESB can be used to coordinate distributed transactions with different services involved. The client has only to  
   mark the begin and end of a transaction.

The coordination work is done by the bus.

4) Background
Basic groundwork was done by Hophe and Woolf with their work on EIP - Enterprise Integration Platform.
They have described common integration scenarios, abstracted them into different patterns, and categorized them into the following 6 categories:

- Message endpoints
All kind of patterns for connecting applications that should integrate with each other.
E.g.:
- Polling consumer - An adapter that polls periodically a data source and consume the data that needs to be
            processed.
         - Service activator - A component that locates a service of an application that then will be called.

- Message construction
All kind of patterns that deal with messages itself.
     E.g.:
- Correlation Identifier  -  The correlation id is used to map messages to a transaction Messages that took part in a
           transaction should have the same correlation id.
  - Message expiration - Message have a period of time in which they need to be processed. If this time passed
           without processing they are dropped by the message broker or moved do a dead letter queue.

- Message channels
All kind of patterns that describe how message are delivered to message endpoints.
E.g.:
 - Guaranteed delivery - Producer can rely to 100% on the fact that if a topic or queue has confirmed the receive of a
           message, that the message will be later on processed. After the confirmation the resources of the JMS client are
           freed and he can continue with further actions/processing.
-  Point-to-point channels - A message channel pattern to realize a synchronous communication based on JMS.
           Sender blocks until receiver of a message has processed and delivered the result to the sender.

- Message routing
All kind of pattern that do not change the semantics of a message - no content change. But based on certain rules
     messages are forwarded to different endpoint.
E.g.:
- Aggregator - Split message are composed to a new resulting message.
- Splitter - A message is split in several parts that result in new messages.

- Message transformation
All kind of patterns that change the content of a message.
E.g.:
- Enricher - A message is enriched with further information from other services, data sources etc.
- Message translator - Transforming a message of a data model A into a message of a data model B without
          changing the semantic of the message.

- System management
All kind of patterns that can't be categorized in the above categories or have general supporting characteristics.