Enabling SOA Using WebSphere Messaging

10 Enabling SOA Using WebSphere Messaging

1.3 Using an enterprise service bus

An enterprise service bus (ESB) provides an infrastructure that removes any

direct connection between service consumers and providers. Consumers

connect to the bus and not the provider that actually implements the service. This

type of connection further decouples the consumer from the provider. A bus also

implements further value add capabilities. For example, security and delivery

assurance can be implemented centrally within the bus instead of having this

buried within the applications.

Integrating and managing services in the enterprise outside of the actual

implementation of the services in this way helps to increase the flexibility and

manageability of SOA. The primary driver for an ESB, however, is that it

increases decoupling between service consumers and providers. Protocols such

as Web services define a standard way of describing the interface to a service

provider that allow some level of decoupling, as the actual implementation details

are hidden. However, the protocols imply a direct connection between the

consumer and provider.

Although it is relatively straightforward to build a direct link between a consumer

and provider, these links can lead to an interaction pattern that consists of

building multiple point-to-point links (Figure 1-3) that perform specific

interactions. With a large number of interfaces this quickly leads to the build up of

a complex spaghetti of links with multiple security and transaction models.

Routing control is distributed throughout the infrastructure, and probably no

consistent approach to logging, monitoring, or systems management is

implemented. This environment is difficult to manage or maintain and inhibits

change.

Figure 1-3 Direct connection integration style

Direct Connection

Service

Consumer

Service

Consumer

Service

Consumer

Service

Provider

Service

Provider

Service

Provider

Chapter 1. Introduction 11

A common approach to reduce this complexity is to introduce a centralized point

through which interactions are routed, as shown in Figure 1-4.

Figure 1-4 Central hub integration style

This hub and spoke architecture is a common approach that is used in

application integration architectures. In a hub, the distribution rules are separated

from applications. The applications connect to the hub and not directly to any

other application. This type of connection allows a single interaction from an

application to be distributed to multiple target applications without the consumer

being aware that multiple providers are involved in servicing the request. This

connection can reduce the proliferation of point-to-point connections.

Note that the benefit of reducing the number of connections only truly emerges if

the application interfaces and connections are genuinely reusable. For example,

consider the case where one application needs to send data to three other

applications. If this is implemented in a hub, the sending application must define

a link to the hub, and the hub must have links that are defined to the three

receiving applications, giving a total of four interfaces that need to be defined. If

the same scenario was implemented using multiple point-to-point links, the

sending application would need to define links to each of the three receiving

applications, giving a total of just three links. A hub only offers the benefit of

reduced links if another application also needs to send data to the receiving

applications and can make use of the same links as those that are already

defined for the first application. In this scenario, the new application only needs

to define a connection between itself and the hub, which can then send the data

correctly formatted to the receiving applications.

Hubs can be federated together to form what is logically a single entity that

provides a single point of control but is actually a collection of physically

distributed components. This is commonly termed a bus

. A bus provides a

Hub and Spoke

Service

Consumer

Service

Consumer

Service

Consumer

Service

Provider

Service

Provider

Service

Provider

Hub:

ESB