The Routing Slip Pattern

The Pattern

Introduction

A routing slip is a configuration that specifies a sequence of processing steps (services). This routing slip must be attached to the message to be processed. Each service (processing step) is designed to receive the message, perform its functionality (based on the configuration) and invoke the next service. In that way, a message gets processed sequentially by multiple services, without the need of a coordinating component. The schema below is taken from Enterprise Integration Patterns.

Some examples of this pattern are:

• BizTalk Server ESB Toolkit Itineraries
• nServiceBus Routing Slip Support
• Routing Slip pattern in Azure Logic Apps

Routing Slip

Routing slips can be configured in any language, JSON or XML are quite popular. An example of a simple routing slip can be found below. The header contains the name of the routing slip and a counter that carries the current step number. Each service is represented by a routing step. A step has its own name to identify the service to be invoked and has a specific key-value configuration pairs.

Remark that this is just one way to represent a routing slip. Feel free to add your personal flavor…

Assign Routing Slip

There are multiple ways to assign a routing slip to a message. Let’s have a look:

• External: the source system already attaches the routing slip to the message
• Static: when a message is received, a fixed routing slip is attached to it
• Dynamic: when a message is received, a routing slip is attached, based on some business logic
• Scheduled: the integration layer has routing slips scheduled that contain also a command to retrieve a message

Service

A service is considered as a “step” within your routing slip. When defining a service, you need to design it to be generic. The executed logic within the service must be based on the configuration, if any is required. Ensure your service has a single responsibility and there’s a clear boundary of its scope.

A service must consist of three steps:

• Receive the message
• Process the message, based on the routing slip configuration
• Invoke the next service, based on the routing slip configuration

There are multiple ways to invoke services:

• Synchronous: the next service is invoked without any persistence in between (e.g. in memory). This has the advantage that it will perform faster.
• Asynchronous: the next service is invoked with persistence in between (e.g. a queue). This has the advantage that reliability increases, but performance degrades.

Think on the desired way to invoke services. If required, a combination of sync and async can be supported.

Advantages

Encourages reuse

Integrations are composed of reusable and configurable building blocks. The routing slip pattern forces you to analyze, develop and operate in a streamlined manner. Reuse is heavily encouraged on different levels: the way analysis is performed, how patterns are implemented, the way releases are rolled out and how operational tasks are performed. One unified way of working, built on reusability.

Configuration based

Your integration is completely driven by the assigned routing slip. There are no hard-coded links between components. This allows you to change its behavior without the need of a re-deployment. This configuration also serves as a great source of documentation, as it explains exactly what message exchanges are running on your middleware and what they exactly do.

Faster release cycles

Once you have set up a solid routing slip framework, you can increase your release cadence. By leveraging your catalogue of reusable services, you heavily benefit from previous development efforts. The focus is only on the specifics of a new message exchange, which are mostly data bound (e.g. mapping). There’s also a tremendous increase of agility, when it comes to small changes. Just update the routing slip configuration and it has an immediate effect on your production workload.

Technology independent

A routing slip is agnostic to the underlying technology stack. The way the routing slip is interpreted, is of course specific to the technology used. This introduces ways to have a unified integration solution, even if it is composed of several different technologies. It enables also cross technology message exchanges. As an example, you can have an order that is received via an AS2 Logic App, being transformed and sent to an on premise BizTalk Server that inserts it into the mainframe, all governed by a single routing slip config.

Provides visibility

A routing slip can introduce more visibility into the message exchanges, for sure from an operational perspective. If a message encounters an issue, operations personnel can immediately consult the routing slip to see where the message comes from, what steps are already executed and where it is heading to. This visibility can be improved, by updating the routing slip with some extra historical information, such as the service start and end time. Why even not including an URL in the routing slip that points to a wiki page or knowledge base about that interface type?

Pitfalls

Not enough reusability

Not every integration project is well-suited to use the routing slip pattern. During analysis phase, it’s important to identity the integration needs and to see if there are a lot of similarities between all message exchanges. When a high level of reusability is detected, the routing slip pattern might be a good fit. If all integrations are too heterogenous, you’ll introduce more overhead than benefits.

Too complex logic

A common pitfall is adding too much complexity into the routing slip. Try to stick as much as possible to a sequential series of steps (services) that are executed. Some conditional decision logic inside a routing slip might be acceptable, but define clear boundaries for such logic. Do not start writing you own workflow engine, with its own workflow language. Keep the routing slip logic clean and simple, to stick to the purpose of a routing slip.

Limited control

In case of maintenance of the surrounding systems, you often need to stop a message flow. Let’s take the scenario where you face the following requirement: “Do not send orders to SAP for the coming 2 hours”. One option is to stop a message exchange at its source, e.g. stop receiving messages from an SFTP server. In case this is not accepted, as these orders are also sent to other systems that should not be impacted, things get more complicated. You can stop the generic service that sends a message to SAP, but then you also stop sending other message types… Think about this upfront!

Hard deployments

A very common pain-point of a high level of reuse, is the impact of upgrading a generic service that is used all over the place. There are different ways to reduce the risks of such upgrades, of which automated system testing is an important one. Within the routing slip, you can specify explicitly the version of a service you want to invoke. In that way, you can upgrade services gradually to the latest version, without the risk of a big bang deploy. Define a clear upgrade policy, to avoid that too many different versions of a service are running side-by-side.

Monitoring

A message exchange is spread across multiple loosely coupled service instances, which could impose a monitoring challenge. Many technologies offer great monitoring insights for a single service instance, but lack an overall view across multiple service instances. Introducing a correlation ID into your routing slip, can highly improve the monitoring experience. This ID can be generated the moment you initialize a routing slip.

Conclusion

Routing slips are a very powerful mechanism to deliver unified and robust integrations in a fast way. The main key take-aways of this blog are:

• Analyze in depth if can benefit from the routing slip pattern
• Limit the complexity that the routing slip resolves
• Have explicit versioning of services inside the routing slip
• Include a unique correlation ID into the routing slip
• Add historical data to the routing slip

Hope this was a useful read!
Toon