Agility and Structural Modularity – part II

In this second Agility and Structural Modularity post we explore the importance of OSGi™; the central role that OSGi plays in realising Java™ structural modularity and the natural synergy between OSGi and the aims of popular Agile methodologies.

But we are already Modular!

Most developers appreciate that applications should be modular. However, whereas the need for logical modularity was rapidly embraced in the early years of Object Orientated programming (see, it has taken significantly longer for the software industry to appreciate the importance of structural modularity; especially the fundamental importance of structural modularity with respect to increasing application maintainability and controlling / reducing  environmental complexity.

Just a Bunch of JARs

In Java Application Architecture, Kirk Knoernschild explores structural modularity and develops a set of best practice structural design patterns. As Knoernschild explains, no modularity framework is required to develop in a modular fashion; for Java the JAR is sufficient.cover-small-229x300

Indeed, it is not uncommon for ‘Agile’ development teams to break an application into a number of smaller JAR’s as the code-base grows. As JAR artifacts increase in size, they are broken down into collections of smaller JAR’s. From a code perspective, especially if Knoernschild’s structural design patterns have been followed, one would correctly conclude that – at one structural layer – the application is modular.

But is it ‘Agile’ ?

From the perspective of the team that created the application, and who are subsequently responsible for its on-going maintenance, the application is more Agile. The team understand the dependencies and the impact of change. However, this knowledge is not explicitly associated with the components. Should team members leave the company, the application and the business are immediately compromised. Also, for a third party (e.g. a different team within the same organisation), the application may as well have remained a monolithic code-base.

While the application has one layer of structural modularity – it is not self-describing. The metadata that describes the inter-relationship between the components is absent; the resultant business system is intrinsically fragile.

What about Maven?

Maven artifacts (Project Object Model – POM) also express dependencies between components. These dependencies are expressed in-terms of the component names.

A Maven based modular application can be simply assembled by any third party. However, as we already know from the first post in this series, the value of name based dependencies is severely limited. As the dependencies between the components are not expressed in terms of Requirements and Capabilities,  third parties are unable to deduce why the dependencies exist and what might be substitutable.

It is debatable whether Maven makes any additional tangible contribution to our goal of application Agility.

The need for OSGi

As Knoernschild demonstrates in his book Java Application Architecture, once structural modularity is achieved, it is trivially easy to move to OSGi – the modularity standard for Java. 

Not only does OSGi help us enforce structural modularity, it provides the necessary metadata to ensure that the Modular Structures we create are also Agile structures

OSGi expresses dependencies in terms of Requirements and Capabilities. It is therefore immediately apparent to a third party which components may be interchanged. As OSGi also uses semantic versioning, it is immediately apparent to a third party whether a change to a component is potentially a breaking change.

OSGi also has a key part to play with respect to structural hierarchy.

At one end of the modularity spectrum we have Service Oriented Architectures, at  the other end of the spectrum we have Java Packages and Classes. However, as explained by Knoernschild, essential layers are missing between these two extremes.


Figure 1: Structural Hierarchy: The Missing Middle (Kirk Knoernschild – 2012).

The problem, this missing middle, is directly addressed by OSGi.


Figure 2: Structural Hierarchy: OSGi Services and Bundles

As explained by Knoernschild the modularity layers provided by OSGi address a number of critical considerations:

  • Code Re-Use: Via the concept of the OSGi Bundle, OSGi enables code re-use.
  • Unit of Intra / Inter Process Re-Use: OSGi Services are light-weight Services that are able to dynamically find and bind to each other. OSGi Services may be collocated within the same JVM, or via use of an implementation of OSGi’s remote service specification, distributed across JVM’s separated by a network. Coarse grained business applications may be composed from a number of finer grained OSGi Services.
  • Unit of Deployment: OSGi bundles provide the basis for a natural unit of deployment, update & patch.
  • Unit of Composition: OSGi bundles and Services are essential elements in the composition hierarchy.

Hence OSGi bundles and services, backed by OSGi Alliance’s open specifications, provide Java with essential – and previously missing – layers of structural modularity. In principle, OSGi technologies enable Java based business systems to be ‘Agile – All the Way Down!’.

As we will now see, the OSGi structures (bundles and services) map well to, and help enable, popular Agile Methodologies.

Embracing Agile

The Agile Movement focuses on the ‘Processes’ required to achieve Agile product development and delivery. While a spectrum of Lean & Agile methodologies exist, each tends to be; a variant of, a blend of, or an extension to, the two best known methodologies; namely Scrum and Kanban

To be effective each of these approaches requires some degree of structural modularity.


Customers change their minds. Scrum acknowledges the existence of ‘requirement churn’ and adopts an empirical ( approach to software delivery. Accepting that the problem cannot be fully understood or defined up front. Scrum’s focus is instead on maximising the team’s ability to deliver quickly and respond to emerging requirements.

Scrum is an iterative and incremental process, with the ‘Sprint’ being the basic unit of development. Each Sprint is a “time-boxed” ( effort, i.e. it is restricted to a specific duration. The duration is fixed in advance for each Sprint and is normally between one week and one month. A Sprint is preceded by a planning meeting, where the tasks for the Sprint are identified and an estimated commitment for the Sprint goal is made. This is followed by a review or retrospective meeting, where the progress is reviewed and lessons for the next Sprint are identified.

During each Sprint, the team creates finished portions of a product. The set of features that go into a Sprint come from the product backlog, which is an ordered list of requirements (

Scrum attempts to encourage the creation of self-organizing teams, typically by co-location of all team members, and verbal communication between all team members.


‘Kanban’ originates from the Japanese word “signboard” and traces back to Toyota, the Japanese automobile manufacturer in the late 1940’s ( see ). Kanban encourages teams to have a shared understanding of work, workflow, process, and risk; so enabling the team to build a shared comprehension of a problems and suggest improvements which can be agreed by consensus.

From the perspective of structural modularity, Kanban’s focus on work-in-progress (WIP), limited pull and feedback are probably the most interesting aspects of the methodology:

  1. Work-In-Process (WIP) should be limited at each step of a multi-stage workflow. Work items are “pulled” to the next stage only when there is sufficient capacity within the local WIP limit.
  2. The flow of work through each workflow stage is monitored, measured and reported. By actively managing ‘flow’, the positive or negative impact of continuous, incremental and evolutionary changes to a System can be evaluated.

Hence Kanban encourages small continuous, incremental and evolutionary changes. As the degree of structural modularity increases, pull based flow rates also increase while each smaller artifact spends correspondingly less time in a WIP state.


An Agile Maturity Model

Both Scrum and Kanban’s objectives become easier to realize as the level of structural modularity increases. Fashioned after the Capability Maturity Model (see – which allows organisations or projects to measure the improvements on a software development process), the Modularity Maturity Model is an attempt to describe how far along the modularity path an organisation or project might be; this proposed by Dr Graham Charters at the OSGi Community Event 2011. We now extend this concept further, mapping an organisation’s level of Modularity Maturity to its Agility.

Keeping in step with the Modularity Maturity Model we refer to the following six levels.

Ad Hoc – No formal modularity exists. Dependencies are unknown. Java applications have no, or limited, structure. In such environments it is likely that Agile Management Processes will fail to realise business objectives.

Modules – Instead of classes (or JARs of classes), named modules are used with explicit versioning. Dependencies are expressed in terms of module identity (including version). Maven, Ivy and RPM are examples of modularity solutions where dependencies are managed by versioned identities. Organizations will usually have some form of artifact repository; however the value is compromised by the fact that the artifacts are not self-describing in terms of their Capabilities and Requirements.

This level of modularity is perhaps typical for many of today’s in-house development teams. Agile processes such are Scrum are possible, and do deliver some business benefit. However ultimately the effectiveness & scalability of the Scrum management processes remain limited by deficiencies in structural modularity; for example Requirements and Capabilities between the Modules usually being verbally communicated. The ability to realize Continuous Integration (CI) is again limited by ill-defined structural dependencies.

Modularity – Module identity is not the same as true modularity. As we’ve seen Module dependencies should be expressed via contracts (i.e. Capabilities and Requirements), not via artifact names. At this point, dependency resolution of Capabilities and Requirements becomes the basis of a dynamic software construction mechanism. At this level of structural modularity dependencies will also be semantically versioned.

With the adoption of a modularity framework like OSGi the scalability issues associated with the Scrum process are addressed. By enforcing encapsulation and defining dependencies in terms of Capabilities and Requirements, OSGi enables many small development teams to efficiently work independently and in parallel. The efficiency of Scrum management processes correspondingly increases. Sprints can be clearly associated with one or more well defined structural entities i.e. development or refactoring of OSGi bundles. Meanwhile Semantic versioning enables the impact of refactoring is efficiently communicated across team boundaries. As the OSGi bundle provides strong modularity and isolation, parallel teams can safely Sprint on different structural areas of the same application.

Services – Services-based collaboration hides the construction details of services from the users of those services; so allowing clients to be decoupled from the implementations of the providers. Hence, Services encourage loose-coupling. OSGi Services‘ dynamic find and bind behaviours directly enable loose-coupling, enabling the dynamic formation, or assembly of, composite applications. Perhaps of greater import, Services are the basis upon which runtime Agility may be realised; including rapid enhancements to business functionality, or automatic adaption to environmental changes.

Having achieved this level of structural modularity an organization may simply and naturally apply Kanban principles and achieve the objective of Continuous Integration.

Devolution – Artifact ownership is devolved to modularity-aware repositories which encourage collaboration and enable governance. Assets may selected on their stated Capabilities. Advantages include:

  • Greater awareness of existing modules
  • Reduced duplication and increased quality
  • Collaboration and empowerment
  • Quality and operational control

As software artifacts are described in terms of a coherent set of Requirements and Capabilities, developers can communicate changes (breaking and non-breaking) to third parties through the use of semantic versioning. Devolution allows development teams to rapidly find third-party artifacts that meet their Requirements. Hence Devolution enables significantly flexibility with respect to how artifacts are created, allowing distributed parties to interact in a more effective and efficient manner. Artifacts may be produced by other teams within the same organization, or consumed from external third parties. The Devolution stage promotes code re-use and efficient, low risk, out-sourcing, crowd-sourcing, in-sources of the artifact creation process.

Dynamism This level builds upon Modularity, Services & Devolution and is the culminatation of our Agile journey.

  • Business applications are rapidly assembled from modular components.
  • As strong structural modularity is enforced (isolation by the OSGi bundle boundary),  components may be efficiently and effectively created and maintained by a number of small – on-shore, near-shore or off-shore developement teams.
  • As each application is self-describing, even the most sophisticated of business systems is simple to understand, to maintain, to enhance.
  • As semantic versioning is used; the impact of change is efficiently communicated to all interested parties, including Governance & Change Control processes.
  • Software fixes may be hot-deployed into production – without the need to restart the business system.
  • Application capabilities may be rapidly extended applied, also without needing to restart the business system.

Finally, as the dynamic assembly process is aware of the Capabilities of the hosting runtime environment, application structure and behavior may automatically adapt to location; allowing transparent deployment and optimization for public Cloud or traditional private datacentre environments.


Figure 3: Modularity Maturity Model

An organization’s Modularisation Migration strategy will be defined by the approach taken to traversing these Modularity levels. Mosts organizations will have already moved from an initial Ad- Hoc phase to Modules. Meanwhile organizations that value a high degree of Agility will wish to reach the endpoint; i.e. Dynamism. Each organisation may traverse from Modules to Dynamism via several paths; adapting migration strategy as necessary.

  • To achieve maximum benefit as soon as possible; an organization may choose to move directly to Modularity by refactor the existing code base into OSGi bundles. The benefits of Devolution and Services naturally follow. This is also the obvious strategy for new greenfield applications.
  • For legacy applications an alternative may be to pursue a Services first approach; first expressing coarse grained software components as OSGi Services; then driving code level modularity (i.e. OSGi bundles) on a Service by Service basis. This approach may be easier to initiate within large organizations with extensive legacy environments.
  • Finally, one might move first to limited Devolution by adoption OSGi metadata for existing artifacts. Adoption of Requirements and Capabilities, and the use of semantic versioning, will clarify the existing structure and impact of change to third parties. While structural modularity has not increased, the move to Devolution positions the organisation for subsequent migration to the Modularity and Services levels.

diverse set of choices and the ability to pursue these choices as appropriate, is exactly what one would hope for, expect from, an increasingly Agile environment!