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Service Decomposition

How can the granularity of a service be increased subsequent to its implementation?

Table 16.5. Profile summary for the Service Decomposition pattern.

Problem

Overly coarse-grained services can inhibit optimal composition design.

Solution

An already implemented coarse-grained service can be decomposed into two or more fine-grained services.

Application

The underlying service logic is restructured, and new service contracts are established. This pattern will likely require Proxy Capability (497) to preserve the integrity of the original coarse-grained service contract.

Impacts

An increase in fine-grained services naturally leads to larger, more complex service composition designs.

Principles

Service Loose Coupling, Service Composability

Architecture

Service

Problem

When modeling services during the initial analysis phases it is common to take practical considerations into account. For example, what may ideally be represented by a set of fine-grained business service candidates is later combined into a smaller number of coarse-grained services primarily due to performance and other infrastructure-related concerns motivated by the need to keep service composition sizes under control.

After a service inventory architecture matures and more powerful and sophisticated technology and runtime products are incorporated, larger, more complex service compositions become a reality. When designing such compositions, it is generally preferable to keep the footprints of individual services as small as possible because only select service capabilities are required to automate a given parent business task. However, when forced to work with overly coarse-grained services, composition performance can be negatively affected, and the overall composition designs can be less than optimal (Figure 16.13).

Figure 16.13

Figure 16.13 An Invoice service with a functional context originally derived from three separate business entities ends up existing as a large software program with a correspondingly large footprint, regardless of which capability a composition may need to compose.

Solution

The coarse-grained service is decomposed into a set of fine-grained services that collectively represent the functional context of the original service but establish distinct functional contexts of their own (Figure 16.14).

Figure 16.14

Figure 16.14 The original, coarse-grained Invoice service is decomposed into three separate services, one of which remains associated with general invoice processing but only encapsulates a subset of the original capabilities.

Application

Carrying out this pattern essentially requires that the existing, coarse-grained service be broken apart and its logic reorganized into new, finer-grained functional boundaries.

Therefore, the first step is usually to revisit the service inventory blueprint and decide how the service can be re-modeled into multiple service candidates. As part of this process, new capability candidates will also need to be defined, especially if Decomposed Capability (504) was not taken into account during the service's original design. After the modeling is completed, the new services are subject to the standard lifecycle phases, beginning with contract design (based on the modeled service candidates) and all the way through to final testing and quality assurance phases (Figure 16.15).

Figure 16.15

Figure 16.15 The new, fine-grained services each provide fewer capabilities and therefore also impose smaller program sizes.

Unless it is decided to also retrofit previous consumer programs that formed dependencies on the original service, Proxy Capability (497) will likely need to be applied to preserve the original service contract for backwards compatibility.

Impacts

The extent to which Service Decomposition can impact a service inventory depends on how established a service is and how many consumer programs have formed relationships on it. The more consumers involved, the more complicated and disruptive this pattern can be.

Because this pattern is commonly applied after an inventory architecture has matured, its application needs to be carefully planned together with the repeated application of Proxy Capability (497).

The preventative use of Decomposed Capability (504) can ease the impact of Service Decomposition and will also result in a cleaner separation of functional service contexts.

Relationships

Service Decomposition has a series of relationships with other service-level patterns, most notably Service Refactoring (484). When a service is upgraded as a result of a refactoring effort, the application of Service Decomposition may very well be the means by which this is carried out.

As explained in the pattern description for Proxy Capability (497), Service Decomposition relies on that pattern to implement the actual partitioning via the redevelopment effort required to turn one or more regular capabilities into proxies. As a result, this pattern shares several of the same patterns as Proxy Capability (497).

Service Decomposition is most frequently applied to agnostic services, therefore tying it to Entity Abstraction (175) and Utility Abstraction (168). However, the result of this pattern can introduce a measure of service redundancy due to the need for Proxy Capability (497) to violate Service Normalization (131) to some extent.

Figure 16.16

Figure 16.16 Service Decomposition is a refactoring-related approach to splitting up service logic that ties into numerous patterns that shape service logic and contracts.

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