There is something terribly wrong with software development in the enterprise application space. No one is able to release working software without coordinating system integration across all product development teams to align the version of every product in the universe, because end-to-end workflows can’t be made to work as products are released on independent life cycles.
I believe we are missing architectural design principles. We talk about forward and backward compatibility of APIs, but I’m not sure the industry deeply understands what that entails. The problem goes beyond teams within an organization, because the software industry doesn’t even understand what compatibility entails.
Horizontal applications lack vertical behavior
The issue lies in how the base application (e.g., product catalog, store front, sales automation, care, order fulfillment, customer and subscription management, charging, billing, revenue management) is horizontal (generic) and hollow, expecting after-market extensibility to provide the vertical behavior that is specialized for the industry and the enterprise’s business model. The intent of the application vendor is to provide a general purpose platform that can be tailored after-market to the peculiarities of any enterprise.
The application will implement an API defined by industry standards (say, tmforum.org for the communications industry) that reflects this general purpose hollowness. The application doesn’t have any real substance until it is customized to model the business. For example, a product catalog would not come populated with 5G mobile product specifications that are branded and priced according to a 5G service provider’s business model).
When extending entities with data that have hidden meaning, implied behavior, constraints, and statefulness (life cycle, workflow), these contribute to the API in ways that were not defined by the original specification. Each new element introduces some degree of incompatibility. Industry standards can never specify in a precise and rigorous manner things they did not foresee.
Stateful behavior is especially troublesome to specify in a manner that ensures compatibility. This includes conversational state and persistent state. Conversational state is where linked information is implicitly kept across multiple requests involved in the same session. A cursor for iterating through a collection of query results is an example of conversational state. Persistent state is durable across transactions, having memory that spans the life of a transaction, a session, a process, and even the life of a compute instance. When methods can only act against objects in certain states, but not others, this constraint must be honored for compatibility across collaborating components.
Objects and attributes are allowed to take on certain values at various points in their life cycles. Transactional behavior and workflow (the steps performed by business processes) are conditional upon the state of these objects. For example, when equipment is installed, it may be in various states of readiness for production use, but when not installed the equipment’s operational characteristics and configuration are irrelevant. Every component with access to that object must understand these semantics and enforce them consistently, otherwise there is no compatibility. Unfortunately, even these very simple conditional constraints and the ones in the previous paragraph are beyond the capability of today’s prevailing interface specification languages and entity modeling frameworks.
Immutability is often conditional on the life cycle state of an entity. For example, an order can be edited during information capture, but its captured intent cannot be edited after the order is firm and in the process of being fulfilled. Again, this constraint cannot be specified in a manner that ensures compatibility across collaborating components.
Methods have failure modes, usually specified as failure responses, error codes, or exceptions. Some kinds of failures are recoverable using techniques like retrying, while others are non-recoverable. This too is usually not expressible for compatibility.
Methods have performance expectations in terms of latency, concurrency, and transaction volume. Methods have resource consumption expectations in terms of memory, cpu, storage, network, and I/O. Methods that involve data sets have expectations about how much data can be passed with corresponding performance and scalability characteristics. This too is usually not expressible for compatibility.
Persistent versus volatile
Objects and their attributes are often persistent on durable storage. Subsets of attributes may be persistent, while others are volatile or derived (computed based on the value of other attributes, such as a rolled-up status or a count of a collection). This too is usually not expressible for compatibility.
Concurrency, availability, and partition tolerance
Methods must trade off concurrency, availability, and partition tolerance. The expectation of what trade offs should be chosen is usually not expressible for compatibility.
Authentication and authorization
Methods expect the caller to be authenticated and they are expected to enforce access control to verify that the caller is authorized. Moreover, the method is expected to enforce data permissions and data privacy. This too is usually not expressible for compatibility.
The list of requirements and constraints that contribute to compatibility goes on. The above is a sampling to give the reader a sense of the problem, not to be comprehensive. The intent is to show how formal specifications are grossly insufficient to ensure a high degree of compatibility across heterogeneous suppliers and independently developed implementations.
Because API compatibility is so unreliable based on specifications and contract testing, the promise of a microservice architecture (within an application) or a service-oriented architecture (for integrating applications across the enterprise) cannot be achieved naively. System integration continues to be plagued by a waterfall model of requiring a complete line-up of application versions to be tested end-to-end, before we have any confidence that they work together. The benefits of agile development and independent life cycles are not achievable, because the pre-requisite compatibility guarantees cannot be met. System integration of enterprise applications remains in the stone age because of this crippling deficiency.