Sunday 27 April 2014

Microservices relax

Recently I blogged explaining that I view microservice architectures as a collection of strategies to independently vary system characteristics. I suggested that these strategies could be framed in terms of a generic architectural constraint:
X can be varied independently of the rest of the system.
Mark Baker pointed out that while I employed the language of architectural constraints, my one "isn't upon components, connectors, and data." He referred me to Roy Fielding's excellent thesis, famous for defining the REST architectural style, for a rigorous description of how architectural constraints can be described.

Thinking about the problem further, I considered what kind of data flows through the microservice systems I've worked with. I realised that many of the qualities that characterise a microservice architecture are not visible during a single system run. Microservice architecture can only be explained in terms of components, connectors and data if the constituent services are themselves regarded as data, and the processes that evolve them as components and connectors.

I don't think that's too much of a stretch. For systems that are deployed via continuous delivery pipelines, the automated deployment infrastructure is part of the system itself. If the build is red, or otherwise unavailable, then the production system loses one of its most important capabilities - to evolve. The manner in which the code/data that specifies the behaviour of the system evolves can be subject to similar constraints to the introduction of novelty into conventional system data.

Viewed that way, it becomes apparent that the constraint I proposed was the inverse of what it should have been, namely:
X's variation must be synchronised with the rest of the system.
Monolithic architectures are subject to this constraint for various values of 'X'. Microservice architectures are produced via the relaxation of this constraint along different axes.

The process of starting with a highly constrained system and gradually relaxing constraints that it is subject to is the opposite of the approach Fielding advocates in his thesis. When deriving REST, Fielding starts with the null style and layers on six constraints to cumulatively define the style.

However, the microservice style is a reaction against highly-constrained monolithic architectures, so it has taken the opposite approach, which is to peel away constraints layer by layer in order to introduce new flexibility into the system.

For example, a monolithic system might consist of five functional areas, all integrated within a single codebase. For a functional change to make its away to production, the data describing the system's behaviour (its source code) is modified, checked-in and moves through the deployment pipeline into production. This must happen for all five functional areas as one, because the transaction of deployment encompasses all five pieces of data.

This system is devolved into a system of five microservices, each living in an independent source control root. A change to any one of those services can be checked in and move through its pipeline to production independently of the others. The deployment transactions are now smaller in scope, encompassing only a single service at a time.

The change in the scope of a single deployment generates options for the maintainers of the system. They can now release changes to one of the five functional areas without downtime to any of the other four.

But, as is the case with real options, this comes at a price. If the five areas interact, their APIs need to be versioned and kept backwards compatible. Changes that crosscut multiple services need to be carefully orchestrated to avoid breaking changes, whereas in the original monolithic system such refactorings might have been made with impunity.

Generally speaking, for each constraint that is relaxed in a microservice architecture, there is a corresponding real option generated and a corresponding coordination cost incurred. For good explanations on real options and how they apply to software development, see the various works of Chris Matts, including the book he wrote with Olav MaassenCommitment.

I think that it's a productive approach to describe microservice architectures in terms of the the relaxation of architectural constraints relative to a monolithic architecture. This provides us with a model not only to analyse the runtime properties of the system, but also its evolution and the options it affords its maintainers.

Sunday 13 April 2014

Review: Adaptive web design

Adaptive web design by Aaron Gustafson is a good book on an important topic. How the web evolves, and how it caters to the broad spectrum of its users, that now make up the majority of the population in affluence societies, is a crucial consideration.

The book is a curious mixture of tips, philosophy and polemic. Gustafason includes specific technical detail, for example microformats, structuring CSS selectors and ARIA roles.

He also treats progressive enhancement as a moral issue, which is both a reasonable point of view and a rhetorical device. For example, he describes the technique of graceful degradation as "fault tolerance's superficial, image-obsessed sister" and that eventually "smart folks working on the web began to realize that graceful degradation's emphasis". He thus frames issues in a way as to try and ensure the reader's conclusion cannot be anything other than agreement, which is a time-honoured technique of polemic.

This book is a good overview of a topic that can fade into the background under the pressure of deadlines and corporate disregard for users, so it's great that Gustafason has put his energy into a book that software developers can rally around.

However it has two serious flaws, one of omission and one of commission, that make it less useful to a contemporary software team than it otherwise might have been.

The flaw of omission is that it does not engage with the recent crop of client-side MVC applications. Adaptive web design was published in 2011, two years after the release of Google's framework AngularJS, but it contains no mention of this style of application development, which challenges some of the fundamental assumptions of the progressive enhancement movement.

Client-side MVC is, literally speaking, progressive enhancement, but it's not of the sort that Gustafason lionises. These applications, usually constructed with the help of extensive JavaScript frameworks, place rich interactivity on top of APIs, where as traditional progressive enhancement layers unobtrusive interactivity on top of HTML.

The major difference with Gustafason's vision is that the HTML comes on top of the JavaScript rather than the other way around. Data is pulled into the browser by Ajax calls, before being rendered using client-side templating engines directly into the DOM. This allows the application to keep a model of the application's state and rules in the browser, and to update that model and apply those rules in the interests of a rich experience without the latency of calls back to the server.

I don't know how such applications should be regarded. They don't, for example, allow users to access content if JavaScript is unavailable. This may or may not have the same urgency as it did when Adaptive web design was written three years ago, but in this book at least, Gustafason is unable to give us guidance.

That substantially diminishes the relevance of this book, as the most pressing question most contemporary teams will have about progressive enhancement is how it relates to the major movement in the JavaScript world towards client-side applications.

The flaw of commission, that makes it difficult to treat Adaptive web design as even a point-in-time reference, is the way that Gustafason uses definitions and terminology.

One such example is the term "accessibility". Gustafason takes the perfectly reasonable view that accessibility is a broader issue than simply people who use assistive technologies. The problem with this definition is that it's incompatible with the common usage of the term.

Interestingly, this is an example of a failure of progressive enhancement of language itself. Specialising normal language into terms of art allows the common punter to at least get the gist of what's being discussed. Expanding the definition of terms is much more confusing, because it leads to sentences where the word simply does not make sense under its old definition.

But all this is bunk when we get to the "Accessibility" chapter itself, where Gustafason reverts to the classic idea of accessibility being about people with physical needs that might impede them from accessing content. I guess Gustafason just found it too difficult to sustain his own revised definition.

But the terminology that most grates with me is the assertion that progressive enhancement can be defined as "fault tolerance". The most obvious issue is that there are many instances of progressive enhancement that don't involve "fault". For example, it doesn't make sense to talk about the fault of my browser not specifically recognising the XFN microformat, but my ability to browse the page regardless is, as Gustafason points out, a classic case of progressive enhancement.

The greater problem is that there are times when fault tolerance and progressive enhancement are actively opposed. For example, when browsers tolerate malformed markup it makes it more difficult for later changes to e.g. HTML to be added that bring new features because they are more likely to clash with the "fault tolerant" HTML implementations of older browsers.

The pity is that I don't think this particular confusion is necessary. Progressive enhancement is a very specific kind of fault tolerance, and Gustafason's attempt to compare progressive enhancement to Galapagos finches is entirely misleading, if entertaining. If Gustafason had chosen an alternative term to "fault tolerance", perhaps "graceful gradation" (to contrast with "graceful degradation") then the same point could have been made with much greater clarity.

Adaptive web design is a solid book, written with energy and with a strong sense of the historical development of the web. If a team reads this book together, it will spark a lot of important discussions, but its occasional problems with terminology and its failure to engage with client-side MVC means it will not settle them.