Wednesday, August 31, 2016

Single Page Applications And The Impending Concurrency Explosion

Ok explosion might be a little dramatic but bear with me for a moment.

Going from a Multi Page Application (MPA, did I just coin a retronym?) to a Single Page Application (SPA) is like going from a single synchronization lock (coarse-grained locking) to multiple synchronization locks (fine-grained locking).

Anyone who has made that journey knows that it is fraught with peril. Even for a solo developer it can be difficult to implement without error. For a team of developers, with varying level of application domain familiarity and developer expertise, it is far more daunting.

How is going from an MPA to an SPA like this concurrency problem?

With an MPA the full page submit is analogous to acquiring a single lock. The state of the app is updated in one fell swoop.

With an SPA the same amount of state will be distributed across separate async requests that fire, largely, in response to user interaction. Since the user drives the requests any ordering requirements must be enforced by the developer(s).

Each of these requests is like acquiring one of many locks (e.g., read-only consumers only need the read lock, which is shareable, whereas read-write consumers need both the read and write lock (not shareable)). On the user interface, independently queryable data can update different UI widgets in parallel*. This increases the perceived interactivity of the web app but comes at the cost of increased complexity.

It can be difficult to coordinate multiple lock scenarios even when all of the developers working on the project are co-located. Part of this difficulty stems from the difficulty modeling concurrency - there are many notations available and none used by everyone.

Throw in developers distributed geographically, varying levels of expertise, even varying languages and the coordination required to prevent lock-related errors becomes quite substantial.

The theory of it is sound but in practice there are often subtle data dependencies that don't reveal themselves when all data is fetched and the page is rendered in one fell swoop. That is, the dependencies don't show up until teasing apart the data needed for the various dynamic portions of the page. In other words, the data access patterns of an SPA are going to be different than the data access patterns of an MPA.

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