This document proposes a design principle that we should apply during the FTP review process:
You only pay for what you use
Specifically, when adding functionality to FIDL, we should evaluate the costs that adding that functionality imposes on people who use FIDL but do not use the new functionality. We should then have a very high bar for accepting functionality that imposes costs on people who do not use the functionality.
One of the most important aspects of FIDL is that Fuchsia uses FIDL pervasively for interprocess communication and therefore for defining the system ABI.
Many use cases for interprocess communication are performance-critical. When people are evaluating technologies for these performance-critical use cases, FIDL is competing against custom message formats and ad-hoc serialization and deserialization routines.
Other uses cases for interprocess communication are flexibility-critical. When people are evaluating technologies for these flexibility-critical use cases, FIDL is competing against protobufs or a host of other networking-oriented message formats.
In order to succeed at being used pervasively throughout Fuchsia, FIDL needs to address both needs. Specifically, protocol designers working in FIDL need the ability to make tradeoffs between performance and flexibility in order to meet their needs.
Adopting the design principle of "you only pay for what you use" lets performance-critical customers avoid paying for functionality that support flexibility whereas its dual, "do pay for what you do use," lets FIDL serve flexibility-critical customers.
This section describes the history of how we arrived at this design principle as well as positive and negative examples.
FIDL is an evolution of the Mojo interprocess communication system. At the time, Mojo was significantly more flexible than FIDL, which worked well for flexibility-critical use cases. However, customers with performance-critical use cases were unwilling to adopt Mojo because the flexibility offered by the system did not meet their needs.
The original design for FIDL2 (the current version iteration of FIDL as of this writing, circa 2017-03-01) picked a different point in the design space. In order to win over performance-critical customers, FIDL2 is significantly less flexible than Mojo, which lets FIDL2 be performance-competitive with custom message formats and ad-hoc serialization and deserialization routines. Some clients still demand ad-hoc serialization and deserialization routines, but FIDL2 has succeeded in being used pervasively for message formats for interprocess communication.
The original design for FIDL2 over-rotated towards performance and needed to be improved to meet the needs of flexibility-critical customers. In order to be successful, FIDL needs to add functionality that supports use cases for flexibility without compromising on performance for customers that do not require flexibility.
Structs and tables
A positive example for "you only pay for what you use" is FTP-007, which introduced tables. Rather than replace structs (which have a fixed size and layout, supporting performance-critical use cases), tables are a separate data type which support flexibility-critical use cases. Protocol designers are able to choose whether to pay for flexibility.
Another important example is FTP-015, which introduces extensible unions. This example illustrates that we should not blindly apply the principle. In that design, there is a choice of whether to introduce extensible unions as a separate concept or whether to replace all non-extensible unions with extensible unions.
This choice boils down to making a value judgment weighing the performance cost of imposing flexibility on all clients of union against the complexity cost of having two largely overlapping constructs (e.g., imposing cognative load on protocol designers to pick the right construct for their use case). In this case, we analyzed the clients of unions and decided that the vast majority of them value flexibility, which means imposing the costs of flexibility upon the vast majority of union client does not cause them to pay for functionality they do not use. For the handful of uses that did not value flexibility, we consulted with the customers and agreed that the extra costs would not be burdensome.
One strategy for designing an interprocess communication system is figure out the ideal balance of all concerns up front and then implement the system. Unfortunately, the concerns involved in designing an interprocess communication system are sufficiently complex that this strategy is beyond human ability. Instead, we are pursuing a strategy by which we do as well as we can today and then iteratively refine the design to better address the needs of our customers.
Broadly speaking, there are two strategies we can use to balance the concerns of performance and flexibility: we can approach the ideal balance from either overemphasizing performance or overemphasizing flexibility.
Another way to interpret this document is as proposing that we structure the engineering program for FIDL to start by overemphasizing performance (as in the original FIDL2 design) and then approach the idea balance between performance and flexibility by adding flexibility while holding the line on performance.
In evaluating changes to FIDL, and as part of the FTP process, we expect this principle to be weighed against other design considerations. When two principles are at odds, there are a number of approaches appropriate to resolving ties: evaluate the impact with potentially affected users, look at prior art (e.g., optimization work of Protobuf, or FlatBuffers design choices), think of who needs to absorb the complexity (e.g., users, language designers, binding authors), consider whether the design puts a limit on the theoretical max performance (even if today's implementation falls short of that). Ultimately, we will need to use our judgment about how best to balance these factors.
Documentation and examples
This document proposes adding the "you only pay for what you use" principle to the list of efficiency goals for FIDL.
This principle is backwards compatible with the current FIDL design and engineering program.
This principle values performance.
This principle could potentially have a negative impact on security because satisfying the principle might result in a more complex system (e.g., that has both structs and tables).
Drawbacks, alternatives, and unknowns
One cost of this proposal is foreclosing design space that could be used to meet flexibility-critical use cases at the expense of performance-critical use cases.
Another cost is adopting this design principle will cause the FIDL system to be more complex than it would otherwise have been. For example, using tables everywhere might be simpler than using structs in some places and tables in other places. This added complexity is a burden both for the FIDL implementation and for developers who use FIDL. To mitigate this drawback, we should consider this complexity cost when applying the principle.
Another strategy for balancing the concerns of performance and flexibility would be to approach the ideal balance by overemphasizing flexibility. The difficulty with this approach is that human beings engineer systems by adding code rather than removing code (e.g., similar to sculpting in clay rather than sculpting in marble). It’s easier to add flexibility by adding code than it is to add performance by adding code.
Prior art and references
Many other languages have adopted the “you only pay for what you use” design principle, including C++  and Rust .
 B. Stroustrup. The Design and Evolution of C++. Addison Wesley, ISBN 0-201-54330-3. March 1994.  J. Orendorff, J. Blandy. Programming Rust. O'Reilly Media, ISBN 9781491927274. https://www.oreilly.com/library/view/programming-rust/9781491927274/ch01.html