Contributed Articles: “Modern Debugging: The Art of Finding a Needle in a Haystack”
Communications of the ACM, November 2018, Vol. 61 No. 11, Pages 124-134
By Diomidis Spinellis
The computing pioneer Maurice Wilkes famously described his 1949 encounter with debugging like this: “As soon as we started programming, […] we found to our surprise that it wasn’t as easy to get programs right as we had thought it would be. […] Debugging had to be discovered. I can remember the exact instant […] when I realized that a large part of my life from then on was going to be spent in finding mistakes in my own programs.”
Seven decades later, modern computers are approximately one million times faster and also have one million times more memory than Wilkes’s Electronic Delay Storage Automatic Calculator, or EDSAC, an early stored-program computer using mercury delay lines. However, in terms of bugs and debugging not much has changed. As developers, we still regularly make mistakes in our programs and spend a large part of our development effort trying to fix them.
Moreover, nowadays, failures can occur nondeterministically in nanosecond time spans within computer systems consisting of thousands of processors spanning the entire planet running software code where size is measured in millions of lines. Failures can also be frighteningly expensive, costing human lives, bringing down entire industries, and destroying valuable property.22 Thankfully, debugging technology has advanced over the years, allowing software developers to pinpoint and fix faults in ever more complex systems.
On the Shoulders of Colleagues
The productivity boost I get as a developer by using the Web is such that I now rarely write code when I lack Internet access. In debugging, the most useful sources of help are Web search, specialized Q&A sites, and source-code repositories. Keep in mind that the terms of a programmer’s work contract might prohibit some of these help options.
Tuning the Software-Development Process
Some elements of a team’s software-development process can be instrumental in preventing and pinpointing bugs. Those I find particularly effective include implementing unit tests, adopting static and dynamic analysis, and setting up continuous integration to tie all these aspects of software development together. Strictly speaking, these techniques aim for bug detection rather than debugging or preventing bugs before they occur, rather than the location of a failure’s root cause. However, in many difficult cases (such as nondeterministic failures and memory corruption), a programmer can apply them as an aid for locating a specific bug. Even if an organization’s software development process does not follow these guidelines, they can be adopted progressively as the programmer hunts bugs.
Making the Software Easier to Debug
Some simple software design and programming practices can make software easier to debug, by providing or configuring debugging functionality, logging and receiving debug data, and using high(er)-level languages. Again, a programmer can selectively adopt these practices during challenging bug-hunting expeditions.
Insights from Data Analytics
Data is the lifeblood of debugging. The more data that is associated with a failure, the easier it is to find the corresponding fault. Fortunately, nowadays, practically limitless secondary storage, ample main memory, fast processors, and broadband end-to-end network connections make it easy to collect and process large volumes of debugging data. The data can come from the development process (such as from revision-control systems and integrated development environments, or IDEs), as well as from program profiling. The data can be analyzed with specialized tools, an editor, command-line tools, or small scripts.
Getting More from a Debugger
Given the propensity of software to attract and generate bugs, it is hardly surprising that the capabilities of debuggers are constantly evolving.
Debugging Distributed Systems
Modern computing rarely involves an isolated process running on a system that matches a programmer’s particular development environment. In many cases, the programmer is dealing with tens to thousands of processes, often distributed around the world and with diverse hardware ranging from resource-constrained Internet of Things (IoT) devices, to smartphones, to experimental and specialized platforms. While these systems are the fuel powering the modern economy, they also present programmers with special challenges. According to the insightful analysis by Ivan Beschastnikh and colleagues at the University of British Columbia, these are heterogeneity, concurrency, distributing state, and partial failures.4 Moreover, following my own experience, add the likely occurrence of events that would be very rare on an isolated machine, the difficulty of correlating logs across several hosts,2 and replicating failures in the programmer’s development environment.