Runtime verification (RV) monitors program executions for conformance with formal specifications (specs). This paper concerns Monitoring-Oriented Programming (MOP), the only RV approach shown to scale to thousands of open-source GitHub projects when simultaneously monitoring passing unit tests against dozens of specs. Explicitly storing traces—sequences of spec-related program events—can make it easier to debug spec violations or to monitor tests against hyperproperties, which requires reasoning about sets of traces. But, most online MOP algorithms are implicit trace, i.e. they work event by event to avoid the time and space costs of storing traces. Yet, TraceMOP, the only explicit-trace online MOP algorithm, is often too slow and often fails.

We propose LazyMOP, a faster explicit-trace online MOP algorithm for RV of tests that is enabled by three simple optimizations. First, whereas all existing online MOP algorithms eagerly monitor all events as they occur, LazyMOP lazily stores only unique traces at runtime and monitors them just before the test run ends. Lazy monitoring is inspired by a recent finding: 99.87% of traces during RV of tests are duplicates. Second, to speed up trace storage, LazyMOP encodes events and their locations as integers, and amortizes the cost of looking up locations across events. Lastly, LazyMOP only synchronizes accesses to its trace store after detecting multi-threading, unlike TraceMOP’s eager and wasteful synchronization of all accesses.

On 179 Java open-source projects, LazyMOP is up to 4.9x faster and uses 4.8x less memory than TraceMOP, finding the same traces (modulo test non-determinism) and violations. We show LazyMOP’s usefulness in the context of software evolution, where tests are re-run after each code change. LazyMOP$^𝑒$ optimizes LazyMOP in this context by generating fewer duplicate traces. Using unique traces from one code version, LazyMOP$^𝑒$ finds all pairs of method 𝑚 and spec 𝑠, where all traces for 𝑠 in 𝑚 are identical. Then, in a future version, LazyMOP$^𝑒$ generates and monitors only one trace of 𝑠 in 𝑚. LazyMOP$^𝑒$ is up to 3.9x faster than LazyMOP and it speeds up two recent techniques that speed up RV during evolution by up to 4.6x with no loss in violations.