UML Sequence Diagram

Show a UML sequence diagram of the calls a selected function, method, procedure, or task makes, in the order they occur.

Each column is an actor — the class, file, or other scope a called entity belongs to — with a vertical lifeline running down it. A solid arrow is a call, landing on an activation box on the callee's lifeline; a dashed arrow returning to the caller is labeled with the callee's return type. An if, switch, or similar conditional is drawn as an alt combined fragment labeled with its condition, with each branch as its own section inside; a for, while, or do while loop is drawn the same way as a loop fragment labeled with its header. These fragments can nest to any depth, following the actual control flow of the function.

Rooted at handle below, the leftmost column is sequence.cpp (the file handle is defined in, since it isn't itself a member of a class), followed by a column for each struct it calls into: Logger, Metrics, Cache, and Store. The calls to log.info and metrics.record happen unconditionally; cache.has(id) is both the condition of the first alt fragment and, since evaluating it is itself a call, the first message shown inside it, followed by metrics.record and cache.get when it's true. store.fetch happens next, followed by a second alt fragment for val < 0 (log.error and metrics.record), and a loop fragment for the retry loop, which calls store.save and checks cache.has(id) again in a nested alt before breaking. Finally, log.info is called once more after the loop.

See the following code and corresponding graph

struct Cache   { bool has(int id); int  get(int id); };
struct Store   { int  fetch(int id); void save(int id, int val); };
struct Logger {
  void info(const char* msg);
  void error(const char* msg);
};
struct Metrics { void record(const char* key, int val); };

int handle(int id, Cache& cache, Store& store,
           Logger& log, Metrics& metrics) {
    log.info("handling");
    metrics.record("requests", 1);

    if (cache.has(id)) {
        metrics.record("hits", 1);
        return cache.get(id);
    }

    int val = store.fetch(id);
    if (val < 0) {
        log.error("fetch failed");
        metrics.record("errors", 1);
        return -1;
    }

    for (int retry = 0; retry < 3; ++retry) {
        store.save(id, val);
        if (cache.has(id))
            break;
    }

    log.info("done");
    return val;
}

UML Sequence Diagram - Classic

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UML Sequence Diagram - Compare

The Compare variant shows how the graph has changed relative to the project's comparison database.

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