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Care and Feeding of Your New Tracing Library

Congratulations! You are now the proud owner of a distributed tracing library.

The primary purpose of this guide is to describe salient features of the library's design. dd-trace-cpp differs considerably from its older sibling and peers.

This guide will also cover operations performed by maintainers of the library, such as scooping the box, applying flea medication, and regular trips to the vet.

Design

Span

class Span is the component with which users will interact the most. Each span:

  • has an "ID,"
  • is associated with a "trace ID,"
  • is associated with a "service," which has a "service type," a "version," and an "environment,"
  • has a "name" (sometimes called the "operation name"),
  • has a "resource name," which is a description of the thing that the span is about,
  • contains information about whether an error occurred during the represented operation, including an error message, error type, and stack trace,
  • includes an arbitrary name/value mapping of strings, called "tags,"
  • has a start time indicating when the represented operation began,
  • has a duration indicating how long the represented operation took to finish.

Aside from setting and retrieving its attributes, Span also has the following operations:

  • parent.create_child(...) returns a new Span that is a child of parent.
  • span.inject(writer) writes trace propagation information to a DictWriter, which is an interface for setting a name/value mapping, e.g. in HTTP request headers.

A Span does not own its data. class Span contains a raw pointer to a class SpanData, which contains the actual attributes of the span. The SpanData is owned by a TraceSegment, which is described in the next section. The Span holds a shared_ptr to its TraceSegment.

By default, a span's start time is when it is created, and its end time (from which its duration is calculated) is when it is destroyed. However, a span's start time can be specified when it is created, via SpanConfig::start (see span_config.h), and a span's end time can be overridden via Span::set_end_time.

When a span is destroyed, it is considered "finished" and notifies its TraceSegment. There is no way to "finish" a span without destroying it. You can override its end time throughout the lifetime of the Span object, but a TraceSegment does not consider the span finished until the Span object is destroyed. This allows us to avoid "finished" Span states.

Along similar lines, class Span is move-only. Its copy constructor is deleted. Functions that produce spans return them by value, but only one copy of a span can exist at a time. In fact, class Span is even more strict than move-only: its assignment operator is deleted, including the move-assignment operator. To see why, consider the following (disallowed) example:

Span span = tracer.create_span();
// ...
// Let's reuse the variable `span`.
span = tracer.create_span();

Move assignment begins with two objects and ends up with one object (and one empty shell of an object).

Since destroying a Span has the side effect of finishing it, one sensible definition of Span::operator=(Span&& other) would be equivalent to:

this->~Span();
new (this) Span(std::move(other));
return *this;

This would have the potentially surprising feature of finishing the first span when you wish to replace it with another, i.e. there would always be two spans.

This could be avoided if we could guarantee that the two Spans belong to the same TraceSegment. Then move-assigning a Span could be defined as move-assigning its SpanData and somehow annotating the moved-from SpanData as being invalid. However, if the two Spans belong to different TraceSegments, then it could be that the moved-to Span's TraceSegment consists of only that one Span. Now we have to account for empty TraceSegment states. This could all be dealt with, but no matter what we decide, it would always be the case that Span::operator=(Span&&) has the effect of making the original span (this) either finish implicitly or disappear entirely, which is at odds with its otherwise RAII nature.

To avoid these issues, assignment to Span objects is disallowed.

Another opinionated property of Span is that it is not an interface, nor does it implement an interface. Usually it is considered polite for a C++ library to deal in handles (unique_ptr or shared_ptr) to interfaces, i.e. classes that contain pure virtual functions. This way, a client of the library can substitute an alternative implementation to the interface(s) for testing or for when the behavior of the library is not desired.

At the risk of being impolite, dd-trace-cpp takes a different approach. Span is a concrete type whose behavior cannot be substituted. Instead, there are other places in the library where dependency injection can be used to restrict or alter the behavior of the library. The trade-off is that Span and related components must always "go through the motions" of their definitions and cannot be completely customized, but in exchange the indirection, pointer semantics, and null states that accompany handle-to-interface are avoided.

Trace Segment

A "trace" is the entire tree of spans having the same trace ID.

Within one process/worker/service, though, typically there is not an entire trace but only part of the trace. Let's call the process/worker/service a "tracer."

One portion of a trace that's passing through the tracer is called a "trace segment." A trace segment begins either at the trace's root span or at a span extracted from trace context, e.g. a span created from the X-Datadog-Trace-Id and X-Datadog-Parent-Id HTTP request headers. The trace segment includes all local descendants of that span, and has as its "boundary" any descendant spans without children or descendant spans that were used to inject trace context out-of-tracer, e.g. in outgoing HTTP request headers.

There might be more than one trace segment for the same trace within a tracer at the same time. Consider the diagram below.

flame graph

If our tracer is "service X," then this trace passes through the tracer twice. We would have two concurrent trace segments for the same trace.

class TraceSegment is defined in trace_segment.h. TraceSegment objects are managed internally by the library. That is to say, a user never creates a TraceSegment.

The library creates a TraceSegment whenever a new trace is created or when trace context is extracted. This is the job of class Tracer, described in the next section.

Primarily, TraceSegment is a bag of spans. It contains a vector<unique_ptr<SpanData>>. Span objects then refer to the SpanData objects via raw pointers. Now that I think about it, deque<SpanData> would work just as well.

When one of a trace segment's spans creates a child, the child is registered with the trace segment. When a span is finished, the trace segment is notified. The trace segment keeps track of how many spans it contains (the size of its vector) and how many spans are finished. When the two numbers are equal, the trace segment is finished.

When a trace segment is finished, it performs some finalization logic in order to prepare its spans for submission to the Collector. Then it moves its spans into the collector via Collector::send, and a short time later the trace segment is destroyed. See TraceSegment::span_finished in trace_segment.cpp. Collector is described in a subsequent section.

A TraceSegment contains shared_ptrs to everything that it needs in order to do its job. Those objects are created by class Tracer when the tracer is configured, and then shared with TraceSegment when the TraceSegment is created.

Tracer

class Tracer is what users configure, and it is how Spans are extracted from trace context or created as a trace's root. See tracer.h.

Tracer has two member functions:

  • create_span(...)
  • extract_span(...)

and another that combines them:

  • extract_or_create_span(...).

All of these result in the creation of a new TraceSegment (or otherwise return an error). The Tracer's data members, which were initialized based on the tracer's configuration, are copied into the TraceSegment so that the TraceSegment can operate independently.

Note how create_span never fails. This is a nice property. extract_span can fail.

The bulk of Tracer's implementation is extract_span. The other substantial work is configuration, which is handled by finalize_config(const TracerConfig&), declared in tracer_config.h. Configuration will be described in more depth in a subsequent section.

Collector

class Collector is an interface for sending a TraceSegment's spans somewhere once they're all done. It's defined in collector.h.

It's just one function: send. More of a callback than an interface.

A Collector is either created by Tracer or injected into its configuration. The Collector instance is then shared with all TraceSegments created by the Tracer. The only thing that a TraceSegment does with the Collector is call send once the segment is finished.

The default implementation is DatadogAgent, which is described in the next section.

DatadogAgent

class DatadogAgent is the default implementation of Collector. It's defined in datadog_agent.h.

DatadogAgent sends trace segments to the Datadog Agent in batches that are flushed periodically. In order to do this, DatadogAgent needs a means to make HTTP requests and a means to set a timer for the flush operation. So, there are two interfaces: HTTPClient and EventScheduler.

The HTTPClient and EventScheduler can be injected as part of DatadogAgent's configuration, which is usually specified via the agent member of Tracer's configuration. If they're not specified, then default implementations are used:

DatadogAgent::flush is periodically called by the event scheduler. flush uses the HTTP client to send a POST request to the Datadog Agent's /v0.4/traces endpoint. It's all callback-based.

HTTPClient

class HTTPClient is an interface for sending HTTP requests. It's defined in http_client.h.

The only kind of HTTP request that the library needs to make, currently, is a POST to the Datadog Agent's traces endpoint. HTTPClient has one member function for each HTTP method needed — so, currently just the one:

virtual Expected<void> post(const URL& url, HeadersSetter set_headers,
                            std::string body, ResponseHandler on_response,
                            ErrorHandler on_error) = 0;

It's callback-based. post returns almost immediately. It invokes set_headers before returning, in order to get the HTTP request headers. The request body is moved elsewhere for later processing. One of on_response or on_error will eventually be called, depending on whether a response was received or if an error occurred before a response was received. If something goes wrong setting up the request, then post returns an error. If post returns an error, then neither of on_response nor on_error will be called.

HTTPClient also has another member function:

virtual void drain(std::chrono::steady_clock::time_point deadline) = 0;

drain waits for any in-flight requests to finish, blocking up until no later than deadline. It's used to ensure "clean shutdown." Without it, on average the last one second of traces would be lost on shutdown. Implementations of HTTPClient that don't have a dedicated thread need not support drain; in those cases, drain returns immediately.

The default implementation of HTTPClient is class Curl : public HTTPClient, which uses libcurl's multi interface together with a dedicated thread as an event loop.

class Curl is also used within NGINX in Datadog's NGINX module, nginx-datadog. This is explicitly discouraged in NGINX's developer documentation, but libcurl-with-a-thread is widely used within NGINX modules regardless. One improvement that I am exploring is to use libcurl's "multi_socket" mode, which allows libcurl to utilize someone else's event loop, obviating the need for another thread. libcurl can then be made to use NGINX's event loop, as is done in an example library.

For now, though, nginx-datadog uses the threaded class Curl.

Envoy's Datadog tracing integration uses a different implementation, class AgentHTTPClient : public HTTPClient, ..., which uses Envoy's built-in HTTP facilities. libcurl is not involved at all.

EventScheduler

As of this writing, class DatadogAgent flushes batches of finished trace segments to the Datadog Agent once every two second by default.

It does this by scheduling a recurring event with an EventScheduler, which is an interface defined in event_scheduler.h.

EventScheduler has one member function:

virtual Cancel schedule_recurring_event(
    std::chrono::steady_clock::duration interval,
    std::function<void()> callback) = 0;

Every interval, the scheduler will invoke callback, starting an initial interval after schedule_recurring_event is called. The caller can invoke the returned Cancel to prevent subsequent invocations of callback.

The default implementation of EventScheduler is class ThreadedEventScheduler : public EventScheduler, which uses a dedicated thread for executing scheduled events at the correct time. It was a fun piece of code to write.

Datadog's NGINX module, nginx-datadog uses a different implementation, class NgxEventScheduler : public EventScheduler, which uses NGINX's own event loop instead of a dedicated thread.

Envoy's Datadog tracing integration also uses a different implementation, class EventScheduler : public EventScheduler, which uses Envoy's built-in event dispatch facilities.

Configuration

There's a good blog post by Alexis King where she makes the case for encoding configuration validation into the type system. Forbid invalid states by making configurable components accept a different type than that which is used to specify configuration.

This is not a new idea. It's been used, for example, to "taint" strings that originate as program inputs. Then you can't accidentally pass user-influenced inputs to, say, std::system, because std::system takes a const char*, not a class UserTaintedString. There's still ample opportunity to cast away the taint and sneak it into some string building operation, but at least class UserTaintedString gives hope that a static analysis tool could be used to fill in some gaps in human code review.

This library adopts that approach for configuration. The configuration of class Tracer is class TracerConfig, but in order to construct a Tracer you must first convert the TracerConfig into a FinalizedTracerConfig by calling finalize_config. If there is anything wrong with the TracerConfig or with environment variables that would override it, finalize_config will return an Error instead of a FinalizedTracerConfig. In that case, you can't create a Tracer at all.

This technique applies to multiple components:

Component Unvalidated Validated Parser
Tracer TracerConfig FinalizedTracerConfig finalize_config in tracer_config.h
DatadogAgent DatadogAgentConfig FinalizedDatadogAgentConfig finalize_config in datadog_agent_config.h
TraceSampler TraceSamplerConfig FinalizedTraceSamplerConfig finalize_config in trace_sampler_config.h
SpanSampler SpanSamplerConfig FinalizedSpanSamplerConfig finalize_config in span_sampler_config.h
multiple double Rate Rate::from in rate.h

An alternative approach, that Caleb espouses, is to accept invalid configuration quietly. When invalid configuration is detected, the library could substitute a reasonable default and then send notice of the configuration issue to Datadog, e.g. as a hidden span tag. That information would then be available to Support should the customer raise an issue due to a difference in behavior between what they see and what they think they configured. This approach is also resilient to dynamic configuration changes. Rather than a "bad config update" causing tracing to cease completely, instead tracing could continue to operate in the defaulted mode.

This library uses the stricter approach. The downside is that a user of the library has to decide what to do when even the slightest part of the configuration or environment is deemed invalid.

One other convention of the library is that FinalizedFooConfig (for some Foo) is never a data member of the configured component class. That is, FinalizedTracerConfig is not stored in Tracer. Instead, a constructor might individually copy the finalized config's data members. This is to prevent eventual intermixing between the "configuration representation" and the "runtime representation." In part, finalize_config already mitigates the problem. Abstaining from storing the finalized config as a data member is a step further.

Error Handling

Most error scenarios within this library are individually enumerated by enum Error::Code, defined in error.h.

As of this writing, some of the error codes are emitted in more than one place, but most of them are emitted in only one place, as is illustrated by the following incomprehensible command shell pipeline:

david@ein:~/src/dd-trace-cpp/src/datadog$ sed -n 's/^\s*\(\w\+\)\s*=\s*[0-9]\+,\?\s*$/{Error::\1/p' error.h | paste -d '|' -s | xargs git grep -P | sed -n 's/.*Error::\(\w\+\),.*/\1/p' | sort | uniq -c | sort -rn
      3 MESSAGEPACK_ENCODE_FAILURE
      2 MAX_PER_SECOND_OUT_OF_RANGE
      2 INVALID_INTEGER
      2 INVALID_DOUBLE
      2 CURL_REQUEST_SETUP_FAILED
      2 CURL_HTTP_CLIENT_ERROR
      1 ZERO_TRACE_ID
      1 URL_UNSUPPORTED_SCHEME
      1 URL_UNIX_DOMAIN_SOCKET_PATH_NOT_ABSOLUTE
      1 URL_MISSING_SEPARATOR
      1 UNKNOWN_PROPAGATION_STYLE
      1 TRACE_SAMPLING_RULES_WRONG_TYPE
      1 TRACE_SAMPLING_RULES_UNKNOWN_PROPERTY
      1 TRACE_SAMPLING_RULES_SAMPLE_RATE_WRONG_TYPE
      1 TRACE_SAMPLING_RULES_INVALID_JSON
      1 TAG_MISSING_SEPARATOR
      1 SPAN_SAMPLING_RULES_WRONG_TYPE
      1 SPAN_SAMPLING_RULES_UNKNOWN_PROPERTY
      1 SPAN_SAMPLING_RULES_SAMPLE_RATE_WRONG_TYPE
      1 SPAN_SAMPLING_RULES_MAX_PER_SECOND_WRONG_TYPE
      1 SPAN_SAMPLING_RULES_INVALID_JSON
      1 SPAN_SAMPLING_RULES_FILE_IO
      1 SERVICE_NAME_REQUIRED
      1 RULE_WRONG_TYPE
      1 RULE_TAG_WRONG_TYPE
      1 RULE_PROPERTY_WRONG_TYPE
      1 RATE_OUT_OF_RANGE
      1 OUT_OF_RANGE_INTEGER
      1 NO_SPAN_TO_EXTRACT
      1 MISSING_TRACE_ID
      1 MISSING_SPAN_INJECTION_STYLE
      1 MISSING_SPAN_EXTRACTION_STYLE
      1 MISSING_PARENT_SPAN_ID
      1 MALFORMED_TRACE_TAGS
      1 DUPLICATE_PROPAGATION_STYLE
      1 DATADOG_AGENT_NULL_HTTP_CLIENT
      1 DATADOG_AGENT_INVALID_FLUSH_INTERVAL
      1 CURL_REQUEST_FAILURE
      1 CURL_HTTP_CLIENT_SETUP_FAILED
      1 CURL_HTTP_CLIENT_NOT_RUNNING

The integer values of the enumerated Error::Codes are intended to be permanent. Given an error message from a log mentioning an error code 24, one can look at the source code for the matching Error::Code (TAG_MISSING_SEPARATOR) and then find all of the possible source origins for that error. At first this seems like a poor substitute for including the source file and line in the error message, but I suspect that this compromise is better.

In addition to an error code, each error condition is associated with a contextual diagnostic message. The diagnostic is not only a description of the error, but also contains runtime context that might help a user or a maintainer identify the underlying issue. For example, a message for TAG_MISSING_SEPARATOR might include the text that was being parsed, or mention the name of the relevant environment variable.

The Error::Code code and std::string message are combined in a struct Error, which is the error type used by this library.

struct Error has a convenience member function, Error with_prefix(StringView) const, that allows context to be added to an error. It's analogous to catching one exception and then throwing another, e.g.

Expected<Cat> adopt_cat(AnimalShelter& shelter) {
  Expected<Cat> result = shelter.adopt(Animals::CAT, 1);
  if (Error *error = result.if_error()) {
    return error->with_prefix("While trying to adopt from AnimalShelter: ");
  }
  return result;
}

struct Error is most often used in conjunction with the Expected class template, which is defined in expected.h.

template <typename T> class Expected is either a T or an Error. It is a wrapper around std::variant<T, Error>. It's inspired by C++23's std::expected, but the two are not compatible.

Functions in this library that intend to return a "Value," but that might instead fail with an Error, return an Expected<Value>.

This library never reports errors by throwing an exception. However, the library will allow exceptions, such as std::bad_alloc and std::bad_variant_access, to pass through it, and does sometimes use exceptions internally. The intention is that a client of this library does not need to write catch. An alternative design would be to use exceptions for their intended purpose. We decided that, for a tracing library embedded in proxies, the ergonomics of error values are a better fit than exceptions.

Expected supports two syntaxes of use. The first is std::get_if-style unpacking with the if_error member function:

Expected<Salad> lunch = go_buy_lunch();
if (Error *error = lunch.if_error()) {
  return error->with_prefix("Probably getting a hotdog instead: ");
}
eat_salad(*lunch); // or, lunch.value()

if_error returns a pointer to the Error if there is one, otherwise it returns nullptr. The body of a conditional statement such as if or while is allowed to contain a variable definition. If the conditional statement contains a variable definition, then the truth value of the resulting variable is used as the condition. So, if there is not an error, then if_error returns nullptr, and so the declared Error* is nullptr, which is false for the purposes of conditionals, and so in that case the body (block) of the conditional statement is skipped.

if_error cannot be called on an rvalue, because allowing this makes it far too easy to obtain a pointer to a destroyed object. I wrote the component and nonetheless made this mistake repeatedly. So, the rvalue-flavored overload of Expected::if_error is deleted.

The other syntax of use supported by Expected is the traditional check-and-accessor pattern:

Expected<Salad> lunch = go_buy_lunch();
if (!lunch) {
  return lunch.error().with_prefix("Probably getting halal instead: ");
}
eat_salad(*lunch); // or, lunch.value()

Expected defines explicit operator bool, which is true if the Expected contains a non-Error, and false if the Expected contains an Error. There is also has_value(), which returns the same thing.

Then the Error can be obtained via error(), or the non-Error can be obtained via value() or operator*().

Why bother with if_error? Because I like it! I'm a sucker for structured bindings and pattern matching.

template <typename T> class Expected has one specialization: Expected<void>. Expected<void> is "either an Error or nothing." It's used to convey the result of an operation that might fail but that doesn't yield a value when it succeeds. It behaves in the same way as Expected<T>, except that value() and operator*() are not defined.

At first I instead used std::optional<Error> directly. The problem there was that explicit operator bool has the opposite meaning as it does in Expected<T>. I wanted error handling code to be the same in the two cases, and so I specialized Expected<void>. Expected<void> is implemented in terms of std::optional<Error>, but inverts the value of explicit operator bool.

Logging

Can we write a tracing library that does not do any logging by itself? The previous section describes how errors are reported by the library, and no logging is involved there. Why not leave it up to the client to decide whether to note error conditions in the log or to proceed silently?

The problem is that the default implementations of HTTPClient (Curl) and EventScheduler (ThreadedEventScheduler) do some of their work on background threads, where error conditions may occur without having a "caller" to notify, and where execution will proceed despite the error. In those cases, should the library squelch the error?

One option is for these async components to accept an on_error(Error) callback that will be invoked whenever an error occurs on the background thread that would not otherwise be reported to client code. What would a client library do in on_error? I think that it would either do nothing or log the error. So, an on_error callback for use in background threads is the same as a logging interface.

It's tempting to omit a logging interface, leaving it to clients to decide whether and how to log. Why have logging and error reporting when you can have just error reporting?

We could do that. Here are three reasons why this library has a logging interface anyway:

  1. Logging a Tracer's configuration when it's initialized is a helpful diagnostic tool. A logging interface allows Tracer to do this explicitly, as opposed to counting on client code to log Tracer::config_json() itself. A client library can still suppress the startup message in its implementation of the logging interface, but this is more opt-out than opt-in.
  2. Along the same lines, reporting errors that occur on a background thread by invoking a logging interface allows for the library's default behavior to be to print an error message to a log, as opposed to having an on_error callback that a client library might choose to log within.
  3. A logging interface allows warnings to be logged, notifying a user of a potentially problematic, but valid, configuration. Sometimes these warnings are a matter of taste, and sometimes they are required by the specification of the feature being configured. Logging is not the only way to handle this — imagine if the return value of finalize_config included a list of warnings. But, as with the previous two points, a logging interface allows for the default behavior to be a logged message.

On the other hand, the primary clients of this library are NGINX and Envoy, where Datadog engineers have a say in how the library is used. So, the distinction is probably not so important.

The logging interface is class Logger, defined in logger.h.

The design of class Logger is informed by three constraints:

  1. Most "warn," "info," "debug," and "trace" severity logging is noise. It is more an artifact of feature development than it is a helpful event with which issues can be diagnosed. The logger should have few severity levels, or ideally only one.
  2. Logging libraries that obscure a conditional branch via the use of preprocessor macros are difficult to understand. Reverse engineering a composition of preprocessor macros is hard, and is not justified by the syntactic convenience that the macros provide.
  3. When client code decides that a message will not be logged, the library should minimize the amount of computation that is done — as close to nothing as is feasible.

(1) and (3) work well together.

On account of (1), Logger has only two "severities": "error" and "startup." log_startup is called once by a Tracer when it is initialized. log_error is called in all other logging circumstances.

On account of (3), Logger must do as little work as possible when an implementer decides that a logging function should not log. If this library were to build diagnostic messages in all cases, and then pass them to the Logger, then the cost of building the message would be paid even when the Logger decides not to log. On account of (2), the library cannot define a DD_LOG macro that hides an if (logger.severity > ...) branch. As a matter of taste, I don't want the library to be littered with such branches explicitly.

The compromise is to have Logger::log_error and Logger::log_startup accept a std::function that, if invoked, does the work of building the diagnostic message. When a Logger implementation decides not to log, the only cost paid at the call site is the construction of the std::function and the underlying capturing lambda expression with which it was likely initialized. The signature of the std::function, aliased as Logger::LogFunc, is void(std::ostream&). I don't know whether this results in a real runtime savings in the not-logging case, but it seems likely.

Logger also has two convenience overloads of log_error: one that takes a const Error& and one that takes a StringView.

The const Error& overload reveals a contradiction in the design. If an Error is produced in a context where its only destiny is to be passed to Logger::log_error(const Error&), then the cost of building Error::message will always be paid, in violation of constraint (3). One way to avoid this would be to have versions of the Error-returning operations that instead accept a Logger& and use Logger::log_error(Logger::LogFunc) directly. I think that the present state of things is acceptable and that such a change is not warranted.

The default implementation of Logger is CerrLogger, defined in cerr_logger.h. CerrLogger logs to std::cerr in both log_error and log_startup.

The Logger used by a Tracer is configured via std::shared_ptr<Logger> TracerConfig::logger, defined in tracer_config.h.

Finally, constraint (1) is still a matter of debate. Support representatives have expressed frustration with this library's, and its predecessor's, lack of a "debug mode" that logs the reason for every decision made throughout the processing of a trace. This issue deserves revisiting. Here are some potential approaches:

  • Introduce a "debug mode" that sends fine-grained internal information to Datadog, rather than to a log, either as hidden tags on the trace or via a different data channel, such as the Telemetry API.
  • Introduce a "trace the tracer" mode that generates additional traces that represent operations performed by the tracer. This technique was prototyped in the david.goffredo/traception branch.
  • Add a Logger::log_debug function that optionally prints fine-grained tracing information to the log, in violation of constraint (1).

Operations

TODO

Testing

TODO

Code Coverage

TODO

Benchmarks

TODO

Continuous Integration

TODO

Releases

TODO

Support

TODO