Finish the tracing core APIs

[wu-sheng]

Concepts

All concepts are from the official SkyWalking definitions.

Span

Span is an important and common concept in distributed tracing system. Learn Span from Google Dapper Paper.
For better performance, we extend the span into 3 kinds.

1. EntrySpan EntrySpan represents a service provider, also the endpoint of server side. As an APM system, we are targeting the application servers. So almost all the services and MQ-consumer are EntrySpan(s).
2. LocalSpan LocalSpan represents a normal Java method, which does not relate to remote service, neither a MQ producer/consumer nor a service(e.g. HTTP service) provider/consumer.
3. ExitSpan ExitSpan represents a client of service or MQ-producer, as named as LeafSpan at early age of SkyWalking. e.g. accessing DB by JDBC, reading Redis/Memcached are cataloged an ExitSpan.

Tag and Log are similar attributes of the span.

• Tag is a key:value pair to indicate the attribute with a string value.
• Log is heavier than tag, with one timestamp and multiple key:value pairs. Log represents an event, typically an error happens.

TracingContext

TracingContext is the context of the tracing process. Span should only be created through context, and be archived into the
context after the span finished.

Injectable

Injectable is used(optional) when the exit span creates. This Injectable received the notification from tracing context,
including the key and value for tracing context across process propagation. Typically, Injectable implementation would
manipulate the RPC header/metadata to make the key/value sent to the server side.

Extractable

Extractable is used(optional) when the entry span creates. The Extractable fetches the value of the given key from the propagated
context. Typically, Extractable implementation would read the RPC header/metadata, which sent from the client side.

APIs

Tracing Core APIs

Tracing core APIs are 100% manual control tracing APIs. Users could use them to trace any process by following SkyWalking
core concepts.

let mut context = TracingContext::new(&reporter).unwrap();
let span1 = context.create_entry_span("op1", None, Some(&MockerHeader {}));
{
    assert_eq!(span1.span_id(), 0);
    let mut span2 = context.create_local_span("op2", Some(&span1));
    span2.tag(Tag::new(String::from("tag1"), String::from("value1")));
    {
        assert_eq!(span2.span_id(), 1);
        let mut span3 = context.create_exit_span("op3", Some(&span2), "127.0.0.1:8080", Some(&HeaderCarrier {}));
        assert_eq!(span3.span_id(), 2);

        context.finish_span(span3);
    }
    context.finish_span(span2);
}
context.finish_span(span1);

reporter.report_trace(context);

---

Notice, report module has not finished. Report module monitors the tracing context finish event and do

1. Service and service instance register
2. Trace report

[kezhenxu94]

There're some warnings in the files changed tab :P

[wu-sheng]

There're some warnings in the files changed tab :P

Yes, that is expected, as I haven't finished the report part, many fields are never used. Rust compiler will send the warnings :)

[hanahmily]

LGTM expect for some compiling warning.

But I suggest we should keep improving below concerns:

• Now span is allocated in the heap instead of the stack, that is not common practice because of the overhead of heap allocation.
• About to close span, users should take care of closing span when they want to return workflow. It might bring potential bugs. Let's provide a closure method to wrap users' logic block into it, that's useful for Marco especially.

[wu-sheng]

Now span is allocated in the heap instead of the stack, that is not common practice because of the overhead of heap allocation.

How do you expect the span is in the stack? Could you provide some examples? The span is unsized because of tags and logs.

About to close span, users should take care of closing span when they want to return workflow. It might bring potential bugs. Let's provide a closure method to wrap users' logic block into it, that's useful for Marco especially.

Do you have an example? I think this could be optional, as a manual API core, I will keep the API as directly as possible without the unnecessary limits. This thing should be more suitable in skywalking-agent . WDYT?