Observability is the ability to measure the internal states of a system by examining its outputs. A good observability system allows to easily identify problem source and troubleshoot them.

Observability extends the concept of monitoring by not just telling when something goes wrong but also providing information about why and how.

OpenTelemetry is an open standard for telemetry collection that was created as a successor to previously existing standards OpenTracing and Opencensus by Cloud Native Computing Foundation (CNCF).

Telemetry Signals

Signals comprise of various system and application monitoring information that we collect as part of our telemetry data. A signal can be a value or a event that you want to be observable.

We can group different signals together to get idea about the health of our system at any particular time. Opentelemetry specification constitutes of the following signals:

• Trace
• Metric
• Log
• Baggage

Traces

A distributed trace or just trace is a representation of a series of causally related events that encode the end-to-end request flow through a distributed system. A distributed trace contains events that cross process, network and security boundaries.

In particular, a Trace can be thought of as a Directed Acyclic Graph (DAG) of Spans.

Span

Spans represent a single logical operation within a trace. For example, a function call during a user request can be represented by a span.

A span object primarily consists of information about time duration, structured logs/events, attributes, parent span (optional), trace id, etc. We can also create casual relationships between spans across different trace using Links.

To create a span we need a tracer which is globally created in your application.

let span = tracer
	.span_builder("Greeter/client")
	.with_kind(SpanKind::Client)
	.with_attributes([KeyValue::new("component", "grpc")])
	.start(&tracer);

We can create four kinds of spans primarily:

• Client: A synchronous outgoing request to another service, e.g. HTTP call, database query, etc.
• Server: An incoming request that needs to be processed synchronously, e.g. HTTP request, RPC call, etc.
• Internal: Anything that doesn’t leave a particular service during operation, e.g. processing data, reading from a file, etc.
• Producer: It can be used for representing creation of a job for asynchronous processing later. The job created can be within or outside the service boundary.
• Consumer: Handling of an asynchronous job which was created by a producer.

A span can have an associated status after execution of operation it can be Ok, Error, or Unset. A span can also hold additional information about it’s operation in the attributes.

"attributes": {
    "net.transport": "IP.TCP",
    "net.peer.ip": "172.17.0.1",
    "net.peer.port": "51820"
}

Span Events are structured log message on a span. They denote a meaningful event that happened at a particular time in the duration of span execution.

"events": [
	{
	  "name": "",
	  "message": "we are done here",
	  "timestamp": "2021-10-22 16:04:01.209512872 +0000 UTC"
	}
]

Span Analysis

There are some common patters in spans to look out for which provide information related to anomalies/shortcomings in your application.

• Span taking to long to complete are cause of latency.
• Spans finishing at the same time can be a sign of timeouts or locks held too long.
• Gap Between spans shows un-instrumented code which can use a better coverage.
• Temporal Coverage is the measure of trace time covered by leaf spans which tells about granularity of coverage.

Metrics

A metric is the measurement of system’s working that provides outlook on its health. A metric event is generated at the time of metric collection consisting of time at which it happened and associated metadata.

Apart from general metrics like Memory consumption, CPU utilization, etc we can instrument our application with custom metrics using Meter Provider, Meter, Metric Instrument and Metric Exporter.

// Create a Histogram Instrument.
let histogram = meter
	.f64_histogram("my_histogram")
	.with_description("My histogram example description")
	.init();

// Record measurements using the histogram with attributes
histogram.record(
	10.5,
	&[
		KeyValue::new("mykey1", "myvalue1"),
		KeyValue::new("mykey2", "myvalue2"),
	],
);

A metric instrument is created with a name, unit, kind and description. Otel supports different kinds of metric instruments with each having a default form of aggregation (combining large number of measurements):

• Counter (Monotonically Increasing) & Asynchronous Counter (Async)
• UpDownCounter (Integral Summation) & Asynchronous UpDownCounter (Async)
• Gauge (Value at Time)
• Histogram (Statistical Measurement)

Logs

A log is an immutable, timestamped record of discrete events that happened over time. It can be in structured or unstructured (plain text) format.

[2021-02-23T13:26:23.505892] INFO rich_obsevability.printer : [6459ffe1-ea53-4044-aaa3-bf902868f730] Started GET "/" for ::1 now processing.

Logs are very easy to generate and can be represented in whatever format we want providing flexibility. The logging libraries have very small memory footprint and are extremely fast.

info!("i responded to the message");
error!("I am not working right now!");

With tracing and metrics otel defined a new standard but for logs given the large landscape of legacy systems it acts as bridge. The raw logs are attached to the executing trace and span with added information about time, severity, resource, etc finally emitting a LogRecord.

Baggage

Baggage is a key value store used to add additional contextual information about the operation and service for access to downstream paths.

To add baggage entries to attributes, you need to explicitly read the data from baggage and add it as attributes to your spans, metrics, or logs.

Instrumentation

We have to instrument our code using the signals described above to make it observable. There are primarily two ways:

• Code based solution
• Zero code solution

Zero code solution provide rich telemetry from libraries you use and/or the environment your application runs in. This means that requests and responses, database calls, message queue calls, and so forth are what gets instrumented. Don’t you love to work a little less :P

Code based solution allow you to generate deeper more custom insights and rich telemetry from your application. We can use instrumentation libraries which provide us easy to use macros which generate spans for us.

#[instrument(
    name = "handle_init_request",
    skip(init_handler, request),
    fields(message_name),
    err(Debug)
)]
fn handle_init_request(
    init_handler: &mut InitHandler,
    request: SignerRequest,
    request_id: u64,
) -> StdResult<(bool, Option<SignerResponse>), Error> {
    let msg = msgs::from_vec(request.message)?;
    Span::current().record("message_name", msg.inner().name());

Conclusion

To develop a highly scalable and distributed system its not just good enought to write good code and tests but also important to ensure monitoring and observability. The system however good is fallible due to its interaction with other components, hardware, user, etc.

At any point of time your system should be able to answer…

Thanks for reading as always expect more stuff coming in on opentelemetry as its peaked my interest recently.