# Payment Service

This service is responsible to process credit card payments for orders. It will
return an error if the credit card is invalid or the payment cannot be
processed.

[Payment service source](https://github.com/open-telemetry/opentelemetry-demo/blob/main/src/payment/)

## Initializing OpenTelemetry

It is recommended to `require` Node.js app using an initializer file that
initializes the SDK and auto-instrumentation. When initializing the
OpenTelemetry Node.js SDK in that module, you optionally specify which
auto-instrumentation libraries to leverage, or make use of the
`getNodeAutoInstrumentations()` function which includes most popular frameworks.
The below example of an initializer file (`opentelemetry.js`) contains all code
required to initialize the SDK and auto-instrumentation based on standard
OpenTelemetry environment variables for OTLP export, resource attributes, and
service name. It then `require`s your app at `./index.js` to start it up once
the SDK is initialized.

```javascript
const opentelemetry = require('@opentelemetry/sdk-node');
const {
  getNodeAutoInstrumentations,
} = require('@opentelemetry/auto-instrumentations-node');
const {
  OTLPTraceExporter,
} = require('@opentelemetry/exporter-trace-otlp-grpc');
const {
  OTLPMetricExporter,
} = require('@opentelemetry/exporter-metrics-otlp-grpc');
const { PeriodicExportingMetricReader } = require('@opentelemetry/sdk-metrics');
const {
  alibabaCloudEcsDetector,
} = require('@opentelemetry/resource-detector-alibaba-cloud');
const {
  awsEc2Detector,
  awsEksDetector,
} = require('@opentelemetry/resource-detector-aws');
const {
  containerDetector,
} = require('@opentelemetry/resource-detector-container');
const { gcpDetector } = require('@opentelemetry/resource-detector-gcp');
const {
  envDetector,
  hostDetector,
  osDetector,
  processDetector,
} = require('@opentelemetry/resources');

const sdk = new opentelemetry.NodeSDK({
  traceExporter: new OTLPTraceExporter(),
  instrumentations: [
    getNodeAutoInstrumentations({
      // only instrument fs if it is part of another trace
      '@opentelemetry/instrumentation-fs': {
        requireParentSpan: true,
      },
    }),
  ],
  metricReader: new PeriodicExportingMetricReader({
    exporter: new OTLPMetricExporter(),
  }),
  resourceDetectors: [
    containerDetector,
    envDetector,
    hostDetector,
    osDetector,
    processDetector,
    alibabaCloudEcsDetector,
    awsEksDetector,
    awsEc2Detector,
    gcpDetector,
  ],
});

sdk.start();
```

You can then use `opentelemetry.js` to start your app. This can be done in the
`ENTRYPOINT` command for the service's `Dockerfile`.

```dockerfile
ENTRYPOINT [ "node", "--require", "./opentelemetry.js", "./index.js" ]
```

## Traces

### Add attributes to auto-instrumented spans

Within the execution of auto-instrumented code you can get current span from
context.

```javascript
const span = opentelemetry.trace.getActiveSpan();
```

Adding attributes to a span is accomplished using `setAttributes` on the span
object. In the `chargeServiceHandler` function an attributes is added to the
span as an anonymous object (map) for the attribute key/values pair.

```javascript
span.setAttributes({
  'app.payment.amount': parseFloat(`${amount.units}.${amount.nanos}`),
});
```

### Span Exceptions and status

You can use the span object's `recordException` function to create a span event
with the full stack trace of a handled error. When recording an exception also
be sure to set the span's status accordingly. You can see this in the
`chargeServiceHandler` function

```javascript
span.recordException(err);
span.setStatus({ code: opentelemetry.SpanStatusCode.ERROR });
```

## Metrics

### Creating Meters and Instruments

Meters can be created using the `@opentelemetry/api-metrics` package. You can
create meters as seen below, and then use the created meter to create
instruments.

```javascript
const { metrics } = require('@opentelemetry/api-metrics');

const meter = metrics.getMeter('payment');
const transactionsCounter = meter.createCounter('app.payment.transactions');
```

Meters and Instruments are supposed to stick around. This means you should get a
Meter or an Instrument once , and then re-use it as needed, if possible.

## Logs

TBD

## Baggage

OpenTelemetry Baggage is leveraged in this service to check if the request is
synthetic (from the load generator). Synthetic requests will not be charged,
which is indicated with a span attribute. The `charge.js` file which does the
actual payment processing, has logic to check the baggage.

```javascript
// check baggage for synthetic_request=true, and add charged attribute accordingly
const baggage = propagation.getBaggage(context.active());
if (
  baggage &&
  baggage.getEntry('synthetic_request') &&
  baggage.getEntry('synthetic_request').value == 'true'
) {
  span.setAttribute('app.payment.charged', false);
} else {
  span.setAttribute('app.payment.charged', true);
}
```