express-monitor

A Prometheus middleware to add basic but very useful metrics for your Express JS app.

Metrics

The only exposed metrics (for now) are the following:

request_seconds_bucket{type, status, isError, errorMessage, method, addr, le}
request_seconds_count{type, status, isError, errorMessage, method, addr}
request_seconds_sum{type, status, isError, errorMessage, method, addr}
response_size_bytes{type, status, isError, errorMessage, method, addr}
dependency_up{name}
dependency_request_seconds_bucket{name, type, status, isError, errorMessage, method, addr, le}
dependency_request_seconds_count{name, type, status, isError, errorMessage, method, add}
dependency_request_seconds_sum{name, type, status, isError, errorMessage, method, add}
application_info{version}

Where, for a specific request, type tells which request protocol was used (grpc, http, etc), status registers the response status, method registers the request method, addr registers the requested endpoint address, version tells which version of your app handled the request, isError lets us know if the status code reported is an error or not, and name register the name of the dependency.

In detail:

1. request_seconds_bucket is a metric defines the histogram of how many requests are falling into the well defined buckets represented by the label le;

2. request_seconds_count is a counter that counts the overall number of requests with those exact label occurrences;

3. request_seconds_sum is a counter that counts the overall sum of how long the requests with those exact label occurrences are taking;

4. response_size_bytes is a counter that computes how much data is being sent back to the user for a given request type. It captures the response size from the content-length response header. If there is no such header, the value exposed as metric will be zero;

5. dependency_up is a metric to register weather a specific dependency is up (1) or down (0). The label name registers the dependency name;

6. The dependency_request_seconds_bucket is a metric that defines the histogram of how many requests to a specific dependency are falling into the well defined buckets represented by the label le;

7. The dependency_request_seconds_count is a counter that counts the overall number of requests to a specific dependency;

8. The dependency_request_seconds_sum is a counter that counts the overall sum of how long requests to a specific dependency are taking;

9. Finally, application_info holds static info of an application, such as it's semantic version number;

How to

Add this package as a dependency:

npm i -P @labbsr0x/express-monitor@2.11.0

HTTP Metrics

Use it as middleware:

const express = require("express");
const { Monitor } = require("@labbsr0x/express-monitor");

const app = express();
Monitor.init(app, true); // the 'true' argument exposes default NodeJS metrics as well

One can optionally define the buckets of observation for the request_second histogram by doing:

...
Monitor.init(app, true, [0.1]); // where only one bucket (of 100ms) will be given as output in the /metrics endpoint

Other optional parameters are also:

1. version: a semantic version string identifying the version of your application. Empty by default.
2. isErrorCallback: an error callback to define what you consider as error. 4** and 5** considered as errors by default;
3. metricsEndpoint: the endpoint where the metrics will be exposed. /metrics by default.

Monitor also comes with a promclient so you can expose your custom prometheus metrics:

// below we define a Gauge metric
var myGauge = new Monitor.promclient.Gauge({
    name: "my_gauge",
    help: "records my custom gauge metric",
    labelNames: [ "example_label" ]
});

...

// and here we add a metric event that will be automatically exposed to /metrics endpoint
myGauge.set({"example_label":"value"}, 220);

It is possible to capture error messages that were saved using res.set("Error-Message", "foo message"). For example:

res.set("Error-Message", "User not found");

Important: This middleware requires to be put first in the middleware execution chain, so it can capture metrics from all possible requests.

Manual request metrics collection

Some cases you want to collect request times from requests not made by express endpoints in this cases you can use the collectRequestTime function.

Eg. Your app is subscribed to receive some request from a message broker and have to process it

const start = process.hrtime();

try {
    internalProcessFunction();
    Monitor.collectRequestTime("amqp", 200, 'queue_to_test', start);
} catch (err) {
    Monitor.collectRequestTime('amqp', 500, 'queue_to_test', start, err);
    throw err;
}

Dependency Metrics

For you to know when a dependency is up or down, just provide a health check callback to be watchDependencies function:

const express = require("express");
const { Monitor } = require("@labbsr0x/express-monitor");

const app = express();
Monitor.init(app, true);

// A RegisterDepedencyMetricsCallback will be automatically injected into the HealthCheckCallback
Monitor.watchDependencies((register) => {
    // here you implement the logic to go after your dependencies and check their health
    register({ name: "Fake dependency 1", up: true});
    register({ name: "Fake dependency 2", up: false});
});

Now run your app and point prometheus to the defined metrics endpoint of your server.

You also can monitor a dependency event. Just call collectDependencyTime and pass the right params.

Monitor.collectDependencyTime(name: string, type: string, statusCode: number, method: string, addr: string, errorMessage: string, start: [number, number])

To properly collect the time of the dependency request, we suggest to define a variable to register the startTime right before calling the dependency, then pass it to the collectDependencyTime method:

const start = process.hrtime()
  try{
    //using a service to create a slow request
    const response = await axios.get('http://slowwly.robertomurray.co.uk/delay/2000/url/http://google.com/')
    const { method, path } = response.request
    
    Monitor.collectDependencyTime("Google", "axios", response.status, method, path, "", start)

⚠️ NOTE: If the errorMessage param is not provided, the isError label is automatically set to false. Thus, is required to pass a value in the errorMessage param, in order to set the isError to true.

More details on how Prometheus works, you can find it here.

Example

In the example folder, you'll find a very simple but useful example to get you started. On your terminal, navigate to the project's root folder and type:

npm i && tsc && npm i && cd example && npm i

and then

npm start

On your browser, go to localhost:3000 and then go to localhost:3000/example/metrics to see the exposed metrics.

Big Brother

This is part of a more large application called Big Brother.