Using Fission's Prometheus Metrics

You can’t improve what you can’t measure. Visibility into metrics is a foundational requirement for much of software operations.

Serverless metrics collection can be tricky. For one, there is no “uptime” to measure. Secondly, with a “pull model” like Prometheus, there is no long-lived service to scrape metrics from. By the time Prometheus scrapes your function’s running instance, it may already be terminated, since functions are short-lived. Though Prometheus has a push gateway, it doesn’t maintain history, which makes it hard to track metrics for multiple concurrent function instances.

Fission lets you avoid this complexity – Fission itself exposes Prometheus metrics for all functions, so you don’t have to build Prometheus integration into any of your application code. You get metrics simply by deploying functions into Fission.

Prometheus Introduction

Prometheus is a popular tool for aggregating, storing, and querying metrics. There are a few crucial concepts to learn with Prometheus.

The Prometheus Data model is built around time series. Each time series has a name and a set of key-value pairs (called labels). You can query time series by these labels.

Prometheus operates on a “pull” model – Fission exposes a set of metrics, and Prometheus samples these metrics at regular intervals (once a minute by default).

Using Fission’s Prometheus Metrics

Fission exposes metrics from its components that Prometheus then scrapes. Prometheus offers a web-based querying UI that you can use to query and graph Fission metrics.

Below we’ll show a few sample queries for common things you may want to monitor from your deployment. First let’s make sure your Prometheus is up and running, and that you can access it.

Setting up Prometheus

Starting in Fission 0.11, an installation of Prometheus is included by default with Fission. With older versions, you can install Prometheus with helm install stable/prometheus.

Accessing your Prometheus server

The Prometheus server has a web-based UI. To access it, you’ll need to setup a local port forwarded to the prometheus service on your cluster:

# Forward local port 9090 to the Prometheus server pod
$ kubectl -n fission port-forward $(kubectl -n fission get pod -l app=prometheus,component=server -o name) 9090

To check that this works, visit http://localhost:9090 in your browser. You should see a page with an empty query window.

Now, let’s see what kind of metrics we can query!

Number of function invocations, and their rate

For a start, try querying the number of function calls, entering into the query box:

fission_function_calls_total

If this query returns no data points, make sure you have at least one function and have invoked it at least once.

Click on the Graph tab. You’ll see one or more line graphs showing the number of invocations.

You can narrow down this query further, for example by function name:

fission_function_calls_total{name="hello"}

This will give you metrics for a function named hello. There still may be multiple lines, for example from different router instances. You can calculate the sum of all these using a sum query:

sum(fission_function_calls_total{name="hello"}) without (cached,job,instance,code,kubernetes_pod_name,pod_template_hash,path,method,application,svc)

This somewhat long query sums over all of the labels that we don’t want to distinguish between.

Note that this metric is a counter. It’s the total number of function calls, cumulative since the beginning of this setup. That means it only ever goes up, never down. To find the rate of function calls instead, you can use the rate function:

rate(fission_function_calls_total{name="func-v1"}[5m]) 

This gives you the number of function calls per second for a function, calculated over a 5 minute sliding window. Read more about rate calculation in the Prometheus docs.

Function success/error rate

The previous section showed the total number of function calls. We also count the number of function calls that resulted in an error. For example, here is the function error metric for a function named “hello”:

fission_function_errors_total{name="hello"}

Function duration percentiles

Fission also tracks how long your functions took to execute. You can query these by quantiles: 50%le (i.e. the median), 90th percentile, and 99-th percentile.

For example, the 90-th percentile duration in seconds for a function “hello”:

fission_function_duration_seconds{name="hello",quantile="0.9"}

Note that this will return two results, one with cached="true" and the other with cached="false". The result with cached="false" measures the 90%le cold-start time. The result with cache="true" shows the function duration when the function was already running. This is a very close measurement of your function’s execution time, since Fission’s overheads in this case are close to zero.

Alerting

Prometheus comes with an alerting service called AlertManager. You can configure Alertmanager to generate alerts for various conditions. For example, if a function’s rate of errors goes beyond a certain threshold, you could send a slack message or a pagerduty page.

For more on this, consult the Alertmanager documentation.

Dashboards

You can use Grafana or other tools for creating dashboards from Prometheus data. Install Grafana on your cluster, and follow instructions to add a Prometheus data source. You can then add graphs based on Prometheus queries.

Conclusion

Fission’s Prometheus metrics integration allows you to query the function call rate, error rate and duration quantiles without writing any Prometheus integration yourself. Using Prometheus' Alertmanager service, you can also set up alerting for cases where you need it.

Check out the Fission installation guide to get started with Fission. Join us on the Fission Slack to chat, or follow us on Twitter at @fissionio.

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