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RFC 3092 - 2020-08-21 - Apache HTTP Server metrics source

rfcs/2020-08-21-3092-apache-metrics-source.md

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RFC 3092 - 2020-08-21 - Apache HTTP Server metrics source

This RFC is to introduce a new metrics source to consume metrics from the Apache HTTP Server (httpd). The high level plan is to implement a scrapper similar to the existing prometheus source that will scrape the Apache HTTP Server stats endpoint (provided by mod_status) on an interval and publish metrics to the defined pipeline.

Scope

This RFC will cover:

  • A new source for Apache metrics

This RFC will not cover:

  • Generating metrics from Apache logs

Motivation

Users running httpd want to collect, transform, and forward metrics to better observe how their webservers are performing.

Internal Proposal

I expect to largely copy the existing prometheus source and modify it to parse the output of the httpd status page which looks like:

text
localhost
ServerVersion: Apache/2.4.46 (Unix)
ServerMPM: event
Server Built: Aug  5 2020 23:20:17
CurrentTime: Friday, 21-Aug-2020 18:41:34 UTC
RestartTime: Friday, 21-Aug-2020 18:41:08 UTC
ParentServerConfigGeneration: 1
ParentServerMPMGeneration: 0
ServerUptimeSeconds: 26
ServerUptime: 26 seconds
Load1: 0.00
Load5: 0.03
Load15: 0.03
Total Accesses: 30
Total kBytes: 217
Total Duration: 11
CPUUser: .2
CPUSystem: .02
CPUChildrenUser: 0
CPUChildrenSystem: 0
CPULoad: .846154
Uptime: 26
ReqPerSec: 1.15385
BytesPerSec: 8546.46
BytesPerReq: 7406.93
DurationPerReq: .366667
BusyWorkers: 1
IdleWorkers: 74
Processes: 3
Stopping: 0
BusyWorkers: 1
IdleWorkers: 74
ConnsTotal: 1
ConnsAsyncWriting: 0
ConnsAsyncKeepAlive: 0
ConnsAsyncClosing: 0
Scoreboard: ________________________________________________________W__________________.....................................................................................................................................................................................................................................................................................................................................

I'll use this to generate the following metrics:

  • apache_up (gauge)
  • apache_uptime_seconds_total (counter)
  • apache_accesses_total (counter; extended)
  • apache_sent_kilobytes_total (counter; extended)
  • apache_duration_seconds_total (counter; extended)
  • apache_cpu_seconds_total{type=(system|user|cpu_children_user|cpu_children_system)} (gauge; extended)
  • apache_cpu_load (gauge; extended)
  • apache_workers{state=(busy|idle)} (gauge)
  • apache_connections{state=(closing|keepalive|writing|total)} (gauge)
  • apache_scoreboard_waiting{state=(waiting|starting|reading|sending|keepalive|dnslookup|closing|logging|finishing|idle_cleanup|open} (gauge)

Metrics labeled extended are only available if ExtendedStatus is enabled for Apache. This is the default in newer versions (>= 2.4; released 2012), but purportedly increases CPU load so some users may turn it off. If it is off, they simply won't have those metrics published.

I figure we probably don't want metrics for:

  • System Load (should be handled by a cpu or similar metrics source)

Metrics will be labeled with:

  • endpoint the full endpoint (sans any basic auth credentials)
  • host the hostname and port portions of the endpoint

Doc-level Proposal

Users will be instructed to setup mod_status and enable ExtendedStatus.

The following additional source configuration will be added:

toml
[sources.my_source_id]
  type = "apache_metrics" # required
  endpoints = ["http://localhost/server-status?auto"] # required, default
  scrape_interval_secs = 15 # optional, default, seconds
  namespace = "apache" # optional, default, namespace to put metrics under

Some possible configuration improvements we could add in the future would be:

  • response_timeout; to cap request lengths
  • tls: settings to allow setting specific chains of trust and client certs
  • basic_auth: to set username/password for use with HTTP basic auth; we'll allow this to be set in the URL too which will work for now

But I chose to leave those out for now given the Prometheus source doesn't support them either. We could add support to both at the same time (see Future Work section below).

Datadog's plugin has numerous more options we could also consider in the future.

The host key will be set to the host parsed out of the endpoint.

Rationale

Apache HTTP Server is a fairly common webserver. If we do not support ingesting metrics from it, it is likely to push people to use another tool to forward metrics from httpd to the desired sink.

As part of Vector's vision to be the "one tool" for ingesting and shipping observability data, it makes sense to add as many sources as possible to reduce the likelihood that a user will not be able to ingest metrics from their tools.

Prior Art

Drawbacks

  • Additional maintenance and integration testing burden of a new source

Alternatives

Having users run telegraf and using Vector's prometheus source to scrape it

We could not add the source directly to Vector and instead instruct users to run Telegraf and point Vector at the exposed Prometheus scrape endpoint. This would leverage the already supported telegraf Apache input plugin

I decided against this as it would be in contrast with one of the listed principles of Vector:

One Tool. All Data. - One simple tool gets your logs, metrics, and traces (coming soon) from A to B.

Vector principles

On the same page, it is mentioned that Vector should be a replacement for Telegraf.

You SHOULD use Vector to replace Logstash, Fluent*, Telegraf, Beats, or similar tools.

If users are already running telegraf though, they could opt for this path.

Have a generic HTTP scrape source

We could model this as a generic http_scrape source that has a type (or codec?) that would determine how what type of endpoint it is scraping.

I would err away from this as I think there is some risk that some "HTTP scrape" endpoints will need source specific configuration. We could do this later if this does not end up being the case and they really are all the same.

One downside of this is that I think it'd be less discoverable than a first-class source for each type of endpoint we support scraping.

Outstanding Questions

  • Do we want to apply any metric labels based on the other information available via the status page? I could see labeling the url at least. Answer: label with host and endpoint as described above.
  • Do we want to have one apache_metrics source able to scrape multiple endpoints? Answer: yes, the config has been updated to allow multiple endpoints.
  • Are there preferences between apache or httpd for the nomenclature? I feel like apache is more well-known though httpd is more accurate. Answer: standardize on apache.
  • Should the host key include the port from the endpoint , if any? Or just the hostname. Answer: include the port.

Plan Of Attack

Incremental steps that execute this change. Generally this is in the form of:

  • Submit a PR with the initial sink implementation

Future Work

Refactor HTTP-scraping-based sources

I think one thing that would make sense would be to refactor the sources based on HTTP scraping to share a base similar to how our sinks that rely on http are factored (splunk_hec, http, loki, etc.). This allows them to share common configuration options for their behavior.

My recommendation is to implement this and the nginx metrics source and then figure out where the seams our to pull out an HttpScrapeSource module that could be used by this source, the nginx source, and the prometheus source.