Prometheus metric library for Nginx

This is a Lua library that can be used with Nginx to keep track of metrics and expose them on a separate web page to be pulled by Prometheus.

Quick start guide

You would need to install nginx package with lua support (nginx-extras on Debian) and make prometheus.lua available in your LUA_PATH (or just point lua_package_path to a directory with this git repo).

To track request latency broken down by HTTP host and request count broken down by host and status, add the following to nginx.conf:

lua_shared_dict prometheus_metrics 10M;
lua_package_path "/path/to/nginx-lua-prometheus/?.lua";
init_by_lua '
  prometheus = require("prometheus").init("prometheus_metrics")
  metric_requests = prometheus:counter(
    "nginx_http_requests_total", "Number of HTTP requests", {"host", "status"})
  metric_latency = prometheus:histogram(
    "nginx_http_request_duration_seconds", "HTTP request latency", {"host"})
';
log_by_lua '
  local host = ngx.var.host:gsub("^www.", "")
  metric_requests:inc(1, {host, ngx.var.status})
  metric_latency:observe(ngx.now() - ngx.req.start_time(), {host})
';

This:

• configures a shared dictionary for your metrics called prometheus_metrics with a 10MB size limit;
• registers a counter called nginx_http_requests_total with two labels: host and status;
• registers a histogram called nginx_http_request_duration_seconds with one label host;
• on each HTTP request measures its latency, recording it in the histogram and increments the counter, setting current HTTP host as host label and HTTP status code as status label.

Last step is to configure a separate server that will expose the metrics. Please make sure to only make it reachable from your Prometheus server:

server {
  listen 9145;
  allow 192.168.0.0/16;
  deny all;
  location /metrics {
    content_by_lua 'prometheus:collect()';
  }
}

Metrics will be available at http://your.nginx:9145/metrics.

Note: using HTTP host as a metric label value on servers that have many virtual hosts has potential performance implications. Please read the caveats section below for more information.

API reference

init()

syntax: require("prometheus").init(dict_name, [prefix])

Initializes the module. This should be called once from the init_by_lua section in nginx configuration.

• dict_name is the name of the nginx shared dictionary which will be used to store all metrics. Defaults to prometheus_metrics if not specified.
• prefix is an optional string which will be prepended to metric names on output

Returns a prometheus object that should be used to register metrics.

Example:

init_by_lua '
  prometheus = require("prometheus").init("prometheus_metrics")
';

prometheus:counter()

syntax: prometheus:counter(name, description, label_names)

Registers a counter. Should be called once from the init_by_lua section.

• name is the name of the metric.
• description is the text description that will be presented to Prometheus along with the metric. Optional (pass nil if you still need to define label names).
• label_names is an array of label names for the metric. Optional.

Naming section of Prometheus documentation provides good guidelines on choosing metric and label names.

Returns a counter object that can later be incremented.

Example:

init_by_lua '
  prometheus = require("prometheus").init("prometheus_metrics")
  metric_bytes = prometheus:counter(
    "nginx_http_request_size_bytes", "Total size of incoming requests")
  metric_requests = prometheus:counter(
    "nginx_http_requests_total", "Number of HTTP requests", {"host", "status"})
';

prometheus:histogram()

syntax: prometheus:histogram(name, description, label_names, buckets)

Registers a histogram. Should be called once from the init_by_lua section.

• name is the name of the metric.
• description is the text description. Optional.
• label_names is an array of label names for the metric. Optional.
• buckets is an array of numbers defining bucket boundaries. Optional, defaults to 20 latency buckets covering a range from 5ms to 10s (in seconds).

Returns a histogram object that can later be used to record samples.

Example:

init_by_lua '
  prometheus = require("prometheus").init("prometheus_metrics")
  metric_latency = prometheus:histogram(
    "nginx_http_request_duration_seconds", "HTTP request latency", {"host"})
  metric_response_sizes = prometheus:counter(
    "nginx_http_response_size_bytes", "Size of HTTP responses", nil,
    {10,100,1000,10000,100000,1000000})
';

prometheus:collect()

syntax: prometheus:collect()

Presents all metrics in a text format compatible with Prometheus. This should be called in content_by_lua to expose the metrics on a separate HTTP page.

Example:

location /metrics {
  content_by_lua 'prometheus:collect()';
  allow 192.168.0.0/16;
  deny all;
}

counter:inc()

syntax: counter:inc(value, label_values)

Increments a previously registered counter. This is usually called from log_by_lua globally or per server/location.

• value is a value that should be added to the counter. Defaults to 1.
• label_values is an array of label values. The number of values should match the number of label names defined when the counter was registered using prometheus:counter(). No label values should be provided for counters with no labels.

Example:

log_by_lua '
  metric_bytes:inc(tonumber(ngx.var.request_length))
  metric_requests:inc(1, {ngx.var.host, ngx.var.status})
';

histogram:observe()

syntax: histogram:observe(value, label_values)

Records a value in a previously registered histogram. Usually called from log_by_lua globally or per server/location.

• value is a value that should be recorded. Required.
• label_values is an array of label values. The number of values should match the number of label names defined when the histogram was registered using prometheus:histogram(). No label values should be provided for histograms with no labels.

Example:

log_by_lua '
  metric_latency:observe(ngx.now() - ngx.req.start_time(), {ngx.var.host})
  metric_response_sizes:observe(tonumber(ngx.var.bytes_sent))
';

Built-in metrics

The module increments the nginx_metric_errors_total metric if it encounters an error (for example, when lua_shared_dict becomes full). You might want to configure an alert on that metric.

Caveats

Please keep in mind that all metrics stored by this library are kept in a single shared dictionary (lua_shared_dict). While exposing metrics the module has to list all dictionary keys, which has serious performance implications for dictionaries with large number of keys (in this case this means large number of metrics OR metrics with high label cardinality). Listing the keys has to lock the dictionary, which blocks all threads that try to access it (i.e. potentially all nginx worker threads).

There is no elegant solution to this issue (besides keeping metrics in a separate storage system external to nginx), so for latency-critical servers you might want to keep the number of metrics (and distinct metric label values) to a minimum.

Development

Install dependencies for testing

• luarocks install luacheck
• luarocks install luaunit

Run tests

• luacheck --globals ngx -- prometheus.lua
• lua prometheus_test.lua

License

Licensed under MIT license.