Issue #000 feat: Ugrading nginx version to 1.15.8

with lua support for prometheus direct metrics scraping

images/proxy/Dockerfile

-FROM nginx:1.13.8-alpine
+FROM alpine:3.9
 
-RUN rm -rf /etc/nginx/conf.d
+LABEL maintainer="NGINX Docker Maintainers <docker-maint@nginx.com>"
 
-RUN rm -rf /usr/share/nginx/html
+ENV NGINX_VERSION 1.15.8
+
+ENV LUAJIT_LIB=/usr/local/lib
+ENV LUAJIT_INC=/usr/local/include/luajit-2.1
+
+COPY nginx_devel_kit /opt/nginx_devel_kit
+COPY luajit /usr/src/luajit
+COPY nginx_lua /opt/nginx_lua
+
+RUN GPG_KEYS=B0F4253373F8F6F510D42178520A9993A1C052F8 \
+    && CONFIG="\
+        --prefix=/etc/nginx \
+        --sbin-path=/usr/sbin/nginx \
+        --modules-path=/usr/lib/nginx/modules \
+        --with-ld-opt='-Wl,-rpath,/usr/local/lib' \
+        --add-module=/opt/nginx_devel_kit \
+        --add-module=/opt/nginx_lua \
+        --conf-path=/etc/nginx/nginx.conf \
+        --error-log-path=/var/log/nginx/error.log \
+        --http-log-path=/var/log/nginx/access.log \
+        --pid-path=/var/run/nginx.pid \
+        --lock-path=/var/run/nginx.lock \
+        --http-client-body-temp-path=/var/cache/nginx/client_temp \
+        --http-proxy-temp-path=/var/cache/nginx/proxy_temp \
+        --http-fastcgi-temp-path=/var/cache/nginx/fastcgi_temp \
+        --http-uwsgi-temp-path=/var/cache/nginx/uwsgi_temp \
+        --http-scgi-temp-path=/var/cache/nginx/scgi_temp \
+        --user=nginx \
+        --group=nginx \
+        --with-http_ssl_module \
+        --with-http_realip_module \
+        --with-http_addition_module \
+        --with-http_sub_module \
+        --with-http_dav_module \
+        --with-http_flv_module \
+        --with-http_mp4_module \
+        --with-http_gunzip_module \
+        --with-http_gzip_static_module \
+        --with-http_random_index_module \
+        --with-http_secure_link_module \
+        --with-http_stub_status_module \
+        --with-http_auth_request_module \
+        --with-http_xslt_module=dynamic \
+        --with-http_image_filter_module=dynamic \
+        --with-http_geoip_module=dynamic \
+        --with-threads \
+        --with-stream \
+        --with-stream_ssl_module \
+        --with-stream_ssl_preread_module \
+        --with-stream_realip_module \
+        --with-stream_geoip_module=dynamic \
+        --with-http_slice_module \
+        --with-mail \
+        --with-mail_ssl_module \
+        --with-compat \
+        --with-file-aio \
+        --with-http_v2_module " \
+    && addgroup -S nginx \
+    && adduser -D -S -h /var/cache/nginx -s /sbin/nologin -G nginx nginx \
+    && apk add --no-cache libgcc \
+    && apk add --no-cache --virtual .build-deps \
+        gcc \
+        libc-dev \
+        make \
+        openssl-dev \
+        pcre-dev \
+        zlib-dev \
+        linux-headers \
+        curl \
+        gnupg1 \
+        libxslt-dev \
+        gd-dev \
+        geoip-dev \
+    && curl -fSL https://nginx.org/download/nginx-$NGINX_VERSION.tar.gz -o nginx.tar.gz \
+    && curl -fSL https://nginx.org/download/nginx-$NGINX_VERSION.tar.gz.asc  -o nginx.tar.gz.asc \
+    && export GNUPGHOME="$(mktemp -d)" \
+    && found=''; \
+    for server in \
+        ha.pool.sks-keyservers.net \
+        hkp://keyserver.ubuntu.com:80 \
+        hkp://p80.pool.sks-keyservers.net:80 \
+        pgp.mit.edu \
+    ; do \
+        echo "Fetching GPG key $GPG_KEYS from $server"; \
+        gpg --keyserver "$server" --keyserver-options timeout=10 --recv-keys "$GPG_KEYS" && found=yes && break; \
+    done; \
+    test -z "$found" && echo >&2 "error: failed to fetch GPG key $GPG_KEYS" && exit 1; \
+    gpg --batch --verify nginx.tar.gz.asc nginx.tar.gz \
+    && rm -rf "$GNUPGHOME" nginx.tar.gz.asc \
+    && mkdir -p /usr/src \
+    && tar -zxC /usr/src -f nginx.tar.gz \
+    && rm nginx.tar.gz \
+    && cd /usr/src/luajit \
+    && export LUAJIT_LIB=/usr/local/lib \
+    && export LUAJIT_INC=/usr/local/include/luajit-2.1 \
+    && cd /usr/src/luajit \
+    && make -j$(getconf _NPROCESSORS_ONLN) \
+    && make install \
+    && ls /usr/local/lib /usr/local/include/luajit-2.1 \
+    && cd /usr/src/nginx-$NGINX_VERSION \
+    && ./configure $CONFIG --with-debug \
+    && make -j$(getconf _NPROCESSORS_ONLN) \
+    && mv objs/nginx objs/nginx-debug \
+    && mv objs/ngx_http_xslt_filter_module.so objs/ngx_http_xslt_filter_module-debug.so \
+    && mv objs/ngx_http_image_filter_module.so objs/ngx_http_image_filter_module-debug.so \
+    && mv objs/ngx_http_geoip_module.so objs/ngx_http_geoip_module-debug.so \
+    && mv objs/ngx_stream_geoip_module.so objs/ngx_stream_geoip_module-debug.so \
+    && ./configure $CONFIG \
+    && make -j$(getconf _NPROCESSORS_ONLN) \
+    && make install \
+    && rm -rf /etc/nginx/html/ \
+    && mkdir /etc/nginx/conf.d/ \
+    && mkdir -p /usr/share/nginx/html/ \
+    && install -m644 html/index.html /usr/share/nginx/html/ \
+    && install -m644 html/50x.html /usr/share/nginx/html/ \
+    && install -m755 objs/nginx-debug /usr/sbin/nginx-debug \
+    && install -m755 objs/ngx_http_xslt_filter_module-debug.so /usr/lib/nginx/modules/ngx_http_xslt_filter_module-debug.so \
+    && install -m755 objs/ngx_http_image_filter_module-debug.so /usr/lib/nginx/modules/ngx_http_image_filter_module-debug.so \
+    && install -m755 objs/ngx_http_geoip_module-debug.so /usr/lib/nginx/modules/ngx_http_geoip_module-debug.so \
+    && install -m755 objs/ngx_stream_geoip_module-debug.so /usr/lib/nginx/modules/ngx_stream_geoip_module-debug.so \
+    && ln -s ../../usr/lib/nginx/modules /etc/nginx/modules \
+    && strip /usr/sbin/nginx* \
+    && strip /usr/lib/nginx/modules/*.so \
+    && rm -rf /usr/src/nginx-$NGINX_VERSION \
+    \
+    # Bring in gettext so we can get `envsubst`, then throw
+    # the rest away. To do this, we need to install `gettext`
+    # then move `envsubst` out of the way so `gettext` can
+    # be deleted completely, then move `envsubst` back.
+    && apk add --no-cache --virtual .gettext gettext \
+    && mv /usr/bin/envsubst /tmp/ \
+    \
+    && runDeps="$( \
+        scanelf --needed --nobanner --format '%n#p' /usr/sbin/nginx /usr/lib/nginx/modules/*.so /tmp/envsubst \
+            | tr ',' '\n' \
+            | sort -u \
+            | awk 'system("[ -e /usr/local/lib/" $1 " ]") == 0 { next } { print "so:" $1 }' \
+    )" \
+    && apk add --no-cache --virtual .nginx-rundeps $runDeps \
+    && apk del .build-deps \
+    && apk del .gettext \
+    && mv /tmp/envsubst /usr/local/bin/ \
+    \
+    # Bring in tzdata so users could set the timezones through the environment
+    # variables
+    && apk add --no-cache tzdata \
+    \
+    # forward request and error logs to docker log collector
+    && ln -sf /dev/stdout /var/log/nginx/access.log \
+    && ln -sf /dev/stderr /var/log/nginx/error.log
+
+COPY nginx.conf /etc/nginx/nginx.conf
+copy prometheus.lua /etc/nginx/lua_modules/
+
+EXPOSE 80
+
+STOPSIGNAL SIGTERM
+
+CMD ["nginx", "-g", "daemon off;"]

images/proxy/Jenkinsfile

@@ -32,10 +32,10 @@ node('build-slave') {
             stage('Build') {
                 env.NODE_ENV = "build"
                 print "Environment will be : ${env.NODE_ENV}"
-                sh("./images/proxy/build.sh ${build_tag} ${env.NODE_NAME} ${hub_org}")
+                sh("cd ./images/proxy && ./build.sh ${build_tag} ${env.NODE_NAME} ${hub_org}")
             }
             stage('ArchiveArtifacts') {
-                archiveArtifacts "metadata.json"
+                archiveArtifacts "images/proxy/metadata.json"
                 currentBuild.description = "${build_tag}"
             }
         }

images/proxy/build.sh

 #!/bin/bash
-# Build script
-set -eo pipefail
+
+# Downloading deps
+wget https://codeload.github.com/simplresty/ngx_devel_kit/tar.gz/v0.3.0 -O ngx_devel_kit_0_3_0.tar.gz
+wget https://codeload.github.com/openresty/luajit2/tar.gz/v2.1-20190626 -O luajit_2_1.tar.gz
+wget https://codeload.github.com/openresty/lua-nginx-module/tar.gz/v0.10.15 -O ngx_lua.tar.gz
+
+# Creating deps directory
+mkdir nginx_devel_kit luajit nginx_lua
+tar --strip-components=1 -xf ngx_devel_kit_0_3_0.tar.gz -C nginx_devel_kit
+tar --strip-components=1 -xf luajit_2_1.tar.gz -C luajit
+tar --strip-components=1 -xf ngx_lua.tar.gz -C nginx_lua
+
+# Creating nginx
 build_tag=$1
 name=proxy
 node=$2
 org=$3
 
-docker build -f ./images/proxy/Dockerfile -t ${org}/${name}:${build_tag} .
+docker build -t ${org}/${name}:${build_tag} .
 echo {\"image_name\" : \"${name}\", \"image_tag\" : \"${build_tag}\", \"node_name\" : \"$node\"} > metadata.json

images/proxy/nginx.conf

+user  nginx;
+worker_processes  1;
+load_module modules/ndk_http_module.so;
+load_module modules/ngx_http_lua_module.so;
+
+
+error_log  /var/log/nginx/error.log warn;
+pid        /var/run/nginx.pid;
+
+events {
+    worker_connections  1024;
+}
+
+
+http {
+    include       /etc/nginx/mime.types;
+    # Don't need complete openresty
+    lua_load_resty_core off;
+    default_type  application/octet-stream;
+
+    log_format  main  '$remote_addr - $remote_user [$time_local] '
+                      '"$request" $status $body_bytes_sent '
+                      '$request_time $upstream_response_time $pipe'
+                      '"$http_referer" "$http_user_agent"';
+
+    access_log  /var/log/nginx/access.log  main;
+
+    # Shared dictionary to store metrics
+    lua_shared_dict prometheus_metrics 10M;
+    lua_package_path "/etc/nginx/lua_modules/?.lua";
+    # Collecting metrics
+	init_by_lua '
+      prometheus = require("prometheus").init("prometheus_metrics")
+      metric_requests = prometheus:counter(
+        "nginx_http_requests_total", "Number of HTTP requests", {"host", "status", "request_method"})
+      metric_latency = prometheus:histogram(
+        "nginx_http_request_duration_seconds", "HTTP request latency", {"host"})
+      metric_connections = prometheus:gauge(
+        "nginx_http_connections", "Number of HTTP connections", {"state"})
+    ';
+
+    # Collecting metrics
+    log_by_lua '
+      metric_requests:inc(1, {ngx.var.server_name, ngx.var.status, ngx.var.request_method})
+      metric_latency:observe(tonumber(ngx.var.request_time), {ngx.var.server_name})
+    ';
+
+
+    sendfile        on;
+    #tcp_nopush     on;
+
+    keepalive_timeout  500s;
+    keepalive_requests 200;
+
+    #gzip  on;
+
+    include /etc/nginx/conf.d/*.conf;
+
+    server {
+      listen 9145;
+      location /metrics {
+        content_by_lua '
+          metric_connections:set(ngx.var.connections_reading, {"reading"})
+          metric_connections:set(ngx.var.connections_waiting, {"waiting"})
+          metric_connections:set(ngx.var.connections_writing, {"writing"})
+          prometheus:collect()
+        ';
+      }
+    }
+}

images/proxy/prometheus.lua

+-- vim: ts=2:sw=2:sts=2:expandtab
+--
+-- This module uses a single dictionary shared between Nginx workers to keep
+-- all metrics. Each counter is stored as a separate entry in that dictionary,
+-- which allows us to increment them using built-in `incr` method.
+--
+-- Prometheus requires that (a) all samples for a given metric are presented
+-- as one uninterrupted group, and (b) buckets of a histogram appear in
+-- increasing numerical order. We satisfy that by carefully constructing full
+-- metric names (i.e. metric name along with all labels) so that they meet
+-- those requirements while being sorted alphabetically. In particular:
+--
+--  * all labels for a given metric are presented in reproducible order (the one
+--    used when labels were declared). "le" label for histogram metrics always
+--    goes last;
+--  * bucket boundaries (which are exposed as values of the "le" label) are
+--    presented as floating point numbers with leading and trailing zeroes.
+--    Number of of zeroes is determined for each bucketer automatically based on
+--    bucket boundaries;
+--  * internally "+Inf" bucket is stored as "Inf" (to make it appear after
+--    all numeric buckets), and gets replaced by "+Inf" just before we
+--    expose the metrics.
+--
+-- For example, if you define your bucket boundaries as {0.00005, 10, 1000}
+-- then we will keep the following samples for a metric `m1` with label
+-- `site` set to `site1`:
+--
+--   m1_bucket{site="site1",le="0000.00005"}
+--   m1_bucket{site="site1",le="0010.00000"}
+--   m1_bucket{site="site1",le="1000.00000"}
+--   m1_bucket{site="site1",le="Inf"}
+--   m1_count{site="site1"}
+--   m1_sum{site="site1"}
+--
+-- "Inf" will be replaced by "+Inf" while publishing metrics.
+--
+-- You can find the latest version and documentation at
+-- https://github.com/knyar/nginx-lua-prometheus
+-- Released under MIT license.
+
+
+-- Default set of latency buckets, 5ms to 10s:
+local DEFAULT_BUCKETS = {0.005, 0.01, 0.02, 0.03, 0.05, 0.075, 0.1, 0.2, 0.3,
+                         0.4, 0.5, 0.75, 1, 1.5, 2, 3, 4, 5, 10}
+
+-- Metric is a "parent class" for all metrics.
+local Metric = {}
+function Metric:new(o)
+  o = o or {}
+  setmetatable(o, self)
+  self.__index = self
+  return o
+end
+
+-- Checks that the right number of labels values have been passed.
+--
+-- Args:
+--   label_values: an array of label values.
+--
+-- Returns:
+--   an error message or nil
+function Metric:check_label_values(label_values)
+  if self.label_names == nil and label_values == nil then
+    return
+  elseif self.label_names == nil and label_values ~= nil then
+    return "Expected no labels for " .. self.name .. ", got " ..  #label_values
+  elseif label_values == nil and self.label_names ~= nil then
+    return "Expected " .. #self.label_names .. " labels for " ..
+           self.name .. ", got none"
+  elseif #self.label_names ~= #label_values then
+    return "Wrong number of labels for " .. self.name .. ". Expected " ..
+           #self.label_names .. ", got " .. #label_values
+  else
+    for i, k in ipairs(self.label_names) do
+      if label_values[i] == nil then
+        return "Unexpected nil value for label " .. k ..  " of " .. self.name
+      end
+    end
+  end
+end
+
+local Counter = Metric:new()
+-- Increase a given counter by `value`
+--
+-- Args:
+--   value: (number) a value to add to the counter. Defaults to 1 if skipped.
+--   label_values: an array of label values. Can be nil (i.e. not defined) for
+--     metrics that have no labels.
+function Counter:inc(value, label_values)
+  local err = self:check_label_values(label_values)
+  if err ~= nil then
+    self.prometheus:log_error(err)
+    return
+  end
+  if value ~= nil and value < 0 then
+    self.prometheus:log_error_kv(self.name, value, "Value should not be negative")
+    return
+  end
+
+  self.prometheus:inc(self.name, self.label_names, label_values, value or 1)
+end
+
+local Gauge = Metric:new()
+-- Set a given gauge to `value`
+--
+-- Args:
+--   value: (number) a value to set the gauge to. Should be defined.
+--   label_values: an array of label values. Can be nil (i.e. not defined) for
+--     metrics that have no labels.
+function Gauge:set(value, label_values)
+  if value == nil then
+    self.prometheus:log_error("No value passed for " .. self.name)
+    return
+  end
+  local err = self:check_label_values(label_values)
+  if err ~= nil then
+    self.prometheus:log_error(err)
+    return
+  end
+  self.prometheus:set(self.name, self.label_names, label_values, value)
+end
+
+
+-- Increase a given gauge by `value`
+--
+-- Args:
+--   value: (number) a value to add to the gauge (a negative value when you
+--     need to decrease the value of the gauge). Defaults to 1 if skipped.
+--   label_values: an array of label values. Can be nil (i.e. not defined) for
+--     metrics that have no labels.
+function Gauge:inc(value, label_values)
+  local err = self:check_label_values(label_values)
+  if err ~= nil then
+    self.prometheus:log_error(err)
+    return
+  end
+  self.prometheus:inc(self.name, self.label_names, label_values, value or 1)
+end
+
+local Histogram = Metric:new()
+-- Record a given value in a histogram.
+--
+-- Args:
+--   value: (number) a value to record. Should be defined.
+--   label_values: an array of label values. Can be nil (i.e. not defined) for
+--     metrics that have no labels.
+function Histogram:observe(value, label_values)
+  if value == nil then
+    self.prometheus:log_error("No value passed for " .. self.name)
+    return
+  end
+  local err = self:check_label_values(label_values)
+  if err ~= nil then
+    self.prometheus:log_error(err)
+    return
+  end
+  self.prometheus:histogram_observe(self.name, self.label_names, label_values, value)
+end
+
+local Prometheus = {}
+Prometheus.__index = Prometheus
+Prometheus.initialized = false
+
+-- Generate full metric name that includes all labels.
+--
+-- Args:
+--   name: string
+--   label_names: (array) a list of label keys.
+--   label_values: (array) a list of label values.
+-- Returns:
+--   (string) full metric name.
+local function full_metric_name(name, label_names, label_values)
+  if not label_names then
+    return name
+  end
+  local label_parts = {}
+  for idx, key in ipairs(label_names) do
+    local label_value = (string.format("%s", label_values[idx])
+      :gsub("[^\032-\126]", "")  -- strip non-printable characters
+      :gsub("\\", "\\\\")
+      :gsub('"', '\\"'))
+    table.insert(label_parts, key .. '="' .. label_value .. '"')
+  end
+  return name .. "{" .. table.concat(label_parts, ",") .. "}"
+end
+
+-- Construct bucket format for a list of buckets.
+--
+-- This receives a list of buckets and returns a sprintf template that should
+-- be used for bucket boundaries to make them come in increasing order when
+-- sorted alphabetically.
+--
+-- To re-phrase, this is where we detect how many leading and trailing zeros we
+-- need.
+--
+-- Args:
+--   buckets: a list of buckets
+--
+-- Returns:
+--   (string) a sprintf template.
+local function construct_bucket_format(buckets)
+  local max_order = 1
+  local max_precision = 1
+  for _, bucket in ipairs(buckets) do
+    assert(type(bucket) == "number", "bucket boundaries should be numeric")
+    -- floating point number with all trailing zeros removed
+    local as_string = string.format("%f", bucket):gsub("0*$", "")
+    local dot_idx = as_string:find(".", 1, true)
+    max_order = math.max(max_order, dot_idx - 1)
+    max_precision = math.max(max_precision, as_string:len() - dot_idx)
+  end
+  return "%0" .. (max_order + max_precision + 1) .. "." .. max_precision .. "f"
+end
+
+-- Extract short metric name from the full one.
+--
+-- Args:
+--   full_name: (string) full metric name that can include labels.
+--
+-- Returns:
+--   (string) short metric name with no labels. For a `*_bucket` metric of
+--     histogram the _bucket suffix will be removed.
+local function short_metric_name(full_name)
+  local labels_start, _ = full_name:find("{")
+  if not labels_start then
+    -- no labels
+    return full_name
+  end
+  local suffix_idx, _ = full_name:find("_bucket{")
+  if suffix_idx and full_name:find("le=") then
+    -- this is a histogram metric
+    return full_name:sub(1, suffix_idx - 1)
+  end
+  -- this is not a histogram metric
+  return full_name:sub(1, labels_start - 1)
+end
+
+-- Makes a shallow copy of a table
+local function copy_table(table)
+  local new = {}
+  if table ~= nil then
+    for k, v in ipairs(table) do
+      new[k] = v
+    end
+  end
+  return new
+end
+
+-- Check metric name and label names for correctness.
+--
+-- Regular expressions to validate metric and label names are
+-- documented in https://prometheus.io/docs/concepts/data_model/
+--
+-- Args:
+--   metric_name: (string) metric name.
+--   label_names: label names (array of strings).
+--
+-- Returns:
+--   Either an error string, or nil of no errors were found.
+local function check_metric_and_label_names(metric_name, label_names)
+  if not metric_name:match("^[a-zA-Z_:][a-zA-Z0-9_:]*$") then
+    return "Metric name '" .. metric_name .. "' is invalid"
+  end
+  for _, label_name in ipairs(label_names or {}) do
+    if label_name == "le" then
+      return "Invalid label name 'le' in " .. metric_name
+    end
+    if not label_name:match("^[a-zA-Z_][a-zA-Z0-9_]*$") then
+      return "Metric '" .. metric_name .. "' label name '" .. label_name ..
+             "' is invalid"
+    end
+  end
+end
+
+-- Initialize the module.
+--
+-- This should be called once from the `init_by_lua` section in nginx
+-- configuration.
+--
+-- Args:
+--   dict_name: (string) name of the nginx shared dictionary which will be
+--     used to store all metrics
+--   prefix: (optional string) if supplied, prefix is added to all
+--   metric names on output
+--
+-- Returns:
+--   an object that should be used to register metrics.
+function Prometheus.init(dict_name, prefix)
+  local self = setmetatable({}, Prometheus)
+  dict_name = dict_name or "prometheus_metrics"
+  self.dict = ngx.shared[dict_name]
+  if self.dict == nil then
+    ngx.log(ngx.ERR,
+      "Dictionary '", dict_name, "' does not seem to exist. ",
+      "Please define the dictionary using `lua_shared_dict`.")
+    return self
+  end
+  self.help = {}
+  if prefix then
+    self.prefix = prefix
+  else
+    self.prefix = ''
+  end
+  self.type = {}
+  self.registered = {}
+  self.buckets = {}
+  self.bucket_format = {}
+  self.initialized = true
+
+  self:counter("nginx_metric_errors_total",
+    "Number of nginx-lua-prometheus errors")
+  self.dict:set("nginx_metric_errors_total", 0)
+  return self
+end
+
+function Prometheus:log_error(...)
+  ngx.log(ngx.ERR, ...)
+  self.dict:incr("nginx_metric_errors_total", 1)
+end
+
+function Prometheus:log_error_kv(key, value, err)
+  self:log_error(
+    "Error while setting '", key, "' to '", value, "': '", err, "'")
+end
+
+-- Register a counter.
+--
+-- Args:
+--   name: (string) name of the metric. Required.
+--   description: (string) description of the metric. Will be used for the HELP
+--     comment on the metrics page. Optional.
+--   label_names: array of strings, defining a list of metrics. Optional.
+--
+-- Returns:
+--   a Counter object.
+function Prometheus:counter(name, description, label_names)
+  if not self.initialized then
+    ngx.log(ngx.ERR, "Prometheus module has not been initialized")
+    return
+  end
+
+  local err = check_metric_and_label_names(name, label_names)
+  if err ~= nil then
+    self:log_error(err)
+    return
+  end
+
+  if self.registered[name] then
+    self:log_error("Duplicate metric " .. name)
+    return
+  end
+  self.registered[name] = true
+  self.help[name] = description
+  self.type[name] = "counter"
+
+  return Counter:new{name=name, label_names=label_names, prometheus=self}
+end
+
+-- Register a gauge.
+--
+-- Args:
+--   name: (string) name of the metric. Required.
+--   description: (string) description of the metric. Will be used for the HELP
+--     comment on the metrics page. Optional.
+--   label_names: array of strings, defining a list of metrics. Optional.
+--
+-- Returns:
+--   a Gauge object.
+function Prometheus:gauge(name, description, label_names)
+  if not self.initialized then
+    ngx.log(ngx.ERR, "Prometheus module has not been initialized")
+    return
+  end
+
+  local err = check_metric_and_label_names(name, label_names)
+  if err ~= nil then
+    self:log_error(err)
+    return
+  end
+
+  if self.registered[name] then
+    self:log_error("Duplicate metric " .. name)
+    return
+  end
+  self.registered[name] = true
+  self.help[name] = description
+  self.type[name] = "gauge"
+
+  return Gauge:new{name=name, label_names=label_names, prometheus=self}
+end
+
+
+-- Register a histogram.
+--
+-- Args:
+--   name: (string) name of the metric. Required.
+--   description: (string) description of the metric. Will be used for the HELP
+--     comment on the metrics page. Optional.
+--   label_names: array of strings, defining a list of metrics. Optional.
+--   buckets: array if numbers, defining bucket boundaries. Optional.
+--
+-- Returns:
+--   a Histogram object.
+function Prometheus:histogram(name, description, label_names, buckets)
+  if not self.initialized then
+    ngx.log(ngx.ERR, "Prometheus module has not been initialized")
+    return
+  end
+
+  local err = check_metric_and_label_names(name, label_names)
+  if err ~= nil then
+    self:log_error(err)
+    return
+  end
+
+  for _, suffix in ipairs({"", "_bucket", "_count", "_sum"}) do
+    if self.registered[name .. suffix] then
+      self:log_error("Duplicate metric " .. name .. suffix)
+      return
+    end
+    self.registered[name .. suffix] = true
+  end
+  self.help[name] = description
+  self.type[name] = "histogram"
+
+  self.buckets[name] = buckets or DEFAULT_BUCKETS
+  self.bucket_format[name] = construct_bucket_format(self.buckets[name])
+
+  return Histogram:new{name=name, label_names=label_names, prometheus=self}
+end
+
+-- Set a given dictionary key.
+-- This overwrites existing values, so it should only be used when initializing
+-- metrics or when explicitely overwriting the previous value of a metric.
+function Prometheus:set_key(key, value)
+  local ok, err = self.dict:safe_set(key, value)
+  if not ok then
+    self:log_error_kv(key, value, err)
+  end
+end
+
+-- Increment a given metric by `value`.
+--
+-- Args:
+--   name: (string) short metric name without any labels.
+--   label_names: (array) a list of label keys.
+--   label_values: (array) a list of label values.
+--   value: (number) value to add (a negative value when you need to decrease
+--     the value of the gauge). Optional, defaults to 1.
+function Prometheus:inc(name, label_names, label_values, value)
+  local key = full_metric_name(name, label_names, label_values)
+  if value == nil then value = 1 end
+
+  local newval, err = self.dict:incr(key, value)
+  if newval then
+    return
+  end
+  -- Yes, this looks like a race, so I guess we might under-report some values
+  -- when multiple workers simultaneously try to create the same metric.
+  -- Hopefully this does not happen too often (shared dictionary does not get
+  -- reset during configuation reload).
+  if err == "not found" then
+    self:set_key(key, value)
+    return
+  end
+  -- Unexpected error
+  self:log_error_kv(key, value, err)
+end
+
+-- Set the current value of a gauge to `value`
+--
+-- Args:
+--   name: (string) short metric name without any labels.
+--   label_names: (array) a list of label keys.
+--   label_values: (array) a list of label values.
+--   value: (number) the new value for the gauge.
+function Prometheus:set(name, label_names, label_values, value)
+  local key = full_metric_name(name, label_names, label_values)
+  self:set_key(key, value)
+end
+
+-- Record a given value into a histogram metric.
+--
+-- Args:
+--   name: (string) short metric name without any labels.
+--   label_names: (array) a list of label keys.
+--   label_values: (array) a list of label values.
+--   value: (number) value to observe.
+function Prometheus:histogram_observe(name, label_names, label_values, value)
+  self:inc(name .. "_count", label_names, label_values, 1)
+  self:inc(name .. "_sum", label_names, label_values, value)
+
+  -- we are going to mutate arrays of label names and values, so create a copy.
+  local l_names = copy_table(label_names)
+  local l_values = copy_table(label_values)
+
+  -- Last bucket. Note, that the label value is "Inf" rather than "+Inf"
+  -- required by Prometheus. This is necessary for this bucket to be the last
+  -- one when all metrics are lexicographically sorted. "Inf" will get replaced
+  -- by "+Inf" in Prometheus:collect().
+  table.insert(l_names, "le")
+  table.insert(l_values, "Inf")
+  self:inc(name .. "_bucket", l_names, l_values, 1)
+
+  local label_count = #l_names
+  for _, bucket in ipairs(self.buckets[name]) do
+    if value <= bucket then
+      -- last label is now "le"
+      l_values[label_count] = self.bucket_format[name]:format(bucket)
+      self:inc(name .. "_bucket", l_names, l_values, 1)
+    end
+  end
+end
+
+-- Prometheus compatible metric data as an array of strings.
+--
+-- Returns:
+--   Array of strings with all metrics in a text format compatible with
+--   Prometheus.
+function Prometheus:metric_data()
+  if not self.initialized then
+    ngx.log(ngx.ERR, "Prometheus module has not been initialized")
+    return
+  end
+
+  local keys = self.dict:get_keys(0)
+  -- Prometheus server expects buckets of a histogram to appear in increasing
+  -- numerical order of their label values.
+  table.sort(keys)
+
+  local seen_metrics = {}
+  local output = {}
+  for _, key in ipairs(keys) do
+    local value, err = self.dict:get(key)
+    if value then
+      local short_name = short_metric_name(key)
+      if not seen_metrics[short_name] then
+        if self.help[short_name] then
+          table.insert(output, string.format("# HELP %s%s %s\n",
+            self.prefix, short_name, self.help[short_name]))
+        end
+        if self.type[short_name] then
+          table.insert(output, string.format("# TYPE %s%s %s\n",
+            self.prefix, short_name, self.type[short_name]))
+        end
+        seen_metrics[short_name] = true
+      end
+      -- Replace "Inf" with "+Inf" in each metric's last bucket 'le' label.
+      if key:find('le="Inf"', 1, true) then
+        key = key:gsub('le="Inf"', 'le="+Inf"')
+      end
+      table.insert(output, string.format("%s%s %s\n", self.prefix, key, value))
+    else
+      self:log_error("Error getting '", key, "': ", err)
+    end
+  end
+  return output
+end
+
+-- Present all metrics in a text format compatible with Prometheus.
+--
+-- This function should be used to expose the metrics on a separate HTTP page.
+-- It will get the metrics from the dictionary, sort them, and expose them
+-- aling with TYPE and HELP comments.
+function Prometheus:collect()
+  ngx.header.content_type = "text/plain"
+  ngx.print(self:metric_data())
+end
+
+return Prometheus