Add basic instrumentation possibilities to pretix (#271)

* add basic instrumentation possibilities to pretix

* make tabs to spaces

* apply flake8

* implement upstreams suggestions, round 1

* adjust naming of redis-connection

* address noredis

* add view for metrics

* implement HTTP basic auth in front of metrics-endpoint

* rename labelset

* make flake8-clean

* implement upstreams suggestions, round 2

* correct minor slipups

* fix missing return

* let isort add an empty line

* implement test for counter

* implement upstream suggestions, round 3

* correct typo

* implement first test for view

* finish view-test

* fix deprecated keyword

* implement upstream-suggestions, round 4

* implement test for gauge

* test exceptions as well

* add db-decorator

src/pretix/base/metrics.py

@@ -0,0 +1,150 @@
+from django.conf import settings
+
+if settings.HAS_REDIS:
+    import django_redis
+    redis = django_redis.get_redis_connection("redis")
+
+REDIS_KEY_PREFIX = "pretix_metrics_"
+
+
+class Metric(object):
+    """
+    Base Metrics Object
+    """
+
+    def __init__(self, name, helpstring, labelnames=None):
+        self.name = name
+        self.helpstring = helpstring
+        self.labelnames = labelnames or []
+
+    def __repr__(self):
+        return self.name + "{" + ",".join(self.labelnames) + "}"
+
+    def _check_label_consistency(self, labels):
+        """
+        Checks if the given labels provides exactly the labels that are required.
+        """
+
+        # test if every required label is provided
+        for labelname in self.labelnames:
+            if labelname not in labels:
+                raise ValueError("Label {0} not specified.".format(labelname))
+
+        # now test if no further labels are required
+        if len(labels) != len(self.labelnames):
+            raise ValueError("Unknown labels used: {}".format(", ".join(set(labels) - set(self.labelnames))))
+
+    def _construct_metric_identifier(self, metricname, labels=None):
+        """
+        Constructs the scrapable metricname usable in the output format.
+        """
+        if not labels:
+            return metricname
+        else:
+            named_labels = []
+            for labelname in self.labelnames:
+                named_labels.append('{}="{}",'.format(labelname, labels[labelname]))
+
+            return metricname + "{" + ",".join(named_labels) + "}"
+
+    def _inc_in_redis(self, key, amount):
+        """
+        Increments given key in Redis.
+        """
+        rkey = REDIS_KEY_PREFIX + key
+        if settings.HAS_REDIS:
+            redis.incrbyfloat(rkey, amount)
+
+    def _set_in_redis(self, key, value):
+        """
+        Sets given key in Redis.
+        """
+        rkey = REDIS_KEY_PREFIX + key
+        if settings.HAS_REDIS:
+            redis.set(rkey, value)
+
+
+class Counter(Metric):
+    """
+    Counter Metric Object
+    Counters can only be increased, they can neither be set to a specific value
+    nor decreased.
+    """
+
+    def inc(self, amount=1, **kwargs):
+        """
+        Increments Counter by given amount for the labels specified in kwargs.
+        """
+        if amount < 0:
+            raise ValueError("Counter cannot be increased by negative values.")
+
+        self._check_label_consistency(kwargs)
+
+        fullmetric = self._construct_metric_identifier(self.name, kwargs)
+        self._inc_in_redis(fullmetric, amount)
+
+
+class Gauge(Metric):
+    """
+    Gauge Metric Object
+    Gauges can be set to a specific value, increased and decreased.
+    """
+
+    def set(self, value=1, **kwargs):
+        """
+        Sets Gauge to a specific value for the labels specified in kwargs.
+        """
+        self._check_label_consistency(kwargs)
+
+        fullmetric = self._construct_metric_identifier(self.name, kwargs)
+        self._set_in_redis(fullmetric, value)
+
+    def inc(self, amount=1, **kwargs):
+        """
+        Increments Gauge by given amount for the labels specified in kwargs.
+        """
+        if amount < 0:
+            raise ValueError("Amount must be greater than zero. Otherwise use dec().")
+
+        self._check_label_consistency(kwargs)
+
+        fullmetric = self._construct_metric_identifier(self.name, kwargs)
+        self._inc_in_redis(fullmetric, amount)
+
+    def dec(self, amount=1, **kwargs):
+        """
+        Decrements Gauge by given amount for the labels specified in kwargs.
+        """
+        if amount < 0:
+            raise ValueError("Amount must be greater than zero. Otherwise use inc().")
+
+        self._check_label_consistency(kwargs)
+
+        fullmetric = self._construct_metric_identifier(self.name, kwargs)
+        self._inc_in_redis(fullmetric, amount * -1)
+
+
+def metric_values():
+    """
+    Produces the scrapable textformat to be presented to the monitoring system
+    """
+    if not settings.HAS_REDIS:
+        return ""
+
+    metrics = {}
+
+    for key in redis.scan_iter(match=REDIS_KEY_PREFIX + "*"):
+        dkey = key.decode("utf-8")
+        _, _, output_key = dkey.split("_", 2)
+        value = float(redis.get(dkey).decode("utf-8"))
+
+        metrics[output_key] = value
+
+    return metrics
+
+
+"""
+Provided metrics
+"""
+http_requests_total = Counter("http_requests_total", "Total number of HTTP requests made.", ["code", "handler", "method"])
+# usage: http_requests_total.inc(code="200", handler="/foo", method="GET")