All articles tagged #promql

Working with real metrics is hard. Metrics are needed to give you an understanding of how your service behaves. That is, by definition, you have some uncertainty about the said behavior. Therefore, you have to be hell certain about your observability part. Otherwise, all sorts of metric misinterpretations and false conclusions will follow.

Here are the things I'm always trying to get confident about as soon as possible:

• How metric collection works - push vs. pull model, aggregation on the client- or server-side?
• How metrics are stored - raw samples or aggregated data, rollup and retention strategies?
• How to query metrics - is my mental model aligned with the actual query execution model?
• How to plot query results - what approximation errors may be induced by the graphing tools?

And even if I have a solid understanding of all of the above stuff, there will be one thing I'm never entirely sure about - the correctness of my query logic. But this one becomes testable once other parts are known.

Recently, I've been through another round of this journey - I was making an acquaintance with Prometheus. Since it was already a third of fourth monitoring system I had to work with, at first, I thought I could skip all the said steps and jump into writing queries to production metrics and reading graphs... The hope was on the knowledge extrapolation. But nope, it didn't work out well. So, I gave up on the idea of cutting corners quickly. That's how I found myself setting up a Prometheus playground, feeding it with some known inputs, observing the outputs, and trying to draw some meaningful conclusion.

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When you have a long series of numbers, such as server memory consumption scraped 10-secondly, it's a natural desire to derive another, probably more meaningful series from it, by applying a moving window function. For instance, moving average or moving quantile can give you much more readable results by smoothing some spikes.

Prometheus has a bunch of functions called <smth>_over_time(). They can be applied only to range vectors. It essentially makes them window aggregation functions. Every such function takes in a range vector and produces an instant vector with elements being per-series aggregations.

For people like me who normally grasp code faster than text, here is some pseudocode of the aggregation logic:

# Input vector example.
range_vector = [
    ({"lab1": "val1", "lab2": "val2"}, [(12, 1624722138), (11, 1624722148), (17, 1624722158)]),
    ({"lab1": "val1", "lab2": "val2"}, [(14, 1624722138), (10, 1624722148), (13, 1624722158)]),
    ({"lab1": "val1", "lab2": "val2"}, [(16, 1624722138), (12, 1624722148), (15, 1624722158)]),
    ({"lab1": "val1", "lab2": "val2"}, [(12, 1624722138), (17, 1624722148), (18, 1624722158)]),
]

# agg_func examples: `sum`, `min`, `max`, `avg`, `last`, etc.

def agg_over_time(range_vector, agg_func, timestamp):
    # The future instant vector.
    instant_vector = {"timestamp": timestamp, "elements": []}

    for (labels, samples) in range_vector:
        # Every instant vector element is 
        # an aggregation of multiple samples.
        sample = agg_func(samples)
        instant_vector["elements"].append((labels, sample))

    # Notice, that the timestamp of the resulting instant vector 
    # is the timestamp of the query execution. I.e., it may not 
    # match any of the timestamps in the input range vector.
    return instant_vector

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PromQL looks neat and powerful. And at first sight, simple. But when you start using it for real, you'll quickly notice that it's far from being trivial. Searching the Internet for query explanation rarely helps - most articles focus on pretty high-level overviews of the language's most basic capabilities. For example, when I needed to match multiple metrics using the common labels, I quickly found myself reading the code implementing binary operations on vectors. Without a solid understanding of the matching rules, I constantly stumbled upon various query execution errors, such as complaints about missing group_left or group_right modifier. Reading the code, feeding my local Prometheus playground with artificial metrics, running test queries, and validating assumptions, finally helped me understand how multiple metrics can be joined together. Below are my findings.

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