How does OpenTSDB downsample data

Question

I have a 2 part question regarding downsampling on OpenTSDB.

The first is I was wondering if anyone knows whether OpenTSDB takes the last end point inclusive or exclusive when it calculates downsampling, or does it count the end data point twice?

For example, if my time interval is 12:30pm-1:30pm and I get DPs every 5 min starting at 12:29:44pm and my downsample interval is summing every 10 minute block, does the system take the DPs from 12:30-12:39 and summing them, 12:40-12:49 and sum them, etc or does it take the DPs from 12:30-12:40, then from 12:40-12:50, etc. Yes, I know my data is off by 15 sec but I don't control that.

I've tried to calculate it by hand but the data I have isn't helping me. The numbers I'm calculating aren't adding up to the above, nor is it matching what the graph is showing. I don't have access to the system that's pushing numbers into OpenTSDB so I can't setup dummy data to check.

The second question is how does downsampling plot its points on the graph from my time range and downsample interval? I set downsample to sum 10 min blocks. I set my range to be 12:30pm-1:30pm. The graph shows the first point of the downsampled graph to start at 12:35pm. That makes logical sense.
I change the range to be 12:24pm-1:29pm and expected the first point to start at 12:30 but the first point shown is 12:25pm.

Hopefully someone can answer these questions for me. In the meantime, I'll continue trying to find some data in my system that helps show/prove how downsampling should work.

Thanks in advance for your help.

Answer 1

Downsampling isn't currently working the way you expect, although since this is a reasonable and commonly made expectations, we are thinking of changing this in a later release of OpenTSDB.

You're assuming that if you ask for a "10 min sum", the data points will be summed up within each "round" (or "aligned") 10 minute block (e.g. 12:30-12:39 then 12:40-12:49 in your example), but that's not what happens. What happens is that the code will start a 10-minute block from whichever data point is the first one it finds. So if the first one is at time 12:29:44, then the code will sum all subsequent data points until 600 seconds later, meaning until 12:39:44.

Within each 600 second block, there may be a varying number of data points. Some blocks may have more data points than others. Some blocks may have unevenly spaced data points, e.g. maybe all the data points are within one second of each other at the beginning of the 600s block. So in order to decide what timestamp will result from the downsampling operation, the code uses the average timestamp of all the data points of the block.

So if all your data points are evenly spaced throughout your 600s block, the average timestamp will fall somewhere in the middle of the block. But if you have, say, all the data points are within one second of each other at the beginning of the 600s block, then the timestamp returned will reflect that by virtue of being an average. Just to be clear, the code takes an average of the timestamps regardless of what downsampling function you picked (sum, min, max, average, etc.).

If you want to experiment quickly with OpenTSDB without writing to your production system, consider setting up a single-node OpenTSDB instance. It's very easy to do as is shown in the getting started guide.