04. 12. 2019 Patrick Zambelli ITOA, NetEye

Downsampling Performance Data in InfluxDB

During the course of a NetEye installation, a wide range of performance data is collected. From latency metrics of ping checks, to the saturation data of disks and memory units, to the collection of utilization metrics of various kinds. For all of this data we’ve adopted the open source time series database InfluxDB.

The time-aligned method of allocating data makes InfluxDB a great choice for comparing data from various sources over a specific time frame. Just such a use case arose during a recent project in the context of an IoT environment, where we wanted to collect performance data from a large number of devices. Given that the number of devices was only expected to grow, the customer desired to keep historical data for a period of at least one year.

Since the IoT data is collected at a resolution rate of 10 seconds, the quantity of stored performance data grows quite quickly. This was the point at which we asked ourselves whether we actually need to keep all that data at such a high resolution for such a long period of time.

We decided to implement a retention management policy to keep only very recent data at full resolution. Data older than one month would instead be kept as a single aggregated average data point per hour.

In order to realize this concept, we chose an implementation path using continuous queries and retention policies. To give a rough understanding of both concepts, I suggest you read about the principles behind a retention policy and how to implement the concept of downsampling, combining retention policies and continuous queries in InfluxDB.

Retention Policy Management

Based on the concepts of defining retention policies, we decided to limit the default retention policy to 1 month, while adding an additional retention policy to store the lower resolution data for 1 year. This latter retention policy represents all data collected for 1 year at the resolution of 1 data point per hour.

For this we first start by implementing a new retention policy acting in addition to the default “autogenerated” retention policy. We create this retention policy with a duration of 365 days:

> CREATE RETENTION POLICY rp_365d ON perfdata DURATION 365d REPLICATION 1 
> ALTER RETENTION POLICY "autogen" ON "perfdata" DURATION 30d SHARD DURATION 2h DEFAULT 

> show retention policies
name    duration  shardGroupDuration replicaN default
----    --------  ------------------ -------- -------
autogen 720h0m0s  2h0m0s             1        true
rp_365d 8760h0m0s 168h0m0s           1        false 

Continuous Queries

Now we have another retention policy within the same database, but data is still collected only within the “autogen” retention policy. Here continuous queries come into play: They ship the downsampled performance data into the new retention policy.

We start with the definition of a create continuous query statement and define it to read the mean() of the “value” of all fields for 1 hour from all measurements available in the current database, and then write it into the new retention policy “rp_365d” while maintaining the name of the original measurement.

HINT: If you omit the “AS value” in the Select section, the newly created field will be named “mean_value” instead of retaining the name “value”. If you don’t do so, visualization in Grafana will be more difficult.

CREATE CONTINUOUS QUERY "cq_measurementXX" ON "measurementXX" 
BEGIN 
   SELECT mean(value) AS value
   INTO "perfdata"."rp_365d".:MEASUREMENT 
   FROM /.*/
   GROUP BY time(60m),* 
END 

Creating Visualizations in Grafana that Adapt Dynamically to a Suitable Retention Policy

We now have performance data collected in InfluxDB according to our two retention policies. Visualization in Grafana should adapt according those retention policies.

Unfortunately, the behavior is not that automatic, and the select statements within the various visualizations act on a specific retention policy (if not specified, they default to “autogen”). But we wanted to see performance data at the highest resolution whenever we are observing data from the most recent 30 days!

In the Grafana user forum I found a rather interesting discussion about implementing dynamic visualization in Grafana: https://github.com/grafana/grafana/issues/4262

Here I’ll explain the approach to you with a simple example:

Step 1: Map your retention policies through 2 data series stored within a single dedicated retention policy:

CREATE RETENTION POLICY "forever" ON perfdata DURATION INF REPLICATION 1 

Step 2: For each retention policy, write a series indicating the start and end times of the corresponding retention policy:

INSERT INTO forever rp_config,idx=1 rp="autogen",start=0i,end=‭2592000000‬i -9223372036854775806
INSERT INTO forever rp_config,idx=2 rp="rp_365d",start=2592000000i,end=‭31536000000‬i -9223372036854775806 

Note: I assumed a day length of ‭86400000. Insert #1 is that value × 30, and Insert #2 is × 365.

Now verify the stored data:

select * from forever.rp_config
name: rp_config
time end idx rp start
---- --- --- -- -----
-9223372036854775806 2592000000 1 autogen 0
-9223372036854775806 3153600000 2 rp_365d 2592000000

Next, create a “variable” in Grafana that provides you with the suitable retention policy for the chosen time frame:

Variable name: rp
Type: Query
Query: SELECT rp FROM forever.rp_config WHERE $__to - $__from > "start" AND $__to - $__from <= "end"

And now, apply this variable to the queries for your Grafana graphs and look at the results:

  • When visualizing the time range now – “last 30 days”, the “autogen” retention policy is automatically selected
  • When visualizing the time range “last 30 days” to “last 365 days” the retention period “rp_365d” is chosen.
Patrick Zambelli

Patrick Zambelli

Project Manager at Würth Phoenix
After my graduation in Applied Computer Science at the Free University of Bolzano I decided to start my professional career outside the province. With a bit of good timing and good luck I went into the booming IT-Dept. of Geox in the shoe district of Montebelluna, where I realized how a big IT infrastructure has to grow and adapt to quickly changing requirements. During this experience I had also the nice possibility to travel the world, while setting up the various production and retail areas of this company. Arrived at Würth Phoenix I started developing on our monitoring solution NetEye. Today, in my position as Consulting an Project Manager I am continuously heading to implement our solutions to meet the expectation of your enterprise customers.

Author

Patrick Zambelli

After my graduation in Applied Computer Science at the Free University of Bolzano I decided to start my professional career outside the province. With a bit of good timing and good luck I went into the booming IT-Dept. of Geox in the shoe district of Montebelluna, where I realized how a big IT infrastructure has to grow and adapt to quickly changing requirements. During this experience I had also the nice possibility to travel the world, while setting up the various production and retail areas of this company. Arrived at Würth Phoenix I started developing on our monitoring solution NetEye. Today, in my position as Consulting an Project Manager I am continuously heading to implement our solutions to meet the expectation of your enterprise customers.

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