EventStore Projections – Calculating daily averages

This projection will calculate the daily average consumption per device. We take our Device streams as input and store an intermediate calculation in the state for each one. This calculation contains the total sum of measurements for the current day. The projection’s result is a new stream for each device which contains a new calculated average for each day.


The $init method initializes the state. Each subsequent MeasurementRead event will invoke the calculate method, which will update the state with the total number of measurements, the sum of the measurements, and the timestamp of the last measurement. When we determine from the timestamp that a full day has passed, we emit an event which contains the date, the sum, and the calculated average, obtained by dividing the sum of the measurements by the number of measurements.


JavaScript implementation

First, we implement a daily timestamp formatter which will handle formatting the date part of a JavaScript Date object. Normally, we’d use an existing library such as momentjs to format the date part, but Event Store doesn’t currently let us include external libraries. One workaround for this would be to inject the library in the projection source when the projection is created. I might cover this option in a future blog post, but for now, we’ll have to roll our own function.

Next, we implement the projection module with the init and the cacluate methods:

Just like previous projections, we instantiate the calculator module once globally and then reference it in the projection’s definition:

Again we use the fromCategory(“Device”) definition to tell the engine that this projection’s events come from all streams in the Device category. Streams are categorized based on the stream name before the final dash by default: for example, Device-0 and Device-1 are both in the Device category. To enable categories, the $stream_by_category and $by_category system projections should be enabled. We delegate $init and MeasurementRead event handling to the calculator module, and we’re done.

Testing the projection

First, we test the formatting of the date part of the timestamp:

Next, we define the scenarios for testing the projection module:

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The first two examples are almost too simple to be worth writing tests for at all, but this example demonstrates how easy it is to test projections with JavaScript and Jasmine. All you have to do is call the projection module with the event stream as its input and then check whether the resulting state matches the expected state. We can also verify the emitted events by testing whether the mocked emit methods are called with the correct events.

Querying the results with the Client API

All new events are emitted to a stream, so reading them is straightforward:

The implementation of the ReadStreamEventsBackward method is explained in the Client API post.


This a fairly naive implementation. First of all, the calculation doesn’t handle processing the same event twice properly: it will just add the value to the total twice, throwing off the average. It also expects events in some semblance of chronological order: if it receives an event whose timestamp indicates that a day has passed before it’s actually processed all the events for the day, then the emitted average event will be erroneous. Finally, it will fail to calculate the average entirely if a device stops sending messages during the day. To fix this, we’ll need to implement some kind of timeout mechanism. I’ll cover how to fix these issues in a future post.

Source code

A working project for this example can be found on github:

Event Store Projections by Example

This post is part of a series:

  1. EventStore Client API Basics (C#)
  2. Counting events of a specific type
  3. Partition events based on data found in previous events
  4. Calculating an average per day
  5. The irresponsible gambler
  6. Distribute events to other streams
  7. Temporal Projection to generate alarms
  8. Projection in C#

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