# Calculate Delta - Aggregate Filter

**URL:** <https://discuss.elastic.co/t/calculate-delta-aggregate-filter/159288>\
**Category:** Logstash\
**Created:** [December 4, 2018, 5:07am UTC](https://discuss.elastic.co/t/calculate-delta-aggregate-filter/159288 "2018-12-04T05:07:29Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![karnamonkster](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/karnamonkster/32/67266_2.png) [@karnamonkster](https://discuss.elastic.co/u/karnamonkster)\
**Post date:** [December 4, 2018, 5:07am UTC](https://discuss.elastic.co/t/calculate-delta-aggregate-filter/159288/1 "2018-12-04T05:07:29Z")

</div>

Hi,

I need to calculate delta consumption from a stream of events generated by multiple devices uniquely identified by an ID.  
Based on some other forums i could see this is possible using Logstash Aggregate filter

Here is my attempt but unsuccessful to calculate delta between two readings for every device.

```
filter {
        aggregate {
        task_id => "%{uniquedeviceid}"
        code => "
                map['next'] ||= 0 ;
                map['next'] -= event.get('reading');
                event.set('delta', (map['next'] - event.get('reading')))"
                timeout_task_id_field => "uniquedeviceid"
        }

```

Need some help in getting towards the solution  
I need to calculate  
`delta = Current Reading - Previous Reading`

---

<div class="post-metadata">

**Author:** ![karnamonkster](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/karnamonkster/32/67266_2.png) [@karnamonkster](https://discuss.elastic.co/u/karnamonkster)\
**Post date:** [December 4, 2018, 9:01am UTC](https://discuss.elastic.co/t/calculate-delta-aggregate-filter/159288/2 "2018-12-04T09:01:50Z")

</div>

Figured out..  
Posting the solution, may help someone.

```
filter {
        aggregate {
        task_id => "%{uniquedeviceid}"
        code => "
                map['next'] ||= event.get('reading');
                event.set('delta', (event.get('reading') - map['next']))"
                timeout_task_id_field => "uniquedeviceid"
        }
```

---

<div class="post-metadata">

**Author:** ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)\
**Post date:** [January 1, 2019, 9:01am UTC](https://discuss.elastic.co/t/calculate-delta-aggregate-filter/159288/3 "2019-01-01T09:01:54Z")

</div>

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