Docs for DPL users (#145)

Author: awegrzyn

doc/DPL.md

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+# Instructions for DPL users
+
+DPL stands for [Data Processing Layer](https://github.com/AliceO2Group/AliceO2/tree/dev/Framework/Core) and it is part of ALICE O<sup>2</sup> Framework.
+
+### Monitoring in DPL
+
+A URL to the monitoring backend can be passed through `monitoring-url` parameter. This will redirect all the metric to the selected backend. The list of supported backends is available in the [Monitoring README](https://github.com/AliceO2Group/Monitoring#monitoring-instance) file.
+
+Then, from the `DataProcessingDevice` source Monitoring instance can be accessed via [Monitoring service](https://github.com/AliceO2Group/AliceO2/tree/dev/Framework/Core#monitoring-service).
+
+### Monitoring backends at CERN
+
+There are so-called _Community Services_ available at CERN GPN network and O<sup>2</sup> developers are welcome to use them in order to store and display metrics.
+
+#### Time-series database
+A time-series database, called [InfluxDB](https://docs.influxdata.com/influxdb/v1.7/), is used for metric storage.
+
+Pass the following Monitoring URL via `--monitoring-url` to your workflow in order to redirect metrics to _Community Services_ instance:
+```
+influxdb-udp://aido2mon-gpn:8089
+```
+
+#### Dashboard
+
+Time-series can be visualised in [Grafana](https://grafana.com) using a web browser.
+
+1. Go to: https://alice-o2-mon.cern.ch:3000
+2. Sign in with CERN SSO
+3. Create a new dashboard, then and a new panel and use [Query Editor](http://docs.grafana.org/features/datasources/influxdb/#query-editor) to visualise previously stored metrics.