ds: hrpd-x histogram-mode ADR

doc/specific_iocs/datastreaming/ADRs/001_histograms.md

@@ -3,7 +3,7 @@
 
 ## Status
 
-Pending discussion with HRPD-X interested parties (including instrument scientists & Mantid).
+Accepted
 
 ## Context
 
@@ -18,15 +18,19 @@ In event mode, over the course of a run, each individual neutron event's detecti
 Event mode data can be later binned to form a histogram, but a histogram cannot be recovered to individual events. In
 other words, histogramming is lossy. The advantage of histogram mode is that it typically produces smaller data volumes.
 
-Histogram mode has historically been used due to hardware limitations in many cases.
+Histogram mode has historically been used because:
+- Hardware limitations in DAE2/3 preventing event-mode acquisition at high rates
+- Analysis routines in Mantid which may perform poorly on event-mode data
 
 ## Decision
 
-For HRPD-x, we will collect all data, including data from neutron monitors, in event mode only. HRPD-x will not support
-histogram mode.
+For HRPD-x, we will initially stream all data, including data from neutron monitors, in event mode. The Kafka streams will only contain event-mode data.
+
+We will write event-mode Nexus files, however HRPD-X also require histogram-mode Nexus files (not containing neutron events) for their analysis routines.
 
 ## Consequences
 
 - Data volumes on HRPD-x will be higher running in event mode compared to histogram mode. This includes both data in-flight
-during networking and Kafka processing, as well as final Nexus file sizes.
+during networking and Kafka processing, as well as final event-mode Nexus file sizes. The histogram file size is independent of number of events.
 - Only considering events will simplify components of the HRPD-x data streaming implementation.
+- There may be two separate files generated per run, `_event.nxs` and `_histogram.nxs`. The exact naming is to be determined.