Streaming Data Types

FlatBuffers is the format chosen for the ESS messaging system.

We would like to be able to read any message in the system at any time, therefore:

All schemas that we use for the transmission of data are collected in this repository.

The names of the schema files in this repository are prefixed by their unique 4-character file_identifier. This file_identifier must be set in the schema definition file as:

file_identifier = "abcd";

The file identifiers (also called "schema id") must be unique on the network. The root_type should include the schema version number. For example, the f144 schema has the root_type:

root_type f144_LogData;

The naming convention for new identifiers and a table of existing identifiers follows later in this README. Please add your own (new schema) with file identifier to that table.

Backwards compatibility

Please, avoid changes which break binary compatibility. FlatBuffers documentation contains good information about how to maintain binary compatibility. If you need to make breaking changes to schemas that are not under development, acquire a new schema id.

Schemas that are under development should be clearly marked as such in the schema file and in the Schema ids below to warn users of possible loss of backwards compatibility.

Not enough file identifiers available?

If you feel that you may need a lot of schema ids, you can use a single schema and work with the flat buffers union data type in your root element.

Schema coding standard

• Completely new schemas should have an ID comprising of two characters plus 00, e.g. hs00
• When updating an existing schema with a breaking change then the new schema should have the same ID but with the number incremented, e.g. hs00 -> hs01
  • For older schema which don't end with two numbers, propose a new name which matches the convention.
• Prefix your schema files in this repository with the chosen schema id to more easily prevent id collision.
• Tables should use UpperCamelCase.
• Fields should use snake_case.
• Try to keep names consistent with equivalent fields in existing schema, e.g.:
  • timestamp for timestamp
  • source_name for a string indicating origin/source of data in flatbuffer
  • service_id for a string indicating the name of the service that created the flatbuffer
• Do not use unsigned integers unless required for your application.

Schema ids

ID	File name	Description
f140	f140_general.fbs	[OBSOLETE] Can encode an arbitrary EPICS PV
f141	f141_ntarraydouble.fbs	[OBSOLETE] A simple array of double, testing file writing
f142	f142_logdata.fbs	For log data, for example forwarded EPICS PV update [superseded by f144]
f143	f143_structure.fbs	[OBSOLETE] Arbitrary nested data
f144	f144_logdata.fbs	Controls related log data, typically from EPICS or NICOS. Note: not to be used for array data
ev42	ev42_events.fbs	Multi-institution neutron event data for a single pulse
ev43	ev43_events.fbs	Multi-institution neutron event data from multiple pulses
ev44	ev44_events.fbs	Multi-institution neutron event data for both single and multiple pulses
an44	an44_events.fbs	ANSTO-specific variant of ev44
ar51	ar51_readout_data.fbs	Streaming raw ESS detector readout data
is84	is84_isis_events.fbs	ISIS specific addition to event messages
ba57	ba57_run_info.fbs	[OBSOLETE] Run start/stop information for Mantid [superseded by pl72]
df12	df12_det_spec_map.fbs	Detector-spectrum map for Mantid
senv	senv_data.fbs	(DEPRECATED) Used for storing for waveforms from DG ADC readout system.
se00	se00_data.fbs	Used for storing arrays with optional timestamps, for example waveform data. Replaces senv.
NDAr	NDAr_NDArray_schema.fbs	(DEPRECATED) Holds binary blob of data with n dimensions
ADAr	ADAr_area_detector_array.fbs	(DEPRECATED) EPICS area detector array data [superseded by ad00]
ad00	ad00_area_detector_array.fbs	EPICS area detector array data
mo01	mo01_nmx.fbs	Daquiri monitor data: pre-binned histograms, raw hits and NMX tracks
ns10	ns10_cache_entry.fbs	NICOS cache entry
ns11	ns11_typed_cache_entry.fbs	NICOS cache entry with typed data
hs00	hs00_event_histogram.fbs	Event histogram stored in n dim array
hs01	hs01_event_histogram.fbs	Event histogram stored in n dim array
dtdb	dtdb_adc_pulse_debug.fbs	Debug fields that can be added to the ev42 schema
ep00	ep00_epics_connection_info.fbs	(DEPRECATED) Status of the EPICS connection
ep01	ep01_epics_connection.fbs	Status or event of EPICS connection. Replaces ep00
json	json_json.fbs	Carries a JSON payload
tdct	tdct_timestamps.fbs	Timestamps from a device (e.g. a chopper)
pl72	pl72_run_start.fbs	File writing, run start message for file writer and Mantid
6s4t	6s4t_run_stop.fbs	File writing, run stop message for file writer and Mantid
answ	answ_action_response.fbs	Holds the result of a command to the filewriter
wrdn	wrdn_finished_writing.fbs	Message from the filewriter when it is done writing a file
x5f2	x5f2_status.fbs	Status update and heartbeat message for any software
rf5k	rf5k_forwarder_config.fbs	(DEPRECATED) Configuration update for Forwarder [superseded by fc00]
fc00	fc00_forwarder_config.fbs	Configuration update for Forwarder
al00	al00_alarm.fbs	Generic alarm schema for EPICS, NICOS, etc.
da00	da00_dataarray.fbs	Pseudo-scipp DataArray with time-dependent and constant Variables

Useful information:

• Have CMake download and compile schema
• Time formats we use and how to convert between them