# Purviews

For the RC to route requests to the appropriate DAPs, it must know what data each DAP contains. Thus, on the RC-DAP handshake, the DAP sends the RC the following information describing the data it contains:

The labels and time range make up a DAP's purview. This is the means by which the RC determines data locality when making routing decisions.

## Labels

You define labels in the assembly configuration file. They are arbitrary key-value pairs that are meant to be user-friendly tags describing the kind of data held in the assembly's DAPs. Labels can be used to target particular assemblies in queries (see: Routing). The examples below show the labels section of multiple assembly files:

# Market data assemblies covering different asset classes, exchanges and countries.
labels:
class: equity
exchange: nyse
country: usa

labels:
class: futures
exchange: nasdaq
country: usa

labels:
class: equity
exchange: tsx

...

# Sensors data assemblies covering different sensor types.
labels:
sensorType: electric

labels:
sensorType: gas

...

# Market data assemblies covering different asset classes, exchanges and countries.
spec:
labels:
class: equity
exchange: nyse
country: usa

spec:
labels:
class: futures
exchange: nasdaq
country: usa

spec:
labels:
class: equity
exchange: tsx

...

# Sensors data assemblies covering different sensor types.
spec:
labels:
sensorType: electric

spec:
labels:
sensorType: gas

...


Labels

• An assembly must have at least one label, but there is no maximum number.
• Two assemblies with the same labels are considered to have the same data (this can be used for redundancy).
• Labels need not have a consistent set of label keys; the RC treats any missing labels as null (see Routing).
• Label keys should not use the names of tables or columns in the assembly's schema.

## Temporal purview

A DAP's temporal purview refers to the time range which it covers. It is expressed as two timestamps.

• startTS - inclusive start time of the data the DAP covers
• endTS - exclusive end time of the data the DAP covers

Unlike labels, the temporal purview of a DAP changes over time as data migrates across tiers at end-of-interval (EOI) and end-of-day (EOD) events. When such events occur, the DAPs automatically update the RC of the change in their temporal purview.