> ## Documentation Index
> Fetch the complete documentation index at: https://opendata.dev/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# OpenData: Reading Data

Reading data from OpenData databases heavily leverages data caches to avoid
unnecessary `GET` API calls to object storage.

<pre className="ascii-art">
  {`
    ┌─────────────────────────────────────────────────────────────────────┐
    │▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ opendata read path ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│
    ├─────────────────────────────────────────────────────────────────────┤
    │                                                                     │
    │                 ╔═ ══ ══ ═╗      ╔═════════╗      ╔═════════╗       │
    │                                  ║         ║▒     ║         ║▒      │
    │ ╔══════════╗    ║In-Memory║      ║In-Memory║▒     ║  NVMe   ║▒      │
    │ ║read(data)║───▶║  Delta  ║─────▶║  Cache  ║─────▶║  Cache  ║▒      │
    │ ╚══════════╝                     ║         ║▒     ║         ║▒      │
    │                 ╚═ ══ ══ ═╝      ╚═════════╝▒     ╚═════════╝▒      │
    │                                   ▒▒▒▒▒▒▒▒▒▒▒      ▒▒▒▒│▒▒▒▒▒▒      │
    │                                       ┌────────────────┘            │
    │                                       │                             │
    │                                       ▼                             │
    │                  ╔═════════════════════════════════════════╗        │
    │                  ║                                         ║▒       │
    │                  ║             Object Storage              ║▒       │
    │                  ║                                         ║▒       │
    │                  ╚═════════════════════════════════════════╝▒       │
    │                   ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒       │
    │                                                                     │
    └─────────────────────────────────────────────────────────────────────┘
    `}
</pre>

Reading data relies on a combination of domain-specific logic as well
as shared storage access primitives. Each database implements its own
indexes and data layouts for optimal read performance, but all use
SlateDB queries to access the data through multiple levels of caches.

### Read Freshness

Read freshness can be measured in terms of object storage round trips between
a write being accepted and it being visible to a query:

| Write Path                                                       | Read Target                                             | S3 Round Trips | Description                                                                                                                                                   |
| ---------------------------------------------------------------- | ------------------------------------------------------- | -------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Direct write to writer                                           | Writer                                                  | **0**          | Data is written to the in-memory Delta and is immediately readable on the writer with no object storage interaction.                                          |
| [Zonal ingest](/overview/architecture#stateless-zonal-ingestion) | Writer                                                  | **1**          | Data is written to object storage by a stateless ingestor and must be picked up by the writer before it is readable.                                          |
| Any                                                              | [Read replica](/overview/architecture#multiple-readers) | **+1**         | Read replicas discover new data by watching object storage for manifest updates, adding one additional round trip on top of whatever the write path requires. |

Each round trip typically adds around 100ms of latency plus the polling
interval, which is configurable. In practice, the polling interval dominates
the freshness delay.