53 lines
2.5 KiB
Markdown
53 lines
2.5 KiB
Markdown
```markdown
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# Sepsis Engine Design Decisions
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## Why Redis for SIRS state, not a PostgreSQL time-range query
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At a medium hospital with 200 concurrent inpatients, each generating five observations
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per patient per minute, the sepsis engine processes approximately 17 observation events
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per second at steady state.
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A PostgreSQL alternative would look like this on every event:
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```sql
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SELECT observation_code
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FROM observations
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WHERE encounter_id = :encounterId
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AND observation_code IN ('TEMP_C', 'HEART_RATE', 'RESP_RATE', 'WBC_K_UL')
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AND recorded_at >= NOW() - INTERVAL '30 minutes'
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AND (
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(observation_code = 'TEMP_C' AND (value > 38.3 OR value < 36.0)) OR
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(observation_code = 'HEART_RATE' AND value > 90) OR
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(observation_code = 'RESP_RATE' AND value > 20) OR
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(observation_code = 'WBC_K_UL' AND (value > 12.0 OR value < 4.0))
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);
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```
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This query hits the `observations` table on every event. Under load it competes for
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I/O with the ingest path writing new rows — both want the same composite index. With
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Redis: four `SET`/`DEL` operations and one `MGET`, all O(1), all in-memory. No disk
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I/O, no lock contention with the write path.
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The TTL enforces the 30-minute sliding window automatically. Without Redis (or an
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equivalent in-memory store), a background job would be needed to clean up stale
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criteria — another failure point, another deployment concern.
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## Why not Apache Flink
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Flink is a distributed stream processor designed for stateful computation at scale
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(millions of events per second across a fleet). It brings real costs:
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- A Flink cluster (JobManager + TaskManagers) is infrastructure that must be deployed,
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monitored, and upgraded independently of the application.
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- Flink state backends (RocksDB, heap) add operational complexity that is not
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justified unless the stream volume saturates what a single consumer thread can handle.
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- Flink's exactly-once semantics require Kafka transactions, which add latency and
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require tuning separate from the rest of the application.
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At a single hospital (200 inpatients, ~17 observations/second), a Kafka consumer +
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Redis state store handles the volume with single-digit millisecond latency per event
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and no additional infrastructure. The trade-off: if this system needed to scale to a
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multi-hospital network with 50,000+ concurrent inpatients (~5,000 observations/second),
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Flink would become the right choice. The architecture decision is correct at this scale
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and defensible at interview with a clear scale inflection point named.
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``` |