Core Concepts

Tiers

Tier	Runtime Component	Purpose
Hot	Storage Engine	Primary write path, low-latency reads, canonical MQL behavior
Cold	Query Engine	Cost-efficient historical/data-lake-style reads
Federated	Coordinator	Merges hot and cold results for a single query response

Query Routing Modes

Thermocline supports per-collection policy-driven routing through the coordinator:

• hotOnly
• coldArchival
• sizeBasedArchival

Policy commands are exposed through gateway-admin command handlers (thermoclineSetPolicy, thermoclineGetPolicy, thermoclineListPolicies, thermoclineDeletePolicy, thermoclineValidatePolicy).

Separation of Responsibilities

• Gateway: protocol and command routing, not query semantics.
• Coordinator: orchestration and merge semantics, not storage persistence.
• Storage Engine: canonical MQL operator semantics and hot-tier persistence.
• Query Engine: cold-tier scanning and operator execution for supported subsets.

This separation is enforced by architecture tests and guardrails:

• make test-architecture
• make check-guardrails

MongoDB Compatibility Model

Thermocline aims for MongoDB wire and MQL compatibility with explicit tier-level truth:

• Fully supported operators/stages
• Partial operators/stages with noted caveats
• Routed behavior where cold paths delegate semantics through hot

See Compatibility for the matrix.

Source of Truth

• services/gateway/src/proxy/handler/policy.rs
• services/coordinator/internal/server/server.go
• services/storage-engine/src/filter/evaluator.rs
• services/query-engine/src/executor/execution.rs