The Federation Tier with AQL

This page is a plain account of the specification FerroFED implements. It cites the sections of the vendored text at docs/specs/federation-spec/ and adds nothing of FerroFED’s own; the next page does that.

Three tiers

The specification adds a Federation Tier to openEHR querying, so that a client can run one query across several openEHR CDRs as though they were a single repository (§1). The system has three tiers (§3.1):

• the application tier, where a client sends AQL and reads the combined result;
• the Federation Tier, one or more gateways that act as the intermediary;
• the node tier, member CDRs that run ordinary openEHR queries scoped to a single EHR and need not know they belong to a federation.

The gateway is a technically transparent intermediary (§3.2). The client interface is a conformant openEHR Query API, and a conformant ITS-REST surface generally, so a basic patient query needs no federation-specific syntax.

The flow

Federation keys on each node’s local ehr_id. Patient identity is resolved outside the query, and each node receives standard AQL scoped to its own ehr_id (§4).

flowchart LR
    C[Client] -->|"AQL with a subject predicate"| G[Gateway]
    G -->|"patient identifier"| X[Cross-reference service]
    X -->|"ehr_id per node, or not found"| G
    G -->|"standard AQL scoped to ehr_id A"| A[Node A]
    G -->|"standard AQL scoped to ehr_id B"| B[Node B]
    A -->|rows| G
    B -->|rows| G
    G -->|"one RESULT_SET with meta.federation"| C

1. The façade. The client sends POST {base}/v1/query/aql and may identify the patient the openEHR way, with WHERE e/ehr_status/subject/external_ref/id/value = <patientId> (§4.1, §7).
2. Resolution. The gateway resolves the identifier to a set of {node, ehr_id} pairs through a localization service, a directory and an identifier cross-reference service. The proposed bindings are IHE XCPD, mCSD and PIXm (§4.1, §5, Annex A). Without a localization service the gateway asks every known node (§4.3).
3. Fan-out. Each node with a resolved ehr_id receives the node AQL: the subject predicate replaced by e/ehr_id/value = '<resolved>', every other patient identifier removed or the query refused, and everything else forwarded unchanged (§7.1).
4. Combine. The gateway concatenates the rows, applies DISTINCT and ORDER BY across nodes, and reports every node it asked under meta.federation.endpoints[] with a status (§9, §11.1).

What the specification insists on

• No identifier leaves the gateway. The patient identifier used for resolution is consumed at the gateway and never reaches a node, in the query, its path or its headers (§5.4, N33).
• A partial answer is never presented as a whole one. When a node that was asked does not answer, the default is to fail the query, and every response carries meta.federation.complete (§3.2, §11.4).
• Consent stays at the node. A node enforces consent before it releases data; a gateway pre-filter is optional and never replaces that check (§1, N27).
• Follow-up reads and writes go to the owning CDR, routed on creating_system_id (§12).
• Some things are refused, not approximated: federated demographics, object creation across nodes, cross-node OFFSET paging and undirected aggregates (§2.3, §3.2).

The specification closes with a consolidated list of numbered conformance points (§17) and a Connectathon-style test approach (§16).