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+# TIKA-4771 sketch — composable Document, coordinator optional
+
+Status: design for discussion (TIKA-4771). A contract proposal, not an
+implementation.
+
+## Where this sits against the ticket
+
+TIKA-4771 as filed proposes Tika brokering registered external parsers, the way
+fetchers and emitters register today, with Parse and ParseStream plugin modes.
+This sketch is the contract-first take on the same goal: fix the envelope and
+the composition rules first, so coordination can live anywhere — a StormCrawler
+bolt today, a one-call service later, and an in-Tika broker, if wanted, becomes
+one implementation of the contract rather than its definition. Round one ships
+no broker.
+
+## Who owns what
+
+- StormCrawler (or any orchestrator): acquisition, exact bytes, provenance,
+  and the composition itself.
+- Tika: the Document and the composition rules.
+- External parsers: their own result schemas.
+- Consumers: unpack only the extension types they understand.
+
+## Flow
+
+    composer (SC bolt now; coordinator service later, same rules)
+        │  bytes
+        ├────► Tika ParseBytes ────► Document (core + extra)
+        ├────► external parser ───► ExtensionResult
+        └─ compose ─► one Document (core + extra + extensions)
+
+## Proto
+
+    message ExtensionResult {
+      string producer_id = 1;           // names the composer's contribution slot,
+                                        // not a service instance: the same service
+                                        // called twice is two slots
+      Status status = 2;                // OK | EMPTY | FAILED | SKIPPED
+      google.protobuf.Any payload = 3;  // producer-owned schema; multiplicity inside
+      repeated string warnings = 4;
+      int64 elapsed_ms = 5;
+    }
+    // in Document:
+    repeated ExtensionResult extensions = 40;   // the number the ticket already
+                                                // names, left unassigned on the
+                                                // 4795 branch
+
+## Rules
+
+1. Ownership is disjoint. core + extra belong to the core parse; extensions are
+   append-only. An extension can't touch core fields.
+2. Identity lives with the composer. Its work-item id names the plan, keys the
+   dedup, and becomes the composed Document's id. The envelope carries no
+   identity: it never travels alone in round one. A correlation field is an
+   additive change the day envelopes go detached.
+3. The composer hashes its input bytes and compares the result with Tika's
+   origin.sha256, when present. A mismatch rejects the merge. External
+   envelopes carry no digest in round one.
+4. Duplicates are deterministic. Same slot, identical contribution: collapse to
+   one. Same slot, different content: a conflict, independent of arrival order —
+   neither is kept, the composer records a FAILED envelope for that slot with a
+   warning, and the core Document stays valid.
+5. One envelope per slot per call. Multi-part output (a preview per page) lives
+   inside the producer's payload schema, not in N envelopes. This also defers
+   streamed or partial contributions — the ticket's ParseStream mode — to a
+   later round.
+6. The composer answers for its whole plan. A producer that replied contributes
+   its envelope; one that timed out or crashed gets a composer-synthesized
+   FAILED envelope; one skipped by policy gets SKIPPED. The composed Document
+   then accounts for every planned slot; the plan itself stays with the
+   composer.
+
+## Runtime
+
+Sequential compose in one StormCrawler bolt: call Tika, call the parser, merge
+in memory, emit. Parallel fan-out is a later round.
+
+## Not in round one
+
+- The in-Tika broker and plugin registration (the ticket's orchestration
+  model): buildable later on these same rules, deliberately not their
+  definition.
+- ParseStream plugin mode: deferred with it, alongside the 4772 streaming
+  conversation.
+- Payload mappings (projecting an Any into a document shape): another day, per
+  Kristian.
+- Schema registry: later layer for discovery and JSON projection. Known types
+  resolve from generated classes; unknown payloads round-trip as bytes.
+- Embedded docs, block/span annotations: blocked on the typed content lane the
+  Document doesn't carry yet. Any must not become the workaround for that.
+- Producer authentication and registration are out of scope.
+
+## Depends on
+
+TIKA-4766 provides the Document contract. TIKA-4795 provides the exact-byte
+path and parity oracle used by the first demo.
+
+## Mermaid twin (GitHub venues only)
+
+```mermaid
+flowchart LR
+    SC[composer: StormCrawler bolt] -->|bytes| T[Tika ParseBytes]
+    SC -->|bytes| P[external parser]
+    T --> D["Document (core + extra)"]
+    P --> E[ExtensionResult]
+    D --> M[compose]
+    E --> M
+    M --> OUT["one Document (core + extra + extensions)"]
+```