horizondb-idiosyncrasies

name

horizondb-idiosyncrasies

description

Work with Azure HorizonDB (preview) — its AI/embedding features, extension allow-listing, AGE/pgvector/pg_durable quirks, and the connection/TLS gotchas discovered while bringing up the horizon-facts incubator. Use when connecting to a HorizonDB cluster, enabling extensions, writing AGE Cypher, wiring in-DB embeddings via pg_durable df.http(), or debugging "extension not allow-listed" / "access to library not allowed" / "self-signed certificate" / Cypher syntax errors.

HorizonDB Idiosyncrasies

Azure HorizonDB is a managed, preview Postgres-17 service (a.k.a. OrionDB internally). It bundles AI/graph/vector/durable-execution extensions but locks them behind an allow-list and several managed-PG restrictions that differ from vanilla Postgres. This skill captures the verified facts and the workarounds.

Reference implementation: incubator/horizon-facts/ — a HorizonDB-only enhanced facts interface. Its integration suite (test/integration/) runs live against a HorizonDB cluster and exercises every capability below.

Quick reference

Symptom	Cause	Fix
self-signed certificate in certificate chain	pg v8 treats sslmode=require as verify-full	append uselibpqcompat=true to the connection string
extension "X" is not allow-listed for "azure_pg_admin" users	empty/insufficient azure.extensions on the parameter group	add X to azure.extensions on a custom parameter group, attach to cluster
access to library "age" is not allowed	age is in shared_preload_libraries; explicit LOAD 'age' is forbidden	tolerate that error — the library is already preloaded
syntax error at or near "WHERE" from cypher_parser.c	AGE doesn't support any(x IN list WHERE pred)	use size([x IN list WHERE pred]) > 0
schema "..." already exists creating an AGE graph	graph name collides with an existing schema (create_graph makes a schema)	give the graph a name distinct from every other schema
relation "<label>" already exists (42P07) or duplicate key … "pg_class_relname_nsp_index" (23505) under concurrent graph writes	AGE creates a label's backing table lazily on first CREATE/MERGE; concurrent first-references race the DDL (two detection points)	re-run the same idempotent statement — the label now exists (see §3 concurrency)
cannot cast type agtype to jsonb	agtype has no jsonb cast	RETURN scalars/label()/type(), parse client-side
ECONNRESET / Connection terminated unexpectedly mid-workload	HorizonDB resets idle pooled TLS connections	guard + bounded retry on a fresh connection (§1.1)
AGE writes are 600 ms–2 s each under concurrency	small connection pool → ops queue (connection-queue wait, not query cost)	raise poolMax for the parallelism (default now 16, HORIZON_POOL_MAX); per-connection AGE setup; fewer round-trips per upsert (§3 performance)

1. Connection & TLS

HorizonDB requires SSL. The cluster's certificate chain is not in Node's default trust store, and modern pg (v8+) / pg-connection-string treat sslmode=require as verify-full, so a plain ?sslmode=require URL fails with:

CONN FAIL: self-signed certificate in certificate chain

Fix: append uselibpqcompat=true, which restores libpq semantics where require means encrypt but don't verify the CA:

postgresql://USER:PW@<cluster>.<id>.<region>.horizondb.azure.com:5432/postgres?sslmode=require&uselibpqcompat=true

For production trust verification, install the Azure CA bundle and use sslmode=verify-full with an explicit ca. For dev/test against preview clusters, uselibpqcompat=true is the pragmatic path. The horizon-facts test harness normalizes this automatically — see normalizeDbUrl() in incubator/horizon-facts/test/integration/_db.mjs.

1.1 Idle pooled connections get reset — guard + retry on a fresh connection

HorizonDB (preview) intermittently drops idle pooled TLS connections. Under a sustained workload this surfaces two ways:

• an uncaught error event on a checked-out client (ECONNRESET) — this crashes a long-running Node process unless you attach a pool/client error handler;
• a rejected query (ENOTCONN / EPIPE / Connection terminated unexpectedly) from pool.query.

Neither partially commits, so re-running on a fresh connection is safe. Pattern (see incubator/horizon-facts/src/db-retry.ts):

• pool.on("error", …) and a process-level guard that swallow only this transient socket class;
• a bounded retry (withDbRetry) that re-acquires a connection and re-runs.

Keep two retry classifiers distinct — they target different recovery actions:

Classifier	Matches	Retry action
isTransientDbError	ECONNRESET/ENOTCONN/EPIPE/Connection terminated + pg 08xxx/57P0x	re-run on a fresh connection
isLabelCreationRaceError	42P07 or 23505 on a pg_catalog index (pg_class_*/pg_type_*/pg_namespace_*)	re-run the same statement (AGE label race, §3)

Do not lump the label race into the connection regex, and do not retry a bare 23505 — a unique violation on a user constraint is a real error; only the system-catalog index variant is the label race.

2. Extension allow-list (the #1 blocker)

HorizonDB rejects CREATE EXTENSION unless the extension name is in the azure.extensions server parameter, even for the admin user:

ERROR: extension "vector" is not allow-listed for "azure_pg_admin" users
        in Azure Database for PostgreSQL  (SQLSTATE 0A000)

The default parameter group (default_pg17) ships with azure.extensions empty — only plpgsql is installed. The extensions exist on the image (check pg_available_extensions), they're just not allowed yet.

Available AI/graph extensions (PG17 image, verified 2026-06)

Extension	Version	Purpose
vector (pgvector)	0.8.0	embedding storage + HNSW ANN
age (Apache AGE)	1.6.0	property graph + Cypher
pg_durable	0.2.1	durable execution + df.http() for in-DB HTTP
pg_textsearch	1.3.0-dev	ranked lexical search
azure_ai	2.2.1	native in-DB model inference (Azure-managed)
pg_diskann	0.7.1	disk-based ANN index (azure.extensions ONLY — NOT preloadable)

Enabling extensions

You cannot set azure.extensions per-session. You must create a custom parameter group, set the allow-list (and shared_preload_libraries for preload-required extensions), and attach it to the cluster. Attaching triggers a cluster restart (~2 min downtime) when shared_preload_libraries changes.

Preload requirements: age and pg_durable must be in shared_preload_libraries. vector and pg_textsearch only need to be in azure.extensions. pg_diskann is azure.extensions-ONLY — it is rejected in shared_preload_libraries (HorizonDB's allowed preload set is fixed and does not include it), yet works without preloading.

Via ARM (az rest)

SUB=<subscription-id>; RG=<resource-group>; API=2026-01-20-preview
CLUSTER=<cluster-name>; PG=<cluster>-ext

# 1. Create the parameter group (sparse list merges into ~426 defaults).
#    NOTE: top-level "location" is REQUIRED or you get LocationRequired.
cat > /tmp/pg-body.json <<EOF
{
  "location": "<region>",
  "properties": {
    "description": "Enable AI/graph extensions",
    "pgVersion": "17",
    "parameters": [
      { "name": "azure.extensions",          "value": "age,azure_ai,pg_durable,pg_textsearch,vector" },
      { "name": "shared_preload_libraries",  "value": "age,pg_durable,pg_textsearch" }
    ]
  }
}
EOF
az rest --method put \
  --url "https://management.azure.com/subscriptions/$SUB/resourceGroups/$RG/providers/Microsoft.HorizonDb/parameterGroups/$PG?api-version=$API" \
  --headers "Content-Type=application/json" --body @/tmp/pg-body.json

# 2. Poll until properties.provisioningState == "Succeeded".

# 3. Attach to the cluster (triggers restart). PATCH only the parameterGroup ref.
cat > /tmp/cluster-patch.json <<EOF
{ "properties": { "parameterGroup": {
  "id": "/subscriptions/$SUB/resourceGroups/$RG/providers/Microsoft.HorizonDb/parameterGroups/$PG" } } }
EOF
az rest --method patch \
  --url "https://management.azure.com/subscriptions/$SUB/resourceGroups/$RG/providers/Microsoft.HorizonDb/clusters/$CLUSTER?api-version=$API" \
  --headers "Content-Type=application/json" --body @/tmp/cluster-patch.json

# 4. Poll the cluster: properties.state goes Updating -> Succeeded.
#    (provisioningState may be empty in steady state; key off `state`.)

Notes:

• The platform re-injects its own system preload libs (azure, orion_storage, pg_availability, pg_qs, pgms_stats, pgms_wait_sampling) on top of yours — you only list the extras.
• The default group default_pg17 is not an ARM resource (GET returns ResourceNotFound). Discover existing user groups with: az graph query -q "resources | where type =~ 'microsoft.horizondb/parametergroups'".
• A good template to clone from is any working group that already enables these (e.g. a fts-age-pgts style group in the same subscription).

Verify after restart

await pool.query(`show shared_preload_libraries`);      // includes age,pg_durable,pg_textsearch
await pool.query(`CREATE EXTENSION IF NOT EXISTS vector`);
await pool.query(`CREATE EXTENSION IF NOT EXISTS age`);
await pool.query(`CREATE EXTENSION IF NOT EXISTS pg_durable`);
// df.http() should now exist:
await pool.query(`select proname from pg_proc
                  where proname='http' and pronamespace::regnamespace::text='df'`);

3. Apache AGE quirks

Don't LOAD 'age' when it's preloaded

On vanilla Postgres you must LOAD 'age' per session. On HorizonDB age is in shared_preload_libraries, so it's already loaded and an explicit LOAD is rejected:

ERROR: access to library "age" is not allowed

Make the load tolerant — attempt it (for portability to vanilla PG) but swallow that one error:

async function loadAge(client: any): Promise<void> {
  try { await client.query(`LOAD 'age'`); }
  catch (err: any) {
    if (!/access to library "age" is not allowed/i.test(String(err?.message ?? ""))) throw err;
    // already preloaded via shared_preload_libraries — nothing to do
  }
}

Still run SET search_path = ag_catalog, "$user", public on every AGE session.

Graph name must differ from every schema name

SELECT create_graph('g') creates a Postgres schema named g. If you also have a facts/data schema named g, you get schema "g" already exists. Always use distinct names (e.g. data schema hf_test_<r>, graph hf_graph_<r>).

Cypher: no any(... WHERE ...)

AGE 1.6.0's Cypher parser rejects the list predicate any(x IN list WHERE pred) with syntax error at or near "WHERE" (cypher_parser.c). Use the list-comprehension + size() form instead:

-- BROKEN on AGE:
WHERE any(a IN e.aliases WHERE toLower(a) CONTAINS 'tgl')
-- WORKS:
WHERE size([a IN e.aliases WHERE toLower(a) CONTAINS 'tgl']) > 0

UNWIND ... WITH x WHERE ... also works. When in doubt, probe syntax against a throwaway graph before committing to a query shape.

Other AGE limitations to design around

• Avoid startNode() / endNode() / UNWIND over variable-length paths. For neighbourhood edges, collect node keys first, then match edges with WHERE a.entity_key IN [...] AND b.entity_key IN [...].
• Cypher cannot be parameterized through pg bind params — values are inlined into the cypher('graph', $$ ... $$) literal. Use a strict escaping helper (see cypherStr / cypherStrList / cypherNum in horizon-facts sql-util.ts) and never interpolate untrusted text without it.
• agtype has no cast to jsonb (cannot cast type agtype to jsonb). Don't return a whole node/edge and cast it — RETURN the scalars you need, or label(x) / type(r), and parse the agtype scalars client-side.

AGE concurrency: lazy label creation races (SQLSTATE 42P07)

AGE creates a label's backing table lazily — on the first CREATE/MERGE that references a label that doesn't exist yet. That internal check-then-CREATE TABLE is not atomic and not serialized. Any concurrent writer (a connection-per-call pool, or an LLM agent issuing parallel tool calls) that races the first reference to the same label hits the collision in one of two forms, depending on where the duplicate is detected:

relation "GraphNode" already exists                                  -- 42P07 duplicate_table
duplicate key value violates unique constraint "pg_class_relname_nsp_index"  -- 23505 unique_violation

The 42P07 form is caught at the relation level; the 23505 form fires when two CREATE TABLE for the same label race into the pg_class system catalog before either registered. Both are the same race and both abort the whole statement (the MERGE/CREATE persisted nothing). Errors are front-loaded — they cluster on the first touch of each label, then disappear once the backing tables exist.

⚠️ The 23505 form is easy to miss: the "already exists" text is in err.detail, not err.message (the message is "duplicate key value …"). A message-only regex will silently let it through. Key off the SQLSTATE + the catalog constraint name (pg_class_* / pg_type_* / pg_namespace_*).

Do not "fix" this in a migration by pre-creating labels. In an open graph the label set (node kinds, edge predicates) is owned by the application/agent layer, not infrastructure — it's unbounded and unknowable at migrate time. Pre-creating a fixed set only masks the case you thought of.

Correct, label-agnostic fix: the statement is idempotent, so on the label race, re-run the same statement — the label now exists and the re-run succeeds. Match both manifestations, and for 23505 retry only when the violated constraint is a system catalog index (never a user constraint):

const CATALOG_RACE_CONSTRAINT = /pg_(class|type|namespace)_/i;
function isLabelCreationRaceError(err: any): boolean {
  if (String(err?.code) === "42P07") return true;                     // duplicate_table
  if (String(err?.code) === "23505") {                                // unique_violation…
    const c = String(err?.constraint ?? ""), m = String(err?.message ?? "");
    if (CATALOG_RACE_CONSTRAINT.test(c) || CATALOG_RACE_CONSTRAINT.test(m)) return true;  // …on a pg_catalog index
  }
  return /already exists/i.test(String(err?.message ?? ""));
}

Implement it at the single point where Cypher is executed, so it covers every label automatically:

// in the one cypher(client, query) helper:
const { rows } = await withDbRetry(
  "graph_label_race",
  () => client.query(sql),
  { isRetryable: isLabelCreationRaceError });   // 42P07 OR 23505-on-catalog

Safe because each cypher() call is its own autocommit statement (a prior failure doesn't poison the connection) and all graph writes are idempotent MERGE.

Ask of AGE/HorizonDB: make first-reference label creation atomic (advisory-lock or true CREATE TABLE IF NOT EXISTS semantics) so callers don't have to retry DDL races on the hot path.

AGE concurrency: no unique constraints → MERGE can duplicate

AGE vertices/edges have no unique indexes/constraints, so a naive MERGE (f:Fact {scope_key}) MERGE (e)-[:EVIDENCED_BY]->(f) can, under concurrency, create duplicate :Fact anchors; a later MERGE then binds every duplicate and multiplies edges. Use read-decide-write instead: check whether the edge already exists, bind exactly one anchor (WITH … LIMIT 1), and only create when none exists (see linkEvidenceAnchor in graph-queries.ts).

AGE performance: write latency = round-trips × RTT

On a remote cluster, AGE write latency is dominated by the number of round trips, not query cost. Measured in a 119-fact agent harvest:

op	p50	why
graph_upsert_edge	~610 ms	endpoint check + existing-edge read + create/reinforce = 3+ statements
graph_upsert_node	~730 ms	existence read + create/update + per-evidence anchor link
single stored proc (facts_set_crawled)	~67 ms	one round trip

A single round trip to the remote cluster is ~45–100 ms, so a multi-statement upsert is a half-second of pure latency. The agent-driven harvest is too noisy to measure tuning changes (every run issues a different number of calls with different concurrency; unrelated ops like facts_similar swing 5× between runs from DB contention). Use a deterministic microbenchmark instead — see incubator/horizon-facts/eval/bench-graph.mjs, which drives a fixed set of upserts directly (no LLM) and reports clean p50/p95 plus the RTT and age-prep primitives.

Three compounding causes and their fixes, biggest lever first:

1. Connection-pool starvation (the dominant cost). The harvester fires graph tool calls in parallel, but a small pool serializes them — the measured per-op p50 is then mostly connection-queue wait, not work. Deterministic sweep (80 nodes + 80 edges, concurrency 8):

   poolMax	node wall	node p50	edge wall
   3 (old default)	6017 ms	407 ms	4517 ms
   8	3415 ms	224 ms	1997 ms
   16	1506 ms	97 ms	2669 ms

   Pool 3→8 is ~45–55% faster; 3→16 is ~75% faster on nodes. The "round-trips/op" estimate (p50 / RTT) collapses from ~9 to ~2 as the pool grows — proving the apparent "9 round trips" was queue wait; the real work is ~2 round trips (existence-check + create). Fix: size the pool for the expected parallelism. horizon-facts now defaults poolMax to 16 (was 3), overridable via HORIZON_POOL_MAX — bounded by the cluster's max_connections.

Migration locking: keep the global lock narrow

The Horizon facts migrator uses a per-schema advisory lock for ordinary schema migrations. Only database-global DDL (for example CREATE EXTENSION and AGE ag_catalog.create_graph) should take the global Horizon DDL lock, and it should be transaction-scoped (pg_try_advisory_xact_lock) so an idle pooled connection cannot retain it after the protected transaction ends. Do not wrap the entire per-schema migration run in a session-level global lock; concurrent SDK --with-horizondb tests create many isolated ps_test_facts_* schemas and a broad global lock becomes a suite-wide bottleneck or stale-lock failure.

2. Per-checkout session setup. Running LOAD 'age' (a wasted failed round trip when preloaded) + SET search_path on every graph-op checkout adds ~2 round trips (measured age-prep p50 ≈ 93 ms). Fix: do AGE session setup once per physical connection — horizon-facts caches prepared connections in a WeakSet<client> in GraphQueries (a reset connection yields a new client object → re-prepared automatically). Measured ~17–18% faster per op. A pg-pool connect-event handler is the equivalent if you don't hold a client wrapper:

   pool.on("connect", (client) => {
     client.query("LOAD 'age'").catch(() => {});              // tolerate preloaded
     client.query(`SET search_path = ag_catalog, "$user", public`).catch(() => {});
   });
   

3. Multi-statement upserts. Collapse existence-check + write into a single MERGE … ON CREATE SET … ON MATCH SET … where it's race-safe, and batch independent upserts. Fewer statements = fewer round trips.

Ask of AGE: the cypher() SRF wrapper carries per-call overhead; a way to issue multiple graph mutations per round trip (or a lighter write path) would materially help write-heavy workloads.

4. AI / embeddings — two paths

HorizonDB offers two ways to generate embeddings in-database. Pick based on what your cluster allows and where the model lives.

Path A — pg_durable df.http() (external endpoint) ✅ verified working

pg_durable exposes df.http(url, method, body, headers jsonb, timeout_seconds) for durable HTTP calls from inside the database. This is the supported in-DB HTTP primitive on HorizonDB — pgsql-http (the http extension) is NOT available; only azure_ai and pg_durable ship.

incubator/horizon-facts/sql/006_embeddings_http.sql + src/http-embedding.ts implement this end-to-end (verified against the live cluster, embedding real 1536-dim vectors). The hard-won mechanics:

1. df.http() is a durable FUTURE, not a synchronous call. It returns a node descriptor ({"node_type":"HTTP",...}), not the response. You run it via df.start(fut, label, database) → instance id, then poll df.wait_for_completion(iid, timeout) and read df.result(iid):

v_iid := df.start(df.http(url,'POST',body,headers,30), 'hz-embed', NULL);
COMMIT;                                  -- ⚠️ see (2)
PERFORM df.wait_for_completion(v_iid, 40);
v_res := df.result(v_iid);               -- text JSON, see (3)

2. Must COMMIT between start and wait → use a PROCEDURE, not a function. The pg_durable worker runs the instance in its OWN connection and can't see the instance row until the enqueuing transaction commits. So you cannot df.start + df.wait_for_completion inside one SQL function (functions can't commit and would deadlock until timeout). Drive it from a plpgsql PROCEDURE that COMMITs after df.start. A FOR row IN SELECT … LOOP … COMMIT … END LOOP with commits inside works on HorizonDB's PG17.

3. df.result() shape: text JSON with keys {ok, body, status, headers, duration_ms}. body is the raw response string. For an AOAI embeddings response: (((df.result(iid)::jsonb->>'body')::jsonb->'data'->0->'embedding')::text)::vector. Check (res::jsonb->>'ok')::boolean first.

4. df.http() enforces an Azure egress ALLOW-LIST. Only approved Azure service domains are permitted. The Azure AI Foundry unified host *.services.ai.azure.com is BLOCKED — the instance fails with "... is not in the allowed endpoint list. Only requests to approved Azure service domains are permitted." The classic Azure OpenAI host *.openai.azure.com points at the same deployment and IS allowed. setupHttpEmbedding auto-rewrites the host (toAllowlistedAzureHost). The cognitiveservices.azure.com host also works. (The Node fallback uses Node fetch and works with either host.)

5. Grant HTTP usage: SELECT df.grant_usage(current_user, true, false); (the true = include_http). Idempotent.

6. Recurring / cron embedder via df.loop (sub-minute cadence). pg_durable exposes futures combinators — df.seq(a,b) (THEN), df.loop(body), df.sleep(seconds), df.sql(query), df.race, df.if*, plus df.cancel, df.signal, df.wait_for_schedule(cron_expr). They serialize to a node tree ({"node_type":"LOOP","left_node":{...}}) and you launch with df.start. For a self-perpetuating recurring job at a fixed interval, wrap a sleep+work sequence in a loop:

-- every N seconds, embed any changed facts (the loop is pure scheduling; the
-- per-fact df.http embedding lives in the CALLed procedure → "nested" durability)
SELECT df.start(
  df.loop(df.seq(df.sleep(5::bigint), df.sql('CALL "schema".embed_new_facts_durable(128, NULL)'))),
  'hz-embed-cron:schema', NULL);

• df.wait_for_schedule takes a real cron expression but only minute granularity — for sub-minute ticks (e.g. every 5s) you MUST use df.sleep.
• A loop calling a procedure that itself starts df.http instances works fine (verified): the loop ticks, the proc runs nested durable HTTP per fact, the pg_durable worker handles both. The loop instance stays pending/running.
• Track/stop by a stable label (e.g. hz-embed-cron:<schema>): look it up in df.instances (PK column is id, plus label, status, created_at; terminal statuses are completed/cancelled/failed, active are pending/running). Make start idempotent by reusing any non-terminal instance with that label; stop = df.cancel(id, reason).

7. Embedding is a PROVIDER-INTERNAL lifecycle, not a contract method. PilotSwarm only knows the EnhancedFactStore interface; its sole embedding responsibility is passing the endpoint config. How/when embeddings are produced is the provider's private business. The HorizonDB provider exposes only a lifecycle on HorizonFactStore (src/horizon-store.ts):

• configureEmbedder(endpoint) — write/replace the embedding_config row (with the df.http allow-list host rewrite).
• startEmbedder({ intervalSeconds = 5, batch = 128 }) — launch the durable df.loop cron; idempotent; returns EmbedderStatus { running, instanceId?, status? }.
• stopEmbedder() — df.cancel the loop.
• embedderStatus() — derived { running } (no df leakage in the shape).

Callers never trigger embedding or wait on a df instance — they write facts and observe the outcome (vector present / semantic search returns the fact).

The two @internal methods _embedPendingNode() and _embedNewFactsInDbOnce() are NOT production paths — they are sanity checks for the df.http path: _embedPendingNode posts the identical request df.http builds (same headers/ body, read from embedding_config) via Node fetch, so it validates the request shape against a Node-reachable stub; _embedNewFactsInDbOnce is a one-shot synchronous trigger of the same procedure the cron CALLs. Both are exercised only by the provider's own integration tests (test/integration/http-embeddings.test.mjs), which legitimately assert on df internals because they test the mechanism. PilotSwarm-level tests must stay outcome-only and never reference df.

Other constraints:

• Store the source-content hash alongside the vector so re-embeds only happen when content changes (last_embedded_hash IS DISTINCT FROM content_hash).
• df.instances (with your label) is the audit trail that a durable HTTP call actually ran; it's global, not schema-scoped.
• Inspect failures via df.instance_nodes(iid, 1) → the node status/result.

Path B — azure_ai (Azure-managed inference)

azure_ai is HorizonDB's native model-management/inference extension. It wires embeddings/completions to Azure-managed model endpoints from inside the DB (no external HTTP plumbing). Cluster-side feature gating (aiModelManagement) may need to be enabled before it works. Prefer this when you want managed, in-region inference and don't want to operate your own endpoint.

Path C — Node-side fallback (embedPending())

For bootstrapping, clusters without a working in-DB HTTP path, or local tests, embed from the application: read pending facts, call the embeddings endpoint from Node, write vector(dim) back. Reference: embedPending() / http-embedding.ts in horizon-facts. Still requires vector to be allow-listed so the embedding vector(dim) column and HNSW index exist.

5. pgvector notes

• CREATE EXTENSION vector after allow-listing; vector 0.8.0 supports hnsw and ivfflat AMs out of the box.
• HNSW index: USING hnsw (embedding vector_cosine_ops).
• DiskANN is a SEPARATE extension (pg_diskann), not pgvector core. On the cluster it shows in pg_available_extensions (0.7.1). It is NOT installed by default and must be allow-listed in azure.extensions. IMPORTANT: unlike Azure Flexible Server, HorizonDB does NOT require (and REJECTS) pg_diskann in shared_preload_libraries — the allowed preload set is fixed (age, auto_explain, azure_storage, pg_cron, pg_durable, pg_partman_bgw, pg_prewarm, pg_stat_statements, pg_textsearch, pgaudit, wal2json); adding pg_diskann there fails the parameter group with ParameterValueInvalid. Put pg_diskann in azure.extensions ONLY — it works without preload (verified 2026-06 on waldemort-cms-2: CREATE EXTENSION pg_diskann + USING diskann (embedding vector_cosine_ops) both succeed). It shares the vector_cosine_ops opclass and the <=> operator with HNSW, so query code is identical. horizon-facts selects the AM via annIndex: "diskann"|"hnsw"|"auto" (env HORIZON_ANN_INDEX); auto (default) tries diskann and falls back to hnsw when it isn't allow-listed (ensureAnnIndex in migrations.ts).
• A jsonb array text "[...]" casts directly to vector — handy when parsing an embeddings HTTP response: (emb_json::text)::vector.
• Embedding dimension must match the endpoint's model exactly, or inserts fail.

6. Networking / firewall

• HorizonDB firewall rules live under clusters/{cluster}/pools/{pool}/firewallRules, but the pool name is not enumerable via ARM/CLI (no list/read; GET on the collection returns MethodNotAllowed). The practical path is the portal Networking blade on the cluster to add your client IP, or open a broad range temporarily.
• Connections still require the TLS handling from §1 once the firewall is open.

7. Working checklist for a fresh cluster

1. Open firewall to your IP (portal Networking blade).
2. Connect with ?sslmode=require&uselibpqcompat=true; confirm select version().
3. Create + attach a parameter group with azure.extensions and shared_preload_libraries for the extensions you need; wait for the restart.
4. CREATE EXTENSION for vector, age, pg_durable, pg_textsearch as needed.
5. For AGE: tolerant LOAD 'age', set search_path, distinct graph vs schema names, avoid any(... WHERE ...).
6. For embeddings: choose df.http() (external endpoint reachable from cluster), azure_ai (managed), or Node-side embedPending(); run generation as a pg_durable activity; hash-guard re-embeds.

See also

• incubator/horizon-facts/README.md — capability overview + design rules
• incubator/horizon-facts/CRAWLER-SPEC.md — embedding pipeline + AGE graph contract
• incubator/horizon-facts/test/integration/ — live HorizonDB tests for all of the above
• repo memory horizondb-incubator-2026-06-02.md — the original bring-up notes