# Health — normalized target spec

Status: **Draft**. The single design the sister projects converge on. Derived from the
three code-verified current-state docs (`../current-state/`). Goal is *path to shared code*
(`../shared-contract/ZB.MOM.WW.Health.md`), so each normalized section maps to a shared library seam.

## 0. Scope

**Normalized here:** the three-tier endpoint convention (`/health/ready`, `/health/active`,
`/healthz`) with canonical tags `ready` / `active` / `live` and their semantics; the canonical
JSON response shape; the `IActiveNodeGate` request-gating seam; a configurable
`AkkaClusterHealthCheck` with two named policy presets that reconcile the diverging Akka logic in
OtOpcUa and ScadaBridge; a role-filtered `ActiveNodeHealthCheck` that unifies OtOpcUa's
`AdminRoleLeaderHealthCheck` and ScadaBridge's `ActiveNodeHealthCheck`; a generic
`DatabaseHealthCheck<TContext>` that covers both apps' EF Core probe patterns; a
`GrpcDependencyHealthCheck` for downstream gRPC reachability.

**Explicitly NOT normalized** (domain-specific — keep per project): which probes each app
registers and how it wires them to tags; orchestrator / Traefik routing rules and routing priorities;
ScadaBridge's `HealthMonitoring/` domain-aggregation pipeline — this is a distributed, actor-based
domain-health telemetry system (background services + Akka actors that aggregate site-cluster signals
into a central health picture) and is **not** an ASP.NET health-probe; it is an independent concern
that happens to share the word "health".

## 1. Tier convention

Three tiers, always served in this order, each filtered to a named tag:

| Tier | Endpoint | Tag | Semantics | Healthy→ | Degraded→ | Unhealthy→ |
|---|---|---|---|---|---|---|
| Ready | `/health/ready` | `ready` | Can this node serve its dependencies? Fails if a DB, gRPC dependency, or cluster membership check is unhealthy. Orchestrators use this to gate traffic. | 200 | 200 | 503 |
| Active | `/health/active` | `active` | Is this the leader / active node? Fails (503) on a standby or role-member-but-not-leader node. Used to route write traffic or admin requests to exactly one node. | 200 | 200 | 503 |
| Live | `/healthz` | `live` | Bare process liveness — is the process alive and not deadlocked? **No probes registered to this tag** (predicate `_ => false`). Always 200 as long as the process can handle HTTP. | 200 | 200 | 200 |

Notes:

- The `live` tier intentionally carries no probes. Registering a probe to `live` is an error —
  a liveness failure that kills the pod should be reserved for total process hangs, not probe failures.
- `Degraded` maps to HTTP 200 (not 503) for the `ready` and `active` tiers. Orchestrators use 503
  to remove a node from load-balancing; Degraded means "still up but degraded" — remove the node
  only on hard failure.
- The tag names (`ready`, `active`, `live`) are declared as constants in `ZbHealthTags` and used
  consistently across all three apps. Per-project probe registrations must filter by these tags.

## 2. Probe catalog

### 2.1 Database probe — `DatabaseHealthCheck<TContext>`

Wraps an EF Core `DbContext` to verify database reachability. Default behavior calls
`context.Database.CanConnectAsync()` — matches ScadaBridge's pattern. An optional delegate
(`Func<TContext, CancellationToken, Task>`) overrides the default for more specific validation
(matches OtOpcUa's "query `Deployments`" pattern). Registered to the `ready` tag.

### 2.2 Akka cluster probe — `AkkaClusterHealthCheck`

Checks the local node's cluster membership status via Akka.Cluster. The status-to-health
mapping is **configurable** through `AkkaClusterStatusPolicy`.

**Two named policy presets reconcile the existing divergence:**

| Preset | Origin | `Up` / `Joining` | `Leaving` / `Exiting` | Other (`WeaklyUp`, `Down`, `Removed`, `Unknown`) |
|---|---|---|---|---|
| `AkkaClusterStatusPolicy.Default` | ScadaBridge `AkkaClusterHealthCheck.cs` | Healthy | Degraded | Unhealthy |
| `AkkaClusterStatusPolicy.OtOpcUaCompat` | OtOpcUa `AkkaClusterHealthCheck.cs` | Healthy (if self is `Up` among reachable members) | Degraded[^1] | Degraded |

[^1]: In the `OtOpcUaCompat` member-scan approach, `Leaving`/`Exiting` statuses also map to
Degraded because a member with those statuses will not appear with `Status == Up` in the reachable
member set — the scan finds self without `Up`, so the result is Degraded.

The `Default` preset is the convergence target. `OtOpcUaCompat` is provided for backward
compatibility during OtOpcUa's migration; it maps any non-`Up`-among-members state to Degraded
rather than Unhealthy. Registered to the `ready` tag.

> **Note on error/exception cases:** in both modes, if the ActorSystem is not yet ready or cluster
> state is inaccessible (e.g. during startup), the check returns Degraded (startup-safety rule).
> The status cells in the table above describe the normal-operation path only; the "—" cells in the
> `OtOpcUaCompat` column refer to states that collapse into Degraded via the member-scan result,
> not to an explicit policy match.

### 2.3 Active / leader probe — `ActiveNodeHealthCheck`

Checks whether this node is the designated leader (active node). Accepts an optional Akka
cluster role name that scopes the check to nodes carrying that role.

**Two behaviors unify the existing divergence:**

| Mode | Role param | Origin | Healthy | Degraded | Unhealthy |
|---|---|---|---|---|---|
| Role-less | `null` | ScadaBridge `ActiveNodeHealthCheck` | Node is `Up` **and** cluster leader | — | Otherwise |
| Role-filtered | e.g. `"admin"` | OtOpcUa `AdminRoleLeaderHealthCheck` | Node does **not** carry the role (not a participant — ignore it) **or** node carries the role and is the role-singleton leader | Carries the role but is **not** the role-singleton leader (role member, not leader) | — |

The role-filtered variant maps "not a member of the role" to Healthy (transparent — the probe
is irrelevant for this node). This is the correct behavior for heterogeneous clusters where not
every node carries every role. Registered to the `active` tag.

### 2.4 gRPC dependency probe — `GrpcDependencyHealthCheck`

Checks that a downstream gRPC channel is reachable by invoking a caller-supplied probe
delegate (`Func<GrpcChannel, CancellationToken, Task<bool>>`). The default probe calls
`GrpcChannel.ConnectAsync`. Used by:

- OtOpcUa — checks the MxAccessGateway gRPC channel.
- MxGateway — checks the x86 worker gRPC channel.

Registered to the `ready` tag.

## 3. Response-writer contract

All health endpoints share one canonical JSON serializer. The shape is lifted from ScadaBridge's
`HealthChecks.UI.Client` style and becomes the library default (replacing per-project divergence).

**Content-type:** `application/json`

**Shape:**

```json
{
  "status": "Healthy",
  "totalDurationMs": 12,
  "entries": {
    "database": {
      "status": "Healthy",
      "description": "SQL Server reachable",
      "durationMs": 12
    },
    "akka-cluster": {
      "status": "Healthy",
      "description": "Member status: Up",
      "durationMs": 0.1
    }
  }
}
```

**Field rules:**

| Field | Type | Notes |
|---|---|---|
| `status` | string | `"Healthy"` \| `"Degraded"` \| `"Unhealthy"` — the aggregate across all filtered checks |
| `totalDurationMs` | long | Total wall-clock time for all probes in this tier, milliseconds |
| `entries` | object | Keyed by check registration name |
| `entries.<name>.status` | string | Per-check status |
| `entries.<name>.description` | string? | Human-readable detail (may be null) |
| `entries.<name>.durationMs` | number | Per-check elapsed time, milliseconds |

The writer is exposed as a static `Task WriteJsonAsync(HttpContext, HealthReport)` so consumers can
plug it into `MapHealthChecks` options and also call it from custom endpoints.

## 4. Active-node gating seam — `IActiveNodeGate`

`IActiveNodeGate` is a single-property interface (`bool IsActiveNode { get; }`) that expresses
whether the current node should accept write / active-role requests. The default implementation,
`AkkaActiveNodeGate`, reads cluster state **directly**: `IsActiveNode` returns `true` iff the
`ActorSystem` is available, `SelfMember.Status == Up`, and the node is the cluster leader. It is
null-guarded and returns `false` when the `ActorSystem` is not yet ready (safe default during
startup). It does **not** resolve `ActiveNodeHealthCheck` from DI. A `RequireActiveNode()` extension on
`IEndpointConventionBuilder` attaches a policy that short-circuits with `503 Service Unavailable`
on standby nodes.

This seam is generalized from ScadaBridge's `ActiveNodeGate.cs`. It is in the core `ZB.MOM.WW.Health`
package (not the Akka satellite) so MxGateway can implement it without an Akka dependency if needed.

## 5. Endpoint registration

`app.MapZbHealth()` maps all three tiers in one call:

```csharp
app.MapZbHealth();                        // all three tiers, defaults
app.MapZbHealth(o => {
    o.ReadyPath   = "/health/ready";      // override paths if needed
    o.ActivePath  = "/health/active";
    o.LivePath    = "/healthz";
    o.ResponseWriter = ZbHealthWriter.WriteJsonAsync;
});
```

The library does **not** call `services.AddHealthChecks()` — that is the app's responsibility, as
the probe set is per-project. `MapZbHealth` only maps the three endpoints with the correct tag
predicates and response writer.

## 6. Migration notes

| Project | Current state | Gap | What normalizes |
|---|---|---|---|
| **OtOpcUa** | All three tiers present (`/health/ready`, `/health/active`, `/healthz`); `DatabaseHealthCheck`, `AkkaClusterHealthCheck`, `AdminRoleLeaderHealthCheck` inline. | Inline probes diverge from the shared policy model; no `IActiveNodeGate`. | Replace inline `AkkaClusterHealthCheck` with shared + `OtOpcUaCompat` preset; replace `AdminRoleLeaderHealthCheck` with shared `ActiveNodeHealthCheck(role: "admin")`; replace inline `DatabaseHealthCheck` with shared generic; call `app.MapZbHealth()`. |
| **ScadaBridge** | `/health/ready` + `/health/active` present; no `/healthz`; `DatabaseHealthCheck`, `AkkaClusterHealthCheck`, `ActiveNodeHealthCheck`, `ActiveNodeGate` inline. | Missing `/healthz` live tier; inline implementations. | Add `/healthz` via `MapZbHealth()`; replace inline probes with shared equivalents (Default policy); replace inline `ActiveNodeGate` with `AkkaActiveNodeGate`. |
| **MxGateway** | Only `/health/live` (custom `GatewayHealthReply`); `AddHealthChecks()` called but zero probes registered. | Missing `ready` and `active` tiers; no probes; not using standard health middleware. | Replace custom endpoint with `app.MapZbHealth()`; register `GrpcDependencyHealthCheck` for the x86 worker channel on the `ready` tag. |

## 7. Acceptance (what "converged" means)

A project is converged when: (a) it calls `app.MapZbHealth()` and exposes all three canonical
endpoints; (b) its Akka probes (if applicable) use the `AkkaClusterHealthCheck` + `ActiveNodeHealthCheck`
from `ZB.MOM.WW.Health.Akka` with the Default policy; (c) its DB probe uses `DatabaseHealthCheck<TContext>`
from `ZB.MOM.WW.Health.EntityFrameworkCore`; (d) its gRPC-dependency probe (if applicable) uses
`GrpcDependencyHealthCheck`; (e) its `IActiveNodeGate` implementation is `AkkaActiveNodeGate`
(or a project-specific implementation of the shared interface); (f) all health endpoints return the
canonical JSON shape defined in §3.
