# Product Overview

XCE is an HPC (High-Performance Computing) cluster management system providing job scheduling, queue management, and cluster resource control. It targets organizations running batch workloads on on-premises or air-gapped compute clusters who need deterministic scheduling, multi-tenant access control, and zero external database dependencies.

## Core Capabilities

1. **Job lifecycle management** — submit, start, stop, restart, kill, and monitor jobs including job arrays. Resource usage data is collected and exposed via API.
2. **Queue and scheduling** — plugin-based scheduling engine for job-to-node placement with queue control (enable/disable) and expression-based resource filtering.
3. **Cluster and host management** — node status monitoring (OK, UNAVAIL, LOCKEDU, LOCKEDW, BUSY, CLOSED, CLOSED_BY_ADMIN), cluster topology tracking, and host control operations.
4. **Multi-tenant RBAC + pluggable auth** — two-part `action:operation` permission IDs (e.g., `job:view`) with a separately-keyed domain scope per role (e.g., `type: self`, `type: self_ug`, `type: all`). Auth backends: configfile, LDAP, OAuth2, PAM. Built-in IP whitelist and rate limiting.
5. **Controller persistence and HA** — WAL + checkpoint-based single-controller persistence with Raft-style quorum election (RequestVote, term-based voting, freshness gates) for standby failover and live replication over TCP (Cap'n Proto serialization).

## Target Use Cases

- **Local HPC batch computing** — long-running scientific/engineering workloads requiring deterministic scheduling, queue priority, and resource isolation.
- **Air-gapped / secure environments** — standalone deployments with file-based auth + SSL, no external cloud dependencies.
- **Multi-team shared clusters** — multiple teams sharing a cluster through RBAC with JWT token authentication.
- **No-shared-storage HPC** — controller uses local filesystem (ext4/xfs) for storage; job files transferred via API upload/download rather than network shares.

## Value Proposition

XCE provides a **fully integrated single-vendor stack** (no database setup, unified build system) with an open, buildable codebase (Nix flakes, AppImage packaging). Compared to Slurm/PBS, it eliminates the external database requirement. Compared to commercial LSF, it is open source and reproducibly buildable.

**Key architectural trade-off**: single-writer model throughout — only the primary writes. HA follows Raft semantics with CP guarantees: consensus election (RequestVote, term-based voting, freshness gates) via the `src/lib/raft/` shared module (`raft.c`/`raft.h`, consumed by the controller through `ha_init_raft`), and majority-ACK log commitment for state-changing writes (WAL entries ≈ Raft log, `committed_lsn` ≈ commitIndex). Operations that cannot reach majority are rejected, never locally degraded. The single-writer constraint is XCE's simplification of Raft — no concurrent client proposals — while commitment and election inherit Raft's safety rules. All controller state is in-memory; persistence is WAL + checkpoint, not a traditional database.

## Active Development Context

The project is in a **persistence and HA transformation** (branch `persistent_ha`). Phases:

| Phase | Goal | Status |
|-------|------|--------|
| P0 | Restart survival (WAL + checkpoint) | Complete |
| P1 | Standby election and takeover (Raft-style) | Complete |
| P2 | Live replication primary → standby (config + WAL) | Complete |
| P2.5 | Worker/scheduler auto-reconnect on failover | In progress |
| P3–P4 | Hardening, metrics, docs | In progress |

## Spec-Driven Development

Features are designed and implemented via the kiro spec system (`.kiro/specs/`).
Each feature follows a 3-phase approval workflow: Requirements → Design → Tasks → Implementation.

**Status as of 2026-07-18**: the ha-consistency-invariants umbrella (ha-commit-engine,
ha-commit-barrier-rollout, ha-single-leader-role, ha-convergence-recovery) is
COMPLETE (closed 2026-07-13). The Raft TLA+ Conformance family (raft-library,
raft-log-replication, raft-commit-safety, raft-sim-conformance) and its
XCE-integration umbrella ha-term-safety are implemented + validate-impl GO.
job-array-single-wal-record shipped complete (2026-07-17, including a
user-directed residual-gap closure pass). A backlog sweep (2026-07-18) ran
`/kiro-validate-impl` on 9 specs whose tasks were 100% done but had never
been formally closed — raft-drive-async, ha-wal-replication,
ha-heartbeat-committed-propagation, ha-prevote, ha-elect-ctx-lifecycle,
ha-entity-snapshots, ha-commit-durability, libxce-failover,
raft-sim-conformance — all now GO. That sweep surfaced one real bug
(bug-1447: same-batch endpoint use-after-free in `io_worker`'s epoll
dispatch loop, see `ha-async-io.md`), root-caused and fixed the same day.

Remaining backlog (requirements-only, design-pending, or partially implemented):
admin-config-tool (spec finalized), config-upgrade-snapshot (tasks generated,
unblocked, implementation nearly complete). scheduler-framework was decomposed
into scheduler-core (implementation-complete), scheduler-allocation, and
scheduler-preemption (both active). See `roadmap.md` for dependency order
and status.

Key design decisions already locked:
- No `fork()` for snapshots (mutex inheritance deadlock in multithreaded controller)
- Raft + CP adopted (2026-06-10, supersedes earlier "No Raft"): Raft is the consensus model for both election and log commitment — majority-ACK commit barrier on all state-changing writes, reject (never degrade) when quorum is unavailable. Single-writer log retained as XCE's simplification: the controller is the sole log writer, avoiding concurrent-proposal complexity while inheriting Raft's safety rules (see `.kiro/specs/ha-consistency-invariants/brief.md`)
- No BoltDB (using dual zarray COW for application-level checkpointing)
- No application-layer encryption (trust OS boundary: chmod 0700, LUKS/dm-crypt)
- No WAL logging for job deletion (deferred to checkpoint time to simplify replay)