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ADR 0016: NAT44 session sync event stream mode

Status​

Accepted

Context​

NAT44SessionSync currently uses a snapshot loop. On every reconcile it dumps selected local conntrack entries with conntrack --dump -o extended -n <snat-address>, builds a delete-then-insert restore script, and sends that script to every target over SSH.

This keeps failover behavior simple and idempotent, but it also creates work on every interval even when only a small number of flows changed. homert02 observations showed the controller taking several seconds per 30 second cycle, with repeated conntrack, ssh, and remote restore command process creation.

Decision​

Add an event stream mode for NAT44 session sync while keeping snapshot as the compatible default.

The event stream mode uses a long-lived local conntrack event reader. The local side consumes conntrack create, update, and destroy events, filters them to the configured SNAT addresses or NAT rules, converts them to restore operations, and sends ordered batches to each target. The first implementation still uses the existing SSH restore path for each batch; a long-lived target transport is left as a follow-up optimization once local snapshot churn has been removed.

The first implementation must preserve these properties:

  • snapshot remains supported and keeps the current behavior.
  • event-stream starts with a full snapshot resync before accepting live events.
  • stream loss forces the worker to restart and perform a snapshot resync before returning to healthy status.
  • target status exposes stream state, last event time, last batch time, last resync time, resync count, queued event count, and last error.
  • all restore operations remain idempotent: duplicate inserts and missing deletes are not fatal.

Transport​

The target transport should reuse SSH because operators already have SSH credentials and privilege boundaries for snapshot sync. To avoid per-cycle handshake cost, the implementation should use either:

  • SSH multiplexing with ControlMaster=auto and ControlPersist, or
  • one long-lived SSH process running a small remote restore loop.

The first implementation reuses the current SSH script path for each event batch to keep the runtime change small and preserve the existing idempotent restore behavior. A long-lived restore loop remains the preferred follow-up because it also avoids starting sh and conntrack for every batch. A later optimization can replace per-entry conntrack calls with a small installed helper if the shell loop becomes the next bottleneck.

Reconciliation Model​

Event stream mode still needs a periodic controller entry point, but that periodic reconcile should supervise long-lived workers rather than perform the full sync itself.

Responsibilities:

  • build desired sync jobs from NAT44SessionSync resources;
  • start, stop, or restart workers when spec, target, or dependency status changes;
  • report worker health into resource status;
  • trigger a full resync when the worker reports an unsafe gap.

The worker owns the hot path:

  • read local conntrack events;
  • filter by SNAT address;
  • coalesce bursts into small batches;
  • send batches to every target;
  • apply backpressure with bounded queues;
  • mark a target Degraded and request resync if it falls behind.

Failure Handling​

Event stream mode is eventually consistent. A target must not be marked Synced until it has completed an initial snapshot and is consuming live events.

Failure cases:

  • local event reader exits: restart reader and resync all targets;
  • target SSH batch fails: mark the batch result as Error or Degraded and continue reporting the failing target status;
  • event stream exits: restart the worker and perform a snapshot resync;
  • restore batch has partial failures: keep idempotent duplicate/missing cases healthy, mark real restore failures as Degraded or Error;
  • resource dependencies become pending: stop workers and report Pending.

Migration Plan​

  1. Keep the existing snapshot implementation as the default.
  2. Add API and validation for mode: event-stream.
  3. Add the worker manager, local event reader, batch restore path, and status reporting behind event-stream.
  4. Test on homert02 with both modes available:
    • compare nat44-session-sync duration;
    • compare 75 second execve counts for ssh and conntrack;
    • verify failover behavior;
    • force SSH disconnect and confirm snapshot resync recovery.
  5. After event stream mode is stable, consider making it the recommended mode for HA routers with high session churn.

Consequences​

Event stream mode reduces steady-state process churn and sync latency, but it adds long-lived worker lifecycle, backpressure, and resync logic. Snapshot mode remains useful as a simple fallback and as the recovery mechanism when stream integrity is uncertain.