Archetype 11 — Control Plane + Local Snapshot #

What this archetype is #

One central config truth publishes versioned snapshots that many local evaluators apply and serve from memory.

Examples: feature flags, config rollout, policy distribution.

We will use feature flags as the running example.

Layer 1: Entities and Postgres table design #

ConfigState
SnapshotState
AppliedVersionState

create table flag_configs (
  flag_key text primary key,
  config jsonb not null,
  version bigint not null,
  updated_at timestamptz not null default now()
);

create table config_snapshots (
  snapshot_id bigserial primary key,
  version bigint not null unique,
  payload jsonb not null,
  created_at timestamptz not null default now()
);

Layer 2: Write path mechanics #

Update config #

begin;

update flag_configs
set config = $2,
    version = version + 1,
    updated_at = now()
where flag_key = $1;

insert into config_snapshots (version, payload)
values ($3, $4);

commit;

Agents then:

• poll latest version
• or watch a notification channel
• fetch snapshot
• atomically swap local snapshot if version is newer

Layer 3: Fault tolerance #

• stale snapshot
• out-of-order apply
• partial rollout
• old config still serving locally

Controls:

• monotonic version checks
• full snapshot fallback
• atomic local apply

Layer 4: Scale #

Default hotspots:

• rollout fanout to many agents
• snapshot distribution bursts
• hot tenant config
• snapshot size

Common mitigations:

• push-version / pull-snapshot
• regional cache/distributor layer
• delta snapshots only when safe