Delta compression

Parsec exposes Centrifuge’s per-channel fossil-delta encoding as an opt-in flag. When enabled, the broker ships only the diff between the previous publication and the current one — the wire payload shrinks dramatically for high-frequency channels with small mutations (scoreboards, ticker feeds, partial state snapshots).

When to enable

Enable delta when all of the following hold:

• The channel publishes frequently (multiple times per second).
• Successive payloads share most of their bytes (incremental counters, small JSON patches, append-only logs).
• Subscribers can tolerate a one-message-out-of-order tail being recomputed from the prior payload.

Do not enable delta for:

• Sparse, infrequent publications (no compression headroom; the encoder overhead is pure cost).
• Mostly-unique payloads (deltas approach the full payload size — slower than no encoding at all).
• Channels where individual messages must remain self-contained on the wire (audit feeds, signed payloads).

Library API

ch, _ := p.OpenPublic("public:scoreboard.counter.global", time.Hour)
_ = p.Manager().SetDelta(ch.Name, true)

SetDelta(name, true) flips the channel’s DeltaEnabled flag in the store. The broker reads the flag on every publish and chooses between the plain and delta wire format. Toggling is hot — no reconnect required.

Channel.DeltaEnabled is exposed on parsec channels get and through the Twirp GetChannel RPC so operators can audit the state.

CLI

parsec channels open public:scoreboard.counter.global --delta
parsec channels create private:agent.analysis.{job}.progress --delta --subject job-42

For an already-open channel:

parsec channels set-delta public:scoreboard.counter.global --on
parsec channels set-delta public:scoreboard.counter.global --off

How it works

Parsec sets centrifuge’s WithDelta(true) publish option and configures the Node with AllowedDeltaTypes: []DeltaType{DeltaTypeFossil}. The encoder is the fossil-delta algorithm shipped in centrifuge OSS — parsec adds the channel-level toggle and persistence; the wire encoding is upstream.

Subscribers negotiate delta support during the connect handshake. A client that does NOT advertise fossil support receives the full payload — the broker degrades gracefully, never producing a delta the client cannot decode.

Manifest exposure

{
  "delta_compression": "supported"
}

The field reports parsec capability, not per-channel state. Operators inspecting individual channels see the flag on Channel.DeltaEnabled.

Multi-node coherence

When the Redis broker is active, the per-channel delta flag rides on the channel registry HASH (parsec:ch:<name> → delta=1). Every node reads the flag before publishing, so a delta channel stays delta across the whole cluster without re-issuing tokens.

The SetDelta event also rides the channel event bus so cached Channel snapshots stay in sync — clients of Manager.Subscribe see a updated event whenever the flag flips.

Observability

parsec_publications_total{visibility="public", delta="true"}
parsec_publications_total{visibility="public", delta="false"}

The label is bounded to true / false so cardinality stays flat.

Caveats

• Delta is per-channel, not per-subscriber. A channel either ships deltas to clients that support fossil OR it does not.
• The encoder holds the previous payload in memory per channel — for very wide payloads (>1 MB) the memory overhead can matter; profile before enabling.
• History playback still ships full payloads — deltas are a transport optimization, not a storage format. Late subscribers receive the ground truth.