Orderbook channel - ParlayX API

The book channel delivers a normalized orderbook: a full snapshot first, then delta frames carrying only the levels that changed. It’s the default channel when you omit channels on subscribe.

Prices and sizes

Every market is normalized to one scale, regardless of venue:

Field	Unit
`price`	Implied probability, a decimal in `[0, 1]` (dollars per share)
`size`	Outcome shares

A Level is { price, size }. A snapshot carries separate bids and asks arrays of levels; a delta carries a flat list of changes, where each change is { side, price, size } and size: 0 means that price level is now gone.

`snapshot` and `delta`

{
  "type": "snapshot", "v": 1, "serverTimestamp": 1748020034210,
  "subscriptionId": "f47ac10b-…", "seq": 4287, "venueTimestamp": 1748020034120,
  "bids": [ { "price": 0.52, "size": 1250 } ],
  "asks": [ { "price": 0.53, "size": 800 } ],
  "status": "live"
}

{
  "type": "delta", "v": 1, "serverTimestamp": 1748020034260,
  "subscriptionId": "f47ac10b-…", "seq": 4288, "venueTimestamp": 1748020034250,
  "changes": [ { "side": "bid", "price": 0.52, "size": 0 } ]
}

A snapshot replaces your local book. A delta mutates it: each change is { side, price, size }, and size: 0 deletes that price level. We deliberately don’t distinguish add / update / remove — it’s always “set this level to this size, where 0 means gone.” venueTimestamp is the venue’s timestamp for the underlying event; serverTimestamp is when we emitted the frame. A minimal apply loop:

const books = new Map();        // subscriptionId -> { bids, asks }

function onSnapshot(f) {
  books.set(f.subscriptionId, { bids: f.bids, asks: f.asks });
}

function onDelta(f) {
  const book = books.get(f.subscriptionId);
  if (!book) return;
  for (const c of f.changes) {
    const side = c.side === "bid" ? book.bids : book.asks;
    const i = side.findIndex((l) => l.price === c.price);
    if (c.size === 0) { if (i >= 0) side.splice(i, 1); }
    else if (i >= 0) side[i].size = c.size;
    else side.push({ price: c.price, size: c.size });
  }
}

Sequencing and resync

Per subscription, seq is a monotonically increasing integer that we assign at ingestion (not the venue). The core guarantee for the book channel:

We never send a delta whose seq is not exactly the previous seq + 1. If we otherwise would, we send a snapshot instead.

How this works:

The first frame on a book subscription is always a snapshot.
On a venue gap or upstream reconnect, you get status: resyncing, then a fresh snapshot, then deltas resume. Drop your book on resyncing and trust the next snapshot.
A mid-stream snapshot (seq jumps via a snapshot rather than a +1 delta) is normal and expected — replace your book and carry on.

Because of this contract, your apply loop needs no gap-handling logic of its own. As a minimal-effort defensive check, track the last seq per subscription; if a delta ever arrives where seq !== last + 1, you can simply wait for the snapshot that the contract guarantees is coming. The trade channel has its own per-market sequence space and no resync mechanics — its seq works differently, so don’t apply book reasoning to it.

​Prices and sizes

​snapshot and delta

​Sequencing and resync

Prices and sizes

`snapshot` and `delta`

Sequencing and resync