FIFO Order Book

Phoenix uses a FIFO order book for normal resting orders. FIFO means:

• better price fills first
• at the same price, older resting liquidity fills before newer resting liquidity

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What a resting order stores

A resting FIFO order stores:

• the maker trader position
• initial and remaining base size
• order flags
• optional conditional order index
• optional slot-based expiry
• initial slot

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Capacity and order limits

Phoenix enforces both per-trader open-order limits and market-level orderbook capacity.

• A trader can have up to 64 resting bid limit orders per market.
• A trader can have up to 64 resting ask limit orders per market.
• The per-trader limit is per market and per side, so one trader can have up to 128 resting FIFO orders on one market: 64 bids and 64 asks.
• IOC and other take-only orders do not count toward this cap because they do not rest on the book.

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Order packet families

At the protocol layer, the core order packet kinds are:

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Important fields

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price_in_ticks

The order price in native book ticks.

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num_base_lots

The total base size to fill or rest.

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last_valid_slot

Optional expiry slot for the order.

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match_limit

Optional cap on how many resting orders an incoming order can match against.

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cancel_existing

Advanced behavior that lets some order paths cancel conflicting resting orders instead of failing a margin check immediately.

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Maker vs taker behavior

• A resting order that provides liquidity is maker flow.
• An incoming aggressive order that removes resting liquidity is taker flow.
• A standard limit order can act as either, depending on whether it crosses the book.

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Post-only behavior

Post-only orders are for adding liquidity only.

• If they would immediately trade, they do not execute as takers.
• Depending on configuration, they may slide to a valid resting price rather than cross.

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Expiry and live L2 views

FIFO order expiry is slot-based. An expired order can remain in the raw book until a matching or maintenance path touches it. Consumers that need a live L2 view should filter expired FIFO orders before aggregating by price.