---
title: "Horizon Pipeline Composition"
description: "Build Horizon strategies by chaining pipeline functions with automatic signature introspection."
icon: "layer-group"
---

Strategies are built by chaining **pipeline functions**. Each function receives the `Context` plus the outputs of previous functions. The final function must return `list[Quote]`.

## Basic Example

```python
# Step 1: Just context, compute fair value
def fair_value(ctx: hz.Context) -> float:
    return ctx.feeds["btc"].price * 0.01

# Step 2: Context + previous output, compute toxicity
def toxicity(ctx: hz.Context) -> float:
    return 0.5  # VPIN, Kyle's lambda, etc.

# Step 3: Context + two previous outputs, generate quotes
def quoter(ctx: hz.Context, fair: float, tox: float) -> list[hz.Quote]:
    spread = 0.02 + tox * 0.04
    return hz.quotes(fair, spread, size=5)

hz.run(pipeline=[fair_value, toxicity, quoter], ...)
```

## How It Works

Horizon inspects each function's **signature** to determine how many arguments it expects. Previous outputs are passed in order:

| Parameters | What's passed |
|-----------|---------------|
| `(ctx)` | Just the context |
| `(ctx, prev)` | Context + last output |
| `(ctx, a, b)` | Context + last two outputs |
| `(ctx, a, b, c)` | Context + last three outputs |

This means you can **insert or remove** pipeline stages without changing downstream functions.

## Signature Introspection

The pipeline runner uses `inspect.signature()` to count parameters:

```python
import inspect

for fn in pipeline:
    sig = inspect.signature(fn)
    n_params = len(sig.parameters)
    # n_params == 1: fn(ctx)
    # n_params == 2: fn(ctx, prev_output)
    # n_params == 3: fn(ctx, output_n-2, output_n-1)
```

<Tip>
  The first parameter is always `ctx: hz.Context`. Additional parameters receive outputs from previous pipeline stages, most recent last.
</Tip>

## Pipeline Rules

1. **At least one function** is required in the pipeline
2. The **final function** must return `list[Quote]` (or a single `Quote`)
3. Each function runs **once per market per cycle**
4. If a function raises an exception, the error is logged and that market is **skipped** for the cycle
5. The pipeline runs for **every market** in the `markets` list

## Result Processing

After the pipeline runs, `_process_result()` handles the output:

1. **Cancel** existing orders for the market (`cancel_market()`)
2. **Tick** the paper exchange with the feed mid price (or quote mid as fallback)
3. **Update mark prices** for unrealized P&L tracking
4. **Submit** new quotes, routed to the correct exchange based on `market.exchange`

<Note>
  Risk rejections during quote submission are logged at `debug` level (not `warning`), since they're expected during normal operation (e.g., position limits hit).
</Note>

## Composability Examples

### Adding a stage

You can add an intermediate stage without modifying existing functions:

```python
# Before: [fair_value, quoter]
# After:  [fair_value, inventory_skew, quoter]

def inventory_skew(ctx: hz.Context, fair: float) -> float:
    """Skew the fair value based on inventory."""
    skew = ctx.inventory.net * 0.001
    return fair - skew

hz.run(pipeline=[fair_value, inventory_skew, quoter], ...)
```

### Multiple data sources

```python
def fair_value(ctx: hz.Context) -> float:
    poly = ctx.feeds.get("poly", hz.context.FeedData())
    kalshi = ctx.feeds.get("kalshi", hz.context.FeedData())
    return (poly.price + kalshi.price) / 2

def volatility(ctx: hz.Context) -> float:
    btc = ctx.feeds.get("btc", hz.context.FeedData())
    return abs(btc.bid - btc.ask) / max(btc.price, 1)

def quoter(ctx: hz.Context, fair: float, vol: float) -> list[hz.Quote]:
    spread = 0.02 + vol * 0.1
    return hz.quotes(fair, spread, size=5)
```

## The `hz.quotes()` Helper

A convenience function to create quotes from a fair value and spread:

```python
quotes = hz.quotes(fair=0.50, spread=0.06, size=5.0)
# → [Quote(bid=0.47, ask=0.53, size=5.0)]
```

Or create quotes manually:

```python
quotes = [
    hz.Quote(bid=0.45, ask=0.55, size=10.0),
    hz.Quote(bid=0.40, ask=0.60, size=5.0),
]
```

## Built-in Pipeline Functions

Horizon provides several ready-to-use pipeline functions that can be composed together:

| Factory | Returns | Description |
|---------|---------|-------------|
| `hz.market_maker(...)` | `list[Quote]` | Avellaneda-Stoikov market making with multi-level quoting |
| `hz.signal_combiner(...)` | `float` | Combine multiple alpha signals into a composite score |
| `hz.kelly_sizer(...)` | `float` | Kelly criterion position sizing |
| `hz.arb_scanner(...)` | `list[Quote] \| None` | Continuous cross-exchange arbitrage scanning |

### Example: Signal → Kelly → Market Maker

```python
hz.run(
    pipeline=[
        hz.signal_combiner([
            hz.price_signal("book", weight=0.5),
            hz.momentum_signal("book", lookback=20, weight=0.3),
            hz.spread_signal("book", weight=0.2),
        ], smoothing=10),
        hz.market_maker(feed_name="book", gamma=0.5, size=5.0),
    ],
    ...
)
```

See [Market Making](/market-making), [Signal Combiner](/signals), [Kelly Criterion](/kelly), and [Arbitrage Executor](/arbitrage) for detailed documentation.
