dask

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# Dask

## Overview

Dask is a Python library for parallel and distributed computing that enables three critical capabilities:
- **Larger-than-memory execution** on single machines for data exceeding available RAM
- **Parallel processing** for improved computational speed across multiple cores
- **Distributed computation** supporting terabyte-scale datasets across multiple machines

Dask scales from laptops (processing ~100 GiB) to clusters (processing ~100 TiB) while maintaining familiar Python APIs.

## When to Use This Skill

This skill should be used when:
- Process datasets that exceed available RAM
- Scale pandas or NumPy operations to larger datasets
- Parallelize computations for performance improvements
- Process multiple files efficiently (CSVs, Parquet, JSON, text logs)
- Build custom parallel workflows with task dependencies
- Distribute workloads across multiple cores or machines

## Core Capabilities

Dask provides five main components, each suited to different use cases:

### 1. DataFrames - Parallel Pandas Operations

**Purpose**: Scale pandas operations to larger datasets through parallel processing.

**When to Use**:
- Tabular data exceeds available RAM
- Need to process multiple CSV/Parquet files together
- Pandas operations are slow and need parallelization
- Scaling from pandas prototype to production

**Reference Documentation**: For comprehensive guidance on Dask DataFrames, refer to `references/dataframes.md` which includes:
- Reading data (single files, multiple files, glob patterns)
- Common operations (filtering, groupby, joins, aggregations)
- Custom operations with `map_partitions`
- Performance optimization tips
- Common patterns (ETL, time series, multi-file processing)

**Quick Example**:
```python
import dask.dataframe as dd

# Read multiple files as single DataFrame
ddf = dd.read_csv('data/2024-*.csv')

# Operations are lazy until compute()
filtered = ddf[ddf['value'] > 100]
result = filtered.groupby('category').mean().compute()
```

**Key Points**:
- Operations are lazy (build task graph) until `.compute()` called
- Use `map_partitions` for efficient custom operations
- Convert to DataFrame early when working with structured data from other sources

### 2. Arrays - Parallel NumPy Operations

**Purpose**: Extend NumPy capabilities to datasets larger than memory using blocked algorithms.

**When to Use**:
- Arrays exceed available RAM
- NumPy operations need parallelization
- Working with scientific datasets (HDF5, Zarr, NetCDF)
- Need parallel linear algebra or array operations

**Reference Documentation**: For comprehensive guidance on Dask Arrays, refer to `references/arrays.md` which includes:
- Creating arrays (from NumPy, random, from disk)
- Chunking strategies and optimization
- Common operations (arithmetic, reductions, linear algebra)
- Custom operations with `map_blocks`
- Integration with HDF5, Zarr, and XArray

**Quick Example**:
```python
import dask.array as da

# Create large array with chunks
x = da.random.random((100000, 100000), chunks=(10000, 10000))

# Operations are lazy
y = x + 100
z = y.mean(axis=0)

# Compute result
result = z.compute()
```

**Key Points**:
- Chunk size is critical (aim for ~100 MB per chunk)
- Operations work on chunks in parallel
- Rechunk data when needed for efficient operations
- Use `map_blocks` for operations not available in Dask

### 3. Bags - Parallel Processing of Unstructured Data

**Purpose**: Process unstructured or semi-structured data (text, JSON, logs) with functional operations.

**When to Use**:
- Processing text files, logs, or JSON records
- Data cleaning and ETL before structured analysis
- Working with Python objects that don't fit array/dataframe formats
- Need memory-efficient streaming processing

**Reference Documentation**: For comprehensive guidance on Dask Bags, refer to `references/bags.md` which includes:
- Reading text and JSON files
- Functional operations (map, filter, fold, groupby)
- Converting to DataFrames
- Common patterns (log analysis, JSON processing, text processing)
- Performance considerations

**Quick Example**:
```python
import dask.bag as db
import json

From Zaoqu-Liu/ScienceClaw, MIT.

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