ETLRelay

ETLRelay is an open-source Python framework for building lightweight, composable ETL pipelines.

It uses a simple pipeline model:

Source → Processor(s) → Sink

A source produces batches of data, processors transform those batches, and a sink consumes the results.

For file-based pipelines, ETLRelay separates storage access from format parsing and serialization:

FileSystem → Reader → Batch → Processor(s) → Writer → FileSystem

This separation allows file sources and sinks to compose storage and format implementations independently. The same CSV reader can work with different storage implementations without needing to know where the data resides.

ETLRelay is being developed incrementally, with test-driven development, explicit component contracts, and a focus on keeping the architecture small.

Installation

Install ETLRelay from PyPI:

pip install etlrelay

Documentation

See the ETLRelay documentation for usage instructions, examples, and API details.

Project Status

Current stage: Core pipeline and local CSV I/O

The foundational components are implemented, including the batch model, pipeline, local filesystem access, CSV reader and writer, file source and sink, and path validation.

Unit tests validate individual components. Integration and functional tests exercise their composition.

The current implementation establishes the foundation for further development. ETLRelay is not yet a complete ETL product, and its public API may evolve before the first stable release.

Implemented Components

Component	Responsibility
Batch	Represents tabular data using Apache Arrow
Source	Defines the contract for producing batches
Processor	Defines the contract for transforming a batch
Sink	Defines the contract for consuming batches
Pipeline	Coordinates source, processor, and sink execution
FileSystem	Defines the interface for opening binary streams
LocalFileSystem	Provides local filesystem access
PathValidator	Enforces local filesystem path containment
Reader	Defines the contract for reading data from a binary stream
CsvReader	Parses CSV data into ETLRelay batches
Writer	Defines the contract for serializing batches to a binary stream
CsvWriter	Serializes ETLRelay batches as CSV
FileSource	Composes a filesystem and reader to produce batches
FileSink	Composes a filesystem and writer to consume batches

Planned Capabilities

The following are potential areas of development, not features currently guaranteed to be available:

• Additional file formats, including Parquet and JSON
• Additional storage backends, including Amazon S3 and HDFS
• A standard library of data transformation processors
• SQL sources and sinks using SQLAlchemy
• Declarative YAML pipeline configuration
• A command-line interface
• Improved configuration, secrets, and operational error handling

Features will be added when requirements justify them. The project will not introduce scheduling, distributed execution, or other orchestration infrastructure merely to anticipate future needs.

Architecture

Pipeline Execution

The pipeline provides the central execution model:

Source
  │
  ▼
Iterable[Batch]
  │
  ▼
Processor 1
  │
  ▼
Processor 2
  │
  ▼
    ...
  │
  ▼
Sink

The pipeline applies processors in their configured order to each batch. With no processors, batches pass through unchanged.

Execution is synchronous. Exceptions propagate to the caller rather than being silently discarded or handled by an elaborate recovery framework.

Separation of Storage and Serialization

ETLRelay keeps resource access independent of file-format handling.

The FileSystem protocol defines operations for opening readable and writable binary streams. LocalFileSystem implements that protocol for local files and uses PathValidator to enforce its permitted filesystem boundary.

Readers and writers operate on streams rather than filesystem paths.

For example, CSV reading follows:

LocalFileSystem.open_read(path)
          │
          ▼
      BinaryIO
          │
          ▼
       CsvReader
          │
          ▼
        Batch

CSV writing follows the reverse path:

        Batch
          │
          ▼
       CsvWriter
          │
          ▼
      BinaryIO
          │
          ▼
LocalFileSystem.open_write(path)

FileSource composes a filesystem and a reader. FileSink composes a filesystem and a writer.

This prevents source and sink implementations from becoming coupled to individual storage technologies or serialization formats.

The current implementation uses the local filesystem and CSV. Additional combinations can be introduced through new implementations of the existing contracts when they are needed.

Common Data Representation

ETLRelay uses Apache Arrow tables as the underlying tabular representation within Batch.

Readers convert external data into batches. Processors operate on batches, and writers serialize batches for output.

This provides a common data boundary between file formats and pipeline components.

Polars is a potential transformation tool for future processors; its use is not required by the current core pipeline implementation.

Component Contracts

ETLRelay uses Python protocols to define component boundaries where substitutability is useful.

The core contracts are deliberately small:

Source
    Produces batches

Processor
    Transforms a batch

Sink
    Consumes batches

For file-based operations:

FileSystem
    Opens binary streams

Reader
    Converts stream data into batches

Writer
    Serializes batches to a stream

Concrete implementations remain independent of higher-level orchestration. The design avoids unnecessary abstract base classes, factories, registries, and inheritance hierarchies.

Development Environment

The development environment is managed with Nix flakes.

Enter the development shell:

nix develop

The environment provides:

• Python 3.12
• uv
• Ruff
• Pyright
• Git

Nix provides system-level development tools and executables, while uv manages the Python project environment and Python dependencies.

The Python virtual environment is managed by uv in .venv/.

The project uses Nix-provided versions of command-line development tools such as Ruff and Pyright to avoid relying on generic Linux executables that may be incompatible with NixOS.

Getting Started

Enter the development shell:

nix develop

Install or synchronize Python dependencies:

uv sync

Run the test suite:

uv run pytest

Run the linter:

ruff check .

Check formatting:

ruff format --check .

Run static type checking:

pyright

Run these checks before committing implementation changes.

Development Principles

Test-Driven Development

Development follows a small, explicit cycle:

Requirement
    ↓
Acceptance Criteria
    ↓
Failing Test
    ↓
Minimal Implementation
    ↓
Passing Test
    ↓
Refactor
    ↓
Validation

Tests establish expected behavior before implementation. Unit tests validate components independently, while integration and functional tests verify that components work together.

Keep Architecture Small

Prefer simple, explicit abstractions over framework machinery.

An abstraction should exist because a concrete requirement justifies it, not because a hypothetical future requirement might eventually need it.

Prefer composition and small protocols over unnecessary inheritance. Add configuration, extension mechanisms, and additional layers only when they solve an identified problem.

Separate Responsibilities

Each component should have one clearly defined responsibility:

• PathValidator checks local path containment.
• LocalFileSystem handles local filesystem access.
• Readers parse external representations.
• Writers serialize batches.
• Sources and sinks compose the relevant components.
• Processors transform batches.
• Pipeline coordinates execution.

Security checks should be implemented at the layer where the relevant risk exists.

Independent Implementation

ETLRelay is independently developed open-source software.

Its implementation, interfaces, documentation, tests, and design decisions are developed from its own requirements and design process.

Security

Security is a design concern from the beginning.

Current filesystem path validation resolves candidate paths and checks that they remain within the permitted base directory. It does not determine whether a path exists; filesystem operations handle existence and access errors.

Further security work will be required as ETLRelay gains more capabilities. Areas for future development include:

• Safe SQL parameterization and identifier validation
• Configuration validation
• Secure credential and secret handling
• Sensitive-data redaction
• Storage-specific authentication and access control
• Appropriate error handling for external resources

The current path-validation component is a filesystem containment control, not a complete filesystem sandbox or authorization system.

Testing

Testing follows the TDD development model.

Unit Tests

Unit tests validate components independently, including:

• Batch representation
• Pipeline execution and processor ordering
• Path containment
• Filesystem interactions
• CSV parsing and serialization
• File source and sink orchestration
• Error propagation and resource cleanup

Integration Tests

Integration tests verify interactions between real components, including local filesystem operations and CSV reading and writing.

Functional Tests

Functional tests exercise complete workflows through the source, processing, and sink boundaries.

The suite should use temporary directories and controlled test inputs rather than relying on repository-local data or external infrastructure unnecessarily.

As additional storage backends and data sources are introduced, their integration tests will be added alongside the corresponding implementations.

Project Structure

The current package is organized around component responsibilities. The structure will evolve as additional features are implemented.

src/
└── etlrelay/
    ├── core/
    │   ├── batch.py
    │   ├── source.py
    │   ├── processor.py
    │   ├── sink.py
    │   ├── reader.py
    │   └── writer.py
    ├── pipeline/
    │   └── pipeline.py
    ├── security/
    │   └── path.py
    ├── storage/
    │   ├── filesystem.py
    │   └── local.py
    ├── readers/
    │   └── csv.py
    ├── writers/
    │   └── csv.py
    ├── sources/
    │   └── file.py
    └── sinks/
        └── file.py

tests/
├── unit/
├── integration/
└── functional/

docs/
examples/

The tree is illustrative of the current component organization. It does not imply that every future feature or module has already been implemented.

Generated files and local development state—including .venv/, Python caches, test output, and local ETL data—should be excluded through .gitignore.

Development Workflow

Each implementation unit follows these steps:

1. Define the requirement.
2. Establish acceptance criteria.
3. Choose the smallest suitable design.
4. Write failing tests.
5. Implement the minimum required behavior.
6. Confirm the tests pass.
7. Refactor where justified.
8. Run the full test suite and quality checks.
9. Update documentation.
10. Commit the completed change.

The project should remain in a working state throughout development.

License

ETLRelay is licensed under the Apache License, Version 2.0.

See LICENSE for the full license text.