feat: Add Microsoft SQL Server (MSSQL) connector

[danghoangnhan]

Context

Pathway currently supports PostgreSQL, MySQL, and other database connectors, but lacks support for Microsoft SQL Server (MSSQL). MSSQL is widely used in enterprise environments and adding native support would expand Pathway's reach for real-time data processing pipelines.

Proposal

Add a new pw.io.mssql module with read() and write() functions for Microsoft SQL Server.

Key features

• Pure Rust TDS implementation via tiberius crate — no ODBC drivers required, works on any Linux environment without additional system dependencies
• Read modes:
  • static — polls the table periodically and diffs against stored state to detect inserts, updates, and deletions
  • streaming — uses MSSQL's Change Data Capture (CDC) for real-time change tracking via the transaction log
• Write modes:
  • stream_of_changes — outputs a log of all data updates with time and diff columns
  • snapshot — maintains current state using atomic MERGE upserts
• Supported MSSQL versions: SQL Server 2017, 2019, 2022, and Azure SQL Edge
• ADO.NET-style connection strings consistent with MSSQL ecosystem conventions

Implementation scope

• Python API: pw.io.mssql.read() and pw.io.mssql.write()
• Rust connector: full TDS protocol support with connection pooling, type mapping, and batch operations
• Integration tests: comprehensive test suite (~778 lines) covering static/CDC reads, writes, snapshot mode, schema handling, and error cases
• Docker Compose example: ready-to-run example with SQL Server container

Types of changes

• New feature (non-breaking change which adds functionality)

Related work

This follows the same connector patterns established by the existing PostgreSQL (pw.io.postgres) and MySQL connectors.

[danghoangnhan]

Implementation Complete — PR #205

The MSSQL connector has been fully implemented in PR #205 (feature/mssql branch). Here's a summary of what's been delivered:

Public API

• pw.io.mssql.read() — Static polling mode (default) + CDC streaming mode
• pw.io.mssql.write() — Stream-of-changes (default) + snapshot mode with MERGE upserts

Technical Details

• Pure Rust TDS via tiberius crate — no ODBC drivers needed
• Compatible with SQL Server 2017, 2019, 2022, and Azure SQL Edge
• 22 files changed, +12,308 lines across Rust connector, Python API, integration tests, and example project
• 778-line integration test suite covering reads, writes, CDC, all data types, and snapshot behavior

Key Design Decisions

1. Static mode as default (vs PostgreSQL's streaming default) — CDC requires Enterprise/Developer edition, so polling is more universally accessible
2. ADO.NET connection strings — consistent with the MSSQL ecosystem
3. MERGE for snapshot upserts — atomic single-statement upserts for data consistency
4. Bracket-quoted identifiers ([dbo].[table]) — proper MSSQL schema qualification

See PR #205 for full details and code review.

[zxqfd555]

The PR has been merged into a separate branch.
The connector has been added to our integration tests runner. I've also applied several code fixes.
It will be released with the next Pathway release.