# soundcloud/lhm

Online MySQL schema migrations

Repository: https://github.com/soundcloud/lhm
Canonical: https://ross.abutalabs.com/products/soundcloud-lhm
Language: Ruby
License: BSD-3-Clause
License Family: permissive
Archived: true
Last push: 2026-07-09T13:43:37+00:00

## Health v2 (maintenance only)
Score: 10/100 (v2, computed 2026-09-02T17:46:02.011165+00:00)
- activity 91, release rhythm 35, longevity 100
- inputs: {"age_days": 5600, "days_push": 55, "days_rel": null, "gap_med": null, "n_releases_24m": 0}
- flags: no_releases, archived
- formula: round(0.45*activity + 0.35*rhythm + 0.20*longevity); archived -> min(score, 10)

## Adoption (not part of the score)
Stars 1852, forks 193 (observed 2026-08-28T04:05:44.460342+00:00)

## What it is
Large Hadron Migrator (LHM) is a Ruby gem for performing online MySQL schema migrations without locking large tables. It works alongside ActiveRecord migrations by copying data into a new table with triggers keeping it in sync while the system stays live.

## Use cases
- run ALTER TABLE on a table with millions of rows without downtime
- add or change indexes on large MySQL tables while the site is live
- evolve a Rails database schema without long table locks
- perform online column additions on busy MySQL tables
- migrate large ActiveRecord-backed tables safely in production

## When to choose
- you have a legacy Rails app on MySQL with very large tables and no other online migration tooling
- you need trigger-based online schema changes with a simple integer id primary key
- you want a lightweight gem instead of running external percona/gh-ost tooling

## When to avoid
- you are starting a new project - the repository is no longer actively maintained and is archived
- you use PostgreSQL or a database other than MySQL
- your tables lack a single auto-increment integer id column or have sparse ids
- you need support for modern Ruby or ActiveRecord versions

## Facets
- artifact type: library
- maturity: abandoned
- function: database, orm
- domain: databases, developer-tools, web-development
- platform: ruby
- tags: mysql, schema-migration, activerecord, online-migration, zero-downtime, archived

## Member repositories
- soundcloud/lhm (main) score 10

## Provenance
- Observed fields: from GitHub, fetched 2026-08-28T04:05:44.460342+00:00.
- Health v2: computed from the inputs above; adoption is never an input.
- Inferred fields (summary, facets, guidance): AI-extracted, prompt v1, taxonomy v1, on 2026-08-30T03:16:53.953043+00:00, confidence not recorded.
  - readme: https://github.com/soundcloud/lhm (fetched 2026-08-28T04:05:44.460342+00:00, sha 185f7f8ef8bf)
- Data as of 2026-08-30T08:39:29.467469+00:00.