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Ghostferry

Ghostferry is a library that enables you to selectively copy data from one mysql instance to another with minimal amount of downtime.

It is inspired by Github's gh-ost, although instead of copying data from and to the same database, Ghostferry copies data from one database to another and has the ability to only partially copy data.

There is an example application called ghostferry-copydb included (under the copydb directory) that demonstrates this library by copying an entire database from one machine to another.

Talk to us on IRC at irc.freenode.net #ghostferry.

Overview of How it Works

An overview of Ghostferry's high-level design is expressed in the TLA+ specification, under the tlaplus directory. It may be good to consult with that as it has a concise definition. However, the specification might not be entirely correct as proofs remain elusive.

On a high-level, Ghostferry is broken into several components, enabling it to copy data. This is documented at https://shopify.github.io/ghostferry/main/technicaloverview.html

Development Setup

Installation

For Internal Contributors

dev up

For External Contributors

  • Have Docker installed
  • Clone the repo
  • docker-compose up -d
  • nix-shell

Testing

Run all tests

  • make test

Run example copydb usage

  • make copydb && ghostferry-copydb -verbose examples/copydb/conf.json
  • For a more detailed tutorial, see the documentation.

Ruby Integration Tests

Kindly take note of following options:

  • DEBUG=1: To see more detailed debug output by Ghostferry live, as opposed to only when the test fails. This is helpful for debugging hanging test.

Examples:

Run all tests

rake test

Run a single file

rake test TEST=test/integration/trivial_test.rb

or

ruby -Itest test/integration/trivial_test.rb

Run a specific test

DEBUG=1 ruby -Itest test/integration/trivial_test.rb -n "TrivialIntegrationTest#test_logged_query_omits_columns"