Did We Forget the Benefits of RDBMS?

An overview of things we need to consider when doing zero downtime web application deployments and how RDBMS can help us.

Transcript

1. Did we forget
 the benefits of RDBMS? Damir Zekić @sidonath
2. A Short History of Rails
5. How to Achieve High Availability? Resilient and scalable server architecture Good software development process Zero-downtime deployments
6. App Deployment Replace old code with the new one Change the database schema (execute migrations)
7. Zero-downtime Deployments "Zero" as in 0, not as in "Coke Zero" (3kc/l) No request is dropped due to code change No request should be fail due to application error (500 status)
8. No Dropped Requests Start workers with the new code Route new requests to the new code Let the old workers finish processing requestes and kill them
9. Unicorn GitHub documented their approach in 2009: https://github.com/blog/517-unicorn
10. Heroku Pre-boot A labs feature: https://devcenter.heroku.com/articles/labs-preboot
11. No Requests Should Fail Why would a request fail? • Codebase/database mismatch • Codebase/user submitted data mismatch (public API change)
13. "What about migrations? Simple: just throw up a “The site is temporarily down for maintenance” page, run the migration, restart Unicorn, then remove the downtime page. Same as it ever was." https://github.com/blog/517-unicorn
14. Codebase/Database Mismatch When do we execute migrations? • Before code deployment • After code deployment • It depends
15. Recipe: Remove a Column 1. Tell Rails not to cache a column (needs a hack in code) 2. Deploy code that doesn't use a column 3. Execute migrations after deployment
16. Recipe: Add a Column 1. Execute the migration* 2. Deploy the new code ! *A nifty helper to execute migrations remotely on Heroku: https://gist.github.com/sidonath/8735211
17. Recipe: Rename a Column 1. Create a new column with the new name 2. Change Rails model to write data to both columns 3. Exclude the old column from the column cache 4. Deploy the changed code 5. Migrate data 6. Remove old column
20. But We Use RDBMS?! ACID: Atomicity, Consistency, Isolation, Durability Stored Procedures Triggers Views
21. Rename a Column #2 1. Write a migration that uses triggers to keep data in sync and migrates the data in a transaction 2. Run the migration 3. Deploy the new code (no hacks necessary!) 4. Run a migration that removes now unnecessary triggers
22. Easy? Easier
23. Step-by-Step Add a column Write a stored procedure (in PL/pgSQL, MySQL, T-SQL, PG/SQL) Create AFTER INSERT/UPDATE/DELETE trigger to run the procedure Execute INSERT/UPDATE to migrate the data COMMIT the transaction
24. Example: Rename a Table
25. create_table :plutus_entries do |t| t.string "description" end ! add_column :plutus_amounts, :entry_id, :integer
26. execute <<-UPDATE_TABLE_TRIGGER_FUNCTION CREATE OR REPLACE FUNCTION copy_transaction_to_entry() RETURNS trigger AS $copy_transaction_to_entry$ BEGIN ! INSERT INTO plutus_entries (id, description) VALUES (NEW.id, NEW.description); ! SELECT setval('plutus_entries_id_seq', NEW.id); ! RETURN NEW; END; $copy_transaction_to_entry$ LANGUAGE plpgsql; UPDATE_TABLE_TRIGGER_FUNCTION
27. execute <<-UPDATE_COLUMN_TRIGGER_FUNCTION CREATE OR REPLACE FUNCTION copy_transaction_id_to_entry_id() RETURNS trigger AS $copy_transaction_id_to_entry_id$ BEGIN ! NEW.entry_id := NEW.transaction_id; RETURN NEW; ! END; $copy_transaction_id_to_entry_id$ LANGUAGE plpgsql; UPDATE_COLUMN_TRIGGER_FUNCTION
28. execute <<-TRIGGERS CREATE TRIGGER replicate_plutus_transactions AFTER INSERT ON plutus_transactions FOR EACH ROW EXECUTE PROCEDURE copy_transaction_to_entry(); ! CREATE TRIGGER replicate_plutus_transaction_id BEFORE INSERT ON plutus_amounts FOR EACH ROW EXECUTE PROCEDURE copy_transaction_id_to_entry_id(); TRIGGERS
29. execute <<-MIGRATEDATA INSERT INTO plutus_entries SELECT * FROM plutus_transactions WHERE id NOT IN (SELECT id FROM plutus_entries); ! SELECT setval('plutus_entries_id_seq', nextval('plutus_transactions_id_seq'), false); ! UPDATE plutus_amounts SET entry_id = transaction_id; MIGRATEDATA
30. Worth it? It depends
31. Thank you! Questions? ! ! Damir Zekić @sidonath