MongoDB World 2019: Distributed Transactions: With Great Power Comes Great Responsibility
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The document discusses the capabilities and features of MongoDB transactions, highlighting their ACID compliance and improvements introduced in MongoDB 4.2, including cross-shard transactions. It outlines best practices for using transactions, such as ensuring they are not overly common operations and implementing retry logic. The document also emphasizes performance improvements when utilizing MongoDB transactions, alongside considerations around read concerns and transaction limits.
![Read Your Own Writes
Operation
Read
TXN
{_id: “OWNER12345”,
name: { first: “Aly”,
last: “Cabral”},
accountType: “Paid”,
pets: [ { type: “Dog”,
name: “Lady”}],
contact: { phoneNumber: 8008675309},
Address: { city: “New York”,
street: “94th”,
zipecode: 10025}}
Uncommitted Writes
street: “92nd”
Address.street = 94th
Address.street = 92nd](https://image.slidesharecdn.com/mdbwtrans-190718211440/85/MongoDB-World-2019-Distributed-Transactions-With-Great-Power-Comes-Great-Responsibility-10-320.jpg)
MongoDB World 2019: Distributed Transactions: With Great Power Comes Great Responsibility
- 1. Alyson Cabral, Product Manager Distributed Transactions: with Great Power Comes Great Responsibility aly_cabral
- 2. I know what you’re thinking…
- 3. But, Really… MongoDB Transactions are ACID Compliant (yes, even across shards)
- 4. Transactions make application development easier • Don’t need to worry about interim states • Either a customer is signed-up or they are not • Rollback/Abort is handled automatically if any single statement fails • Point-in-time consistency across reads/writes
- 5. def insertDocuments(s): s.client.db.col.insert_one({'abc': 1}, session=s) s.client.db.col.insert_one({'xyz': 999}, session=s) with client.start_session() as s: s.with_transaction(insertDocuments); New API in MongoDB 4.2! New in MongoDB 4.2 -- Cross Shard Transactions
- 8. All or Nothing Execution TXN Application MongoDB Uncommitted Writes { Document 1 } { Document 2 } { Document 3 } Committed Writes
- 9. Snapshot Isolation 100 101 102 103 104 105 106 107 108 109 110 111 Snapshot 102 Snapshot 108
- 10. Read Your Own Writes Operation Read TXN {_id: “OWNER12345”, name: { first: “Aly”, last: “Cabral”}, accountType: “Paid”, pets: [ { type: “Dog”, name: “Lady”}], contact: { phoneNumber: 8008675309}, Address: { city: “New York”, street: “94th”, zipecode: 10025}} Uncommitted Writes street: “92nd” Address.street = 94th Address.street = 92nd
- 12. 1) All data modeling rules still apply 2) Transactions should not be the most common operation (sanity check for rule 1) 3) Pass in the session to all statements 4) Implement retry logic, transactions can always abort 5) Don’t unnecessarily leave snapshots open 6) To trigger write conflicts, make sure you’re doing writes 7) Plan for DDL Operations
- 14. Majority Writes ------------ What promises do majority writes give you? No Arbiters!
- 15. Participant any shard that executes operations on behalf of a given transaction Coordinator A single shard that is responsible for coordinating the commit across shards for a given transaction
- 16. P S S S P S Query Router Insert {_id : “abc”}Insert {_id : “xyz”} Coordinator! Commit transaction Show that the commit happens across all replicas
- 17. “ We see at least 2x performance improvement when running MongoDB transactions vs logic on our own, in- app” Imagine a world without transactions
- 18. Even better perf results Collocate your data
- 19. Even better perf results Write to a single shard
- 20. READ CONCERN = READ ISOLATION Options: LOCAL, MAJORITY, SNAPSHOT, LINEARIZABLE Fun Fact: ReadConcern: Majority does not need to coordinate across nodes to return results!
- 21. Read Concern: Snapshot P S S P S S P S S time: 102 Majority or Local time: 102 time: 110 time: 104
- 22. Snapshot Isolation is expensive All shards need to coordinate the time The same snapshot is used throughout the duration of the transaction Even once a snapshot time is selected, some shards may not have that time
- 23. … … keys K K K KVV V V txn time data next Update Structure txn time data next WiredTiger Cache
- 24. More New Functionality in 4.2
- 25. Transactions Larger than 16MB { <statement 1>, <statement 2>, <statement 3>, <statement 4>} Prior to MongoDB 4.2:
- 26. This does not mean you can have infinitely large transactions -- WT still has to maintain the history since the snapshot time Transactions automatically abort after 60 seconds – TUNABLE. transactionLifetimeLimitSeconds
- 27. Mutable Shard Key Values Tiered storage: Aging out older documents to low cost storage shard Global-redistribution: Rehoming documents to a new region
- 28. Use MongoDB 4.2 drivers Transactions targeting a single shard will be faster than those spanning multiple shards Best practice: No more than 1,000 documents modified in a single transaction No arbiters Chunk migrations will block behind running transactions
- 29. MONGODB4DOT2
- 30. • Free ▪ Online ▪ 9 Chapters • Register Now! - https://university.mongodb.com/courses/M042/about