MongoDB .local Bengaluru 2019: Distributed Transactions: With Great Power Comes Great Responsibility
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The document discusses the capabilities of MongoDB's transaction system, emphasizing its ACID compliance and benefits for application development, such as automatic rollback and point-in-time consistency. It outlines the new features introduced in MongoDB 4.2, including cross-shard transactions, guidelines for effective use, and performance improvements. Additionally, it highlights best practices, limitations, and offers incentives for users to explore MongoDB Atlas.
![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/mongodb-190911165106/85/MongoDB-local-Bengaluru-2019-Distributed-Transactions-With-Great-Power-Comes-Great-Responsibility-11-320.jpg)
![• To trigger write conflicts, make sure you’re doing
writes
{_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}}
street: “92nd”
street: “94th”
• Plan for DDL Operations](https://image.slidesharecdn.com/mongodb-190911165106/85/MongoDB-local-Bengaluru-2019-Distributed-Transactions-With-Great-Power-Comes-Great-Responsibility-16-320.jpg)
MongoDB .local Bengaluru 2019: Distributed Transactions: With Great Power Comes Great Responsibility
- 2. Distributed Transactions: with Great Power Comes Great Responsibility Ankur Raina, Sr. TSE @ MongoDB
- 3. But, Really… MongoDB Transactions are ACID Compliant (yes, even across shards)
- 4. Relational vs document data modeling
- 5. 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
- 6. 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
- 9. All or Nothing Execution TXN Application MongoDB Uncommitted Writes { Document 1 } { Document 2 } { Document 3 } Committed Writes
- 10. Snapshot Isolation 100 101 102 103 104 105 106 107 108 109 110 111 Snapshot 102 Snapshot 108
- 11. 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
- 13. • All data modeling rules still apply • Transactions should not be the most common operation • Pass in the session to all statements 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);
- 14. • Implement retry logic, transactions can always abort • Don’t unnecessarily leave snapshots open
- 15. … … keys K K K KVV V V txn time data next Update Structure txn time data next WiredTiger Cache
- 16. • To trigger write conflicts, make sure you’re doing writes {_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}} street: “92nd” street: “94th” • Plan for DDL Operations
- 18. 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
- 19. 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
- 20. Imagine a world without transactions “ We see at least 2x performance improvement when running MongoDB transactions vs logic on our own, in-app”
- 21. Even better perf results Collocate your data
- 22. Even better perf results Write to a single shard
- 23. READ CONCERN = READ ISOLATION Options: LOCAL, MAJORITY, SNAPSHOT, LINEARIZABLE Fun Fact: ReadConcern: Majority does not need to coordinate across nodes to return results!
- 24. Read Concern: Snapshot P S S P S S P S S time: 102 Majority or Local time: 102 time: 110 time: 104
- 25. 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
- 26. More New Functionality in 4.2
- 27. Transactions Larger than 16MB { <statement 1>, <statement 2>, <statement 3>, <statement 4>} Prior to MongoDB 4.2:
- 28. 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
- 29. Mutable Shard Key Values Tiered storage: Aging out older documents to low cost storage shard Global-redistribution: Rehoming documents to a new region
- 30. • 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 You can get started with $200 of free credits on MongoDB Atlas (https://cloud.mongodb.com) Use code MONGODB4DOT2, until the end of 2019. Remember
- 31. Thank You!
- 32. • Free Online • Register Now! - https://university.mongodb.com/courses/M042/about