A year ago we launched replica-set transactions in MongoDB 4.0. We've now expanded transactions to span across shards, making development against MongoDB even easier. Snapshot isolation, write atomicity, distributed commit – we'll touch on it all. You'll learn all you need to know about distributed transactions before you push to prod.

1. Distributed Transactions: with Great
Power Comes Great Responsibility
Alyson Cabral, Product Manager
@aly_cabral

2. I know what you’re thinking…
@aly_cabral

3. But, Really…
MongoDB Transactions are ACID Compliant
(yes, even across shards)
@aly_cabral

4. Transactions make application
development easier
1. Don’t need to worry about interim states
2. Rollback/Abort is handled automatically
3. Point-in-time consistency
@aly_cabral

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!
@aly_cabral

6. Transactions Guarantees
@aly_cabral

7. Multi- Collection
Database
Statement
Document
Shard
@aly_cabral

8. All or Nothing Execution
TXN
Application
MongoDB
Uncommitted Writes
{ Document 1 } { Document 2 }
{ Document 3 }
Committed Writes
@aly_cabral

9. Snapshot Isolation
100 101 102 103 104 105 106 107 108 109 110 111
Snapshot
102
Snapshot
108
@aly_cabral

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
@aly_cabral

11. Transactions Guidelines
@aly_cabral

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
@aly_cabral

13. Distributed Transactions Workflow
@aly_cabral

14. Majority Writes
No Arbiters!
@aly_cabral
P
S
S
S
S

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
@aly_cabral

16. P
S
S
S
P
S
Query Router
Insert {_id : “abc”}Insert {_id : “xyz”}
Coordinator!
Commit transaction
@aly_cabral

17. “ We see at least 2x performance
improvement when running MongoDB
transactions vs logic on our own, in-
app”
Imagine a world without transactions
@aly_cabral

18. For even better perf results… collocate the data
@aly_cabral

19. If you cannot collocate, write to a single shard
@aly_cabral
READS WRITESREADS

20. Read Isolation
@aly_cabral

21. READ CONCERN = READ ISOLATION
Options: LOCAL, MAJORITY,
SNAPSHOT, LINEARIZABLE
Fun Fact: ReadConcern: Majority does not need to
coordinate across nodes to return results!
@aly_cabral

22. Read Concern: Snapshot
P
S
S
P
S
S
P
S
S
time: 102
Majority
or
Local
time: 102 time: 110 time: 104
@aly_cabral

23. Locking
@aly_cabral

24. @aly_cabral
TXN
Write
TXN
Write
{Doc 1}
TXN write is unable to obtain a write lock!
TXN is ABORTED
rolledback entirely

25. @aly_cabral
TXN
Write
Operation
Non-Transactional Write
{Doc 1}
Operation Blocks Behind TXN Commit
Infinitely retries with Backoff Logic until MaxTimeMS is reached
Operation non-transactional write is unable to obtain a write lock!

26. @aly_cabral
When non-transactional writes encounter
write conflicts
Operation Blocks Behind TXN Commit
Infinitely retries with Backoff Logic until MaxTimeMS is reached
When transactions encounter write
conflicts
Transactions abort
rollback entirely

27. @aly_cabral
TXN
Read
TXN
Write
{Doc 1}
Reads do not lock a document from being modified
Only writes make write conflicts

28. More New Functionality in 4.2
@aly_cabral

29. Transactions Larger than 16MB
In MongoDB 4.0:
@aly_cabral
{<statement 1>}
{<statement 2>}
{<statement 3>}
{<statement 4>}
In MongoDB 4.2:
{<statement 1>,
<statement 2>,
<statement 3>,
<statement 4>}

30. This does not mean you can have
infinitely large transactions --
• WT still has to maintain the history since the snapshot
time
• Transactions have the same snapshot throughout the
duration of the transaction
• Transactions automatically abort after 60 seconds –
TUNABLE. transactionLifetimeLimitSeconds
@aly_cabral

31. Mutable Shard Key Values
Tiered storage: Aging out older documents to low cost
storage shard
• Global-redistribution: Rehoming documents to a new
region
@aly_cabral
ShardShard

32. • 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
@aly_cabral
Parting Wisdom

33. ATLAS CODE:
MONGODB4DOT2
@aly_cabral

34. • Free ▪ Online ▪ 9 Chapters
• Register Now! - https://university.mongodb.com/courses/M042/about
@aly_cabral

35. Thank You!