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. #MDBlocal
Distributed Transactions
… with Great Power Comes
Great Responsibility.
Boris Bialek, Head of Innovation EMEA
MUNICH

2. Seriously ?

3. Transactions can be cool ….

4. … or make you a
melted gummi bear in
seconds.

5. #MDBLocal
Transactions Use Cases
Many to Many
Relationships
Positions and Trades
Event Processing
Account Creation
Application Event
Logging
Application Auditing

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

7. #MDBLocal
session = db.getMongo().startSession( { readPreference: { mode: "primary" } } );
coll1 = session.getDatabase(”accounts").payable;
coll2 = session.getDatabase(”billing").invoices;
session.startTransaction( { readConcern: { level: "local" }, writeConcern: { w: "majority" } } );
try {
coll1.insertOne( { ledger: 102010, payee: “Joe Some”, IBAN: 12234, amount: 200 }
);
coll2.insertOne( { invoice: 999, status: “PAID” } ); }
catch (error) {
session.abortTransaction();
throw error; }
session.commitTransaction();
session.endSession();
“OLD SCHOOL VERSION”

8. #MDBLocal
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);
SMART AND FAST in MongoDB 4.2!

9. Transactions Guarantees

10. #MDBLocal
Multi- Collection
Database
Statement
Document
Shard

11. #MDBLocal
All or Nothing Execution
TXN
Application
MongoDB
Uncommitted Writes
{ Document 1 } { Document 2 }
{ Document 3 }
Committed Writes

12. #MDBLocal
Snapshot Isolation
100 101 102 103 104 105 106 107 108 109 110 111
Snapshot
102
Snapshot
108

13. Transactions Guidelines
(…remember the gummi bears)

14. #MDBLocal
1) All existing 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 (at least think what
happens if…)
5) Don’t unnecessarily leave snapshots
open
6) To trigger write conflicts, make sure
you’re doing writes
7) Plan for DDL Operations

15. Distributed Transactions
Workflow

16. #MDBLocal
Majority Writes
No Arbiters!
P
S
S
S
S

17. #MDBLocal
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

18. #MDBLocal
P
S
S
S
P
S
Query Router
Insert {_id : “abc”}Insert {_id : “xyz”}
Coordinator!
Commit transaction

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

20. #MDBLocal
For even better perf results… collocate the data

21. #MDBLocal
If you cannot collocate, write to a single shard
READS WRITESREADS

22. Read Isolation

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

24. #MDBLocal
Read Concern: Snapshot
P
S
S
P
S
S
P
S
S
time: 102
Majority
or
Local
time: 102 time: 110 time: 104

25. Locking

26. #MDBLocal
TXN
Write
TXN
Write
{Doc 1}
TXN write is unable to obtain a write lock!
TXN is ABORTED
rolledback entirely
Double Transaction

27. #MDBLocal
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!
Transaction and an Op

28. #MDBLocal
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

29. #MDBLocal
TXN
Read
TXN
Write
{Doc 1}
Reads do not lock a document from being modified
Only writes make write conflicts
Read meets Write

30. More New Functionality in
MongoDB 4.2

31. #MDBLocal
Transactions Larger than 16MB
In MongoDB 4.0:
{<statement 1>}
{<statement 2>}
{<statement 3>}
{<statement 4>}
In MongoDB 4.2:
{<statement 1>,
<statement 2>,
<statement 3>,
<statement 4>}

32. #MDBLocal
This does not mean you can have infinitely large
transactions --
• WiredTiger Storage Engine 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

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

34. #MDBLocal
• 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
Famous Final Comments

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