1. Update Anatomy
The guide to how an update works

2. Me = Scott Hernandez
● Worked on Morphia (+java), community
● Over 3 years with MongoDB
● Worked on update re-write in 2.6
● Working on replication team now
● Lots of experience with many parts

3. User Components
update({f:1}, {$set: {a: 2}}, {multi:true})
● Query
● Update Modifiers (or replacement doc)
● Options
o multi, upsert
o writeConcern

4. New Code (in 2.6)
● New update and query code
● Replaced all modifier implementations
● New validation and error messages
● Using new “MutableBSON” document class
● New oplog entry generation code

5. Internal Components
● Update Command (-> request)
● Update Executor: Runner + Driver
● *Modifiers (Inc, Set, Unset, Bit, …)
● Validation, Modification, and Replication

6. Two Paths To Update
Wire Protocol Op
User
Write Commands
Internal Update
Request

7. … don’t forget
FindAndModify ;)

8. Update Request
1. Create
o Validate args
o Prepare what we can (reduce lock time)
o Record system state (repl, sharding, etc)
2. Send for execution

9. What is a modifier
{$set: {a: 1} }, {$min: {c: “b”} }
● Modifier: “set”, “min”
● Field Path: “a”, “c” (+ “b”)
● Value: 1, “b”

10. Implementing a modifier
db/ops/modifier_interface.h:
● init ( modExpr, opts ) // validation
● prepare ( doc, fieldName, execInfo ) // work
● apply ( ) // update document
● log ( ) // oplog entry created

11. Modifier::init
● Validate form of update, and args
● Parse path
● Indicate if positional, and supported

12. Modifier::prepare
● Work with doc, but don’t change it
● Validate field, type, operation
● Substitute positional path element, if appl.
● Do work, stored to the side
● Return ExecInfo (no-op, fields affected)

13. Modifier::apply
● Apply prepared work to doc

14. Modifier::log
● Calculate transformation logic for oplog entry
● Update idempotent oplog entry
o Set
o Unset
o Full Replacement

15. db/ops/modifier_* + tests
● add_to_set.h/cpp
● bit.h/cpp
● compare.h/cpp
● current_date.h/cpp
● inc.h/cpp
● obj_replace.h/cpp
● pop.h/cpp
● pull.h/cpp
● pull_all.h/cpp
● push.h/cpp
● rename.h/cpp
● set.h/cpp
● unset.h/cpp

16. Update Executor
● Finish prep
● If no docs + upsert, prepare insert doc
● Run driver (mods) against each doc
● Ensure, validate, log for replication
● Return UpdateResult

17. Update Flow
Query + Update
Cursor
● Init
● Prepare
● Apply
{ $inc: {a:1}, { $inc: {b:1}}
● Init
● Prepare
● Apply
{ $set: {c: {e:1} }}
● Log
● Log
Entry
Oplog
No results,
upsert:true
Insert
Entry

18. Update Flow Notes
● Yields based on timing/in-memory
● Each document update is atomic
● Periodically allows journal to flush, if needed

19. Questions?
Round 1: No, then we have more slides

20. 2.6 Stuff
● New modifiers
● Improvements
● Behavioral Changes

21. New Modifiers
● Mul(tiply) existing field by constant: $mul
{a:1} + {$mul:{a:10}} -> {a:10}
● Min/Max of existing field w/constant: $min/$max
{a:1} + {$min:{a:100}} -> {a:1} (no-op)
● Store current date/ts in a field: $currentDate
{} + {$currentDate:{ dt: true}} -> {dt: new Date()}

22. Improvements
● $push
o $sort elements (works with numbers/strings/etc)
o $sort and/or $slice (doesn’t require $sort for $slice)
o $slice off either end (no middle yet)
o $position lets you specify where in the array
o $each, $sort, $slice no longer order dep.
● $bit
o xor support

23. Improvements
● Error messages with context
o The field 'l' must be an array but is of type
NumberLong in document {_id:
ObjectId('52a0a61672a5f2f771099b54')}
o Cannot apply $inc to a value of non-integral type.
{_id: ObjectId('52a0a61672a5f2f771099b54')} has
the field 'd' of non-integral type Date

24. Improvements
● No reordering of fields (except: _id is first)
● Better building of doc during insert
o update({a:1, b:1}, {$inc:{c:1}}, 0,1) -> {a:1, b:1, c:1}
o now uses parsed query tree to get equalities
● Will now return new _id for all types
● Accurate tracking of no-ops, modified docs

25. Improvements
● Validation
o Immutable Fields (_id, shard keys)
o Storage restrictions (no $field, “.”, _id field types)
o DBRef validation
● Can now query + set _id/shard-key fields
o update({_id:1}, {$set:{_id:1, ...}}) // ok
o update({_id:1}, {_id:2, ...}) // error
● Cleanup bad data ($unset/$rename etc)

26. Questions?
No, well thanks for coming!