Inside MongoDB: the Internals of an Open-Source Database
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The document discusses MongoDB, including how it stores and indexes data, handles queries and replication, and supports sharding and geospatial indexing. Key points covered include how MongoDB stores data in BSON format across data files that grow in size, uses memory-mapped files for data access, supports indexing with B-trees, and replicates operations through an oplog.
Inside MongoDB: the Internals of an Open-Source Database
- 1. Mike Dirolf -‐ @mdirolf -‐ 10gen, Inc. http://www.mongodb.org
- 3. a word of warning this talk might be a bit “hard”, but MongoDB is really easy: http://try.mongodb.org
- 5. _id if not specified drivers will add default: ObjectId("4bface1a2231316e04f3c434") timestamp machine id process id counter http://www.mongodb.org/display/DOCS/Object+IDs
- 6. BSON Encoding {_id: ObjectId(XXXXXXXXXXXX), hello: “world”} x27x00x00x00x07 _ i dx00 X X X X X X X X X X X Xx02 h e l l ox00x06x00 x00x00 w o r l dx00x00 http://bsonspec.org
- 7. Insert Message (TCP/IP) message length request id response id op code (insert) x68x00x00x00 xXXxXXxXXxXX x00x00x00x00 xd2x07x00x00 reserved collection name document(s) x00x00x00x00 f o o . t e s t x00 BSON Data http://www.mongodb.org/display/DOCS/Mongo+Wire+Protocol
- 8. Data File Allocation $ ls -‐sk /data/db/ } 16384 foo.ns allocated per (up to 2 gigs){ double in size 65536 foo.0 131072 foo.1 database 16384 bar.ns ...
- 10. Extent Allocation foo.0 allocated per namespace: foo.1 foo.test 00000000000 foo.bar foo.baz 00000000000 00000000000 foo.$freelist 00000000000 0000 preallocated space foo.2 00000000000 00000000000 ns details stored in foo.ns 00000000000 00000000000
- 11. Record Allocation ... Header (Size, Offset, Next, Prev) BSON Data Padding Deleted Record (Size, Offset, Next) ...
- 12. Indexing B-‐Tree indexes, stored in own namespaces > db.system.namespaces.find() { "name" : "foo.system.indexes" } { "name" : "foo.test" } { "name" : "foo.test.$_id_" } http://www.mongodb.org/display/DOCS/Indexes
- 14. Query Language “query by example” plus $ modifiers: {first_name: “Mike”, age: {$gte: 20, $lt: 40}} http://www.mongodb.org/display/DOCS/Advanced+Queries
- 15. Cursors > var c = db.test.find({x: 20}).skip(20).limit(10) > c.next() > c.next() ... query first N results + cursor id getMore w/ cursor id next N results + cursor id or 0 ...
- 16. Query Optimizer find({x: 10, y: “foo”}) scan terminate index on x index on y remember
- 17. db.foo.drop()
- 18. Commands drop, count, copydb, findAndModify, serverStatus, ... http://www.mongodb.org/display/DOCS/Commands
- 19. db.foo.drop(); = db.foo.runCommand({drop: "foo"}); = db.$cmd.findOne({drop: "foo"}); = db.$cmd.find({drop: "foo"}).limit(-‐1);
- 20. Capped Collections preallocated auto LRI age-‐out no default _id index always in insertion order http://www.mongodb.org/display/DOCS/Capped+Collections
- 21. Replication Oplog > use local switched to db local > use foo > db.oplog.$main.find() switched to db foo {ts: ..., op: "n", ns: "", o: {}} {ts: ..., op: "n", ns: "", o: {}} > db.test.insert({x: 1, url: {ts: ..., op: "i", ns: "foo.test", "http://dirolf.com"}); o: {_id: ObjectId("..."), x: 1, url: "http://dirolf.com"}} {ts: ..., op: "n", ns: "", o: {}} > db.test.update({url: "http:// {ts: ..., op: "u", ns: "foo.test", dirolf.com"}, {$inc: {x: 1}}); o2: {_id: ObjectId("...")}, o: {$set: {x: 2}}} http://www.mongodb.org/display/DOCS/Replication
- 22. Replication Topology master slave master master slave slave slave slave master master slave master http://www.mongodb.org/display/DOCS/Replication
- 23. Auto-Sharding Shards mongod mongod mongod ... Config mongod mongod mongod Servers mongod mongod mongod mongos mongos ... client http://www.mongodb.org/display/DOCS/Sharding
- 24. Geohashing (20, 10) (0001 0100, 0000 1010) 0000 0010 0110 0100 maps close coordinates (21, 9) to close hashes: 0000 0010 0110 0011 tricky part happens at bit-‐flips (127 vs 128) http://www.mongodb.org/display/DOCS/Geospatial+Indexing
- 25. Download MongoDB http://www.mongodb.org these slides are available at http://dirolf.com