Download - Schema Tricks & Tips
Technical Director, 10gen
@jonnyeight [email protected] alvinonmongodb.com
Alvin Richards
#MongoDBdays
Schema Design 4 Real World Use Cases
One size fits all?
Single Table En
Agenda
• Why is schema design important
• 4 Real World Schemas – Inbox – History – Indexed Attributes – Multiple Identities
• Conclusions
Why is Schema Design important?
• Largest factor for a performant system
• Schema design with MongoDB is different • RBMS – "What answers do I have?" • MongoDB – "What question will I have?"
#1 - Message Inbox
Let’s get Social
Sending Messages
?
Design Goals
• Efficiently send new messages to recipients
• Efficiently read inbox
Reading my Inbox
?
3 Approaches (there are more)
• Fan out on Read
• Fan out on Write
• Fan out on Write with Bucketing
// Shard on "from" db.shardCollection( "mongodbdays.inbox", { from: 1 } ) // Make sure we have an index to handle inbox reads db.inbox.ensureIndex( { to: 1, sent: 1 } ) msg = { from: "Joe", to: [ "Bob", "Jane" ],
sent: new Date(), message: "Hi!",
} // Send a message db.inbox.save( msg ) // Read my inbox db.inbox.find( { to: "Joe" } ).sort( { sent: -1 } )
Fan out on read
Fan out on read – Send Message
Shard 1 Shard 2 Shard 3
Send Message
Fan out on read – Inbox Read
Shard 1 Shard 2 Shard 3
Read Inbox
Considerations
• 1 document per message sent
• Multiple recipients in an array key
• Reading an inbox is finding all messages with my own name in the recipient field
• Requires scatter-gather on sharded cluster
• Then a lot of random IO on a shard to find everything
// Shard on “recipient” and “sent” db.shardCollection( "mongodbdays.inbox", { ”recipient”: 1, ”sent”: 1 } ) msg = { from: "Joe”, to: [ "Bob", "Jane" ],
sent: new Date(), message: "Hi!",
} // Send a message for ( recipient in msg.to ) {
msg.recipient = recipient db.inbox.save( msg );
} // Read my inbox db.inbox.find( { recipient: "Joe" } ).sort( { sent: -1 } )
Fan out on write
Fan out on write – Send Message
Shard 1 Shard 2 Shard 3
Send Message
Fan out on write– Read Inbox
Shard 1 Shard 2 Shard 3
Read Inbox
Considerations
• 1 document per recipient
• Reading my inbox is just finding all of the messages with me as the recipient
• Can shard on recipient, so inbox reads hit one shard
• But still lots of random IO on the shard
Fan out on write with buckets
• Each “inbox” document is an array of messages
• Append a message onto “inbox” of recipient
• Bucket inbox documents so there’s not too many per document
• Can shard on recipient, so inbox reads hit one shard
• A few documents to read the whole inbox
// Shard on “owner / sequence” db.shardCollection( "mongodbdays.inbox", { owner: 1, sequence: 1 } ) db.shardCollection( "mongodbdays.users", { user_name: 1 } ) msg = { from: "Joe", to: [ "Bob", "Jane" ],
sent: new Date(), message: "Hi!",
} // Send a message for( recipient in msg.to) { count = db.users.findAndModify({ query: { user_name: msg.to[recipient] }, update: { "$inc": { "msg_count": 1 } }, upsert: true, new: true }).msg_count; sequence = Math.floor(count / 50);
db.inbox.update( { owner: msg.to[recipient], sequence: sequence }, { $push: { "messages": msg } }, { upsert: true } );
} // Read my inbox db.inbox.find( { owner: "Joe" } ).sort ( { sequence: -1 } ).limit( 2 )
Fan out on write – with buckets
Bucketed fan out on write - Send
Shard 1 Shard 2 Shard 3
Send Message
Bucketed fan out on write - Read
Shard 1 Shard 2 Shard 3
Read Inbox
#2 – History
Design Goals
• Need to retain a limited amount of history e.g. – Hours, Days, Weeks – May be legislative requirement (e.g. HIPPA, SOX, DPA)
• Need to query efficiently by – match – ranges
3 Approaches (there are more)
• Bucket by Number of messages
• Fixed size Array
• Bucket by Date + TTL Collections
db.inbox.find() { owner: "Joe", sequence: 25, messages: [ { from: "Joe", to: [ "Bob", "Jane" ], sent: ISODate("2013-03-01T09:59:42.689Z"), message: "Hi!" }, … ] } // Query with a date range db.inbox.find ( { owner: "friend1", messages: { $elemMatch: { sent: { $gte: ISODate("2013-04-04…") }}}}) // Remove elements based on a date db.inbox.update( { owner: "friend1" }, { $pull: { messages: { sent: { $gte: ISODate("2013-04-04…") } } } } )
Inbox – Bucket by # messages
Considerations
• Shrinking documents, space can be reclaimed with – db.runCommand ( { compact: '<collection>' } )
• Removing the document after the last element in the array as been removed – { "_id" : …, "messages" : [ ], "owner" : "friend1", "sequence" : 0 }
msg = { from: "Your Boss", to: [ "Bob" ],
sent: new Date(), message: "CALL ME NOW!"
} // 2.4 Introduces $each, $sort and $slice for $push db.messages.update(
{ _id: 1 }, { $push: { messages: { $each: [ msg ],
$sort: { sent: 1 }, $slice: -50 } }
} )
Maintain the latest – Fixed Size Array
Considerations
• Need to compute the size of the array based on retention period
// messages: one doc per user per day
db.inbox.findOne() {
_id: 1, to: "Joe", sequence: ISODate("2013-02-04T00:00:00.392Z"), messages: [ ] }
// Auto expires data after 31536000 seconds = 1 year db.messages.ensureIndex( { sequence: 1 }, { expireAfterSeconds: 31536000 } )
TTL Collections
#3 – Indexed Attributes
Design Goal
• Application needs to stored a variable number of attributes e.g. – User defined Form – Meta Data tags
• Queries needed – Equality – Range based
• Need to be efficient, regardless of the number of attributes
2 Approaches (there are more)
• Attributes
• Attributes as Objects in an Array
db.files.insert( { _id: "local.0", attr: { type: "text", size: 64, created: ISODate("2013-03-01T09:59:42.689Z" } } )
db.files.insert( { _id:"local.1", attr: { type: "text", size: 128} } )
db.files.insert( { _id:"mongod", attr: { type: "binary", size: 256, created: ISODate("2013-04-01T18:13:42.689Z") } } )
// Need to create an index for each item in the sub-document db.files.ensureIndex( { "attr.type": 1 } ) db.files.find( { "attr.type": "text"} )
// Can perform range queries db.files.ensureIndex( { "attr.size": 1 } ) db.files.find( { "attr.size": { $gt: 64, $lte: 16384 } } )
Attributes as a Sub-Document
Considerations
• Each attribute needs an Index
• Each time you extend, you add an index
• Lots and lots of indexes
db.files.insert( { _id: "local.0", attr: [ { type: "text" }, { size: 64 }, { created: ISODate("2013-03-01T09:59:42.689Z" } ] } )
db.files.insert( { _id: "local.1", attr: [ { type: "text" }, { size: 128 } ] } )
db.files.insert( { _id: "mongod", attr: [ { type: "binary" }, { size: 256 }, { created: ISODate("2013-04-01T18:13:42.689Z") } ] } )
db.files.ensureIndex( { attr: 1 } )
Attributes as Objects in Array
// Range queries db.files.find( { attr: { $gt: { size:64 }, $lte: { size: 16384 } } } )
db.files.find( { attr: { $gte: { created: ISODate("2013-02-01T00:00:01.689Z") } } } )
// Multiple condition – Only the first predicate on the query can use the Index // ensure that this is the most selective. // Index Intersection will allow multiple indexes, see SERVER-3071
db.files.find( { $and: [ { attr: { $gte: { created: ISODate("2013-02-01T…") } } }, { attr: { $gt: { size:128 }, $lte: { size: 16384 } } } ] } )
// Each $or can use an index db.files.find( { $or: [ { attr: { $gte: { created: ISODate("2013-02-01T…") } } }, { attr: { $gt: { size:128 }, $lte: { size: 16384 } } } ] } )
Queries
#4 – Multiple Identities
Design Goal
• Ability to look up by a number of different identities e.g. • Username • Email address • FB Handle • LinkedIn URL
2 Approaches (there are more)
• Identifiers in a single document
• Separate Identifiers from Content
db.users.findOne() { _id: "joe", email: "[email protected], fb: "joe.smith", // facebook li: "joe.e.smith", // linkedin other: {…} }
// Shard collection by _id db.shardCollection("mongodbdays.users", { _id: 1 } )
// Create indexes on each key db.users.ensureIndex( { email: 1} ) db.users.ensureIndex( { fb: 1 } ) db.users.ensureIndex( { li: 1 } )
Single Document by User
Read by _id (shard key)
Shard 1 Shard 2 Shard 3
find( { _id: "joe"} )
Considerations
• Lookup by shard key is routed to 1 shard
• Lookup by other identifier is scatter gathered across all shards
• Secondary keys cannot have a unique index
// Create unique index db.identities.ensureIndex( { identifier : 1} , { unique: true} ) // Create a document for each users document db.identities.save( { identifier : { hndl: "joe" }, user: "1200-42" } ) db.identities.save( { identifier : { email: "[email protected]" }, user: "1200-42" } ) db.identities.save( { identifier : { li: "joe.e.smith" }, user: "1200-42" } ) // Shard collection by _id db.shardCollection( "mongodbdays.identities", { identifier : 1 } )
// Create unique index db.users.ensureIndex( { _id: 1} , { unique: true} )
// Create a docuemnt that holds all the other user attributes db.users.save( { _id: "1200-42", ... } )
// Shard collection by _id db.shardCollection( "mongodbdays.users", { _id: 1 } )
Document per Identity
Read requires 2 reads
Shard 1 Shard 2 Shard 3
db.identities.find({"identifier" : { "hndl" : "joe" }})
db.users.find( { _id: "1200-42"} )
Solution
• Lookup to Identities is a routed query
• Lookup to Users is a routed query
• Unique indexes available
Conclusion
Summary
• Multiple ways to model a domain problem
• Understand the key uses cases of your app
• Balance between ease of query vs. ease of write
• Random IO should be avoided
Technical Director, 10gen
@jonnyeight [email protected] alvinonmongodb.com
Alvin Richards
#MongoDBdays
Thank You
