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Technical Director, 10gen @jonnyeight [email protected] alvinonmongodb.com Alvin Richards #MongoDBdays Schema Design 3 Real World Use Cases
| Date post: | 25-May-2015 |
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Technical Director, 10gen
@jonnyeight [email protected] alvinonmongodb.com
Alvin Richards
#MongoDBdays
Schema Design3 Real World Use Cases
I'm planning a Trip to LA…
Single Table En
Agenda
• Why is schema design important
• 3 Real World Schemas– Inbox– 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?"
• Must consider use case with schema
#1 - Message Inbox
Let’s getSocial
Sending Messages
?
Reading my Inbox
?
Design Goals
• Efficiently send new messages to recipients
• Efficiently read inbox
3 Approaches (there are more)• Fan out on Read
• Fan out on Write
• Fan out on Write with Bucketing
Fan out on read – Send Message
Shard 1 Shard 2 Shard 3
Send Message
db.inbox.save( { to: [ "Bob", "Jane" ], … } )
Fan out on read – Inbox Read
Shard 1 Shard 2 Shard 3
Read Inbox
db.inbox.find( { to: "Bob" } )
// Shard on "from"db.shardCollection( "mongodbdays.inbox", { from: 1 } )
// Make sure we have an index to handle inbox readsdb.inbox.ensureIndex( { to: 1, sent: 1 } )
msg = { from: "Joe", to: [ "Bob", "Jane" ],
sent: new Date(), message: "Hi!",
}
// Send a messagedb.inbox.save( msg )
// Read my inboxdb.inbox.find( { to: "Bob" } ).sort( { sent: -1 } )
Fan out on read
Considerations
1 document per message sent Multiple recipients in an array key Reading inbox finds 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
Fan out on write – Send Message
Shard 1 Shard 2 Shard 3
Send Message
db.inbox.save( { to: "Bob", …} )
Fan out on write– Read Inbox
Shard 1 Shard 2 Shard 3
Read Inbox
db.inbox.find( { to: "Bob" } )
// Shard on “recipient” and “sent” db.shardCollection( "mongodbdays.inbox", { ”recipient”: 1, ”sent”: 1 } )
msg = { from: "Joe”, recipient: [ "Bob", "Jane" ],
sent: new Date(), message: "Hi!",
}
// Send a messagefor ( recipient in msg.recipient ) {
msg.to = recipientdb.inbox.save( msg );
}
// Read my inboxdb.inbox.find( { to: "Joe" } ).sort( { sent: -1 } )
Fan out on write
Considerations
✖1 document per recipient per messageReading inbox is finding all of the
messages with me as the recipientCan 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
Bucketed fan out on write - Send
Shard 1 Shard 2 Shard 3
Send Message
db.inbox.update( { to: "Bob"}, { $push: { msg: … } })
Bucketed fan out on write - Read
Shard 1 Shard 2 Shard 3
Read Inbox
db.inbox.find( { to: "Bob" } )
// Shard on “owner / sequence”db.shardCollection( "mongodbdays.inbox", { owner: 1, sequence: 1 } )db.shardCollection( "mongodbdays.users", { user_name: 1 } )msg = { from: "Joe", to: [ "Bob", "Jane" ],
sent: new Date(), message: "Hi!",
}// Send a messagefor( recipient in msg.to) { count = db.users.findAndModify({ query: { user_name: msg.to[recipient] }, update: { "$inc": { "msg_count": 1 } }, upsert: true, new: true }).msg_count; sequence = Math.floor(count / 50);
db.inbox.update( { to: msg.to[recipient], sequence: sequence },
{ $push: { "messages": msg } },
{ upsert: true } );}// Read my inboxdb.inbox.find( { to: "Joe" } ).sort ( { sequence: -1 } ).limit( 2 )
Fan out on write – with buckets
Considerations
Fewer documents per recipientReading inbox is just finding a few bucketsCan shard on recipient, so inbox reads hit
one shard
✖But still some random IO on the shard
But…
• What if I do not / cannot retain all history?
– Space limited: Hours, Days, Weeks, $$$– Legislative limited: HIPPA, SOX, DPA
3 Approaches (there are more)• Bucket by Number of messages – just
seen that
• Fixed size Array
• Bucket by Date + TTL Collections
// Query with a date rangedb.inbox.find ( { owner: "Joe", messages: { $elemMatch: { sent: { $gte: ISODate("2013-04-04…") }}}})
// Remove elements based on a datedb.inbox.update( { owner: "Joe" }, { $pull: { messages: { sent: { $gte: ISODate("2013-04-04…") } } } } )
Inbox – Bucket by # messages
Considerations
Limited to a known range of messages
✖Shrinking documents• space can be reclaimed withdb.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 $pushdb.messages.update(
{ _id: 1 }, { $push: { messages: { $each: [ msg ],
$sort: { sent: 1 },
$slice: -50 }
} })
Maintain the latest – Fixed Size Array
Push this object onto the array
Sort the resulting array
by "sent"
Limit the array to 50 elements
Considerations
Limited to a known # of messages
✖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 yeardb.messages.ensureIndex( { sequence: 1 }, { expireAfterSeconds: 31536000 } )
TTL Collections
Considerations
Limited to a known range of messages Automatic purge of expired data
No need to have a CRON task, etc. to do this
✖ Per Collection basis
#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
// Flexible set of attributes
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-documentdb.files.ensureIndex( { "attr.type": 1 } )db.files.find( { "attr.type": "text"} )
// Can perform range queriesdb.files.ensureIndex( { "attr.size": 1 } )db.files.find( { "attr.size": { $gt: 64, $lte: 16384 } } )
Attributes
Considerations
Attributes can be queried via an IndexEquality & Range queries supported
✖Each attribute needs an Index
✖Each time you extend, you add an index
✖Single index is used (unless you have $or)
// Flexible set of attributes, each attribute is an object
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 queriesdb.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 indexdb.files.find( { $or: [ { attr: { $gte: { created: ISODate("2013-02-01T…") } } }, { attr: { $gt: { size:128 }, $lte: { size: 16384 } } } ] } )
Queries
Considerations
Attributes can be queried via a Single index
New attributes do not need extra Indexes Equality & Range queries supported
✖ $and can only use a Single Index
#3 – 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)• Multiple 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 _iddb.shardCollection("mongodbdays.users", { _id: 1 } )
// Create indexes on each keydb.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 a document that holds all the other user attributesdb.users.save( { _id: "1200-42", ... } )
// Shard collection by _iddb.shardCollection( "mongodbdays.users", { _id: 1 } )
// Create a document for each users documentdb.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 _iddb.shardCollection( "mongodbdays.identities", { identifier : 1 } )
// Create unique indexdb.identities.ensureIndex( { identifier : 1} , { unique: true} )db.users.ensureIndex( { _id: 1} , { unique: true} )
Document per Identity
Read requires 2 queries
Shard 1 Shard 2 Shard 3
db.identities.find({"identifier" : { "hndl" : "joe" }})
db.users.find( { _id: "1200-42"} )
Considerations
Multiple queries, but always routed 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
• Avoid Random IO
• Avoid Scatter / Gather query pattern
Technical Director, 10gen
@jonnyeight [email protected] alvinonmongodb.com
Alvin Richards
#MongoDBdays
Thank You
