MongoDB Schema Design: Four Real-World Examples

Perl Engineer & Evangelist, 10gen
Mike Friedman
#MongoDBdays
Schema Design
Four Real-World Use
Cases

Single Table En
Agenda
• Why is schema design important
• 4 Real World Schemas
– Inbox
– History
– IndexedAttributes
– Multiple Identities
• Conclusions

Why is Schema Design
important?
• Largest factor for a performant system
• Schema design with MongoDB is different
• RDBMS – "What answers do I have?"
• MongoDB – "What question will I have?"

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

// 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
Read
Inbox

Considerations
• One document per message sent
• Reading an inbox means 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",
sent: new Date(),
message: "Hi!",
}
// Send a message
for ( recipient in msg.to ) {
msg.recipient = msg.to[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
Send
Message

Fan out on write– Read Inbox
Read
Inbox

Considerations
• One 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

// Shard on “owner / sequence”
db.shardCollection( "mongodbdays.inbox", { owner: 1, sequence: 1 } )
db.shardCollection( "mongodbdays.users", { user_name: 1 } )
msg = {
from: "Joe",
sent: new Date(),
message: "Hi!",
}
Fan out on write with buckets

// 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 )

• Each “inbox” document is an array of messages
• Append a message onto “inbox” of recipient
• Bucket inboxes so there’s not too many
messages per document
• Can shard on recipient, so inbox reads hit one
shard
• 1 or 2 documents to read the whole inbox

Fan out on write with buckets -
Send
Send
Message

Fan out on write with buckets -
Read
Read
Inbox

MongoDB Schema Design: Four Real-World Examples

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

more)
• Bucket by Number of messages
• Fixed size Array
• Bucket by Date + TTL Collections

db.inbox.find()
{ owner: "Joe", sequence: 25,
messages: [
{ from: "Joe",
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("…") }}}})
// Remove elements based on a date
db.inbox.update({owner: "friend1" },
{ $pull: { messages: {
sent: { $gte: ISODate("…") } } } } )
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

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

more)
• Attributes as Embedded Document
• Attributes as Objects in an Array

db.files.insert( { _id: "local.0",
attr: { type: "text", size: 64,
created: ISODate("..." } } )
attr: { type: "text", size: 128} } )
db.files.insert( { _id: "mongod",
attr: { type: "binary", size: 256,
created: ISODate("...") } } )
// 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("...") } ] } )
attr: [ { type: "text" },
{ size: 128 } ] } )
db.files.insert( { _id: "mongod",
attr: [ { type: "binary" },
{ size: 256 },
{ created: ISODate("...") } ] } )
db.files.ensureIndex( { attr: 1 } )
Attributes as Objects in Array

Considerations
• Only one index needed on attr
• Can support range queries, etc.
• Index can be used only once per query

Design Goal
• Ability to look up by a number of different
identities e.g.
• Username
• Email address
• FB Handle
• LinkedIn URL

more)
• Identifiers in a single document
• Separate Identifiers from Content

db.users.findOne()
{ _id: "joe",
email: "joe@example.com,
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)
find( { _id: "joe"} )

Read by email (non-shard
key)
find ( { email: joe@example.com }
)

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: "joe@abc.com" }, user: "1200-42" } )
db.identities.save(
{ identifier : { li: "joe.e.smith" }, user: "1200-42" } )
db.shardCollection( "mydb.identities", { identifier : 1 } )
// Create unique index
db.users.ensureIndex( { _id: 1} , { unique: true} )
db.shardCollection( "mydb.users", { _id: 1 } )
Document per Identity

Read requires 2 reads
db.identities.find({"identifier" : {
"hndl" : "joe" }})
db.users.find( { _id: "1200-42"}
)

Considerations
• Lookup to Identities is a routed query
• Lookup to Users is a routed query
• Unique indexes available

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

Perl Engineer & Evangelist, 10gen
Mike Friedman
#MongoDBdays
Thank You

MongoDB Schema Design: Four Real-World Examples

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