Flowdock's full-text search with MongoDB

Full-text search with
MongoDB
Otto Hilska, @mutru
@ﬂowdock

Thursday, July 7, 2011 1


APIdock.com is one of the services we’ve created for the Ruby community: a social
documentation site.


- We did some “research” about real-time web back in 2008.
- At the same time, did software consulting for large companies.
- Flowdock is a product spinoff from our consulting company. It’s Google Wave done right,
with focus on technical teams.


Flowdock combines a group chat (on the right) to a shared team inbox (on the left).

Our promise: Teams stay up-to-date, react in seconds instead of hours, and never forget
anything.


Flowdock gets messages from various external sources (like JIRA, Twitter, Github, Pivotal
Tracker, emails, RSS feeds) and from the Flowdock users themselves.


All of the highlighted areas are objects in the “messages” collection. MongoDB’s document
model is perfect for our use case, where various data formats (tweets, emails, ...) are stored
inside the same collection.


This is how a typical message looks like.

{
   "_id":ObjectId("4de92cd0097580e29ca5b6c2"),
   "id":NumberLong(45967),
   "app":"chat",
   "flow":"demo:demoflow",
   "event":"comment",
   "sent":NumberLong("1307126992832"),
   "attachments":[

   ],
   "_keywords":[
      "good",
      "point", ...
   ],
   "uuid":"hC4-09hFcULvCyiU",
   "user":"1",
   "content":{
      "text":"Good point, I'll mark it as deprecated.",
      "title":"Updated  JIRA integration API"
   },
   "tags":[
      "influx:45958"
   ]
}


This is how a typical message looks like.

Browser

jQuery (+UI)
Comet impl.
MVC impl.

Rails app Scala backend
Website Messages
Admin Who’s online
Payments API
Account mgmt RSS feeds
SMTP server
Twitter feed

PostgreSQL MongoDB


An overview of the Flowdock architecture: most of the code is JavaScript and runs inside the
browser.

The Scala (+Akka) backend does all the heavy lifting (mostly related to messages and online
presence), and the Ruby on Rails application handles all the easy stuff (public website,
account management, administration, payments etc).

We used PostgreSQL in the beginning, and migrated messages to MongoDB. Otherwise there
is no particular reason why we couldn’t use MongoDB for everything.


One of the key features in Flowdock is tagging. For example, if I’m doing some changes to
our production environment, I mention it in the chat and tag it as #production. Production
deployments are automatically tagged with the same tag, so I can easily get a log of
everything that’s happened.

It’s very easy to implement with MongoDB, since we just index the “tags” array and search
using it.

db.messages.ensureIndex({ﬂow: 1, tags: 1, id: -1});



using it.

db.messages.ensureIndex({flow: 1, tags: 1, id: -1});

db.messages.find({flow: 123,
tags: {$all: [“production”]})
.sort({id: -1});


using it.

https://jira.mongodb.org/browse/SERVER-380


There’s a JIRA ticket about full-text search for MongoDB.
Users have built lots of their own implementations, but the discussion continues.

Library support
• Stemming
• Ranked probabilistic search
• Synonyms
• Spelling corrections
• Boolean, phrase, word proximity queries


These are some of the features you might see in an advanced full-text search
implementation. There are libraries to do some parts of this (like libraries speciﬁc to
stemming), and more advanced search libraries like Lucene and Xapian.

Lucene is a Java library (also ported to C++ etc.), and Xapian is a C++ library.

Many of these are hackable with MongoDB by expanding the query.

Standalone server Standalone server Standalone server
Lucene based Lucene queries MySQL integration
Rich document REST/JSON API Real-time indexing
support Real-time indexing Distributed
Result highlighting Distributed searching
Distributed


You can use the libraries directly, but they don’t do anything to guarantee replication &
scaling.

Standalone implementations usually handle clustering, query processing and some more
advanced features.

Things to consider
• Data access patterns
• Technology stack
• Data duplication
• Use cases: need to search Word
documents? Need to support boolean
queries? ...


When choosing your solution, you’ll want to keep it simple, consider how write-heavy your
app is, what special features do you need, can you afford to store the data 3 times in a
MongoDB replica set + 2 times in a search server etc.

Real-time sear
ch
Performance


There are tons of use cases where search doesn’t need to be real-time. It’s a requirement
that will heavily impact your application.

KISS


As a lean startup, we can’t afford to spend a lot of time with technology adventures. Need to
measure what customers want.
Many of the features are possible to achieve with MongoDB.
Facebook messages search also searches exact word matches (=it sucks), and people don’t
complain.

So we built a minimal implementation with MongoDB. No stemming or anything, just a
keyword search, but it needs to be real-time.

KISS
Even Facebook does.


As a lean startup, we can’t afford to spend a lot of time with technology adventures. Need to
measure what customers want.
Many of the features are possible to achieve with MongoDB.
Facebook messages search also searches exact word matches (=it sucks), and people don’t
complain.

So we built a minimal implementation with MongoDB. No stemming or anything, just a
keyword search, but it needs to be real-time.

“Good point. I’ll mark it as deprecated.”

_keywords: [“good”, “point”, “mark”, “deprecated”]


You need client-side code for this transformation.
What’s possible: stemming, search by beginning of the word
What’s not possible: intelligent ranking on the DB side (which is ok for us, since we want to
sort results by time anyway)

db.messages.ensureIndex({
ﬂow: 1,
_keywords: 1,
id: -1});


Simply build the _keywords index the same way we already had our tags indexed.

db.messages.ﬁnd({
ﬂow: 123,
_keywords: {
$all: [“hello”, “world”]}
}).sort({id: -1});


Search is also trivial to implement. As said, our users want the messages in chronological
order, which makes this a lot easier.

That’s it! Let’s take it to production.


A minimal search implementation is the easy part. We faced quite a few operational issues
when deploying it to production.

Index size:
2500 MB per 1M messages


As it turns out, the _keywords index is pretty big.

10M messages: Size in gigabytes

20.00

15.00

10.00

5.00

0
Messages Index: Keywords Index: Tags Index: Others


Would be great to ﬁt indices to the memory. Now it obviously doesn’t. Stemming will reduce
the index size.
Has implications for example to insert/update performance.

Option #1:
Just generate _keywords and build
the index in background.


The naive solution: try to do it with no downtime. Didn’t work, site slowed down too much.

Option #2:
Try to do it during a 6 hour
service break.


It worked much faster when our users weren’t constantly accessing the data. But 6 hours
during a weekend wasn’t enough, and we had to cancel the migration.

Option #3:
Delete _keywords, build the index
and re-generate keywords in the background.


Generating an index is much faster when there is no data to index. Building the index was
ﬁne, but generating keywords was very slow and took the site down.

Option #4:
As previously, but add sleep(5).


You know you’re a great programmer when you’re adding sleep()s to your production code.

Option #5:
As previously, but add Write Concerns.


Let the queries block, so that if MongoDB slows down, the migration script doesn’t ﬂood the
server.

Yeah, it would’ve taken a month, or it would’ve slowed down the service.

Option #6:
Shard.


Would have been a solution, but we didn’t want to host all that data in-memory, since it’s not
accessed that often.

Option #7:
SSD!


We had the possibility to try it on a SSD disk.

This is not a viable alternative to AWS users, but AWS users could temporarily shard their data
to a large number of high-memory instances.


My reactions to using SSD. Decided to benchmark it.

10M messages
in 100 “flows”,
Messages 100k each

Total size 19.67 GB

_id: 1
flow: 1, app: 1, id: -1
flow: 1, event: 1, id: -1
flow: 1, id: -1
Indices flow: 1, tags: 1, id: -1
flow: 1, _keywords: 1, id: -1

Total size 22.03 GB


This is the starting point for my next benchmark. Wanted to test it with a real-size database,
instead of starting from scratch.

mongorestore time in minutes

300.00

225.00

150.00

75.00

0
SSD SATA


First used mongorestore to populate the test database.
133 vs. 235 minutes, and index generation is mostly CPU-bound.
Doesn’t really beneﬁt from the faster seek times.

Insert performance test

A total of 100 workspaces
And 3 workers each accessing 30 workspaces
Performing 1000 inserts to each

= 90 000 inserts, as quickly as possible


insert benchmark: time in minutes

200.00

150.00

100.00

50.00

0
SSD SATA


4.25 vs 155. That’s 4 minutes vs. 2.5 hours.

9.67 inserts/sec
vs.

352.94 inserts/sec


The same numbers as inserts/sec.

36x

36x performance improvement with SSD. So we ended up using it in production.


Works well, searches from all kinds of content (here Git commit messages and deployment
emails), queries typically take only tens of milliseconds max.

Questions / Comments?
@flowdock / otto@flowdock.com


This was a very specific full-text search implementation. The fact that we didn’t need to rank
search results made it trivial.

I’m happy to discuss other use cases. Please share your thoughts and experiences.

Flowdock's full-text search with MongoDB

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