What to expect from MongoDB 2.4

Authored by: Jonathan Freeman, Associate Developer
Today, 10gen released version 2.3 of its document database, MongoDB. Version 2.3 is the development branch of MongoDB 2.4 and an exciting prelude of what is to come on March 5. Since the release date is rapidly approaching here’s a quick overview of some notable changes in MongoDB 2.4.

New JavaScript engine

Google’s V8 will be replacing Mozilla’s SpiderMonkey as the underlying JavaScript engine throughout the application. While there may be some benefits in specific cases (deep recursion), there may not be any noticeable differences for most MongoDB users. The idea for the switch was to help out with parallelism, as SpiderMonkey was restricting MongoDB to a single thread of execution for each mongos in a sharded environment. This may be barely noticeable to most users, thanks to the combination of high-speed modern processors and the short length of the operations sent from MongoDB. It will be nice to see some benchmarking done to see where V8 becomes the clear victor over SpiderMonkey.

Three(read ‘one’) new types of indexes

You’ll now be able to use three new types of indexes to optimize your queries, but you’ll probably only ever use one of the three. A hash-based index is new, but it's been mainly added to ease use of the new hash-based sharding(see below). Hashed-based indexing has little to no benefit over using a regular index on an ObjectId field, so unless you’re planning on using hash-based sharding, there’s nothing to see here.

A new geospatial index helps out with spherical geometry. By a show of hands who needs better indexing for their geo queries involving spherical geometry? For the three of you that raised your hands, you’ll be using GeoJSON to define your geo fields, but only a small subset is supported for this release. You also have the ability to use the new query operator $geoIntersects, although the release notes don’t go into much detail about this. When I was at MongoDB Chicago, there was talk about being able to query a collection with a specific point where the returned documents would have polygons that contained that point. If the $geoIntersects query operator turns out to do this, it looks like a nice solution to find out which restaurants will deliver to me, but only more detailed documentation will clarify.

The new index you’ll most likely be able use starting today is the “text” index, and accompanying query. You can specify one or more fields to index, and each of those fields can be assigned a weight MongoDB will use to sort your search results. You’re not very limited to the types of fields you can specify, either, so long as the field has a string, or is an array of strings, or is a sub-document that has one of those two options, it's fair to build an index on it. This will make it tremendously easy to do a full-text search that includes your user’s ‘About Me’ field and all of their comments and weigh the results based on what you think is more important. Here’s an example of creating a text-based index:

aboutMe: “text”,
“comments.body”: “text”
name: “TextIndex,
aboutMe: 5,
“comments.body”: 2

The example above creates a text index that includes two fields: aboutMe and comments.body. Note the double-quotes when indexing on the field of a sub-document. The second argument names the index, because the default isn’t that great: “aboutMe_text_comments.body_text”. Imagine using 6 fields in your text index. You’ll quickly be running up on the index name length restriction of 128 characters. Even with only a few fields in your index, the default isn’t very friendly, so get into the habit of naming these. I’ve also put weights on the indexes, which will be reflected in the relevant score(position) of each document in the result set.

The text query structure is different than what you may be used to, and you’re returned a document representing the results as opposed to a cursor, so it may not integrate as nicely as it could with your existing code. A query on the index I created above would look like:

db.collection.runCommand( “text”,
search: “\”prototypal inheritance\” -JavaScript”
filter: {experiencePoints: { $gt: 1000 },
limit: 15

The above query will search your text index for documents that have the exact phrase “prototypal inheritance” and that do not include the word “JavaScript”. Notice the escaped double-quotes around the exact phrase string we want, and the hyphen to represent a negation. The results will then be filtered by documents that have an experiencePoints field that is greater than 1000, and only 15 documents will be returned. Again, the result of a text query will not return a cursor, but a special document. The mongo shell is a great place to explore this new functionality. All things considered, the new text indexes and text queries are simple and powerful additions that will enjoy widespread adoption.

Hash-based Sharding

Particularly beneficial for highly active production environments, hashed-based sharding provides a more reliable way to evenly distribute data across your shards. This means you can spend less time worrying about rebalancing your data at the end of the day. To aid the ease of use, the MongoDB team implemented a new "hashed" index type, which makes it trivial to add this type of sharding on a particular collection. The simple two-step process looks like:

// Create your hashed index beforehand if your collection already has data
db.collection.ensureIndex( { someField: “hashed” } );

// Shard your collection, using the new key
sh.shardCollection( “db.collection”, { someField: “hashed” } );

Now at some point there was a problem with people hashing their own keys using MD5: the data was so evenly distributed that read queries were taking too long to scan through the shards. I haven’t seen any benchmarks here, but there is a specific commit on this issue that mentions patching a function whose job it is to calculate the relevant shards for a given query. Hopefully that solves the problem of the longer queries while not touching the nice, even distribution.

For the most part, there are some incremental performance changes and improvements that help niche markets. Full-text search is going to be the big deal here superficially: it’s easy to set up and a handy feature. Be that as it may, hash-based sharding has the potential to take the cake for the most valuable addition: it could help MongoDB continue to be adopted in the enterprise world, where high-volume is a given and rebalancing is a costly procedure to be avoided at all costs. If you want to get an early look at what MongoDB 2.4, check out the 2.3 development branch, scheduled to be released today.


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