Lots of MongoDB users enjoy the flexibility of custom shard keys in organizing a sharded collection’s documents. For certain common workloads though, like key/value lookup, using the natural choice of _id as a shard key isn’t optimal because default ObjectId’s are ascending, resulting in poor write distribution. Creating randomized _ids or choosing another well-distributed field is always possible, but this adds complexity to an app and is another place where something could go wrong.

To help keep these simple workloads simple, in 2.4 MongoDB added the new Hash-based shard key feature. The idea behind Hash-based shard keys is that MongoDB will do the work to randomize data distribution for you, based on whatever kind of document identifier you like. So long as the identifier has a high cardinality, the documents in your collection will be spread evenly across the shards of your cluster. For heavy workloads with lots of individual document writes or reads (e.g. key/value), this is usually the best choice. For workloads where getting ranges of documents is more important (i.e. find recent documents from all users), other choices of shard key may be better suited.

Hash-based sharding in an existing collection

To start off with Hash-based sharding, you need the name of the collection you’d like to shard and the name of the hashed “identifier” field for the documents in the collection. For example, we might want to create a sharded “mydb.webcrawler” collection, where each document is usually found by a “url” field. We can populate the collection with sample data using:

shell$ wget http://en.wikipedia.org/wiki/Web_crawler -O web_crawler.html shell$ mongo connecting to: /test > use mydb switched to db mydb > cat("web_crawler.html").split("

").forEach( function(line){ ... var regex = /a href="([^"]*)"/; if (regex.test(line)) { db.webcrawler.insert({ "url" : regex.exec(line)[1] }); }}) > db.webcrawler.find() ... { "_id" : ObjectId("5162fba3ad5a8e56b7b36020"), "url" : "/wiki/OWASP" } { "_id" : ObjectId("5162fba3ad5a8e56b7b3603d"), "url" : "/wiki/Image_retrieval" } { "_id" : ObjectId("5162fba3ad5a8e56b7b3603e"), "url" : "/wiki/Video_search_engine" } { "_id" : ObjectId("5162fba3ad5a8e56b7b3603f"), "url" : "/wiki/Enterprise_search" } { "_id" : ObjectId("5162fba3ad5a8e56b7b36040"), "url" : "/wiki/Semantic_search" } ...

Just for this example, we multiply this data ~x2000 (otherwise we won’t get any pre-splitting in the collection because it’s too small):

> for (var i = 0; i < 12; i++) { db.webcrawler.find().toArray().forEach( function(doc) { db.webcrawler.insert({ url : doc.url }) }) }

Next, we create a hashed index on this field:

> db.webcrawler.ensureIndex({ url : "hashed" })

As usual, the creation of the hashed index doesn’t prevent other types of indices from being created as well.

Then we shard the “mydb.webcrawler” collection using the same field as a Hash-based shard key:

> db.printShardingStatus(true) --- Sharding Status --- sharding version: { "_id" : 1, "version" : 3, "minCompatibleVersion" : 3, "currentVersion" : 4, "clusterId" : ObjectId("5163032a622c051263c7b8ce") } shards: { "_id" : "test-rs0", "host" : "test-rs0/nuwen:31100,nuwen:31101" } { "_id" : "test-rs1", "host" : "test-rs1/nuwen:31200,nuwen:31201" } { "_id" : "test-rs2", "host" : "test-rs2/nuwen:31300,nuwen:31301" } { "_id" : "test-rs3", "host" : "test-rs3/nuwen:31400,nuwen:31401" } databases: { "_id" : "admin", "partitioned" : false, "primary" : "config" } { "_id" : "mydb", "partitioned" : true, "primary" : "test-rs0" } mydb.webcrawler shard key: { "url" : "hashed" } chunks: test-rs0 4 { "url" : { "$minKey" : 1 } } -->> { "url" : NumberLong("-4837773290201122847") } on : test-rs0 { "t" : 1, "i" : 3 } { "url" : NumberLong("-4837773290201122847") } -->> { "url" : NumberLong("-2329535691089872938") } on : test-rs0 { "t" : 1, "i" : 4 } { "url" : NumberLong("-2329535691089872938") } -->> { "url" : NumberLong("3244151849123193853") } on : test-rs0 { "t" : 1, "i" : 1 } { "url" : NumberLong("3244151849123193853") } -->> { "url" : { "$maxKey" : 1 } } on : test-rs0 { "t" : 1, "i" : 2 }

you can see that the chunk boundaries are 64-bit integers (generated by hashing the “url” field). When inserts or queries target particular urls, the query can get routed using the url hash to the correct chunk.

Sharding a new collection

Above we’ve sharded an existing collection, which will result in all the chunks of a collection initially living on the same shard. The balancer takes care of moving the chunks around, as usual, until we get an even distribution of data.

Much of the time though, it’s better to shard the collection before we add our data - this way MongoDB doesn’t have to worry about moving around existing data. Users of sharded collections are familiar with pre-splitting - where empty chunks can be quickly balanced around a cluster before data is added. When sharding a new collection using Hash-based shard keys, MongoDB will take care of the presplitting for you. Similarly sized ranges of the Hash-based key are distributed to each existing shard, which means that no initial balancing is needed (unless of course new shards are added).

Let’s see what happens when we shard a new collection webcrawler_empty the same way:

> sh.stopBalancer() Waiting for active hosts... Waiting for the balancer lock... Waiting again for active hosts after balancer is off... > db.webcrawler_empty.ensureIndex({ url : "hashed" }) > sh.shardCollection("mydb.webcrawler_empty", { url : "hashed" }) { "collectionsharded" : "mydb.webcrawler_empty", "ok" : 1 } > db.printShardingStatus(true) --- Sharding Status --- ... mydb.webcrawler_empty shard key: { "url" : "hashed" } chunks: test-rs0 2 test-rs1 2 test-rs2 2 test-rs3 2 { "url" : { "$minKey" : 1 } } -->> { "url" : NumberLong("-6917529027641081850") } on : test-rs0 { "t" : 4, "i" : 2 } { "url" : NumberLong("-6917529027641081850") } -->> { "url" : NumberLong("-4611686018427387900") } on : test-rs0 { "t" : 4, "i" : 3 } { "url" : NumberLong("-4611686018427387900") } -->> { "url" : NumberLong("-2305843009213693950") } on : test-rs1 { "t" : 4, "i" : 4 } { "url" : NumberLong("-2305843009213693950") } -->> { "url" : NumberLong(0) } on : test-rs1 { "t" : 4, "i" : 5 } { "url" : NumberLong(0) } -->> { "url" : NumberLong("2305843009213693950") } on : test-rs2 { "t" : 4, "i" : 6 } { "url" : NumberLong("2305843009213693950") } -->> { "url" : NumberLong("4611686018427387900") } on : test-rs2 { "t" : 4, "i" : 7 } { "url" : NumberLong("4611686018427387900") } -->> { "url" : NumberLong("6917529027641081850") } on : test-rs3 { "t" : 4, "i" : 8 } { "url" : NumberLong("6917529027641081850") } -->> { "url" : { "$maxKey" : 1 } } on : test-rs3 { "t" : 4, "i" : 9 }

As you can see, the new empty collection is already well-distributed and ready to use. Be aware though - any balancing currently in progress can interfere with moving the empty initial chunks off the initial shard, balancing will take priority (hence the initial stopBalancer step). Like before, eventually the balancer will distribute all empty chunks anyway, but if you are preparing for a immediate data load it’s probably best to stop the balancer beforehand.

That’s it - you now have a pre-split collection on four shards using Hash-based shard keys. Queries and updates on exact urls go to randomized shards and are balanced across the cluster:

> db.webcrawler_empty.find({ url: "/wiki/OWASP" }).explain() { "clusteredType" : "ParallelSort", "shards" : { "test-rs2/nuwen:31300,nuwen:31301" : [ ... ] ...

However, the trade-off with Hash-based shard keys is that ranged queries and multi-updates must hit all shards:

> db.webcrawler_empty.find({ url: /^\/wiki\/OWASP/ }).explain() { "clusteredType" : "ParallelSort", "shards" : { "test-rs0/nuwen:31100,nuwen:31101" : [ ... ], "test-rs1/nuwen:31200,nuwen:31201" : [ ... ], "test-rs2/nuwen:31300,nuwen:31301" : [ ... ], "test-rs3/nuwen:31400,nuwen:31401" : [ ... ] ...

…

Manual chunk assignment and other caveats

The core benefits of the new Hash-based shard keys are:

Easy setup of randomized shard key

Automated pre-splitting of empty collections

Better distribution of chunks on shards for isolated document writes and reads

The standard split and moveChunk functions do work with Hash-based shard keys, so it’s still possible to balance your collection’s chunks in any way you like. However, the usual “find” mechanism used to select chunks can behave a bit unexpectedly since the specifier is a document which is hashed to get the containing chunk. To keep things simple, just use the new “bounds” parameter when manually manipulating chunks of hashed collections (or all collections, if you prefer):

> use admin > db.runCommand({ split : "mydb.webcrawler_empty", bounds : [{ "url" : NumberLong("2305843009213693950") }, { "url" : NumberLong("4611686018427387900") }] }) > db.runCommand({ moveChunk : "mydb.webcrawler_empty", bounds : [{ "url" : NumberLong("2305843009213693950") }, { "url" : NumberLong("4611686018427387900") }], to : "test-rs3" })

There are a few other caveats as well - in particular with tag-aware sharding. Tag-aware sharding is a feature we released in MongoDB 2.2, which allows you to attach labels to a subset of shards in a cluster. This is valuable for “pinning” collection data to particular shards (which might be hosted on more powerful hardware, for example). You can also tag ranges of a collection differently, such that a collection sharded by { “countryCode” : 1 } would have chunks only on servers in that country.

Hash-based shard keys are compatible with tag-aware sharding. As in any sharded collection, you may assign chunks to specific shards, but since the chunk ranges are based on the value of the randomized hash of the shard key instead of the shard key itself, this is usually only useful for tagging the whole range to a specific set of shards:

> sh.addShardTag("test-rs2", "DC1") sh.addShardTag("test-rs3", "DC1")

The above commands assign a hypothetical data center tag “DC1” to shards -rs2 and -rs3, which could indicate that -rs2 and -rs3 are in a particular location. Then, by running:

> sh.addTagRange("mydb.webcrawler_empty", { url : MinKey }, { url : MaxKey }, "DC1" )

we indicate to the cluster that the mydb.webcrawler_empty collection should only be stored on “DC1” shards. After letting the balancer work:

> db.printShardingStatus(true) --- Sharding Status --- ... mydb.webcrawler_empty shard key: { "url" : "hashed" } chunks: test-rs2 4 test-rs3 4 { "url" : { "$minKey" : 1 } } -->> { "url" : NumberLong("-6917529027641081850") } on : test-rs2 { "t" : 5, "i" : 0 } { "url" : NumberLong("-6917529027641081850") } -->> { "url" : NumberLong("-4611686018427387900") } on : test-rs3 { "t" : 6, "i" : 0 } { "url" : NumberLong("-4611686018427387900") } -->> { "url" : NumberLong("-2305843009213693950") } on : test-rs2 { "t" : 7, "i" : 0 } { "url" : NumberLong("-2305843009213693950") } -->> { "url" : NumberLong(0) } on : test-rs3 { "t" : 8, "i" : 0 } { "url" : NumberLong(0) } -->> { "url" : NumberLong("2305843009213693950") } on : test-rs2 { "t" : 4, "i" : 6 } { "url" : NumberLong("2305843009213693950") } -->> { "url" : NumberLong("4611686018427387900") } on : test-rs2 { "t" : 4, "i" : 7 } { "url" : NumberLong("4611686018427387900") } -->> { "url" : NumberLong("6917529027641081850") } on : test-rs3 { "t" : 4, "i" : 8 } { "url" : NumberLong("6917529027641081850") } -->> { "url" : { "$maxKey" : 1 } } on : test-rs3 { "t" : 4, "i" : 9 } tag: DC1 { "url" : { "$minKey" : 1 } } -->> { "url" : { "$maxKey" : 1 } }

Again, it doesn’t usually make a lot of sense to tag anything other than the full hashed shard key collection to particular shards - by design, there’s no real way to know or control what data is in what range.

Finally, remember that Hash-based shard keys can (right now) only distribute documents based on the value of a single field. So, continuing the example above, it isn’t directly possible to use “url” + “timestamp” as a Hash-based shard key without storing the combination in a single field in your application, for example:

url_and_ts : { url : <url>, timestamp : <timestamp> }

The sub-document will be hashed as a unit.

If you’re interested in learning more about Hash-based sharding, register for the Hash-based sharding feature demo on May 2.

