Moving from MongoDB to Couchbase server

This is a developer-focused guide to moving your application's data store from MongoDB to Couchbase Server, following on from Laurent's guide to making the move from PostgreSQL.

While it doesn't cover every corner-case, it does offer pointers to what you should consider when planning your migration.

Versions

This guide is written for Couchbase Server 4.1 and MongoDB 3.2.

Key differences

Couchbase Server and MongoDB are both document stores that can operate on one or many servers. However, they approach things in some significantly different ways.

When you begin your migration from MongoDB to Couchbase Server, you'll need to consider the following differences:

  Couchbase Server MongoDB
Data models Document, key-value Document
Query N1QL (SQL for JSON), map/reduce views, key-value Ad-hoc query, map/reduce aggregation
Concurrency Optimistic and pessimistic locking
Optimistic and pessimistic locking, with WiredTiger
Scaling model Distributed master-master Master-slave with replica sets

Data model

Couchbase Server is both a document store and a key-value store. 

Everything starts with key-value, as every document has a key that you can use to get and set the document's contents. However, you can use Couchbase Server as a document database by indexing and querying document contents.

Differences between BSON and JSON

It's likely that your application stores JSON-style documents in MongoDB, so we'll start there.

MongoDB stores data in the BSON format, which is a binary JSON-like format. The key difference for us is that BSON records additional type information.

When you export data from MongoDB using a tool such as mongoexport the tool will produce JSON that preserves that type information in a format called Extended JSON.

Let's take a look at an example. First in standard JSON:

And now in Extended JSON:

As you can see, the Extended JSON is still valid JSON. That means you can store, index, query and retrieve it using Couchbase Server. However, you'll need to maintain that additional type information on the application layer.

Alternatively, you could convert the Extended JSON to standard JSON before you import it into Couchbase Server.

Non-JSON data

Both MongoDB and Couchbase Server can store opaque binary data.

Although the internal representation of the binary data differs greatly between the two, you can continue to store in Couchbase Server the binary data that you've been storing in MongoDB.

The main difference is that Couchbase Server can store binaries of up to 20MB in size, while MongoDB offers a convenience layer for chunking very large files into multiple documents.

There are strong arguments against storing large binaries in your database. When you have many very large binaries, you might consider using a dedicated object storage service to store them while using Couchbase to hold metadata on those binaries.

Architecture

Sharding

Couchbase Server shards data, and scales horizontally, by automatically distributing a hash space amongst nodes in a cluster.

It then uses the key of your document to decide where in the hash space — and so on which node in the cluster — that document resides. As a developer, this is abstracted away for you by the client SDK.

With MongoDB you need to choose a sharding method and a shard key. The shard key is an indexed field inside your document that determines where in the cluster the document resides.

The primary difference here is that Couchbase Server automatically handles all sharding for you, whereas MongoDB gives you the choice of sharding method and the shard key. If your applications relies on a particular distribution of data across the cluster then you'll need to adjust it to allow for the hash-based distribution used by Couchbase.

Hash-based sharding greatly simplifies scaling compared to MongoDB. As a developer that might seem more like an issue for your ops team. However, it does mean that it's easier to rely on Couchbase Server should usage of your software grow.

Replication and consistency

Three Couchbase Server nodesCouchbase Server maintains a single active copy of each document and then up to two replicas; you can configure the number of replicas on a bucketwide or per operation basis.

This means that, during normal operation, you deal with the same live copy each time you write into and read from a document. As a result, you don't have to handle conflicting versions of documents. The replicas come into play only when the active copy is unavailable.

As a developer, the distribution and replication of documents abstracted away for you. You write, read and query using your connection to the bucket and the SDK handles precisely where that data is stored. There's no need to factor in replica sets or sharding schemes.

If you know the CAP theorem then you also know that favouring consistency has an effect on availability. In the event of a node failure, its active documents will be unavailable for a short time to allow the cluster to promote the appropriate replicas to active status. In your code, all you need to do is retry a failed operation.

Indexing

Couchbase Server offers two broad types of index:

  • GSI: global secondary indexes
  • views: generated by map-reduce queries.

The difference is more than an implementation detail, in that you create and manage both types of index differently. Mostly, you'll use GSI indexes to replicate your MongoDB indexes.

MongoDB Couchbase Server
Single field GSI
Compound index GSI
Multi-key index GSI
Geospatial index Spatial index in views
Text index Couchbase free-text search coming in Couchbase Server next release

Node types

When you grow beyond one MongoDB server you need to introduce router processes and config servers.

With Couchbase Server, both these functions are found in the client SDK. When you connect to the cluster from your application, the SDK receives a map of where in the cluster each shard lives. Couchbase Server then automatically updates the cluster map each time the shape of the cluster changes. Each request then happens directly from the application server to the relevant Couchbase node.

Once your cluster grows, you can choose to run specialised data, query and indexing nodes. Read more about multi-dimensional scaling.

This all happens transparently to you as the developer.

Buckets and collections

Both Couchbase Server and MongoDB allow you to divide your dataset into groups of documents: Couchbase has buckets and MongoDB has collections.

While MongoDB collections are of an equivalent scope to relational tables, Couchbase Server buckets are perhaps more the equivalent of a relational database.

That distinction matters because usually you'd want no more than ten buckets in a single Couchbase cluster. This makes them somewhat unsuitable as namespaces and instead they serve as a way to share configuration decisions between similar types of documents.

This has two main consequences:

  • you need another way to namespace your documents
  • you need to think about when it's appropriate to create a new bucket.

When to use multiple buckets

First, we need to think about how you can allocate resources to buckets. The two big considerations are:

  • RAM
  • views and indexes.

When you create a bucket, you allocate to it a portion of each machine's RAM. The RAM you give to a bucket should be big enough to store the working set of that data plus the few bytes of metadata associated with each document.

This means you can allocate different amounts of RAM appropriately to different datasets based on how you access them.

Similarly, Couchbase views and indexes run across the documents inside a bucket, much as a MongoDB map-reduce query runs across a single collection.

If you have some documents that don't need indexing — because you only ever access them through their key — and you have some groups of documents that have different velocities, you can see that it would be prudent never to run indexers on the first set of data and to run the indexers with appropriate intervals across the rest.

Dividing our data into different buckets lets us accomplish both good use of RAM and of the CPU time needed by the indexers.

Let's look at the example of an ecommerce application, the data you'd store, its profile and how you respond to that in your bucket configuration.

Data type Data profile Bucket profile
Sessions Fast responses, key-value access, predictable concurrent sessions RAM to fit typical number of live sessions, no indexing
User profiles Fast responses while users active, data changes slowly RAM to fit user profiles for typical number of live sessions, indexing on
Order data Read-heavy after initial creation, short lifetime RAM to fit orders for typical number of live sessions, indexing on
Product data Fast responses needed, read heavy RAM to fit entire catalogue, indexing on

It's a little more involved than deciding whether indexing is on or off. Rather, you choose the types of index and the velocity of updates decides how often the indexers run. Mixing slow and fast moving data could be inefficient because a bucket's indexers run across all documents in that bucket, including those that are unchanged.

Namespacing documents

If we can't use buckets as namespaces, how do we easily distinguish different types of document?

You should use a combination of:

  • key naming
  • using a “type” in your JSON document.

Using semantic prefixes and suffixes in your key names is an easy way to namespace your documents, particularly when you're using Couchbase for key-value.

Requiring a type in your document schemas gives you the data you need to create queries that apply only to certain types of document.

Programming model

There are three ways of working with Couchbase Server:

  • simple key-value access: strong consistency, sub-millisecond responses
  • views: generated by map-reduce queries
  • N1QL: SQL-like querying with JOINs.

Coming from MongoDB, you might be tempted to translate all of your MongoDB queries into N1QL. However, it's worth thinking about the relative merits of each and then using the mix that suits your needs.

You can get a long way with key-value access, such as by using manual secondary indexes.

Query

For ad-hoc query, Couchbase Server offers N1QL. N1QL is a SQL-like language and so is quite different from MongoDB's query.

Let's look at an example where we return the name of employees from the London office who have worked there for two years or more, ordered by start date:

As you can see, N1QL is very familiar if you've ever worked with SQL. You'll be able to translate your MongoDB queries to N1QL with relatively little effort.

Before you begin rewriting your queries, you should consider one major advantage that N1QL offers: you can perform JOINs across documents. Let's take our query above and also return the name of each person's manager.

Read more about N1QL and about views.

Concurrency

In Couchbase Server, locking always happens at the document level and there are two types:

  • pessimistic: no other actor can write to that document until it is released or a timeout is hit
  • optimistic: use CAS values to check if the document has changed since you last touched it and the act accordingly.

In a distributed database, optimistic locking is a much more neighbourly approach. Read more about choosing the right type of lock.

Libraries and integrations

There are officially supported SDKs for all major languages, including Java, .NET, NodeJS, Python, Go, Ruby and C. You'll also find community-developed client libraries for languages including Erlang.

Similarly there are official integrations with Spring Data, Spark, Hadoop, Elasticsearch, .NET's Linq and there's a NodeJS ODM called Ottoman.

Conclusion

Moving from one document store to another is relatively straightforward, as the broad shape of your data doesn't need to change all that much.

As a developer porting an application from MongoDB to Couchbase Server, your main considerations are that you need to:

  • replace collection namespacing with key naming and document types
  • simplify your queries by using N1QL JOINs
  • consider where key-value access can be the best choice.

If you're moving from MongoDB to Couchbase Server, you certainly aren't the first. That's great news for you because you'll find people who've made the switch before in our forums.

Posted by Matthew Revell, Lead Developer Advocate, EMEA, Couchbase

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