MongoDB Datastores

DataNucleus supports persisting/retrieving objects to/from MongoDB datastores (using the datanucleus-mongodb plugin, which utilises the Mongo Java driver). Simply specify your "connectionURL" as follows

datanucleus.ConnectionURL=mongodb:[{server}][/{dbName}] [,{server2}[,server3}]]

For example, to connect to a local server, with database called "myMongoDB"

datanucleus.ConnectionURL=mongodb:/myMongoDB

If you just specify the URL as mongodb then you have a local MongoDB datastore called "DataNucleus", otherwise it tries to connect to the datastore {dbName} at {server}. The multiple {server} option allows you to run against MongoDB replica sets. You then create your PMF/EMF as normal and use JDO/JPA as normal.

The jars required to use DataNucleus MongoDB persistence are datanucleus-core, datanucleus-api-jdo/datanucleus-api-jpa, datanucleus-mongodb and mongo-java-driver

There are tutorials available for use of DataNucleus with MongoDB for JDO and for JPA

Things to bear in mind with MongoDB usage :-

  • Creation of a PMF/EMF will create a MongoClient. This will be closed then the PMF/EMF is closed.
  • Creation of a PM/EM and performing an operation will obtain a DB object from the MongoClient. This is pooled by the MongoClient so is managed by MongoDB. Closing the PM/EM will stop using that DB
  • You can set the number of connections per host with the persistence property datanucleus.mongodb.connectionsPerHost
  • Querying can be performed using JDOQL or JPQL. Some components of a filter are handled in the datastore, and the remainder in-memory. Currently any expression of a field (in the same table), or a literal are handled in-datastore, as are the operators &&, ||, >, >=, <, <=, ==, and !=. Note that if something falls back to being evaluated in-memory then it can be much slower, and this will be noted in the log, so people are advised to design their models and queries to avoid that happening if performance is a top priority.
  • If you want a query to be runnable on a slave MongoDB instance then you should set the query extension (JDO) / hint (JPA) slave-ok as true, and when executed it can be run on a slave instance.
  • All objects of a class are persisted to a particular "document" (specifiable with the "table" in metadata), and a field of a class is persisted to a particular "field" ("column" in the metadata).
  • Relations : DataNucleus stores the id of the related object(s) in a field of the owning object. When a relation is bidirectional both ends of the relation will store the relation information.
  • Capped collections : you can specify the extension metadata key mongodb.capped.size as the number of bytes of the size of the collection for the class in question.
  • If you want to specify the max number of connections per host with MongoDB then set the persistence property datanucleus.mongodb.connectionsPerHost
  • If you want to specify the MongoDB threadsAllowedToBlockForConnectionMultiplier, then set the persistence property datanucleus.mongodb.threadsAllowedToBlockForConnectionMultiplier

Mapping : Embedded Persistable fields

When you have a field in a class that is of a persistable type you sometimes want to store it with the owning object. In this case you can use JDO / JPA embedding of the field. DataNucleus offers two ways of performing this embedding

  • The default is to store the object in the field as a sub-document (nested) of the owning document. Similarly if that sub-object has a field of a persistable type then that can be further nested.
  • The alternative is to store each field of the sub-object as a field of the owning document (flat embedding). Similarly if that sub-object has a field of a persistable type then it can be flat embedded in the same way

For JDO this would be defined as follows (for JPA just swap @PersistenceCapable for @Entity)

@PersistenceCapable
public class A
{
    @Embedded
    B b;

    ...
}

This example uses the default embedding, using a nested document within the owner document, and could look something like this

{ "name" : "A Name" ,
  "id" : 1 ,
  "b" : { "name" : "B name" ,
          "description" : "the description"}
}

The alternative for JDO would be as follows (for JPA just swap @PersistenceCapable for @Entity)

@PersistenceCapable
public class A
{
    @Embedded
    @Extension(vendorName="datanucleus", key="nested", value="false")
    B b;

    ...
}

and this will use flat embedding, looking something like this

{ "name" : "A Name" ,
  "id" : 1 ,
  "b_name" : "B name" ,
  "b_description" : "the description"
}

Mapping : Embedded Collection elements

When you have a field in a class that is of a Collection type you sometimes want to store it with the owning object. In this case you can use JDO/ JPA embedding of the field. So if we have

@PersistenceCapable
public class A
{
    @Element(embedded="true")
    Collection<b> bs;

    ...
}

and would look something like this

{ "name" : "A Name" ,
  "id" : 1 ,
  "bs" :
      [
        { "name" : "B Name 1" ,
          "description" : "desc 1"} ,
        { "name" : "B Name 2" ,
          "description" : "desc 2"} ,
        { "name" : "B Name 3" ,
          "description" : "desc 3"}
      ]
}

References

Below are some references using this support