How do I avoid n+1 queries with Spring Data Rest?

Question

Question. How do I avoid n+1 queries with Spring Data REST?

Background. When querying Spring Data REST for a list of resources, each of the resulting top-level resources has links to the associated resources, as opposed to having the associated resources embedded directly in the top-level resources. For example, if I query for a list of data centers, the associated regions appear as links, like this:

{
  "links" : [ {
    "rel" : "self",
    "href" : "http://localhost:2112/api/datacenters/1"
  }, {
    "rel" : "datacenters.DataCenter.region",
    "href" : "http://localhost:2112/api/datacenters/1/region"
  } ],
  "name" : "US East 1a",
  "key" : "amazon-us-east-1a"
}

It is pretty typical, however, to want to get the associated information without having to do n+1 queries. To stick with the example above, I might want to display a list of data centers and their associated regions in a UI.

What I've tried. I created a custom query on my RegionRepository to get all the regions for a given set of data center keys:

@RestResource(path = "find-by-data-center-key-in")
Page<Region> findByDataCentersKeyIn(
    @Param("key") Collection<String> keys,
    Pageable pageable);

Unfortunately the links this query generates don't overlap with the links that the data center query above generates. Here are the links I get for the custom query:

http://localhost:2112/api/regions/search/find-by-data-center-key-in?key=amazon-us-east-1a&key=amazon-us-east-1b

{
  "links" : [ ],
  "content" : [ {
    "links" : [ {
      "rel" : "self",
      "href" : "http://localhost:2112/api/regions/1"
    }, {
      "rel" : "regions.Region.datacenters",
      "href" : "http://localhost:2112/api/regions/1/datacenters"
    }, {
      "rel" : "regions.Region.infrastructureprovider",
      "href" : "http://localhost:2112/api/regions/1/infrastructureprovider"
    } ],
    "name" : "US East (N. Virginia)",
    "key" : "amazon-us-east-1"
  }, {
    "links" : [ {
      "rel" : "self",
      "href" : "http://localhost:2112/api/regions/1"
    }, {
      "rel" : "regions.Region.datacenters",
      "href" : "http://localhost:2112/api/regions/1/datacenters"
    }, {
      "rel" : "regions.Region.infrastructureprovider",
      "href" : "http://localhost:2112/api/regions/1/infrastructureprovider"
    } ],
    "name" : "US East (N. Virginia)",
    "key" : "amazon-us-east-1"
  } ],
  "page" : {
    "size" : 20,
    "totalElements" : 2,
    "totalPages" : 1,
    "number" : 1
  }
}

The challenge seems to be that the data center query returns links that aren't particularly informative once you already understand the shape of the data. For example, I already know that the region for data center 1 is at /datacenters/1/region, so if I want actual information about which specific region is involved, I have to follow the link to get it. In particular I have to follow the link to get the canonical URI that shows up in the bulk queries that would allow me to avoid n+1 queries.

Answer 1

The reason Spring Data REST works like this is the following: by default, we assume every application repository a primary resource of the REST service. Thus, if you expose a repository for an entity's related object you get links rendered to it and we expose the assignment of one entity to another via a nested resource (e.g. foo/{id}/bar).

To prevent this, annotate the related repository interface with @RestResource(exported = false) which prevents the entities managed by this repository from becoming top level resources.

The more general approach to this is starting with Spring Data REST letting you expose the resources you want to get managed and default rules applied. You can then customize the rendering and links by implementing ResourceProcessor<T> and registering your implementation as Spring bean. The ResourceProcessor will then allow you to customize the data rendered, links added to the representation etc.

For everything else, manually implement controllers (potentially blending into the URI space of the default controllers) and add links to those through ResourceProcessor implementations. An example for this can be seen in the Spring RESTBucks sample. The sample project uses Spring Data REST to manage Order instances and implements a custom controller to implement the more complex payment process. Beyond that it adds a link to the Order resource to point to the manually implemented code.

Answer 2

Spring Data REST will only create the representation you describe if the serializer that is configured inside the Jackson ObjectMapper is triggered by seeing a PersistentEntityResource, which is a special kind of Resource that is used inside Spring Data REST.

If you create a ResourceProcessor<Resource<MyPojo>> and return a new Resource<MyPojo>(origResource.getContent(), origResource.getLinks()), then the default Spring Data REST serialization machinery will not be triggered and Jackson's normal serialization rules will apply.

Note, however, that the reason Spring Data REST does associations the way it does is because it's very difficult to arbitrarily stop traversing an object graph when serializing to JSON. By handling associations the way it does, it guarantees that the serializer won't start traversing an object graph that is N levels deep and become much slower in performance and in the performance of the representation going over-the-wire.

Ensuring that Jackson does not try to serialize a PersistentEntityResource, which is what it's doing in the default configuration, will ensure that none of the Spring Data REST handling of associations is triggered. The down side to this, of course, is that none of Spring Data REST's helpers will be triggered. If you still want links to the associated resources, you'll have to make sure you create those yourself and add them to the outgoing plain Resource.