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Flakiness in REST requests is a common issue. A request can get a 200 OK in one scenario and a 409 next time. Sometimes a request can even succeed and fail intermittently on the same exact request. In short, working over HTTP can be a bit of a mess without solid tooling.

Also, while it’s easy enough to debug these issues locally when developing the application, we’re talking about production here - we can’t afford the downtime while you’re stepping in and out of code. Uptime is kind of the whole point.

With Lightrun, you can get the same level of access you get with a local debugger or profiler - no downtime required. You can add logs, metrics, and snapshots (think breakpoints, but without stopping the running service), in a safe and read-only manner - without redeploying, restarting, or even stopping the running service. Performance and security are maintained throughout the process.

Learn how to debug a live REST API (built with Spring, of course), using Lightrun, in this 5-minute tutorial:

>> Debugging REST Requests in Spring-Based applications using the Lightrun Platform

1. Overview

This tutorial will focus on the implementation of pagination in a REST API using Spring MVC and Spring Data.

Further reading:

Pagination with Spring REST and AngularJS table

An extensive look at how to implement a simple API with pagination with Spring and how to consume it with AngularJS and UI Grid.

JPA Pagination

Pagination in JPA - how to use JQL and the Criteria API to do pagination correctly.

REST API Discoverability and HATEOAS

HATEOAS and Discoverability of a REST Service - driven by tests.

2. Page as Resource vs Page as Representation

The first question when designing pagination in the context of a RESTful architecture is whether to consider the page an actual Resource or just a Representation of Resources.

Treating the page itself as a resource introduces a host of problems, such as no longer being able to uniquely identify resources between calls. This, coupled with the fact that, in the persistence layer, the page isn't a proper entity but a holder that's constructed when necessary, makes the choice straightforward; the page is part of the representation.

The next question in the pagination design in the context of REST is where to include the paging information:

  • in the URI path: /foo/page/1
  • the URI query: /foo?page=1

Keeping in mind that a page isn't a Resource, encoding the page information in the URI isn't an option.

We'll use the standard way of solving this problem by encoding the paging information in a URI query.

3. The Controller

Now for the implementation. The Spring MVC Controller for pagination is straightforward:

@GetMapping(params = { "page", "size" })
public List<Foo> findPaginated(@RequestParam("page") int page, 
  @RequestParam("size") int size, UriComponentsBuilder uriBuilder,
  HttpServletResponse response) {
    Page<Foo> resultPage = service.findPaginated(page, size);
    if (page > resultPage.getTotalPages()) {
        throw new MyResourceNotFoundException();
    }
    eventPublisher.publishEvent(new PaginatedResultsRetrievedEvent<Foo>(
      Foo.class, uriBuilder, response, page, resultPage.getTotalPages(), size));

    return resultPage.getContent();
}

In this example, we're injecting the two query parameters, size and page, in the Controller method via @RequestParam.

Alternatively, we could have used a Pageable object, which maps the pagesize, and sort parameters automatically. In addition, the PagingAndSortingRepository entity provides out-of-the-box methods that support using Pageable as a parameter.

We're also injecting the Http Response and the UriComponentsBuilder to help with Discoverability, which we're decoupling via a custom event. If that's not a goal of the API, we can simply remove the custom event.

Finally, note that the focus of this article is only the REST and web layer; to go deeper into the data access part of pagination, we can check out this article about Pagination with Spring Data.

4. Discoverability for REST Pagination

Within the scope of pagination, satisfying the HATEOAS constraint of REST means enabling the client of the API to discover the next and previous pages based on the current page in the navigation. For this purpose, we'll use the Link HTTP header, coupled with the “next,” “prev,” “first,” and “last” link relation types.

In REST, Discoverability is a cross-cutting concern, applicable not only to specific operations, but to types of operations. For example, each time a Resource is created, the URI of that Resource should be discoverable by the client. Since this requirement is relevant for the creation of ANY Resource, we'll handle it separately.

We'll decouple these concerns using events, as we discussed in the previous article focusing on Discoverability of a REST Service. In the case of pagination, the event, PaginatedResultsRetrievedEvent, is fired in the controller layer. Then we'll implement discoverability with a custom listener for this event.

In short, the listener will check if the navigation allows for nextpreviousfirst and last pages. If it does, it'll add the relevant URIs to the response as a ‘Link' HTTP Header.

Now let's go step by step. The UriComponentsBuilder passed from the controller contains only the base URL (the host, the port and the context path). Therefore, we'll have to add the remaining sections:

void addLinkHeaderOnPagedResourceRetrieval(
 UriComponentsBuilder uriBuilder, HttpServletResponse response,
 Class clazz, int page, int totalPages, int size ){

   String resourceName = clazz.getSimpleName().toString().toLowerCase();
   uriBuilder.path( "/admin/" + resourceName );

    // ...
   
}

Next, we'll use a StringJoiner to concatenate each link. We'll use the uriBuilder to generate the URIs. Let's see how we proceed with the link to the next page:

StringJoiner linkHeader = new StringJoiner(", ");
if (hasNextPage(page, totalPages)){
    String uriForNextPage = constructNextPageUri(uriBuilder, page, size);
    linkHeader.add(createLinkHeader(uriForNextPage, "next"));
}

Let's have a look at the logic of the constructNextPageUri method:

String constructNextPageUri(UriComponentsBuilder uriBuilder, int page, int size) {
    return uriBuilder.replaceQueryParam(PAGE, page + 1)
      .replaceQueryParam("size", size)
      .build()
      .encode()
      .toUriString();
}

We'll proceed similarly for the rest of the URIs that we want to include.

Finally, we'll add the output as a response header:

response.addHeader("Link", linkHeader.toString());

Note that, for brevity, only a partial code sample is included, and the full code is here.

5. Test Driving Pagination

Both the main logic of pagination and discoverability are covered by small, focused integration tests. As in the previous article, we'll use the REST-assured library to consume the REST service and verify the results.

These are a few examples of pagination integration tests; for a full test suite, check out the GitHub project (link at the end of the article):

@Test
public void whenResourcesAreRetrievedPaged_then200IsReceived(){
    Response response = RestAssured.get(paths.getFooURL() + "?page=0&size=2");

    assertThat(response.getStatusCode(), is(200));
}
@Test
public void whenPageOfResourcesAreRetrievedOutOfBounds_then404IsReceived(){
    String url = getFooURL() + "?page=" + randomNumeric(5) + "&size=2";
    Response response = RestAssured.get.get(url);

    assertThat(response.getStatusCode(), is(404));
}
@Test
public void givenResourcesExist_whenFirstPageIsRetrieved_thenPageContainsResources(){
   createResource();
   Response response = RestAssured.get(paths.getFooURL() + "?page=0&size=2");

   assertFalse(response.body().as(List.class).isEmpty());
}

6. Test Driving Pagination Discoverability

Testing that pagination is discoverable by a client is relatively straightforward, although there's a lot of ground to cover.

The tests will focus on the position of the current page in navigation, and the different URIs that should be discoverable from each position:

@Test
public void whenFirstPageOfResourcesAreRetrieved_thenSecondPageIsNext(){
   Response response = RestAssured.get(getFooURL()+"?page=0&size=2");

   String uriToNextPage = extractURIByRel(response.getHeader("Link"), "next");
   assertEquals(getFooURL()+"?page=1&size=2", uriToNextPage);
}
@Test
public void whenFirstPageOfResourcesAreRetrieved_thenNoPreviousPage(){
   Response response = RestAssured.get(getFooURL()+"?page=0&size=2");

   String uriToPrevPage = extractURIByRel(response.getHeader("Link"), "prev");
   assertNull(uriToPrevPage );
}
@Test
public void whenSecondPageOfResourcesAreRetrieved_thenFirstPageIsPrevious(){
   Response response = RestAssured.get(getFooURL()+"?page=1&size=2");

   String uriToPrevPage = extractURIByRel(response.getHeader("Link"), "prev");
   assertEquals(getFooURL()+"?page=0&size=2", uriToPrevPage);
}
@Test
public void whenLastPageOfResourcesIsRetrieved_thenNoNextPageIsDiscoverable(){
   Response first = RestAssured.get(getFooURL()+"?page=0&size=2");
   String uriToLastPage = extractURIByRel(first.getHeader("Link"), "last");

   Response response = RestAssured.get(uriToLastPage);

   String uriToNextPage = extractURIByRel(response.getHeader("Link"), "next");
   assertNull(uriToNextPage);
}

Note that the full low-level code for extractURIByRel, responsible for extracting the URIs by rel relation, is here.

7. Getting All Resources

On the same topic of pagination and discoverability, the choice must be made if a client is allowed to retrieve all the Resources in the system at once, or if the client must ask for them paginated.

If it's decided that the client can't retrieve all Resources with a single request, and pagination is required, then several options are available for the response to get a request. One option is to return a 404 (Not Found) and use the Link header to make the first page discoverable:

Link=<http://localhost:8080/rest/api/admin/foo?page=0&size=2>; rel=”first”, <http://localhost:8080/rest/api/admin/foo?page=103&size=2>; rel=”last”

Another option is to return a redirect, 303 (See Other), to the first page. A more conservative route would be to simply return to the client a 405 (Method Not Allowed) for the GET request.

8. REST Paging With Range HTTP Headers

A relatively different way of implementing pagination is to work with the HTTP Range headers, Range, Content-Range, If-Range, Accept-Ranges, and HTTP status codes, 206 (Partial Content), 413 (Request Entity Too Large), and 416 (Requested Range Not Satisfiable).

One view of this approach is that the HTTP Range extensions aren't intended for pagination, and they should be managed by the Server, not by the Application. Implementing pagination based on the HTTP Range header extensions is technically possible, although not nearly as common as the implementation discussed in this article.

9. Spring Data REST Pagination

In Spring Data, if we need to return a few results from the complete data set, we can use any Pageable repository method, as it will always return a Page. The results will be returned based on the page number, page size, and sorting direction.

Spring Data REST automatically recognizes URL parameters like page, size, sort etc.

To use paging methods of any repository, we need to extend PagingAndSortingRepository:

public interface SubjectRepository extends PagingAndSortingRepository<Subject, Long>{}

If we call http://localhost:8080/subjects, Spring automatically adds the page, size, sort parameter suggestions with the API:

"_links" : {
  "self" : {
    "href" : "http://localhost:8080/subjects{?page,size,sort}",
    "templated" : true
  }
}

By default, the page size is 20, but we can change it by calling something like http://localhost:8080/subjects?page=10.

If we want to implement paging into our own custom repository API, we need to pass an additional Pageable parameter and make sure that API returns a Page:

@RestResource(path = "nameContains")
public Page<Subject> findByNameContaining(@Param("name") String name, Pageable p);

Whenever we add a custom API, a /search endpoint gets added to the generated links. So if we call http://localhost:8080/subjects/search, we'll see a pagination capable endpoint:

"findByNameContaining" : {
  "href" : "http://localhost:8080/subjects/search/nameContains{?name,page,size,sort}",
  "templated" : true
}

All APIs that implement PagingAndSortingRepository will return a Page. If we need to return the list of the results from the Page, the getContent() API of Page provides the list of records fetched as a result of the Spring Data REST API.

10. Convert a List into a Page

Let's suppose that we have a Pageable object as input, but the information that we need to retrieve is contained in a list instead of a PagingAndSortingRepository. In these cases, we may need to convert a List into a Page.

For example, imagine that we have a list of results from a SOAP service:

List<Foo> list = getListOfFooFromSoapService();

We need to access the list in the specific positions specified by the Pageable object sent to us. So let's define the start index:

int start = (int) pageable.getOffset();

And the end index:

int end = (int) ((start + pageable.getPageSize()) > fooList.size() ? fooList.size()
  : (start + pageable.getPageSize()));

Having these two in place, we can create a Page to obtain the list of elements between them:

Page<Foo> page 
  = new PageImpl<Foo>(fooList.subList(start, end), pageable, fooList.size());

That's it! We can now return page as a valid result.

And note that if we also want to give support for sorting, we need to sort the list before sub-listing it.

11. Conclusion

This article illustrated how to implement Pagination in a REST API using Spring, and discussed how to set up and test Discoverability.

If we want to go in depth on pagination in the persistence level, we can check out the JPA or Hibernate pagination tutorials.

The implementation of all these examples and code snippets can be found in the GitHub project – this is a Maven-based project, so it should be easy to import and run as it is.

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