How to build an OData Service with Olingo V4

Part 5.1: System Query Options $top, $skip, $count

Introduction

In the present tutorial, we’ll learn how to implement system query options. Query options are used to refine the result of a query. The OData V4 specification document gives the following definition:

“System query options are query string parameters that control the amount and order of the data returned for the resource identified by the URL. The names of all system query options are prefixed with a dollar ($) character.”

Query options are not part of the resource path, they’re appended to the URL after the ?. As an example the URL http://localhost:8080/my/page?example=true has example as query option with the value true.

As an example for a system query option in Odata: When querying the list of products, the order of the returned entries is defaulted by the OData service. However, the user can change the order of the list by specifying the query option $orderby.

Examples for system query options that are commonly used:

The present tutorial focuses on the first three query options: $top, $skip and $count

Examples

The following example calls are based on our sample service and illustrate the usage of these 3 query options.

First, just to remember, the “normal” query of the product without query options: http://localhost:8080/DemoService/DemoService.svc/Products

AllProductsNoQueryOption

The following URL provides only the first 2 entries and ignores all the rest: http://localhost:8080/DemoService/DemoService.svc/Products?$top=2

ProductsWith$top

The following request returns the products starting with the 3rd and ignores the first 2 entries: http://localhost:8080/DemoService/DemoService.svc/Products?$skip=2

ProductsWith$skip

The following request returns the total number of products and includes it in the payload: http://localhost:8080/DemoService/DemoService.svc/Products?$count=true

ProductsWith$count

Note: TThe final source code can be found in the project git repository. A detailed description how to checkout the tutorials can be found here. This tutorial can be found in subdirectory \samples\tutorials\p5_queryoptions-tcs

Disclaimer: Again, in the present tutorial, we’ll focus only on the relevant implementation, in order to keep the code small and simple. The sample code shouldn’t be reused for advanced scenarios.

Table of Contents

  1. Prerequisites
  2. Preparation
  3. Implementating system query options
    1. Implement $count
    2. Implement $skip
    3. Implement $top
  4. Run the implemented service
  5. Summary
  6. Links

1. Prerequisites

Same prerequisites as in Tutorial Part 1: Read Entity Collection and Tutorial Part 2: Read Entity as well as basic knowledge about the concepts presented in both tutorials.


2. Preparation

Follow Tutorial Part 1: Read Entity Collection and Tutorial Part 2: Read Entity or as shortcut import the project attached to Tutorial Part 2 into your Eclipse workspace.

Afterwards do a Deploy and run: it should be working.


3. Implementing system query options

The system query options we’re focusing on are applied to the entity collection only (for example, it doesn’t make sense to apply a $top to a READ request of a single entity)

Therefore our implementation for all three query options is done in the class myservice.mynamespace.service.DemoEntityCollectionProcessor

The general sequence of the implementation remains unchanged:

  1. Analyze the URI
  2. Fetch data from backend
  3. Serialize
  4. Configure the response

The only difference is that we apply the query options after getting the data from the backend (our database-mock). So the procedure will be:

  1. Analyze the URI
  2. Fetch data from backend
  3. Apply all system query options
  4. Serialize
  5. Configure the response

The following sections describe how such system query options are implemented. The procedure will be similar in all 3 cases:

  1. Get the query option from the UriInfo. If null is returned then nothing has to be done.
  2. Get the value from the query option
  3. Analyze the value
  4. Modify the EntityCollection

3.1. Implement $count

Background The $count allows users to request a count of the matching resources. The number will be included with the resources in the response (see screenshot above).

The user specifies the $count as follows: $count=true $count=false

If the value of $count is set to false, then no number is returned, the same like if $count is not specified at all. However, this case has to be considered in our code as well.

Note: For those who are used to OData V2: In V2, the query option $inlinecount has now been replaced in V4 by $count=true. In V2, the /$count that was part of the resource path, has now been removed in V4.

There’s one more important detail that we have to consider before writing the code: $count always returns the original number of entities, without considering $top and $skip. This is specified by the OData V4 specification:

“The $count system query option ignores any $top, $skip, or $expand query options, and returns the total count of results across all pages including only those results matching any specified $filter and $search.”

Therefore, in our sample code, the $count will be the first to be implemented, to make sure that the data provided by the backend is not modified at the moment when we "count" it.

Implementation As in the previous tutorials, the data is fetched from the backend. It is provided as EntityCollection which we can ask for the list of contained Entity instances. The size of this genuine list is the relevant information for our $count. Furthermore, we create a new instance of an EntityCollection object, which will carry the modified list of entities after applying all the query options.

EntityCollection entityCollection = storage.readEntitySetData(edmEntitySet);
List<Entity> entityList = entityCollection.getEntities();
EntityCollection returnEntityCollection = new EntityCollection();

Then we proceed with the 4 steps as described above:

  1. Get the query option from the UriInfo. If null is returned then nothing has to be done.
  2. Get the value from the query option
  3. Analyze the value
  4. Modify the EntityCollection

And this is the sample code:

CountOption countOption = uriInfo.getCountOption();
if (countOption != null) {
    boolean isCount = countOption.getValue();
    if(isCount){
        returnEntityCollection.setCount(entityList.size());
    }
}

Note: We don’t need to check if the value of the $count is incorrect (e.g. $count=xxx), as this is handled by the Olingo OData V4 library.

One additional step has to be considered: As we know, if $count=true is specified, the structure of the response payload is different. So we have to inform the serializer.that $count has to be considered. So we have to modify the line of code, where the EntityCollectionSerializerOptions is created:

EntityCollectionSerializerOptions opts = EntityCollectionSerializerOptions.with()
                                         .contextURL(contextUrl)
                                         .id(id)
                                         .count(countOption)
                                         .build();

Furthermore, we have to change the following line, because the EntityCollection to be returned is now different;

SerializerResult serializerResult = serializer.entityCollection(serviceMetadata,
                                    edmEntityType,
                                    returnEntityCollection,
                                    opts);

3.2. Implement $skip

Background With the query option $skip, the user of an OData service can specify the number of entries that should be ignored at the beginning of a collection. So if a user specifies $skip=n then our OData service has to return the list of entries starting at position n+1

One important rule that we have to consider is described by the OData V4 specification:

“Where $top and $skip are used together, $skip MUST be applied before $top, regardless of the order in which they appear in the request.”

This means for us that we add the code for $skip before the code for $top.

Implementation

Again we follow the 4 mentioned steps. We get the SkipOption object from the UriInfo. If the SkipOption is null, then it hasn’t been specified by the user. Since it is not mandatory to specify any query option, we can ignore the case of SkipOption being null. We ask the SkipOption object for the value that has been specified by the user. Since the user might give invalid numbers, we have to check that and throw an exception with HTTP status as “Bad Request”. Then we can do the actual job, which is adapting the backend-data according to the specified $skip.

SkipOption skipOption = uriInfo.getSkipOption();
if (skipOption != null) {
    int skipNumber = skipOption.getValue();
    if (skipNumber >= 0) {
        if(skipNumber <= entityList.size()) {
            entityList = entityList.subList(skipNumber, entityList.size());
        } else {
            // The client skipped all entities
            entityList.clear();
        }
    } else {
        throw new ODataApplicationException("Invalid value for $skip", HttpStatusCode.BAD_REQUEST.getStatusCode(), Locale.ROOT);
    }
}

After applying the query option, we have the desired set of entities in the variable entityList. Now we have to populate the EntityCollection instance, that we created in the section above, with these entities, before we can pass it to the serializer:

for(Entity entity : entityList){
    returnEntityCollection.getEntities().add(entity);
}

3.3. Implement $top

Background With the query option $top, the user of an OData service can specify the maximum number of entries that should be returned, starting from the beginning.

Implementation

Again we follow the 4 mentioned steps, the code is very similar, only the logic for reducing the entityList is different:

TopOption topOption = uriInfo.getTopOption();
if (topOption != null) {
    int topNumber = topOption.getValue();
    if (topNumber >= 0) {
        if(topNumber <= entityList.size()) {
            entityList = entityList.subList(0, topNumber);
        }  // else the client has requested more entities than available => return what we have
    } else {
        throw new ODataApplicationException("Invalid value for $top", HttpStatusCode.BAD_REQUEST.getStatusCode(), Locale.ROOT);
    }
}

So now we can finally have a look at the full implementation of the readEntityCollection() method, containing all the three query options:

public void readEntityCollection(ODataRequest request, ODataResponse response, UriInfo uriInfo, ContentType responseFormat)
                                throws ODataApplicationException, SerializerException {

    // 1st retrieve the requested EntitySet from the uriInfo
    List<UriResource> resourcePaths = uriInfo.getUriResourceParts();
    UriResourceEntitySet uriResourceEntitySet = (UriResourceEntitySet) resourcePaths.get(0);
    EdmEntitySet edmEntitySet = uriResourceEntitySet.getEntitySet();

    // 2nd: fetch the data from backend for this requested EntitySetName
    EntityCollection entityCollection = storage.readEntitySetData(edmEntitySet);

    // 3rd: apply System Query Options
    // modify the result set according to the query options, specified by the end user
    List<Entity> entityList = entityCollection.getEntities();
    EntityCollection returnEntityCollection = new EntityCollection();

    // handle $count: return the original number of entities, ignore $top and $skip
    CountOption countOption = uriInfo.getCountOption();
    if (countOption != null) {
        boolean isCount = countOption.getValue();
        if(isCount){
            returnEntityCollection.setCount(entityList.size());
        }
    }

    // handle $skip
    SkipOption skipOption = uriInfo.getSkipOption();
    if (skipOption != null) {
        int skipNumber = skipOption.getValue();
        if (skipNumber >= 0) {
            if(skipNumber <= entityList.size()) {
                entityList = entityList.subList(skipNumber, entityList.size());
            } else {
                // The client skipped all entities
                entityList.clear();
            }
        } else {
            throw new ODataApplicationException("Invalid value for $skip", HttpStatusCode.BAD_REQUEST.getStatusCode(), Locale.ROOT);
        }
    }

    // handle $top
    TopOption topOption = uriInfo.getTopOption();
    if (topOption != null) {
        int topNumber = topOption.getValue();
        if (topNumber >= 0) {
            if(topNumber <= entityList.size()) {
                entityList = entityList.subList(0, topNumber);
            }  // else the client has requested more entities than available => return what we have
        } else {
            throw new ODataApplicationException("Invalid value for $top", HttpStatusCode.BAD_REQUEST.getStatusCode(), Locale.ROOT);
        }
    }

    // after applying the query options, create EntityCollection based on the reduced list
    for(Entity entity : entityList){
        returnEntityCollection.getEntities().add(entity);
    }

    // 4th: create a serializer based on the requested format (json)
    ODataSerializer serializer = odata.createSerializer(responseFormat);

    // and serialize the content: transform from the EntitySet object to InputStream
    EdmEntityType edmEntityType = edmEntitySet.getEntityType();
    ContextURL contextUrl = ContextURL.with().entitySet(edmEntitySet).build();

    final String id = request.getRawBaseUri() + "/" + edmEntitySet.getName();
    EntityCollectionSerializerOptions opts = EntityCollectionSerializerOptions.with()
                                                            .contextURL(contextUrl)
                                                            .id(id)
                                                            .count(countOption)
                                                            .build();
    SerializerResult serializerResult = serializer.entityCollection(serviceMetadata, edmEntityType,
                                                                    returnEntityCollection, opts);

    // 5th: configure the response object: set the body, headers and status code
    response.setContent(serializedContent);
    response.setStatusCode(HttpStatusCode.OK.getStatusCode());
    response.setHeader(HttpHeader.CONTENT_TYPE, responseFormat.toContentTypeString());
}

4. Run the implemented service

After building and deploying your service to your server, you can try the following URLs:


5. Summary

In this tutorial we have learned how enhance our OData service to support system query options. In a first step, we’ve covered $top, $skip and $count. More system query options will be treated in the subsequent tutorials.


6. Links

Tutorials

Code and Repository

Further reading

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