I'm reviewing OData again as I would like to use it in a new Rest project with EF but I have the same concerns I had a few years back.
Exposing a general IQueryable can be quite dangerous. Restricting potentially expensive queries must be done elsewhere. DB, connection level.
OData doesn't allow any interception/customisation of the behaviour by developers as it sits outside the interface.
OData doesn't play well with DI in general. While it is possible to DI an alternative IQueryable you can't intercept the OD calls and check, amend or modify.
My suggestion is that the tool be broken down into more distinct elements to allow far greater customisation and re-use. Break open the black box :) It would also be better in terms of single responsibility. Would it be possible to have components that did the following
Expression generators from urls. Converts OData urls extensions into typed expressions usable with an IQueryable but independent of it. Generate T => Expression<Func<T, bool>> for a where for example. This would be a super useful stand alone component and support OData url formats being used more widely as a standard.
An EF Adaptor to attach the expressions to an EF context. An EF Adaptor to attach the expressions to an EF context or use in any other DI'ed code. So rather than exposing a public IQueryable the service can encapsulate an interface and get the benefits of OData functionality. Rest Get -> Expression Generation -> Map to IQueryable.
This approach would allow developers to intercept the query calls and customise the behaviour if required while maintaining the ease of use for simple cases. We could embed OData and EF within repository patterns where we add our own functionality.
CodePudding user response：
There is a lot of misunderstanding in your post, it's not really well suited to this site, but it is a recurring line of speculation that does need to be addressed.
- OData doesn't play well with DI in general. While it is possible to DI an alternative IQueryable you can't intercept the OD calls and check, amend or modify.
This statement is just not accurate at all, not on the DI topic or the query interception. To go into detail is too far out of scope as there are many different ways to achieve this, it would be better to post a specific scenario that you are challenged by and we can post a specific solution.
- Exposing a general IQueryable can be quite dangerous. Restricting potentially expensive queries must be done elsewhere. DB, connection level.
Exposing raw IQueryable as a concept has some inherent dangers if you do not put in any restrictions, but in OData we are not exposing the IQueryable to the public at all, only a cut-down subset of it and we get granularity over which fields are available for projecting, filtering or sorting. The IQueryable method response is itself fundamental to many of the features that drive us to choose OData in the first place.
However, you do not need to expose IQueryable at all! You can return IEnumerable instead, but by doing so you will need to load enough data into memory to satisfy the query request, if you want to fulfil it that is. There are extension points to help you do this as well as tools to parse the URL query parameters into simple strings or an expression tree that you can apply to your own data models.
The EnableQueryAttribute is an Action Filter that will compose a LINQ query over the results from your controller endpoints to apply any $filter criteria or $select/$expand projections or even $apply aggregations.
- OData doesn't allow any interception/customisation of the behaviour by developers as it sits outside the interface.
EnableQueryAttribute is about as close to a Black Box as you can find in OData, but the OData Libraries are completely open source and you can extend or override the implementation or omit the attribute altogether. If you do so (omit it), you will then need to process and format the response to be OData compliant. The specification allows for a high degree of flexibility, the major caveat is that you need to make sure the $metadata document describes the inputs and outputs.
- The very nature of the ASP request processing pipeline means that we can inject all sorts of middleware implementations at many different points, we can even implement our own custom query options or we pass the query through the request body if we need to.
If your endpoints do NOT return IQueryable, then the LINQ composition in the EnableQueryAttribute can only operate over the data that is in the IEnumerable feed. A simple example of the implication of this is if the URL Query includes a $select parameter for a single field, something like this:
http://my.service.net/api/products(101)?$select=Description
If you are only exposing IEnumerable, then you must manually load the data from the underlying store. You can use the ODataQueryOptions class to access the OData arguments through a structured interface, the specific syntax will vary based on your DAL, ORM and the actual Model of course. However, like most Repository or MVC implementations, many implementations that do not use IQueryable will default to simply loading the entire object into memory instead of the specifically requested fields, they might end up loading the results from this comparative SQL query:
SELECT * FROM Product WHERE Id = @Id
If this Product has 20 fields, then all that data will be materialised into memory to service the request, even though only 1 field was requested. Even without using IQueryable, OData still has significant benefits here by reducing the bytes being sent across the wire to the client application. This reduces costs but also the time it will take to fulfill a request.
By comparison, if the controller method returned an IQueryable expression that had been deferred or not yet materialised, then the final SQL that gets executed could be something much more specific:
SELECT Description FROM Product WHERE Id = @Id
This can have significant performance benefits, not just in the SQL execution but in the transport between the data store and the service layer as well as the serialization of the data that is received.
- Serialization is often taken for granted as a necessary aspect of API development, but that doesn't mean there is no room to improve the process. In the cloud age where we pay for individual CPU cycles there is a lot of wasted processing that we can reclaim by only loading the information that we need, when we need it.
To fully realise the performance gains requires selective data calls from the Client. If the end client makes a call to explicitly request all fields, then there should be no difference between OData and a traditional API approach, but with OData the potential is there to be realized.
If the controller is exposing a complex view, so not a traditional table, then there is even more significance in supporting IQueryable. For custom business DTOs (views) that do not match the underlying storage model we are often forced to compromise between performance practicalities and data structures. Without OData that allows for the caller to trim the data schema, it is common for APIs to either implement some fully dynamic endpoints, or to see a sprawl of similar DTO models that have restricted scope or potentially single purpose. OData provides a mechanism to expose a single common view that has more metadata than all callers need, while still allowing individual callers to only retrieve the sub-set that they need.
In aggregate views you can end up with some individual columns adding significant impact on the overall query execution, in traditional REST APIs this becomes a common justification for having similar DTO models, with OData we can define the view once and give the callers flexibility to choose when the extra data, that comes with a longer response wait time, should be queried, and when it should not.
- OData provides a way to balance between being 100% generic with your DTOs or resorting to single use DTOs.
The flexibility provided by OData can significantly reduce the overall time to market by reducing the iterative evolution of views and complex types that often comes up as the front-end development teams start to consume your services. The nature of IQueryable and the conventions offered by the OData standard means that there is potential for front-end work to begin before the API is fully implemented
This was a very simple and contrived example, we didn't yet cover $expand or $apply that can lead to very memory intensive operations to support. I will however quickly talk about $count, it is a seemingly simple requirement, to return a count of all records for a specific criteria or for no criteria at all. An OData IQueryable implementation requires no additional code and has almost zero processing to service this request as it can be passed entirely to the underlying data store in the form of a SELECT COUNT(*) FROM...
- With OData and the OData Libraries, we get a lot of functionality and flexibility OOTB, but the default functionality is just the start, you can extend your controllers with additional Functions and Actions and views as you need to.
Regarding the Dangers of IQueryable...
A key argument against exposing IQueryable from the DbContext is that it might allow callers to access more of your database than you might have intended. OData has a number of protections against this. The first is that for each field in the entire schema you can specify if the field is available at all, can be filtered, or can be sorted.
The next level of protection is that for each endpoint we can specify the overall expansion depth, by default this is 2.
It is worth mentioning that it is not necessary to expose your data model directly through OData, if your domain model is not in-line with your data model, it may be practical to only expose selected views through the OData API, or only a sub-set of tables in your schema.
Opening the Black Box
Expression generators from urls. Converts OData urls extensions into typed expressions usable with an IQueryable but independent of it. Generate T => Expression<Func<T, bool>> for a where for example.
This is a problematic concept, if you're not open to IQueryable ... That being said, you can use open types and can have a completely dynamic schema that you can validate in real-time or derived from the query routes entirely. There is not a lot of published documentation on this, mainly due to the scenarios where you want to implement this are highly specific, but it's not hard to sort out. With a specific scenario in mind we can post specific implementation advice...
An EF Adaptor to attach the expressions to an EF context. An EF Adaptor to attach the expressions to an EF context or use in any other DI'ed code. So rather than exposing a public IQueryable the service can encapsulate an interface and get the benefits of OData functionality. Rest Get -> Expression Generation -> Map to IQueryable.
To configure OData, you need to specify the structure of the Entities that the OData Model exposes. There are convention based mappers provided OOTB that can help you to expose an OData model that is close to 1:1 representation of an Entity Framework DbContext model with minimal code, but OData is not dependant on EF at all. The only requirement is that you define the DTO models, including the actions and functions, but how you do this is up to you.
I wouldn't recommend it, but I have seen some implementations that use AutoMapper to map between the EF Model to DTOs, and then the DTOs are mapped to the OData Entity model.
The usual approach when you want to Support OData Query Options in ASP.NET Web API 2 without fully implementing the OData API is to use the EnableQueryAttribute in your standard API, it is after all just an Action Filter... and an example of how the OData libraries are already packaged in a way that you can implement OData query conventions within other API patterns.
CodePudding user response：
Thanks for the reply. Your time is much appreciated. This isn't really an answer rather a design discussion. Apologies if it's not appropriate here.
I'm no expert with this tech so I may also be missing options. Manually exposing IEnumerables instead of IQuerable seems to require much more coding if I'm reading that suggestion correctly. It would also lead to service processing of the data after the database queries. The idea of custom actions on a custom IQuerable may also be worth some investigation.
An example of not playing well with DI...If we have an http context user with the token who has some limits re the queries they can do. We would like to take the user info from the http context and restrict the db context queries they do. Maybe the user can only see certain business units or certain clients. There are many other use-cases.
It should be possible to append/amend queries before presented to the database with the extra user context. This is where the decomposition idea comes in. If OData could generate a structure (lamda expression may not be good here either) that can be manipulated we can have the best of both by manipulating before the query execution.
The $filter, $extends concepts could be added more generally to interfaces that would allow the database to be better encapsulated. The interface applies the filter behind the scenes. The OData implementation could make the filters available on the context rather than applying the results 'outside' of the controller.
It would be interesting to know what you mean by "a problematic concept". This model seems so natural to me. I'm amazed it doesn't work this way already.