Sunday, 12 May 2019

Designing an API with Web API on Azure

.NET core is great for creating a Web API project to provide an API.
Tip: Swagger is a great tool that has an editor to specify your API.  With the OpenAPI specification for your API, you can generate documentation, or use the code generation feature to generate tests or C# code.
Tip:  Use the Swashbuckle.AspNetCore Nugget package to generate an OpenAPI specification for your API if you use a code first approach.

Do not trust incoming parameters, you must validate.  Always respond with generic header error messages so you don't give away back-end information.  Ensure you log all errors and review them.

Preferred High Level Design I like for Web API projects

The API needs to return Status Codes to allow consumers/clients to know how there operations are doing so I design with these standard HTTP status codes:

CodeCode Status Meaning
200OK (GET)
201Created (POST or PUT)
202Accepted.  Used for long running operations.
204Resource has been deleted (DELETE)
304Not Modified
400Bad Request
401Not Authorized
403Forbidden
404Not Found
422Entity Validation Issue
429Too many Requests (Implement at APIM layer only)
500Internal Error
    



  • The bold codes are the minimum set I will use for a simple API.
  • Name EndPoints with nouns and I always use lowercase e.g. /api/customers is better than verb noun which was common a few years ago, e.g., /api/GetAllCustomers
  • Always use HTTPS.
  • Sensitive information must be in the header or body, not in the URL as it is not encrypted.

HTTP Methods to Support
GETReturn the current value of an object
PUTReplace or update an object
DELETEDelete an object
POSTCreate a new object.  Return 201 for created and 202 for long running operations.
* PATCH - is also used for updating objects but normally a subsets of existing data

Versioning
API's need to change but there can be multiple client applications consuming my OpenAPI.  We use versioning to ensure I don't break existing implementations.  There are different strategies for versioning API's:
  • No versioning:   https://pbeck.co.uk/customers/35
  • URI versioning: https://pbeck.co.uk/v2/customers/35
  • Query string versioning: https://pbeck/customers/35?version=2
  • Header Versioning:  Not in the URL but passed in the header (my preferred option)
APIM has a great implementation for versioning.  Generally, adding optional data is pretty safe for API versioning.  You need to give 3rd parties using your API's time to implement the new version before deprecating and versions.

Tip: Enforce versions are present in the header.  No version return bad request.

JSON
JSON property names SHOULD be camelCased.

Query Params
Allow filtering using query param e.g. https://pbeck.co.uk/customers?firstname='paul'
Allow sorting for the most common properties

Paging
Requests must have max return limit e.g. 100 items.  Specifying the limit param if you want to change the size of the dataset to return.

CORS
Cross Origin Request will need to be supported, reply on OAuth for who can use the API.

HATEOAS 
Good way to navigate around your API.  And a principle worth adhering to where possible.  

Authentication and Authorization
API's access should be via OAuth2.0.  It is very easy to hook APIM and underlying App Service (Web API) up to AAD B2C.  Scopes are used for authorization.

Code Review/Quality
"Linting is the automated checking of your source code for programmatic and stylistic errors. This is done by using a lint tool (otherwise known as linter)".   Examples of Linting Tools are Resharper, StyleCopSonarQube, ...

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