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There are two main types of web-services: REST (REpresentational State Transfer) and WSDL (Web Services Description Language) /SOAP.

REST services

REST services use the HTTP protocol. As described on wikipedia, a common mapping of services is:

Resource

GET

PUT

POST

DELETE

Collection URI, such as http://example.com/resources/

List the URIs and perhaps other details of the collection's members.

Replace the entire collection with another collection.

Create a new entry in the collection. The new entry's URL is assigned automatically and is usually returned by the operation.

Delete the entire collection.

Element URI, such as http://example.com/resources/item17

Retrieve a representation of the addressed member of the collection, expressed in an appropriate Internet media type.

Replace the addressed member of the collection, or if it doesn't exist, create it.

Treat the addressed member as a collection in its own right and create a new entry in it.

Delete the addressed member of the collection.

POST is also commonly used as a means of performing a complex get or search operation. The POSTed data specifies the parameters to the search

Tools to build REST services

The Java API for XML Web Services (JAX-WS) provides full support for building and deploying REST Web services. It is tightly integrated with the Java Architecture for XML Binding (JAXB) for binding XML to Java data and is included in Java 6.

Some guidelines on building REST Web services can be found online.

WSDL/SOAP Web services

Web Service Description Language (WSDL) Web services operate by exchanging Simple Object Access Protocol (SOAP) messages with clients over HTTP.

WSDL Web services are defined by their WSDL document, an XML format that represents an interface to a Web service. WSDL is machine-readable description of the operations (functions) and parameters offered by the service, i.e. XML message types that the service receives and produces and that get wrapped in SOAP messages exchanged between the client and the service.

WSDL binding describes how a particular Web service is bound to the underlying SOAP messaging protocol (or any other protocol used as a carrier). A WSDL-SOAP binding can be either a Remote Procedure Call (RPC)-style binding or a document-style binding. A SOAP binding can also have an encoded use or a literal use. This gives four style/use models:

  • RPC/encoded
  • RPC/literal
  • Document/encoded
  • Document/literal

There is also a fifth binding style that is commonly referred to as the document/literal wrapped. Thus, developers have five binding styles to choose from when creating a WSDL file. A good description of the differences between these styles can be found online.

Web services in eScience

     “Use of standard data file formats should be encouraged; use of SOAP-based Web Services should be encouraged/enforced; they need to be fine-grained: the XML tags should reflect the structure of the data, making the output ‘parsed on arrival’;  the use of standard data types (tool input and output) should be encouraged, simplifying workflow generation.”
EU ELIXIR ESFRI Project Report http://www.elixir-europe.org

Common service creation mistakes

"Press a button" service publishing

It is very easy to think that providing a service can be done by simply running a tool over existing code. This may "tick the box" of making a service available, but the service is likely to be almost unusable by service consumers. When generating a service, it is necessary to think about what the service consumer needs. The best way to do this is to create the services "contract first" i.e. design the service's WSDL or REST and associated document formats, then from those generate the classes and methods for the service; the service methods are then implemented by calls to the existing code. Alternatively (and more commonly) the existing code should be wrapped and annotated so that consumer-oriented services are generated.

Assumption of knowledge

It is easy for the service provider to assume that service consumers will know almost as much about the service as they do. This assumption leads to lack of documentation, strange features (parameters that change meaning or that should not be used together), changes in the semantics of output and input data.

The service provider should assume that the service consumer's knowledge of the service is minimal. The service consumer merely wants to achieve the task exposed via the service.

Lack of documentation

Many services are poorly described. Even when the services are registered with, for example, the BioCatalogue, only minimal effort is put into documenting them. An undocumented service is an unusable service. The documentation should include, at least,

  • description of the task that the service performs - what it does from a service consumer's point of view
  • what can be used as input, with example values and description of what happens if the value is not specified
  • what will be returned as output, including example values
  • possible error messages, including what they mean from the point of view of the input data and the intended task. The error message should not be described by relation to what has gone wrong in the provider's code

It is very useful if the documentation includes:

  • example workflows (including example input data and results) that use the service
  • other services that work well with the service

Wrong granularity

Services should expose consumer-oriented tasks i.e. what a service consumer wants to do. Services should not expose provider-implemented operations i.e. what the service provider's code does to perform a task.

If the service that is being exposed corresponds to methods or functions within the service provider's code, then the service consumer is forced to use it in a specific order to achieve a task and to process implementation-specific data.

Exposure of implementation data

The internal details of the service implementation should be hidden, as far as possible. For example, just returning a serialization of a Java object is not a sensible or usable response to a service call.

The exposure of even internal ids can cause problems as the services may assume that an internal id is passed in. How is the service consumer meant to know the id?

Wherever possible, implementation classes, data and ids should not be exposed by the service. The service provider should look at what is actually returned by some example service calls and consider if they are actually usable.

Do not return HTML unless it is HTML data

REST GET services are very similar to fetching pages for a web browser. Some service providers simply deliver an HTML document corresponding to what they would return when the corresponding web page is fetched.

Unless the service is requesting a HTML document, the service provider should not return an HTML document. The service should probably return the data that underlies the HTML document.

Because it is possible to use web-content providers as if they are REST services, it is possible that apparent REST services within a workflow will actually be fetching HTML documents. Wherever possible it should be made clear that it is not a "proper" REST service.

Do not specify the format as a parameter of a REST service

The HTTP protocol used by REST has a mechanism for specifying in the message header the format that the data should be returned in. For example:

REST services should not specify the format as part of the URL or message body e.g.

Such a resource should be requested from http://www.example.com/fred with Accept header application/xml.

The use of a format parameter can cause problems. For example, if the service consumer requests "http://www.example.com/fred?format=xml" with "Accept: application/json" then it should be impossible for the transaction to take place.

Be consistent with what is returned

Some REST services return JSON for certain resources, a choice of JSON or XML for other resources, and XML for other resources. This is very confusing for service consumers. Services should be consistent in the formats in which they can return data.

Specify the format

If a service is returning XML then the service provider should specify the xsd or dtd to which the data conforms. Similarly, if the service takes XML then the xsd or dtd of the data should be documented.

If the service is a WSDL service, then the data format should be specified via inclusion and referencing of the xsd within the WSDL. It is not correct to just specify xsd:any.

Try to re-use formats

There are a large number of existing formats in which data can be specified. Wherever possible an existing format should be used by the service. The minimal advantage to the service provider of defining their own format is normally outweighed by maintenance costs; there are also inevitable costs for the service consumer in understanding and processing different formats.

Return the correct errors

Many service providers do not return the correct errors. For example, it is not uncommon for HTTP 500 code (Internal server error) to be returned when attempting to access data that does not exist. Service providers should not only document what errors are returned, but also check (as far as feasible) what happens when unexpected requests are received.

Avoid polymorphism

Polymorphism is when the task performed by the service is determined by one of the parameters to the service call. For a polymorphic service, the validity, meaning or permitted values for other parameters may depend upon the value of the controlling parameter. It is possible to have polymorphic WSDL or REST services.

For example, consider a polymorphic service that allows you to query one of several databases, books or films, then a query may appear as:

where it is not valid to specify the director for a book and the semantics of author switches from writer to screenwriter depending upon the database.

It is far better to have separate resources corresponding to, in this case, the separate databases with the correct parameters specified for the resources.

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