Usage of HTTP Methods

Last modified 13 Mar 2024 13:32 +01:00

Use of HTTP Verbs

MidPoint REST API tries to adhere to the usual HTTP verbs and tries to maintain their original meaning:

Verb	Meaning and usage
GET	Information retrieval. Returns the state of the web resource. Also used to execute searches.
PUT	Create new web resource. This usually means creation of a new object. However, the client has to provide a complete object, including the identifier (URI). Which is not entirely practical, as this means that the client needs to know how to properly construct the URI. Therefore this operation is seldom used.
POST	POST is used for several purposes: Create new web resources without the need to know the URI. POST to a collection web resource will result that a new sub-resource is created. As an equivalent for PATCH operation (see below). Some HTTP clients have problems when using non-standard verbs. Therefore we provide POST as an alternative way to modify objects. To pass data to an RPC-like operation.
PATCH	Modification of existing web resource. Usually modification of existing midPoint object. This is non-standard HTTP verb. The usage of PATCH is preferred because its meaning is much more clear than the desperately overloaded meaning of POST. However, POST is provided as an equivalent to PATCH for HTTP clients that have problems with non-standard verbs.
DELETE	Delete existing resource. This is used mostly to delete objects.

Verb

Meaning and usage

GET

Information retrieval. Returns the state of the web resource. Also used to execute searches.

PUT

Create new web resource. This usually means creation of a new object. However, the client has to provide a complete object, including the identifier (URI). Which is not entirely practical, as this means that the client needs to know how to properly construct the URI. Therefore this operation is seldom used.

POST

POST is used for several purposes:

Create new web resources without the need to know the URI. POST to a collection web resource will result that a new sub-resource is created.
As an equivalent for PATCH operation (see below). Some HTTP clients have problems when using non-standard verbs. Therefore we provide POST as an alternative way to modify objects.
To pass data to an RPC-like operation.

PATCH

Modification of existing web resource. Usually modification of existing midPoint object. This is non-standard HTTP verb. The usage of PATCH is preferred because its meaning is much more clear than the desperately overloaded meaning of POST. However, POST is provided as an equivalent to PATCH for HTTP clients that have problems with non-standard verbs.

DELETE

Delete existing resource. This is used mostly to delete objects.

Creation of New Resources

According to REST architectural style the web resources should be created by PUT operation. However, this assumes that the client knows the URI and therefore it is not very practical. E.g. URI of most midPoint objects contains Object ID, which may or may not be provided by the client. In case that the OID is server-generated the PUT operation is out of question. But even if the OID is client-generated this would expect that the client knows how to construct a URI, which is not very RESTful. Therefore the PUT operation is not very practical.

The usual way how to create a new object in midPoint REST API is to use POST operation to a collection resource. E.g. new user is created by posting the user object to "users" collection (…/ws/rest/users). MidPoint will react by creating a new user object and publishing that as a new web resource (with new URI). MidPoint REST API will respond with and redirection HTTP status (3xx) pointing client to the new URI.

Synchronous and Asynchronous Operations

Most midPoint operations are currently implemented in a synchronous fashion. When talking about REST API this means that they will not return HTTP response until they are finished. When the operation is finished, midPoint will respond with success (2xx) or error (4xx, 5xx). If new resource is created, redirect status (3xx) is indicated. Retrieving that resource should result in success (2xx).

There are also some operations that are executed asynchronously. In that case midPoint returns HTTP response as soon as the operation is initiated. The status in the response will indicate redirection (3xx). The redirection target is a task that can be used to track operation progress.

Return Values

GET operations are supposed to return data. GET invoked on REST-like resources returns what anyone would probably expect: either individual midPoint object or a collection of midPoint objects. All such resources are supposed to behave in the same way, regardless of their type. PUT/POST/PATCH/DELETE for those resources also works in the same way. It returns HTTP status code or indicates an error or asynchronous operation (see below).

However, the situation is very different for the RPC-like resources. The indication of error and asynchronous processing is the same here. But all the rest is different. Those operations usually take data using the body of a POST request. And the data format may be completely different for each operation. The output also differs a lot. Success/failure is always indicated by the HTTP error code. But the data returned on success vary according to the definition of the specific web resource that is used. This is quite expected as those are essentially procedure calls.

Operation success and errors are always indicated by the HTTP error code. MidPoint REST API maintains the original HTTP meaning of the error code classes:

Error code	Meaning
1xx	Information. Stay tuned, operation is in progress.
2xx	Success. Operation finished successfully. There are two custom codes: 250 for partial error which means that during processing some error occurred but some changes was executed. 240 for handled error which means that there was originally error, but midPoint was able to fix this using its consistency mechanism. In this two cases, midPoint returns the OperationResult where there details of executed operations can be found.
3xx	Redirection or "in progress". This is returned mostly to indicate two cases: Operation has finished, but the results are not in this resource. Redirection is issues to point the client to the results. Typical example is creation of an object with POST to a collection web resource. In this case a new object is created, new URI is assigned and the client is redirected to this URI. Retrieving that URI provides created object and 200 status code, indicating that the operation is finished. Operation is in progress (asynchronous operation). In this case midPoint API redirects the client to a task object that can be used to track progress of the operation.
4xx	Client errors. The client has sent request that cannot be processed. This indicates usual situation that are well handled by the midPoint code. Maybe the client request to create conflicting object, delete non-existent object, modification that violates the schema and so on. The OperationResult structure is usually provided in the response to provide more details about the situation.
5xx	Server errors. Situations that the server cannot handle and where the cause is unknown. This usually means bugs in the code, insufficient resources such as RAM or disk space, unforeseen failures in the infrastructure and so on. The OperationResult structure might or might not be provided in this case. Some errors are so severe that the structured error information might not be available.

Error code

Meaning

1xx

Information. Stay tuned, operation is in progress.

2xx

Success. Operation finished successfully. There are two custom codes:

250 for partial error which means that during processing some error occurred but some changes was executed.
240 for handled error which means that there was originally error, but midPoint was able to fix this using its consistency mechanism.

In this two cases, midPoint returns the OperationResult where there details of executed operations can be found.

3xx

Redirection or "in progress". This is returned mostly to indicate two cases:

Operation has finished, but the results are not in this resource. Redirection is issues to point the client to the results. Typical example is creation of an object with POST to a collection web resource. In this case a new object is created, new URI is assigned and the client is redirected to this URI. Retrieving that URI provides created object and 200 status code, indicating that the operation is finished.
Operation is in progress (asynchronous operation). In this case midPoint API redirects the client to a task object that can be used to track progress of the operation.

4xx

Client errors. The client has sent request that cannot be processed. This indicates usual situation that are well handled by the midPoint code. Maybe the client request to create conflicting object, delete non-existent object, modification that violates the schema and so on. The OperationResult structure is usually provided in the response to provide more details about the situation.

5xx

Server errors. Situations that the server cannot handle and where the cause is unknown. This usually means bugs in the code, insufficient resources such as RAM or disk space, unforeseen failures in the infrastructure and so on. The OperationResult structure might or might not be provided in this case. Some errors are so severe that the structured error information might not be available.

Status codes and the indication of errors and asynchronous processing applies uniformly to all midPoint web resources (both RESTful and RPC).

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