This page is part of the FHIR Specification (v3.0.2: STU 3). The current version which supercedes this version is 5.0.0. For a full list of available versions, see the Directory of published versions . Page versions: R5 R4B R4 R3 R2

2.21.0 RESTful API

FHIR Infrastructure Work Group

Maturity Level: 5

Ballot Status: Trial Use

FHIR is described as a 'RESTful' specification based on common industry level use of the term REST. In practice, FHIR only supports Level 2 of the REST Maturity model as part of the core specification, though full Level 3 conformance is possible through the use of extensions. Because FHIR is a standard, it relies on the standardization of resource structures and interfaces. This may be considered a violation of REST principles but is key to ensuring consistent interoperability across diverse systems.

Each "resource type" has the same set of interactions defined that can be used to manage the resources in a highly granular fashion. Applications claiming conformance to this framework claim to be conformant to "RESTful FHIR" (see Conformance).

Note that in this RESTful framework, transactions are performed directly on the server resource using an HTTP request/response. The API does not directly address authentication, authorization, and audit collection - for further information, see the Security Page.

The API describes the FHIR resources as a set of operations (known as "interactions") on resources where individual resource instances are managed in collections by their type. Servers can choose which of these interactions are made available and which resource types they support. Servers SHALL provide a Capability Statement that specifies which interactions and resources are supported.

The following logical interactions are defined:

In addition to these interactions, there is an operations framework, which includes endpoints for validation, messaging and Documents.

Style Guide

The interactions on this page are defined like this:

  VERB [base]/[type]/[id] {?_format=[mime-type]}

• The first word is the HTTP verb used for the interaction
• Content surrounded by [] is mandatory, and will be replaced by the string literal identified. Possible insertion values:
  • base: The Service Base URL
  • mime-type: The Mime Type
  • type: The name of a resource type (e.g. "Patient")
  • id: The Logical Id of a resource
  • vid: The Version Id of a resource
  • compartment: The name of a compartment
  • parameters: URL parameters as defined for the particular interaction
• Content surrounded by {} is optional

Implementations constructing URLs using these patterns SHOULD conform to RFC 3986 Section 6 Appendix A which requires percent-encoding for a number of characters that occasionally appear in the URLs (mainly in search parameters).

This specification uses the underscore as a prefix to disambiguate reserved names from other names in 3 cases:

• To differentiate system wide history and search interactions from interactions on Resource Types
• To differentiate search, history and similar interactions from instances of a resource type
• To differentiate search parameters defined for all resources from those defined for specific resource types

In addition, the character $ is used as a prefix to operation names that are RPC-like additions to the base API defined either by this specification or by implementers.

2.21.0.1 Service Base URL

The Service Base URL is the address where all of the resources defined by this interface are found. The Service Base URL takes the form of

http(s)://server{/path}

The path portion is optional, and does not include a trailing slash. Each resource type defined in this specification has a manager (or "entity set") that lives at the address /[type] where the [type] is the name of the resource type. For instance, the resource manager for the type Patient will live at:

https://server/path/Patient

All the logical interactions are defined relative to the service root URL. This means that if the address of any one FHIR resource on a system is known, the address of other resources may be determined.

Note: All URLs (and ids that form part of the URL) defined by this specification are case sensitive. Clients SHOULD encode URLs using UTF-8, and servers SHOULD decode them assuming they are UTF-8 (for background, see here ).

Note that a server may use a path of the form http://server/...[xx]... where the [xx] is some variable portion that identifies a particular instantiation of the FHIR API. Typically, the variable id identifies a patient or a user, and the underlying information is completely compartmented by the logical identity associated with [xx]. In this case, the FHIR API presents a patient or user centric view of a record, where authentication/authorization is explicitly granted to the URL, on the grounds that some identifiable user is associated with the logical identity. It is not necessary to explicitly embed the patient id in the URL - implementations can associate an FHIR end-point with a particular patient or provider by using an OAuth login. See Compartments for the logical underpinning.

Identity

Systems often need to compare two URLs to determine whether they refer to the same underlying object or not. For the purposes of this specification, the following rules apply:

• The query part of the URL (anything after ?) is ignored
• The comparison of the document portion of the URL (i.e. not the server/port) is case sensitive
• The protocols http: and https: SHALL NOT be used to refer to different underlying objects
• If a port is specified, then the ports must be identical or the objects are different (due to the prevalence of port mapping and/or interface engines running on different ports). Ports should only be explicit when they have explicit meaning to the server

For example: http://myserver.com/Patient/1 and https://myserver.com/Patient/1 refer to the same underlying object, while http://myserver.com:81/Patient/1 is a distinct entity from either of the above. This does not mean that the two addresses need to be treated the same, or that a server must serve both addresses, or that the content from the two addresses must be identical, but just that if these two addresses have the same identity, and if both are served, they must both represent the same underlying object. Systems are not required to check that this is true. Note: the identity comparison for protocols other than http:/https: is undefined.

2.21.0.2 Resource Metadata and Versioning

Each resource has an associated set of resource metadata elements. These map to the HTTP request and response using the following fields:

Note that the Version Id is considered a "weak" ETag and ETag headers should be prefixed with W/ and enclosed in quotes, for example:

ETag: W/"3141"

2.21.0.3 Security

Using HTTPS is optional, but all production exchange of healthcare data SHOULD use SSL and additional security as appropriate. See HTTP Security for further information. Most operations will require user authentication, and all operations that do are subject to RBAC and/or ABAC , and some operations may depend on appropriate consent being granted.

See the HTTP Security guidance on how to handle access denial response.

Note: to support browser-based client applications, servers SHOULD implement cross-origin resource sharing for the interactions documented here. Experience shows that this is an area where ongoing issues may be expected as security holes are found and closed on an ongoing basis.

2.21.0.4 HTTP Status Codes

This specification makes rules about the use of specific HTTP status codes in particular circumstances where the status codes SHALL map to particular states correctly, and only where the correct status code is not obvious. Other HTTP status codes may be used for other states as appropriate, and this particularly includes various authentication related status codes and redirects. Authentication redirects should not be interpreted to change the location of the resource itself (a common web programming error).

FHIR defines an OperationOutcome resource that can be used to convey specific detailed processable error information. For some combinations of interactions and specific return codes, an OperationOutcome is required to be returned as the content of the response. The OperationOutcome may be returned with any HTTP 4xx or 5xx response, but this is not required - many of these errors may be generated by generic server frameworks underlying a FHIR server.

2.21.0.5 Managing Return Content

In the interests of managing band-width, this specification allows clients to specify what kind of content to return.

2.21.0.5.1 conditional read

Clients may use the If-Modified-Since, or If-None-Match HTTP header on a read request. If so, they SHALL accept either a 304 Not Modified as a valid status code on the response (which means that the content is unchanged since that date) or full content (either the content has changed, or the server does not support conditional request).

Servers can return 304 Not Modified where content is unchanged because the If-Modified-Since date-time or the If-None-Match ETag was specified, or they can return the full content as normal. This optimisation is relevant in reducing bandwidth for caching purposes and servers are encouraged but not required to support this. If servers don't support conditional read, they just return the full content.

2.21.0.5.2 create/update/patch/transaction

These interactions are performed using POST, PUT or PATCH, and it may be appropriate for a server to return either only a status code, or also return the entire resource that is the outcome of the create or update (which may be different to that provided by the client). In the case of transactions this means returning a Bundle with just the Bundle.entry.response populated for each entry, and not the Bundle.entry.resource values.

The client can indicate whether the entire resource is returned using the HTTP return preference :

Prefer: return=minimal
Prefer: return=representation
Prefer: return=OperationOutcome

The first of these asks to return no body. The second asks to return the full resource. The third asks the server to return an OperationOutcome resource containing hints and warnings about the operation rather than the full resource. Servers SHOULD honor this header. In the absence of the header, servers may choose whether to return the full resource or not (but not the OperationOutcome; that should only be returned if explicitly requested). Note that this setting only applies to successful interactions. In case of failure, servers SHOULD always return a body that contains an OperationOutcome resource.

2.21.0.6 Content Types and encodings

The formal MIME-type for FHIR resources is application/fhir+xml or application/fhir+json. The correct mime type SHALL be used by clients and servers:

• XML: application/fhir+xml
• JSON: application/fhir+json
• RDF: text/turtle (only the Turtle format is supported)

Servers SHALL support server-driven content negotiation as described in section 12 of the HTTP specification.

Note: between FHIR DSTU2 and STU3, the correct mime type was changed from application/xml+fhir and application/json+fhir to application/fhir+xml and application/fhir+json. Servers MAY also support the older mime types, and are encouraged to do so to smooth the transtion process.

In order to support various implementation limitations, servers SHOULD support the optional _format parameter to specify alternative response formats by their MIME-types. This parameter allows a client to override the accept header value when it is unable to set it correctly due to internal limitations (e.g. XSLT usage). For the _format parameter, the values xml, text/xml, application/xml, and application/fhir+xml SHALL be interpreted to mean the XML format, the codes json, application/json and application/fhir+json SHALL be interpreted to mean the JSON format, and the codes ttl and text/turtle SHALL be interpreted to mean the Turtle RDF format. In addition, the values html and text/html are allowed.

FHIR uses UTF-8 for all request and response bodies. Since the HTTP specification (section 3.7.1) defines a default character encoding of ISO-8859-1, requests and responses SHALL explicitly set the character encoding to UTF-8 using the charset parameter of the MIME-type in the Content-Type header. Requests MAY also specify this charset parameter in the Accept header and/or use the Accept-Charset header.

Note: the _format parameter does not override the content-type header.

The content type application/x-www-form-urlencoded is also accepted for posting search requests.

If neither the accept header nor the _format parameter are specified, the MIME-type of the content returned by the server is undefined and may vary.

Implementation Note: If a client provides a generic mime type in the Accept header (application/xml, text/json, or application/json), the server SHOULD respond with the requested mime type, using the XML or JSON formats described in this specification as the best representation for the named mime type (though see the note on the Binary resource).

2.21.0.7 Support for Versions

Servers that support this API SHOULD provide full version support - that is, populate and track versionId correctly, support vread, and implement version aware updates. Supporting versions like this allows for related systems to track the correct version of information, and to keep integrity in clinical records. However, many current operational systems do not do this, and cannot easily be re-engineered to do so.

For this reason, servers are allowed to not provide versioning support and this API does not enforce that versioning is supported. Clients may elect to only interact with servers that do provide full versioning support. Systems declare their support for versioning in their Capability Statements, where they can indicate one of three levels for versioning support:

• no-version: Versioning and meta.version is not supported (server) or used (client)
• versioned: Versioning and meta.version is supported (server) or used (client)
• versioned-update: Versioning and meta.version is supported, and version aware updates are used - Version ID must be correct for updates (server) or will be specified (If-match header) for updates (client)

Server should always return the default timezone for date searches in the HTTP Response headers using the Date header. Note: Servers are not required to have a default timezone.

2.21.0.8 read

The read interaction accesses the current contents of a resource. The interaction is performed by an HTTP GET command as shown:

  GET [base]/[type]/[id] {?_format=[mime-type]}

This returns a single instance with the content specified for the resource type. This url may be accessed by a browser. The possible values for the Logical Id ("id") itself are described in the id type. The returned resource SHALL have an id element with a value that is the [id]. Servers SHOULD return an ETag header with the versionId of the resource (if versioning is supported) and a Last-Modified header.

Note: Unknown resources and deleted resources are treated differently on a read: a GET for a deleted resource returns a 410 status code, whereas a GET for an unknown resource returns 404. Systems that do not track deleted records will treat deleted records as an unknown resource. Since deleted resources may be brought back to life, servers MAY include an ETag on the error response when reading a deleted record to allow version contention management when a resource is brought back to life.

In addition, the search parameter _summary can be used when reading a resource:

  GET [base]/[type]/[id] {?_summary=text}

This requests that only a subset of the resource content be returned, as specified in the _summary parameter, which can have the values true, false, text & data. Note that a resource that only contains a subset of the data is not suitable for use as a base to update the resource, and may not be suitable for other uses. The same applies to the _elements parameter - both that it should be supported, and the subset implications. Servers SHOULD define a Resource.meta.tag with the SUBSETTED as a Simple Tag to explicitly mark such resources.

2.21.0.9 vread

The vread interaction preforms a version specific read of the resource. The interaction is performed by an HTTP GET command as shown:

  GET [base]/[type]/[id]/_history/[vid] {?_format=[mime-type]}

This returns a single instance with the content specified for the resource type for that version of the resource. The returned resource SHALL have an id element with a value that is the [id], and a meta.versionId element with a value of [vid]. Servers SHOULD return an ETag header with the versionId (if versioning is supported) and a Last-Modified header.

The Version Id ("vid") is an opaque identifier that conforms to the same format requirements as a Logical Id. The id may have been found by performing a history interaction (see below), by recording the version id from a content location returned from a read or from a version specific reference in a content model. If the version referred to is actually one where the resource was deleted, the server should return a 410 status code.

Servers are encouraged to support a version specific retrieval of the current version of the resource even if they do not provide access to previous versions. If a request is made for a previous version of a resource, and the server does not support accessing previous versions (either generally, or for this particular resource), it should return a 404 Not Found error, with an operation outcome explaining that history is not supported for the underlying resource type or instance.

2.21.0.10 update

The update interaction creates a new current version for an existing resource or creates an initial version if no resource already exists for the given id. The update interaction is performed by an HTTP PUT command as shown:

  PUT [base]/[type]/[id] {?_format=[mime-type]}

The request body SHALL be a Resource with an id element that has an identical value to the [id] in the URL. If no id element is provided, or the value is wrong, the server SHALL respond with an HTTP 400 error code, and SHOULD provide an OperationOutcome identifying the issue. If the request body includes a meta, the server SHALL ignore the provided versionId and lastUpdated values. If the server supports versions, it SHALL populate the meta.versionId and meta.lastUpdated with the new correct values. Servers are allowed to review and alter the other metadata values, but SHOULD refrain from doing so (see metadata description for further information). Note that there is no support for updating past versions - see notes on the history interaction.

A server SHOULD accept the resource as submitted when it accepts the update, and return the same content when it is subsequently read. However systems may not be able to do this; see the note on transactional integrity for discussion. Also, see Variations between Submitted data and Retrieved data for additional discussion around update behavior. Note that update generally updates the whole content of the resource. For partial updates, see patch below.

If the interaction is successful, the server SHALL return either a 200 OK HTTP status code if the resource was updated, or a 201 Created status code if the resource was created, with a Last-Modified header, and an ETag header which contains the new versionId of the resource. If the resource was created (i.e. the interaction resulted in a 201 Created), the server SHOULD return a Location header (this is for HTTP conformance; it's not otherwise needed).

Note: Servers MAY choose to preserve XML comments, instructions, and formatting or JSON whitespace when accepting updates, but are not required to do so. The impact of this on digital signatures may need to be considered.

Note that servers MAY choose to allow clients to PUT a resource to a location that does not yet exist on the server - effectively, allowing the client to define the id of the resource. Whether a server allows this is a deployment choice based on the nature of its relationships with the clients. While many servers will not allow clients to define their ids, there are several reasons why it may be necessary in some configurations:

• client is reproducing an existing data model on the server, and needs to keep original ids in order to retain ongoing integrity
• client is a server doing push based pub/sub (this is a special case of the first reason)
• multiple clients doing push in the context of agreed data model shared across multiple servers where ids are shared across servers

Alternatively, clients may be sharing an agreed identification model (e.g. key server, scoped identifiers, or UUIDs) where clashes do not arise.

Servers can choose whether or not to support client defined ids, and indicate such to the clients using CapabilityStatement.rest.resource.updateCreate.

2.21.0.10.1 Rejecting Updates

Servers are permitted to reject update interactions because of integrity concerns or other business rules, and return HTTP status codes accordingly (usually a 422).

Common HTTP Status codes returned on FHIR-related errors (in addition to normal HTTP errors related to security, header and content type negotiation issues):

• 400 Bad Request - resource could not be parsed or failed basic FHIR validation rules (or multiple matches were found for conditional criteria)
• 401 Not Authorized - authorization is required for the interaction that was attempted
• 404 Not Found - resource type not supported, or not a FHIR end-point
• 405 Method Not allowed - the resource did not exist prior to the update, and the server does not allow client defined ids
• 409/412 - version conflict management - see below
• 422 Unprocessable Entity - the proposed resource violated applicable FHIR profiles or server business rules

Any of these errors SHOULD be accompanied by an OperationOutcome resource providing additional detail concerning the issue.

For additional information on how systems may behave when processing updates, refer to the Variations between Submitted data and Retrieved data page.

2.21.0.10.2 Conditional update

The conditional update interaction allows a client to update an existing resource based on some identification criteria, rather than by logical id. To accomplish this, the client issues a PUT as shown:

  PUT [base]/[type]?[search parameters]

When the server processes this update, it performs a search using its standard search facilities for the resource type, with the goal of resolving a single logical id for this request. The action it takes depends on how many matches are found:

• No matches: The server performs a create interaction
• One Match: The server performs the update against the matching resource
• Multiple matches: The server returns a 412 Precondition Failed error indicating the client's criteria were not selective enough

This variant can be used to allow a stateless client (such as an interface engine) to submit updated results to a server, without having to remember the logical ids that the server has assigned. For example, a client updating the status of a lab result from "preliminary" to "final" might submit the finalized result using PUT path/Observation?identifier=http://my-lab-system|123

Note that transactions and conditional create/update/delete are complex interactions and it is not expected that every server will implement them. Servers that don't support the conditional update should return an HTTP 400 error and an OperationOutcome.

2.21.0.11 Managing Resource Contention

Lost Updates , where two clients update the same resource, and the second overwrites the updates of the first, can be prevented using a combination of the ETag and If-Match header. This is also known as 'Optimistic Locking'.

To support this usage, servers SHOULD always return an ETag header with each resource:

HTTP 200 OK
Date: Sat, 09 Feb 2013 16:09:50 GMT
Last-Modified: Sat, 02 Feb 2013 12:02:47 GMT
ETag: W/"23"
Content-Type: application/fhir+json

If provided, the value of the ETag SHALL match the value of the version id for the resource. Servers are allowed to generate the version id in whatever fashion that they wish, so long as they are valid according to the id data type, and are unique within the address space of all versions of the same resource. When resources are returned as part of a bundle, there is no ETag, and the versionId of the resource is used directly.

If the client wishes to request a version aware update, it submits the request with an If-Match header that quotes the ETag from the server:

PUT /Patient/347 HTTP/1.1
If-Match: W/"23"

If the version id given in the If-Match header does not match, the server returns a 409 Conflict status code instead of updating the resource.

Servers can require that clients provide an If-Match header by returning 412 Pre-condition failed status codes when no If-Match header is found.

2.21.0.12 patch

As an alternative to updating an entire resource, clients can perform a patch operation. This can be useful when a client is seeking to minimise its bandwidth utilization, or in scenarios where a client has only partial access or support for a resource. The patch interaction is performed by an HTTP PATCH command as shown:

  PATCH [base]/[type]/[id] {?_format=[mime-type]}

The body of a PATCH operation SHALL be either:

• a JSON Patch document with a content type of application/json-patch+json
• an XML Patch document with a content type of application/xml-patch+xml
• a FHIRPath Patch parameters resource with FHIR Content Type

In either case, the server SHALL process its own copy of the resource in the format indicated, applying the operations specified in the document, following the relevant PATCH specification. When the operations have all been processed, the server processes the resulting document as an Update operation; all the version and error handling etc. applies as specified, as does the Prefer Header.

Processing PATCH operations may be very version sensitive. For this reason, servers SHALL support conditional PATCH, which works exactly the same as specified for update in Concurrency Management. Clients SHOULD always consider using version specific PATCH operations so that inappropriate actions are not executed. In addition, servers SHALL support Conditional PATCH, which works exactly as described for Conditional Update.

The server SHALL ensure that the narrative in a resource is not clinically unsafe after the PATCH operation is performed. Exactly how this is defined and can be achieved depends on the context, and how narrative is being maintained, but servers may wish to consider:

• If the existing narrative has a status != generated, the server could reject the PATCH operation
• The server could regenerate the narrative once the operation has been applied to the data
• In some limited circumstances, an XML PATCH operation could update the narrative
• The server could delete the narrative, on the basis that some later process will be able to populate it correctly

Processing XML Patch documents is tricky because of namespace handling. Servers SHALL handle namespaces correctly, but note that FHIR resources only contain two XML namespaces, for FHIR (http://hl7.org/fhir) and XHTML (http://www.w3.org/1999/xhtml).

For PATCH Examples, see (link todo).

Patch operations may be performed as part of Batch or Transaction Operations using the FHIRPath Patch format.

2.21.0.13 delete

The delete interaction removes an existing resource. The interaction is performed by an HTTP DELETE command as shown:

  DELETE [base]/[type]/[id]

A delete interaction means that subsequent non-version specific reads of a resource return a 410 HTTP status code and that the resource is no longer found through search interactions. Upon successful deletion, or if the resource does not exist at all, the server should return either a 200 OK if the response contains a payload, or a 204 No Content with no response payload.

Whether to support delete at all, or for a particular resource type or a particular instance is at the discretion of the server based on the business rules that apply in its context. If the server refuses to delete resources of that type as a blanket policy, then it should return the 405 Method not allowed status code. If the server refuses to delete a resource because of reasons specific to that resource, such as referential integrity, it should return the 409 Conflict status code. Performing this interaction on a resource that is already deleted has no effect, and the server should return either a 200 OK if the response contains a payload, or a 204 No Content with no response payload. Resources that have been deleted may be "brought back to life" by a subsequent update interaction using an HTTP PUT.

Many resources have a status element that overlaps with the idea of deletion. Each resource type defines what the semantics of the deletion interactions are. If no documentation is provided, the deletion interaction should be understood as deleting the record of the resource, with nothing about the state of the real-world corresponding resource implied.

For servers that maintain a version history, the delete interaction does not remove a resource's version history. From a version history respect, deleting a resource is the equivalent of creating a special kind of history entry that has no content and is marked as deleted. Note that there is no support for deleting past versions - see notes on the history interaction.

Since deleted resources may be brought back to life, servers MAY include an ETag on the delete response to allow version contention management when a resource is brought back to life.

2.21.0.13.1 Conditional delete

The conditional delete interaction allows a client to delete an existing resource based on some selection criteria, rather than by a specific logical id. To accomplish this, the client issues an HTTP DELETE as shown:

  DELETE [base]/[type]/?[search parameters]

When the server processes this delete, it performs a search as specified using the standard search facilities for the resource type. The action it takes depends on how many matches are found:

• No matches or One Match: The server performs an ordinary delete on the matching resource
• Multiple matches: Servers may choose to delete all the matching resources, or it may choose to return a 412 Precondition Failed error indicating the client's criteria were not selective enough. A server indicates whether it can delete multiple resources in its Capability Statement (.rest.resource.conditionalDelete). If there are multiple matches, either all must be deleted, or the server SHALL return an error

This variant can be used to allow a stateless client (such as an interface engine) to delete a resource on a server, without having to remember the logical ids that the server has assigned. For example, a client deleting a lab atomic result might delete the resource using DELETE /Observation?identifier=http://my-lab-system|123.

Note that transactions and conditional create/update/delete are complex interactions and it is not expected that every server will implement them. Servers that don't support the conditional delete should return an HTTP 400 error and an OperationOutcome.

2.21.0.14 create

The create interaction creates a new resource in a server-assigned location. If the client wishes to have control over the id of a newly submitted resource, it should use the update interaction instead. The create interaction is performed by an HTTP POST command as shown:

  POST [base]/[type] {?_format=[mime-type]}

The request body SHALL be a FHIR Resource. The resource does not need to have an id element (this is one of the few cases where a resource exists without an id element). If an id is provided, the server SHALL ignore it. If the request body includes a meta, the server SHALL ignore the existing versionId and lastUpdated values. The server SHALL populate the id, meta.versionId and meta.lastUpdated with the new correct values. Servers are allowed to review and alter the other metadata values, but SHOULD refrain from doing so (see metadata description for further information).

A server SHOULD otherwise accept the resource as submitted when it accepts the create, and return the same content when it is subsequently read. However some systems may not be able to do this; see the note on transactional integrity for discussion.

The server returns a 201 Created HTTP status code, and SHALL also return a Location header which contains the new Logical Id and Version Id of the created resource version:

  Location: [base]/[type]/[id]/_history/[vid]

where [id] and [vid] are the newly created id and version id for the resource version. Servers SHOULD return an ETag header with the versionId (if versioning is supported) and a Last-Modified header.

When the resource syntax or data is incorrect or invalid, and cannot be used to create a new resource, the server returns a 400 Bad Request HTTP status code. When the server rejects the content of the resource because of business rules, the server returns a 422 Unprocessable Entity error HTTP status code. In either case, the server SHOULD include a response body containing an OperationOutcome with detailed error messages describing the reason for the error.

Common HTTP Status codes returned on FHIR-related errors (in addition to normal HTTP errors related to security, header and content type negotiation issues):

• 400 Bad Request - resource could not be parsed or failed basic FHIR validation rules
• 404 Not Found - resource type not supported, or not a FHIR end-point
• 422 Unprocessable Entity - the proposed resource violated applicable FHIR profiles or server business rules. This should be accompanied by an OperationOutcome resource providing additional detail

Note: Servers MAY choose to preserve XML comments, instructions, and formatting or JSON whitespace when accepting creates, but are not required to do so. The impact of this on digital signatures may need to be considered.

For additional information on how systems may behave when processing updates, refer to the Variations between Submitted data and Retrieved data page.

2.21.0.14.1 Conditional create

The conditional create interaction allows a client to create a new resource only if some equivalent resource does not already exist on the server. The client defines what equivalence means in this case by supplying a FHIR search query using an HL7 defined extension header "If-None-Exist" as shown:

  If-None-Exist: [search parameters]

The parameter just contains the search parameters (what would be in the URL following the "?").

When the server processes this create, it performs a search as specified using its standard search facilities for the resource type. The action it takes depends on how many matches are found:

• No matches: The server processes the create as above
• One Match: The server ignore the post and returns 200 OK
• Multiple matches: The server returns a 412 Precondition Failed error indicating the client's criteria were not selective enough

This variant can be used to avoid the risk of two clients creating duplicate resources for the same record. For example, a client posting a new lab result might specify If-None-Exist: identifier=http://my-lab-system|123 to ensure it is does not create a duplicate record.

Note that transactions and conditional create/update/delete are complex interactions and it is not expected that every server will implement them. Servers that don't support the conditional create should return an HTTP 412 error and an OperationOutcome.

2.21.0.15 search

This interaction searches a set of resources based on some filter criteria. The interaction can be performed by several different HTTP commands.

  GET [base]/[type]{?[parameters]{&_format=[mime-type]}}

This searches all resources of a particular type using the criteria represented in the parameters.

Because of the way that some user agents and proxies treat GET and POST requests, in addition to the get based search method above, servers that support search SHALL also support a POST based search:

POST  [base]/[type]/_search{?[parameters]{&_format=[mime-type]}}

This has exactly the same semantics as the equivalent GET command. All these search interactions take a series of parameters that are a series of name=value pairs encoded in the URL (or as an application/x-www-form-urlencoded submission for a POST). (See W3C HTML forms ).

Note: application/x-www-form-urlencoded is supported for POST so that invoking a search by GET or POST can be done from HTML forms in a browser (though considerable active content might be required in the browser), although this is not the main usage.

Searches are processed as specified for the Search handling mechanism.

If the search succeeds, the server SHALL return a 200 OK HTTP status code and the return content SHALL be a Bundle with type = searchset containing the results of the search as a collection of zero or more resources in a defined order. The result collection can be long, so servers may use paging. If they do, they SHALL use the method described below (adapted from RFC 5005 (Feed Paging and Archiving ) for breaking the collection into pages if appropriate. The server MAY also return an OperationOutcome resource within the searchset Bundle entries that contains additional information about the search; if one is sent it SHALL NOT include any issues with a fatal or error severity, and it SHALL be marked with a Bundle.entry.search.mode of outcome.

If the search fails (cannot be executed, not that there is no matches), the return value is a status code 4xx or 5xx with an OperationOutcome.

Common HTTP Status codes returned on FHIR-related errors (in addition to normal HTTP errors related to security, header and content type negotiation issues):

• 400 Bad Request - search could not be processed or failed basic FHIR validation rules
• 401 Not Authorized - authorization is required for the interaction that was attempted
• 404 Not Found - resource type not supported, or not a FHIR end-point

2.21.0.15.1 Variant Searches

To search a compartment, for either all possible resources or for a particular resource type, respectively:

  GET [base]/[Compartment]/[id]/{*?[parameters]{&_format=[mime-type]}}
  GET [base]/[Compartment]/[id]/[type]{?[parameters]{&_format=[mime-type]}}

For example, to retrieve all the observation resources for a particular LOINC code associated with a particular encounter:

  GET [base]/Encounter/23423445/Observation?code=2951-2  {&_format=[mime-type]}

Note that there are specific operations defined to support fetching an entire patient record or all record for an encounter.

It is also possible to search across multiple resource types:

  GET [base]?_type=Condition,Observation&[parameters]{&_format=[mime-type]}

This is a request to search on both Condition and Observation. In this case, the only parameters that can be used are those defined for both Condition and Observation (using SearchParameter.base - see Cross-resource Search Parameters), or the parameters defined for all resources. If a search lists types not listed in SearchParameter.base for any of the parameters, this is an error, and a server SHOULD return a 400 status. It is also possible to search on all types at once:

  GET [base]?[parameters]{&_format=[mime-type]}

When searching all resources at once, only the parameters defined for all resources can be used.

2.21.0.16 capabilities

The capabilities interaction retrieves the server's Capability Statement that defines how it supports resources. The interaction is performed by an HTTP GET command as shown:

  GET [base]/metadata {?_format=[mime-type]}

In addition, there is another method to get the capability statement, using the HTTP OPTIONS command:

  OPTIONS [base] {?_format=[mime-type]}

However as of STU3, this method is deprecated, and will be removed in a future version. Using OPTIONS like this is not conformant with HTTP, and creates challenges for cross-origin resource sharing support.

Applications SHALL return a Capability Statement that specifies which resource types and interactions are supported for the GET command. If a 404 Unknown is returned from the GET, FHIR is not supported on the nominated service url. An ETag header SHOULD be returned with the CapabilityStatement. The value of the header SHALL change if the CapabilityStatement itself changes. Additional parameters that are required to be returned with the OPTIONS command are defined in the OMG hData RESTful Transport specification.

The Capability statement returned typically has an arbitrary id, and no meta element, though it is not prohibited. Capability statements can become quite large; servers are encouraged to support the _summary and _elements parameters on the capabilities interaction, though this is not required. In addition, servers are encouraged to implement the $subset and $implements operations to make it easy for a client to check conformance.

In addition to this capabilities interaction, a server may also choose to provide the standard set of interactions (read, search, create, update) defined on this page for the CapabilityStatement Resource end-point. This is different from the capabilities interaction:

All servers are required to support the capabilities interaction, but servers may choose whether they wish to support the CapabilityStatement end-point, just like any other end-point.

Implementation Note: In DSTU 2 and earlier, the resource that this interaction returned was named "Conformance". Clients often connect to a server, and use the capabilities interaction to check whether they are version and/or feature compatible with the server. Such clients should be able to process either a Conformance or a CapabilityStatement resource.

2.21.0.17 batch/transaction

The batch and transaction interactions submit a set of actions to perform on a server in a single HTTP request/response. The actions may be performed independently as a "batch", or as a single atomic "transaction" where the entire set of changes succeed or fail as a single entity. Multiple actions on multiple resources of the same or different types may be submitted, and they may be a mix of other interactions defined on this page (e.g. read, search, create, update, delete, etc.), or using the operations framework.

The transaction mode is especially useful where one would otherwise need multiple interactions, possibly with a risk of loss of referential integrity if a later interaction fails (e.g. when storing a Provenance resource and its corresponding target resource, or, on document repositories, a document index entry and its accompanying document).

Note that transactions and conditional create/update/delete are complex interactions and it is not expected that every server will implement them.

A batch or transaction interaction is performed by an HTTP POST command as shown:

  POST [base] {?_format=[mime-type]}

The content of the post submission is a Bundle with Bundle.type = batch or transaction. Each entry carries request details (Bundle.entry.request) that provides the HTTP details of the action in order to inform the system processing the batch or transaction what to do for the entry (note: the request is optional, but SHOULD be present). If the HTTP command is a PUT or POST, then the entry SHALL contain a resource for the body of the action. The resources in the bundle are each processed separately as if they were an individual interaction or operation as otherwise described on this page, or the Operations framework. The actions are subject to the normal processing for each, including the meta element, verification and version aware updates, and transactional integrity.

Examples:

• Transaction Example with Matching Response
• Batch request to fetch Meds & Allergies with Response
• Batch request to fetch simple Patient Summary with Response

2.21.0.17.1 Batch Processing Rules

For a batch, there SHALL be no interdependencies between the different entries in the Bundle that cause change on the server. The success or failure of one change SHOULD not alter the success or failure or resulting content of another change. Servers SHOULD validate that this is the case. Note that it is considered that servers execute the batch in the same order as that specified below for transactions, though the order of execution should not matter given the previous rule.

Static references within a Bundle.entry.resource to another Bundle.entry.resource that is being created within the batch are considered to be non-conformant.

2.21.0.17.2 Transaction Processing Rules

For a transaction, servers SHALL either accept all actions and return a 200 OK, along with a response bundle (see below), or reject all resources and return an HTTP 400 or 500 type response. It is not an error if the submitted bundle has no resources in it. The outcome of the processing the transaction SHALL NOT depend on the order of the resources in the transaction. A resource can only appear in a transaction once (by identity).

Because of the rules that a transaction is atomic where all actions pass or fail together and the order of the entries doesn't matter, there is a particular order in which to process the actions:

1. Process any DELETE interactions
2. Process any POST interactions
3. Process any PUT interactions
4. Process any GET interactions

If any resource identities (including resolved identities from conditional update/delete) overlap in steps 1-3, then the transaction SHALL fail.

STU Note: clients are able to request that operations be executed as part of a transaction. Some transactions can cause side effects, such as the creation of new resources or other actions that may be difficult to fit into a transaction framework. Input regarding this issue is sought during the STU period.

Provide feedback here .

A transaction may include references from one resource to another in the bundle, including circular references where resources refer to each other. If the server assigns a new id to any resource in the bundle as part of the processing rules above, it SHALL also update any references to that resource in the same bundle as they are processed. References to resources that are not part of the bundle are left untouched. Version-specific references should remain as version-specific references after the references have been updated. Servers SHALL replace all matching links in the bundle, whether they are found in the resource ids, resource references, url elements, or <a href="" & <img src="" in the narrative.

When processing a "POST" (create), the full URL is treated as the id of the resource on the source, and is ignored; the server generates an id for the resource. For updates, the server performs a mapping between the fullUrl specified and the local URL the server knows that instance as, if possible. If the server does not have a mapping for the fullUrl, the server ignores the base URL and attempts an update assuming the base is the same as the server base. This allows the same transaction bundle to be sent to multiple systems without changing the fullUrls for each target.

When processing a batch or transaction, a server MAY choose to honor existing logical ids (e.g. Observation/1234 remains as Observation/1234 on the server), but since this is only safe in controlled circumstances, servers may choose to assign new ids to all submitted resources, irrespective of any claimed logical id in the resource, or fullUrl on entries in the batch/transaction.

Note: this behavior is subject to verification based on implementation experience and may change.

Provide feedback here .

Conditional References

When constructing the bundle, the client may not know the logical id of a resource, but it may know identifying information - e.g. an identifier. This situation arises commonly when building transactions from v2 messages. The client could resolve that identifier to a logical id using a search, but that would mean that the resolution to a logical id does not occur within the same transaction as the commit (as well as significantly complicating the client). Because of this, in a transaction (and only in a transaction), references to resources may be replaced by a search URI that describes how to find the correct reference:

 <Bundle xmlns="http://hl7.org/fhir">
   <id value="20160113160203" />
   <type value="transaction" />
   <entry>
     <fullUrl value="urn:uuid:c72aa430-2ddc-456e-7a09-dea8264671d8" />
     <resource>
       <Observation>
         <subject>
            <reference value="Patient?identifier=12345"/>
         </subject>
         <--! rest of resource omitted -->
       </Observation>
     </resource>
     <request>
       <method value="POST" />
     </request>
   </entry>
 <Bundle>

The search URI is relative to the server's [base] path, and always starts with a resource type: [type]:?parameters.... Only filtering parameters are allowed; none of the parameters that control the return of resources are relevant.

When processing transactions, servers SHALL:

• check all references for search URIs
• For search URIs, use the search to locate matching resources
• if there are no matches, or multiple matches, the transaction fails, and an error is returned to the user
• if there is a single match, the server replaces the search URI with a reference to the matching resource

2.21.0.17.3 Batch/Transaction Response

For a batch, or a successful transaction, the response the server SHALL return a Bundle with type set to batch-response or transaction-response that contains one entry for each entry in the request, in the same order, with the outcome of processing the entry. For a failed transaction, the server returns a single OperationOutcome instead of a Bundle.

A client may use the returned Bundle to track the outcomes of processing the entry, and the identities assigned to the resources by the server. Each entry element SHALL contain a response element which details the outcome of processing the entry - the HTTP status code, and the location and ETag header values, which are used for identifying and versioning the resources. In addition, a resource may be included in the entry, as specified by the Prefer header.

2.21.0.17.4 Accepting other Bundle types

A server may choose to accept bundle types other than batch or transaction when POSTed to the [base] URL.

Bundles of type history inherently have the same structure as a transaction, and can be treated as either a transaction or batch, so servers SHOULD accept a history Bundle - this makes it possible to replicate data from one server to another easily using a pub/sub model. Note, however, that the original transaction boundaries may not be represented in a history list, and a resource may occur more than once in a history list, so servers processing history bundles must have some strategy to manage this.

For other Bundle types, should the server choose to accept them, there will be no request element (note that every entry will have a resource). In this case, the server treats the entry as either a create or an update interaction, depending on whether it recognises the identity of the resource - if the identity of the resource refers to a valid location on the server, it should treat it as an update to that location. Note: this option allows a client to delegate the matching process to the server.

2.21.0.18 history

The history interaction retrieves the history of either a particular resource, all resources of a given type, or all resources supported by the system. These three variations of the history interaction are performed by HTTP GET command as shown:

  GET [base]/[type]/[id]/_history{?[parameters]&_format=[mime-type]}
  GET [base]/[type]/_history{?[parameters]&_format=[mime-type]}
  GET [base]/_history{?[parameters]&_format=[mime-type]}

The return content is a Bundle with type set to history containing the specified version history, sorted with oldest versions last, and including deleted resources. Each entry SHALL minimally contain either a resource which holds the resource as it is at the conclusion of the interaction, or a request with entry.request.method The request provides information about the interaction that occurred to cause the new version, and allows, for instance, a subscriber system to differentiate between create and update interactions. The principal reason a resource might be missing is that the resource was changed by some other channel rather than via the RESTful interface. If the entry.request.method is a PUT or a POST, the entry SHALL contain a resource.

The interactions create, update, and delete create history entries. Other interactions do not (note that these operations may produce side-effects such as new AuditEvent resources; these are represented as create interactions in their own right). New resources or updates to existing resources that are triggered by operations also appear in the history, as do updates to the resources that result from interactions outside the scope of the RESTful interface.

A create interaction is represented in a history interaction in the following way:

  <entry>
    <resource>
      <Patient>
        <!-- the id of the created resource -->
        <id value="23424"/>
        <!-- snip -->
      </Patient>
    </resource>
    <request>
      <!-- POST: this was a create -->
      <method value="POST"/>
      <url value="Patient"/>
    </request>
  </entry>

Note that conditional creates, updates and deletes are converted to direct updates and deletes in a history list.

In addition to the standard _format parameter, the parameters to this interaction may also include:

The history list can be restricted to a limited period by specifying a _since parameter which contains a full date time with time zone. Clients should be aware that due to timing imprecision, they may receive notifications of a resource update on the boundary instant more than once. Servers are not required to support a precision finer than by second.

The updates list can be long, so servers may use paging. If they do, they SHALL use the method described below for breaking the list into pages if appropriate, and maintain the specified _count across pages.

The history interaction can be used to set up a subscription from one system to another, so that resources are synchronized between them. Refer to the Subscription resource for an alternate means of system synchronization.

Additional Notes about maintaining a history of resources:

• The history is a record version history on a per-resource basis. It is not intended to support concurrent versions, or multi-branch version history
• Accordingly, there is no way to update or delete past versions of the record, except that the metadata can be modified (mainly for access control purposes)
• All past versions of a resource are considered to be superceded, and no longer active, but retained for audit/integrity purposes
• In the case that a past version of a resource needs to be explicitly documented as 'entered-in-error', use a Provenance resource pointing to the past version of the resource
• When tracing the history of a specific resource, applications should retrieve any provenance resources relating to the resource or its past versions
• If a request is made for a history that is not available (e.g. the system does not keep a history for the type, or the particular instance), the server should return a 404 Not Found along with an OperationOutcome explaining the problem

2.21.0.19 Transactional Integrity

When processing create and update interactions, a FHIR server is not obliged to accept the entire resource as it is; when the resource is retrieved through a read interaction subsequently, the resource may be different. The difference may arise for several reasons:

• The server merged updated content with existing content
• The server applied business rules and altered the content
• The server does not fully support all the features or possible values of the resource

Note that there is no general-purpose method to make merging with existing content or altering the content by business rules safe or predictable - what is possible, safe and/or required is highly context dependent. These kinds of behaviors may be driven by security considerations. With regard to incomplete support, clients can consult the server's base CapabilityStatement profile references to determine which features or values the server does not support.

To the degree that the server alters the resource for any of the 3 reasons above, the FHIR server will create implementation consequences for the eco-system that it is part of, which will need to be managed (i.e. it will cost more). For this reason, servers SHOULD change the resource as little as possible, given the constraints of the system exposing the FHIR resource. However due to the variability that exists within healthcare, this specification allows that servers MAY alter the resource on create/update.

Similarly, to the degree that an implementation context makes special rules about merging content or altering the content, that context will become more expensive to maintain.

Although these rules are stated with regard to servers, a similar concept applies to clients - to the degree that different client systems interacting with the server do not support the same feature set, the clients and/or the server will be forced to implement custom logic to prevent information from being lost or corrupted.

Some of these problems can be mitigated by following a pattern built on top of version-aware updates. In this pattern:

• The server provides a read interaction for any resource it accepts update interactions on
• Before updating, the client reads the latest version of the resource
• The client applies the changes it wants to the resource, leaving other information intact (note the extension related rules around this)
• The client writes the result back as an update interaction, and is able to handle a 409 or 412 response (usually by trying again)

If clients follow this pattern, then information from other systems that they do not understand will be maintained through the update.

Note that it is possible for a server to choose to maintain the information that would be lost, but there is no defined way for a server to determine whether the client omitted the information because it wasn't supported (perhaps in this case) or whether it wishes to delete the information.

2.21.0.19.1 Conformance

Both client and server systems SHOULD clearly document how transaction integrity is handled.

STU Note: For now, the only way to document how transaction integrity is handled is as text in the narrative portions of the CapabilityStatement resource. Feedback during the Trial use period on which - if any - of this information should be computable is welcome.

Provide feedback here .

2.21.0.20 Paging

If servers provide paging for the results of a search or history interaction, they SHALL conform to this method (adapted from RFC 5005 (Feed Paging and Archiving) for sending continuation links to the client when returning a Bundle (e.g. with history and search). If the server does not do this then there is no way to continue paging.

This example shows the third page of a search result:

<Bundle xmlns="http://hl7.org/fhir">
  <!-- snip metadata -->
  <!-- This Search url starts with base search, and adds the effective
    parameters, and additional parameters for search state. All searches
    SHALL return this value.

	  In this case, the search continuation method is that the server
    maintains a state, with page references into the stateful list.
	-->
  <link>
    <relation value="self">
    <url value="http://example.org/Patient?name=peter&stateid=23&page=3"/>
  </link>
  <!-- 4 links for navigation in the search. All of these are optional, but recommended -->

  <link>
    <relation value="first"/>
    <url value="http://example.org/Patient?name=peter&stateid=23&page=1"/>
  </link>
  <link>
    <relation value="previous"/>
    <url value="http://example.org/Patient?name=peter&stateid=23&page=2"/>
  </link>
  <link>
    <relation value="next"/>
    <url value="http://example.org/Patient?name=peter&stateid=23&page=4"/>
  </link>
  <link>
    <relation value="last"/>
    <url value="http://example.org/Patient?name=peter&stateid=23&page=26"/>
  </link>

  <!-- then the search results... -->
</Bundle>

The server need not use a stateful paging method as shown in this example - it is at the discretion of the server how to best ensure that the continuation retains integrity in the context of ongoing changes to the resources. An alternative approach is to use version specific references to the records on the boundaries, but this is subject to continuity failures when records are updated.

A server MAY add additional state tracking parameters to the links, as shown in the example above. The client must use the server supplied links in order to traverse the pages. A server MAY inform the client of the total number of resources returned by the interaction for which the results are paged using the Bundle.total.

Note that for search, where _include can be used to return additional related resources, the total number of resources in the feed may exceed the number indicated in totalResults.

2.21.0.21 Intermediaries

The HTTP protocol may be routed through an HTTP proxy such as squid. Such proxies are transparent to the applications, though implementers should be alert to the effects of caching, particularly including the risk of receiving stale content. See the HTTP specification for further detail

Interface engines may also be placed between the consumer and the provider. These differ from proxies because they actively alter the content and/or destination of the HTTP exchange and are not bound the rules that apply to HTTP proxies. Such agents are allowed, but SHALL mark the HTTP header to assist with troubleshooting.

Any agent that modifies an HTTP request or response content other than under the rules for HTTP proxies SHALL add a stamp to the HTTP headers like this:

  request-modified-[identity]: [purpose]
  response-modified-[identity]: [purpose]

The identity SHALL be a single token defined by the administrator of the agent that will sufficiently identify the agent in the context of use. The header SHALL specify the agent's purpose in modifying the content. End point systems SHALL not use this header for any purpose. Its aim is to assist with system troubleshooting.

2.21.0.22 OMG hData RESTful Transport

This RESTful specification described here is based on the OMG Health RESTful specification (HData) . In this regard, FHIR functions as a Record Format Profile as described in that specification. Note the following significant factors to be aware of:

• FHIR maps the hData sections to resource types, and hData documents to resource instances. There are no subsections, and client systems are not able to create new sections, though compartments behave somewhat like sections
• Because clients cannot submit new sections (POST to service URL), POST to the service URL has been re-used for the transaction interaction (difference under review)
• FHIR does not (yet) define a root document. When defined, it will contain information about what the FHIR server has done (as opposed to a Capability Statement, which describes what it is capable of doing)
• Note that this specification does support the hData RESTful Transport OPTIONS command on the service URL

2.21.0.23 Summary

These tables present a summary of the interactions described here. Note that all requests may include an optional Accept header to indicate the format used for the response (this is even true for DELETE since an OperationOutcome may be returned).

Notes:

• N/A = not present, R = Required, O = optional
• For operations defined on all resources, including direct access to the meta element, see Resource Operations

Note: this table lists the status codes described here, but other status codes are possible as described by the HTTP specification. Additional codes that are likely are server errors and various codes associated with authentication protocols.