This page is part of the Smart App Launch Implementation Guide (v2.2.0-ballot: STU 2.2 Ballot 1) based on FHIR (HL7® FHIR® Standard) R4. The current version which supersedes this version is 2.1.0. For a full list of available versions, see the Directory of published versions

Persisting App State (Experimental)

smart-app-state capability

This experimental capability allows apps to persist a small amount of configuration data to an EHR’s FHIR server. Conformance requirements described below apply only to software that implements support for this capability. Example use cases include:

• App with no backend storage persists user preferences such as default screens, shortcuts, or color preferences. Such apps can save preferences to the EHR and retrieve them on subsequent launches.

• App maintains encrypted external data sets. Such apps can persist access keys to the EHR and retrieve them on subsequent launches, allowing in-app decryption and display of external data sets.

Apps SHALL NOT use smart-app-state when data being persisted could be managed directly using FHIR domain models. For example, an app would never persist clinical diagnoses or observations using smart-app-state. Such usage is prohibited because the standard FHIR API provides safer and more interoperable mechanisms for clinical data sharing.

Apps SHALL NOT expect the EHR to use or interpret data stored via smart-app-state unless specific agreements are made outside the scope of this capability.

Note that while state can be associated with a user account or with a patient record, this capability does not enable state associated with (user, patient) pairs. For instance, there is no defined mechanism to save information like “Practitioner A prefers her diabetes management app to start with the ‘recent labs’ screen for Patient A, and the ‘in-range glucose time’ screen for Patient B.”

Formal definitions

The narrative documentation below provides formal requirements, usage notes, and examples.

In addition, the following conformance resources can support automated processing in FHIR R4:

• CapabilityStatement/smart-app-state-server
• StructureDefinition/smart-app-state-basic

Discovery

EHRs supporting this capability SHALL advertise support by including "smart-app-state" in the capabilities array of their FHIR server’s .well-known/smart-configuration file (see section Conformance).

EHRs supporting this capability MAY include a smart_app_state_endpoint property if they want to maintain App State management functionality at a location distinct form the core EHR’s FHIR endpoint (see section Design Notes).

The EHR’s “App State FHIR endpoint” is defined as:

1. The value in smart_app_state_endpoint, if present
2. The EHR’s primary FHIR endpoint, otherwise

Example discovery document

Consider a FHIR server with base URL https://ehr.example.org/fhir.

The discovery document at https://ehr.example.org/fhir/.well-known/smart-configuration might include:

{
  "capabilities": [
    "smart-app-state",
    // <other capabilities snipped>
  ],
  "smart_app_state_endpoint": "https://ehr.example.org/appstate"
  // <other properties snipped>
}

App State Interactions

The EHR’s App State FHIR endpoint SHALL provide support for:

1. POST /Basic
2. PUT /Basic/[id] requiring the presence of If-Match to prevent contention
3. DELETE /Basic/[id] requiring the presence of If-Match to prevent contention
4. GET /Basic?code={}&subject={}
5. GET /Basic?code={}&subject:missing=true // for global app config

The semantics of these FHIR API interactions are defined in the core FHIR specification. In the case of discrepancies, the core specification takes precedence.

This specification does not impose additional requirements on the App State FHIR endpoint.

Managing app state (CRUDS)

App State data can include details like encryption keys. EHRs SHALL evaluate storage requirements and MAY store App State data separately from their routine FHIR Resource storage space.

App State is always associated with a “state code” (Basic.code.coding) and optionally associated with a subject (Basic.subject).

EHRs SHALL allow at least one smart-app-state resource per state code, and at least one smart-app-state resource per (state code, subject) tuple. EHRs SHOULD describe applicable limits in their developer documentation.

EHRs SHOULD retain app state data for as long as the originating app remains actively registered with the EHR. EHRs MAY establish additional retention policies in their developer documentation.

Create

To create app state, an app submits to the EHR’s App State endpoint:

POST /Basic

The request body is a Basic resource where:

1. Total resource size as serialized in the POST body SHALL NOT exceed 256KB unless the EHR’s documentation establishes a higher limit
2. Basic.id SHALL NOT be included
3. Basic.meta.versionId SHALL NOT be included
4. Basic.subject.reference is optional, associating App State with at most one subject. When omitted, global configuration can be stored in App State. When present, this SHALL be an absolute reference to a resource in the EHR’s primary FHIR server. The EHR SHALL support at least Patient, Practitioner, PractitionerRole, RelatedPerson, Person. The EHR’s documentation MAY establish support for a broader set of resources.
5. Basic.code.coding[] SHALL include exactly one app-specified Coding
6. Basic.extension Extensions SHALL be limited to the valueString type unless the EHR’s documentation establishes a broader set of allowed extension types

If the EHR accepts the request, the EHR SHALL persist the submitted resource including:

• a newly generated server-side unique value in populated in Basic.id
• a value populated in Basic.meta.versionId
• the Reference present in Basic.subject, if any
• the Coding present in Basic.code.coding
• all top-level extensions present in the request

If the EHR cannot meet all of these obligations, it SHALL reject the request.

The response headers include an ETag header following FHIR Core “Managing Resource Contention”.

The response body is a Basic resource representing the state persisted by the EHR.

Updates

To update app state, an app submits to the EHR’s App State endpoint:

PUT /Basic/[id]

The request SHALL include an If-Match header with an ETag to prevent contention as defined in FHIR Core “Managing Resource Contention”.

The payload is a Basic resource where:

1. Basic.id SHALL be present, matching the [id] in the request URL
2. Basic.subject and Basic.code SHALL NOT change from the previously-persisted values

EHR servers SHALL return a 412 Precondition Failed response if the version specified in If-Match header’s ETag does not reflect the most recent version stored in the server, or if the Basic.subject or Basic.code does not match previously-persisted value.

The successful response headers include an ETag header following FHIR Core “Managing Resource Contention”.

The response body is a Basic resource representing the state persisted by the EHR.

Deletes

To delete app state, an app submits to the EHR’s App State endpoint:

DELETE /Basic/[id]

The request SHALL include an If-Match header with an ETag to prevent contention as defined in FHIR Core “Managing Resource Contention”.

EHR servers SHALL return a 412 Precondition Failed response if the version specified in If-Match header’s ETag does not reflect the most recent version stored in the server.

After successfully deleting state, an app SHALL NOT submit subsequent requests to modify the same state by Basic.id. Servers SHALL process any subsequent requests about this Basic.id as a failed precondition.

Querying app state

An app SHALL query for state by supplying a state type code and optionally a subject:

• GET /Basic?code={}&subject={} for state associated with a specific subject
• GET /Basic?code={}&subject:missing=true for global state associated with the app overall

The EHR SHALL support the following query parameters:

• ?code , restricted to fixed state codes that exactly match Basic.code.coding[0] (i.e., ${system} + | + ${code})
• ?subject, restricted to absolute references that exactly match Basic.subject.reference, with support for the :missing=true modifier to find global state

The response body is a FHIR Bundle where each entry is a Basic resource as persisted by the EHR.

The EHR MAY support additional queries, and an app MAY issue additional queries when they are supported.

API Examples

Example 1: App-specific User Preferences

The following example POST body shows how an app might persist a user’s app-specific preferences:

POST /Basic

Authorization: Bearer <snipped>

{
  "resourceType": "Basic",
  "subject": {"reference": "https://ehr.example.org/fhir/Practitioner/123"},
  "code": {
    "coding": [
      // app-specific designation; the EHR does not need to understand
      // the meaning of this concept, but SHALL persist it and MAY
      // use it for access control (e.g., using SMART on FHIR scopes
      // or other controls; see below)
      { 
        "system": "https://myapp.example.org",
        "code": "display-preferences"
      }
    ]
  },
  "extension": [
    // app-managed state; the EHR does not need to understand
    // the meaning of these values, but SHALL persist them
    {
      "url": "https://myapp.example.org/display-preferences-v2.0.1",
      "valueString": "{\"defaultView\":\"problem-list\",\"colorblindMode\":\"D\",\"resultsPerPage\":150}"
    }
  ]
}

The API response populates id and meta.versionId, like:

Location: https://ehr.example.org/appstate/Basic/1000
ETag: W/"a"

{
  "resourceType": "Basic",
  "id": "1000",
  "meta": {"versionId": "a"},
  "subject": {"reference": "https://ehr.example.org/fhir/Practitioner/123"},
  ...<snipped for brevity>

To query for these data, an app could invoke the following operation (newlines added for clarity):

GET /Basic
  subject=https://ehr.example.org/fhir/Practitioner/123&
  code=https://myapp.example.org|display-preferences

Authorization: Bearer <snipped>

… which returns a Bundle including the “Example 1” payload.

Example 2: Access Keys to an external data set

The following POST body shows how an app might persist access keys for an externally managed encrypted data set:

POST /Basic

Authorization: Bearer <snipped>

{
  "resourceType": "Basic",
  "subject": {"reference": "https://ehr.example.org/fhir/Patient/123"},
  "code": {
    "coding": [
      // app-specific designation; the EHR does not need to understand
      // the meaning of this concept, but SHALL persist it and MAY
      // use it for access control (e.g., using SMART on FHIR scopes
      // or other controls; see below)
      { 
        "system": "https://myapp.example.org",
        "code": "encrypted-phr-access-keys"
      }
    ]
  },
  "extension": [
    // app-managed state; the EHR does not need to understand
    // the meaning of these values, but SHALL persist them
    {
      "url": "https://myapp.example.org/encrypted-phr-access-keys",
      "valueString": "eyJhbGciOiJSU0EtT0FFUCIsImVuYyI6IkEyNTZH...<snipped>"
    }
  ]
}

The EHR responds with a Basic resource representing the new SMART App State object as it has been persisted. The API response populates id and meta.versionId, like:

Location: https://ehr.example.org/appstate/Basic/1001
ETag: W/"a"

{
  "resourceType": "Basic",
  "id": "1001",
  "meta": {"versionId": "a"},
  "subject": {"reference": "https://ehr.example.org/fhir/Practitioner/123"},
  ...<snipped for brevity>

Example 3: Updating the Access Keys from Example 2

The following POST body shows how an app might update persisted access keys for an externally managed encrypted data set, based on the response to Example 2.

PUT /Basic/1001

Authorization: Bearer <snipped>
If-Match: W/"a"

{
  "resourceType": "Basic",
  "id": "1001",
  "subject": {"reference": "https://ehr.example.org/fhir/Patient/123"},
  "code": {
    "coding": [
      { 
        "system": "https://myapp.example.org",
        "code": "encrypted-phr-access-keys"
      }
    ]
  },
  "extension": [
    {
      "url": "https://myapp.example.org/encrypted-phr-access-keys",
      "valueString": "eyJhbGc<updated value, snipped>"
    }
  ]
}

The EHR responds with a Basic resource representing the updated SMART App State object as it has been persisted. The API response includes an updated meta.versionId, like:

{
  "resourceType": "Basic",
  "id": "1001",
  "meta": {"versionId": "b"},
  ...<snipped for brevity>

Example 4: Deleting the Access Keys from Example 3

The following POST body shows how an app might delete persisted access keys for an externally managed encrypted data set, based on the response to Example 3.

DELETE /Basic/1001

Authorization: Bearer <snipped>
If-Match: W/"b"

{
  "resourceType": "Basic",
  "id": "1001",
  "subject": {"reference": "Patient/123"},
  "code": {
    "coding": [
      { 
        "system": "https://myapp.example.org",
        "code": "encrypted-phr-access-keys"
      }
    ]
  }
}

The EHR responds with a 200 OK or 204 No Content message to indicate a successful deletion.

Security and Access controls

Apps SHALL NOT use data from Extension.valueString without validating or sanitizing the data first. In other words, app developers need to consider a threat model where App State values have been populated with arbitrary content. (Note that EHRs are expected to simply store and return such data unmodified, without “using” the data.)

The EHR SHALL enforce access controls to ensure that only authorized apps are able to perform the FHIR interactions described above. From the app’s perspective, these operations are invoked using a SMART on FHIR access token in an Authorization header. Every App State request SHALL include an Authorization header with an access token issued by the EHR’s authorization server.

This means the EHR tracks (e.g., in some internal, implementation-specific format) sets of Codings representing the SMART App State types (i.e., Basic.code.coding) that the app is allowed to

• query, when the subject is the in-context patient
• modify, when the subject is the in-context patient
• query, when the subject is the in-context app user
• modify, when the subject is the in-context app user
• query, with subject:missing=true (for global app config)
• modify, with subject:missing=true (for global app config)

EHRs SHALL only associate state codes with an app if the app is trusted to access those data. These decisions can be made out-of-band during or after the app registration process. A recommended default is to allow apps to register only state codes where the system matches the app’s verified origin. For instance, if the EHR has verified that the app developer manages the origin https://app.example.org, the app could be associated with SMART App State types like https://app.example.org|user-preferences or https://app.example.org|phr-keys. If an app requires access to other App State types, these could be reviewed through an out-of-band process. This situation is expected when one developer supplies a patient-facing app and another developer supplies a provider-facing “companion app” that needs to query state written by the patient-facing app.

Further consideration is required when granting an app the ability to modify global app state (i.e., where Basic.subject is absent). Such permissions should be limited to administrative apps. See section global app configuration.

Where appropriate, the EHR MAY expose these controls using SMART scopes as follows.

Granting access at the user level

An app writing user-specific state (e.g., user preferences) or writing patient-specific state on behalf of an authorized user can request SMART scopes like:

user/Basic.cuds // query and modify state

This scope would allow the app to manage any app state that the user is permitted to manage. Note that without further refinement, this scope could allow an app to see and manage app state from other apps in addition to its own. This can be a useful behavior in scenarios where sets of apps have a mutual understanding of certain state values (e.g. a suite of apps offered by a single developer).

Granting access at the patient level

An app writing patient-specific state (e.g., access keys for an externally managed encrypted data set) can request a SMART scope like:

patient/Basic.cuds // query and modify state

This scope would allow the app to manage any app state that the user is permitted to manage on the in-context patient record. Note that without further refinement, this scope could allow an app to see and manage app state written by other apps. This can be a useful behavior in scenarios where sets of apps have a mutual understanding of certain state values (e.g., a patient-facing mobile app that creates an encrypted data payload and a provider-facing “companion app” that decrypts and displays the data set within the EHR).

For the scenario of a patient-facing mobile app that works in tandem with provider-facing “companion app” in the EHR, scopes can be narrowed to better reflect minimum necessary access (note the use of more specific codes).

Patient-facing mobile app

patient/Basic.cuds?code=https://myapp.example.org|encrypted-phr-access-keys

Provider-facing “companion app” in the EHR

patient/Basic.s?code=https://myapp.example.org|encrypted-phr-access-keys

Global app configuration

Some apps might leverage “global” (i.e., hospital-wide) state stored in the EHR. Such access could be requested for an administrative end-user with scopes of the form:

user/Basic.cuds // query and modify global app state
user/Basic.s // query global app state

For example, a family of apps could be deployed with read access to a “configuration” state type, with a management app authorized to modify this state type. Using this pattern, a local site administrator could establish settings across the family of apps.

A management app might create such hospital-defined configuration with:

user/Basic.cuds?code=https://myapp.example.org|hospital-config

And the related family of apps installed at a hospital might access such hospital-defined configuration with:

user/Basic.s?code=https://myapp.example.org|hospital-config

Explaining access controls to end users

In the case of user-facing authorization interactions (e.g., for a patient-facing SMART App), it’s important to ensure that such scopes can be explained in plain language. For example, it is important to explain to users that any information their app submits to the EHR may be considered part of the health record and may be accessible to their healthcare provider team. To support authorization, EHRs may need to gather additional information from app developers at registration time with explanations, or may apply additional protections to facilitate access control decisions.

Implementation considerations for EHRs

EHRs can implement support for smart-app-state using their internal resource server infrastructure, or can deploy a decoupled resource server that communicates with the EHR authorization server using SMART on FHIR Token Introspection, signed tokens, or some other mechanism. While some access control decisions can be made directly based on standardized scopes, other decisions may require additional policy information. See “scopes are used to delegate access” for background on this point.

Consider the following examples:

• a token granting patient/Basic.s?code=https://app|1 with a context of "patient": "a" would be authorized to query Basic?subject=Patient/a&code=https://app|1

• a token granting user/Basic.s?code=https://app|1 with a context of "fhirUser": "Practitioner/b" would be authorized to query Basic?subject=Practitioner/b&code=https://app|1

• a token granting user/Basic.s?code=https://app|1 with no patient context and a user context of "fhirUser": "Practitioner/b" would require additional policy information to decide whether to allow Basic?subject=Patient/a&code=https://app|1. A server MAY simply reject queries like this as unauthorized, while still supporting many use cases. To properly authorize this request, a resource server would need to determine whether EHR policy allows Practitioner B to access Patient A’s records. Such policy information could be static (e.g., a simple system could maintain a policy like “all Practitioner users can access all patient records”) or dynamically available in EHR-specific ways such as FHIR Groups, CareTeams, PractitionerRoles, or non-FHIR APIs such as SCIM or others. Such policy details are beyond the scope of SMART on FHIR.

Design Notes

Implementers may wonder why the SMART App State capability defines a FHIR Base URL that might be distinct from the EHR’s main FHIR base URL. During the design and prototype phase, implementers identified requirements that led to this design:

• Ensure that EHRs can identify App State requests via path-based HTTP request evaluation. This allows EHRs to route App State requests to a purpose-built underlying service. Such identification is not possible when the only path information is /Basic (e.g., for POST /Basic on the EHR’s main FHIR endpoint), since non-App-State CRUD operations on Basic would use this same path. Providing the option for a distinct FHIR base URL allows EHRs to establish predictable and distinct paths when this is desirable.

• Ensure that App State can be persisted separately from other Basic resources managed by the EHR. This stems from the fact that App State is essentially opaque to the EHR and should not be returned in general-purpose queries for Basic content (which may be used to surface first-class EHR concepts).