APIs: Query Parameters

(c) AMWA 2016, CC Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0)

The Query API supports a range of query string parameters which MAY be used as part of GET requests, or within WebSocket subscriptions.

The following document describes the expected usage and behaviour of these query parameters alongside the RAML specification in order to aid implementers. A description of each individual query parameter is included within the RAML.

Pagination

Query APIs SHOULD support pagination of their API resources where the paged trait is specified in the RAML documentation. A 501 (Not Implemented) HTTP status code MUST be returned where pagination is attempted against a Query API which does not implement it. Pagination is not used by WebSocket subscriptions.

Query API clients MUST detect whether pagination is being used by examining the HTTP response headers for X-Paging-Limit which MUST be returned in all cases where pagination is in use.

The registry which backs the Query API SHOULD maintain a creation and update timestamp alongside each registered resource. These values SHOULD NOT be returned to API clients in the response body, but will be made available via headers and used as pagination cursors.

To ensure that pagination does not result in resources being skipped, there SHOULD NOT be duplicate creation or update timestamps stored against resources of the same type (Node, Source, Flow, etc.). It is RECOMMENDED that these timestamps are stored with nanosecond resolution using a TAI timebase, which will allow clients to navigate collections based on a common understanding of time.

The choice to page based on creation or update timestamp depends on the client's intended use of the data:

Paging based on creation time from the start of a collection provides the best mechanism to identify all resources held in a registry.
Paging based on update time from the end of a collection provides the best mechanism to watch for changing resources in the case that WebSocket subscriptions are not available.

Paging through a collection for a second time, using the same cursors as previously SHOULD NOT result in new data appearing in the response payloads when using creation ordering, provided the requested paging times are less than or equal to the maximum paging time held in the registry. However, note that as the registry is dynamic and resources could be updated or deleted a given cursor MAY return fewer results than it did previously. When using update ordering and a query filter, new data MAY appear when cursors are used for a second time.

When query parameters which perform filtering are used at the same time as paging, the filters MUST be applied by the implementation before applying paging parameters to the resulting data set.

A server MAY choose its own default value for paging limit (see Example 1).

Where both since and until parameters are specified, the since value takes precedence where a resulting data set is constrained by the server's value of limit (see Example 5).

Servers SHOULD return a Link header with prev and next links in all responses and MAY include first and last links.

Examples

The following examples show pagination for a set of registered data. In order to avoid displaying full resource representations, the only data listed here is the update timestamp associated with each registered record. The same procedures can be applied where creation timestamps and the paging.order=create query parameter are used instead.

Where a paging limit is not specified in a request the server's default is used.

Sample Data: Registered Node Update Timestamps (Comma-Separated)

[0:1, 0:2, 0:3, 0:4, 0:5, 0:6, 0:7, 0:8, 0:9, 0:10, 0:11, 0:12, 0:13, 0:14, 0:15, 0:16, 0:17, 0:18, 0:19, 0:20]

Each of the above corresponds to the update timestamp of a corresponding Node, in the format <seconds>:<nanoseconds> and displayed in ascending order. These will be used throughout the following examples.

Response payloads in the examples will show these values, but in a real implementation would be replaced by the corresponding JSON objects for the Nodes or other resources being queried.

Example 1: Initial /nodes Request

In this example there are no query parameters used in the request, but as the Query API supports pagination it returns a subset of the results with headers identifying how to page further into the collection.

Request

GET /x-nmos/query/v1.1/nodes

Response

Headers

Link: <http://api.example.com/x-nmos/query/v1.1/nodes/?paging.since=0:20&paging.limit=10>; rel="next", <http://api.example.com/x-nmos/query/v1.1/nodes/?paging.until=0:10&paging.limit=10>; rel="prev"
X-Paging-Limit: 10
X-Paging-Since: 0:10
X-Paging-Until: 0:20

Payload Resources

[
  0:20,
  0:19,
  0:18,
  0:17,
  0:16,
  0:15,
  0:14,
  0:13,
  0:12,
  0:11
]

Notes

The data set returned when no paging.since or paging.until parameters are specified MUST be from the most recently updated (or created) resources in the collection, returned in descending order.
The X-Paging- headers identify properties of the collection of data returned in the response. These parameters can be used to construct a URL which would return the same set of bounded data on consecutive requests, for example:

/x-nmos/query/v1.1/nodes?paging.since=0:10&paging.until=0:20

The Link header identifies the next and prev cursors which an application can use to make its next requests. An implementation MAY also provide first and last cursors to identify the beginning and end of the collection. The last cursor URL returns the most recently updated (or created) resources, as when no paging.since or paging.until parameters are specified. The first cursor URL returns the least recently updated (or created) resources, as when paging.since=0:0 is specified.

Example 2: Request With Custom Limit

This request is similar to Example 1, but the client has chosen to use a custom page size limit.

Request

GET /x-nmos/query/v1.1/nodes?paging.limit=5

Response

Headers

Link: <http://api.example.com/x-nmos/query/v1.1/nodes/?paging.since=0:20&paging.limit=5>; rel="next", <http://api.example.com/x-nmos/query/v1.1/nodes/?paging.until=0:15&paging.limit=5>; rel="prev"
X-Paging-Limit: 5
X-Paging-Since: 0:15
X-Paging-Until: 0:20

Payload Resources

[
  0:20,
  0:19,
  0:18,
  0:17,
  0:16
]

Notes

In this case the server has accepted the client's paging size limit request. If the client had requested a page size which the server was unable to honour, the actual page size used would be returned in X-Paging-Limit.

Example 3: Request With Since Parameter

Request

GET /x-nmos/query/v1.1/nodes?paging.since=0:4

Response

Headers

Link: <http://api.example.com/x-nmos/query/v1.1/nodes/?paging.since=0:14&paging.limit=10>; rel="next", <http://api.example.com/x-nmos/query/v1.1/nodes/?paging.until=0:4&paging.limit=10>; rel="prev"
X-Paging-Limit: 10
X-Paging-Since: 0:4
X-Paging-Until: 0:14

Payload Resources

[
  0:14,
  0:13,
  0:12,
  0:11,
  0:10,
  0:9,
  0:8,
  0:7,
  0:6,
  0:5
]

Example 4: Request With Until Parameter

Request

GET /x-nmos/query/v1.1/nodes?paging.until=0:16

Response

Headers

Link: <http://api.example.com/x-nmos/query/v1.1/nodes/?paging.since=0:16&paging.limit=10>; rel="next", <http://api.example.com/x-nmos/query/v1.1/nodes/?paging.until=0:6&paging.limit=10>; rel="prev"
X-Paging-Limit: 10
X-Paging-Since: 0:6
X-Paging-Until: 0:16

Payload Resources

[
  0:16,
  0:15,
  0:14,
  0:13,
  0:12,
  0:11,
  0:10,
  0:9,
  0:8,
  0:7
]

Example 5: Request With Since & Until Parameters

Request

GET /x-nmos/query/v1.1/nodes?paging.since=0:4&paging.until=0:16

Response

Headers

Link: <http://api.example.com/x-nmos/query/v1.1/nodes/?paging.since=0:14&paging.limit=10>; rel="next", <http://api.example.com/x-nmos/query/v1.1/nodes/?paging.until=0:4&paging.limit=10>; rel="prev"
X-Paging-Limit: 10
X-Paging-Since: 0:4
X-Paging-Until: 0:14

Payload Resources

[
  0:14,
  0:13,
  0:12,
  0:11,
  0:10,
  0:9,
  0:8,
  0:7,
  0:6,
  0:5
]

Notes

Whilst both since and until are specified, as this server example has a default paging limit of 10, the since parameter takes precedence. As a result of this the value of X-Paging-Until is lower than requested and a further request can be made to retrieve any remaining data.

Edge Cases

When a client requests data which falls at the extreme ends of the stored data set it can be less clear what values are expected to be returned in the X-Paging-Limit and X-Paging-Since headers. The following examples are intended to clarify these cases.

Where a paging limit is not specified in a request the server's default is used.

Example 1: Client request occurs at the beginning of the data set

Request

GET /x-nmos/query/v1.1/nodes?paging.until=0:20

In this case, assume that there are stored records for 0:21 and 0:22 but no earlier.

Response

Headers

Link: <http://api.example.com/x-nmos/query/v1.1/nodes/?paging.since=0:20&paging.limit=10>; rel="next", <http://api.example.com/x-nmos/query/v1.1/nodes/?paging.until=0:0&paging.limit=10>; rel="prev"
X-Paging-Limit: 10
X-Paging-Since: 0:0
X-Paging-Until: 0:20

Payload Resources

[]

Example 2: Client request occurs at the end of the data set

Request

GET /x-nmos/query/v1.1/nodes?paging.since=0:20

In this case, assume that there are stored records for 0:19 and 0:20 but no later.

Response

Headers

Link: <http://api.example.com/x-nmos/query/v1.1/nodes/?paging.since=0:20&paging.limit=10>; rel="next", <http://api.example.com/x-nmos/query/v1.1/nodes/?paging.until=0:20&paging.limit=10>; rel="prev"
X-Paging-Limit: 10
X-Paging-Since: 0:20
X-Paging-Until: 0:20

Payload Resources

[]

Notes:

In this situation the client is expected to re-perform the same request (as specified in the next cursor) until data is returned. If the client were to increment the value of since requested it would be in danger of moving ahead of the current time and missing records.

Example 3: Client request includes a query parameter resulting in a single result, but no paging parameters

Request

GET /x-nmos/query/v1.1/nodes?label=My%20Node

In this case, assume that the most recently created or updated Node held in the registry has a paging value of 0:20 associated with it. Also assume that the Node with label My Node has a paging time of 0:15.

Response

Headers

Link: <http://api.example.com/x-nmos/query/v1.1/nodes/?label=My%20Node&paging.since=0:20&paging.limit=10>; rel="next", <http://api.example.com/x-nmos/query/v1.1/nodes/?label=My%20Node&paging.until=0:0&paging.limit=10>; rel="prev"
X-Paging-Limit: 10
X-Paging-Since: 0:0
X-Paging-Until: 0:20

Payload Resources

[
  0:15
]

Example 4: Client request includes a query parameter resulting in a no result, with no paging parameters

Request

GET /x-nmos/query/v1.1/nodes?label=My%20Invalid%20Node

In this case, assume that the most recently created or updated Node held in the registry has a paging value of 0:20 associated with it.

Response

Headers

Link: <http://api.example.com/x-nmos/query/v1.1/nodes/?label=My%20Invalid%20Node&paging.since=0:20&paging.limit=10>; rel="next", <http://api.example.com/x-nmos/query/v1.1/nodes/?label=My%20Invalid%20Node&paging.until=0:0&paging.limit=10>; rel="prev"
X-Paging-Limit: 10
X-Paging-Since: 0:0
X-Paging-Until: 0:20

Payload Resources

[]

Downgrade Queries

Query APIs SHOULD support downgrade queries against their API resources where the downgrade trait is specified in the RAML documentation. A 501 (Not Implemented) HTTP status code MUST be returned where a downgrade query is attempted against a Query API which does not implement it.

In order to streamline upgrades from one API version to another a Query API MAY sit in front of a registry which holds registered data matching multiple API versions' schemas. By default the Query API MUST only return data matching the API version specified in the request URL, however downgrade queries permit old-versioned responses to be provided to clients which are confident that they can handle any missing attributes between the specified API versions.

Downgrades MUST only be performed between minor API versions as major versions might remove or re-purpose attributes. Clients which support multiple major API versions SHOULD retrieve this data via multiple HTTP requests or WebSocket subscriptions.

Examples

Example 1: No Downgrade Parameter

Request

GET /x-nmos/query/v1.1/nodes

Response

Returns all Nodes held in the registry which match the v1.1 schema.
Returns Nodes with versions greater than or equal to v1.2, and less than v2.0. These Nodes MUST have all non-v1.1 keys stripped by the Query API before they are returned.

Example 2: Downgrade From v1.1 to v1.0

Request

GET /x-nmos/query/v1.1/nodes?query.downgrade=v1.0

Response

Returns all Nodes held in the registry which match the v1.1 schema.
Returns Nodes with versions greater than or equal to v1.2, and less than v2.0. These Nodes MUST have all non-v1.1 keys stripped by the Query API before they are returned.
Returns all Nodes which are registered in v1.0 format.

Example 3: Downgrade From v1.3 to v1.1

Request

GET /x-nmos/query/v1.3/flows?query.downgrade=v1.1

Response

Returns all Flows held in the registry which match the v1.3 schema.
Returns Flows with versions greater than or equal to v1.4, and less than v2.0. These Flows MUST have all non-v1.3 keys stripped by the Query API before they are returned.
Returns all Flows which are registered in v1.2 or v1.1 format.

Invalid Examples

Invalid Example 1: Downgrade Between Major API Versions

Request

GET /x-nmos/query/v3.0/flows?query.downgrade=v1.0

Response

Returns an HTTP 400 (Bad Request) error code as downgrade queries MUST NOT be performed between major API versions.

Basic Queries

Query APIs SHOULD support basic queries against their API resources. A 501 (Not Implemented) HTTP status code MUST be returned where a basic query is attempted against a Query API which does not implement it.

Basic queries make use of standard HTTP GET query parameters in the form ?key1=value1&key2=value2. Keys match any attribute which the API schemas indicate could be returned by a given resource.

Any attribute which could be returned by a particular API resource SHOULD be available to use as a query parameter, however the RAML documentation only explicitly identifies core attributes under queryParameters.

If a query parameter is requested which does not match an attribute found in any resource, an empty result set MUST be returned.

Querying Within Objects
Querying using attributes of objects is permitted via the use of a . separator. For example, the /receivers resource can be queried with:
```
?subscription.sender_id=2683ad14-642f-459d-a169-ef91c76cec6b
```
Querying Within Arrays
Querying using attributes of objects held within arrays is permitted via the use of a . separator. For example, the /nodes resource can be queried with:
```
?services.type=urn:x-manufacturer:service:tally
```
More advanced queries within arrays might require use of the Resource Query Language in() operator.

Examples

Example 1: Basic Query Using One Parameter

Request

GET /x-nmos/query/v1.0/senders?transport=urn:x-nmos:transport:rtp

Response

Returns all Sender objects which have an attribute transport which exactly matches the string urn:x-nmos:transport:rtp.

Example 2: Basic Query Using Two Parameters

Request

GET /x-nmos/query/v1.0/sources?format=urn:x-nmos:format:video&device_id=9126cc2f-4c26-4c9b-a6cd-93c4381c9be5

Response

Returns all Source objects which have an attribute of format which exactly matches urn:x-nmos:format:video and a device_id attribute which exactly matches 9126cc2f-4c26-4c9b-a6cd-93c4381c9be5.

Example 3: Querying Within Objects

Request

GET /x-nmos/query/v1.0/flows?tags.studio=HQ1

Response

Returns all Flows which have a tags attribute with a key of studio. The value of tags.studio (which is an array in this case, see Flow schema) MUST contain HQ1 as one of its entries.

Example 4: Querying Within Arrays

Request

GET /x-nmos/query/v1.0/nodes?services.type=urn:x-manufacturer:service:myservice

Response

The schema defines a Node's services as an array of objects, where type is a key in these inner objects. As a result, this query returns all Nodes where one of these service objects has a type of urn:x-manufacturer:service:myservice.

Invalid Examples

Invalid Example 1: Duplicate Query Parameters

Request

GET /x-nmos/query/v1.0/flows?tags.location=Salford&tags.location=London

Response

This query specifies two matching query parameters with different values. As different programming frameworks might not make both of these values available, this is an invalid query for which the response is not defined and might vary by implementation. The next section provides a means of achieving the desired query.

Advanced (RQL) Queries (OPTIONAL)

Query APIs MAY support Resource Query Language (RQL, https://github.com/persvr/rql/) queries against their API resources where the rql trait is specified in the RAML documentation. A 501 (Not Implemented) HTTP status code MUST be returned where an RQL query is attempted using RQL functions or operators which are not supported by a Query API.

RQL SHOULD be formatted in the normalised form as opposed to using FIQL syntax, and passed via the query string using a query.rql=... query parameter.

Querying within objects and arrays is performed as in the Basic Queries case by using the . separator.

When an RQL query is specified, the Basic Queries format MAY be ignored in order to simplify implementation.

Note that as RQL permits the definition of very complex queries, the server MAY return a 400 (Bad Request) error code to indicate that it is refusing to action the client's request due to the query's complexity.

Constraints

Not all RQL operators will be suitable for use with a Query API. It is suggested that the following operators are supported, however these are not mandated and implementations MAY choose to support their own subset.

Functions: in(), out(), select()
Logical Operators: and(), or(), not()
Relational Operators: eq(), ne(), gt(), ge(), lt(), le()

Note: It is suggested that sort() and limit() are not implemented as paging and associated limits are specified directly by other query parameters.

Examples

Example 1: Simple Query

Request

GET /x-nmos/query/v1.1/senders?query.rql=eq(transport,urn%3Ax-nmos%3Atransport%3Artp)

Response

Returns all Sender objects which have an attribute transport which exactly matches the string urn:x-nmos:transport:rtp. This response matches that of Example 1 in basic querying.

Example 2: Advanced Query

Request

GET /x-nmos/query/v1.1/sources?query.rql=and(eq(format,urn%3Ax-nmos%3Aformat%3Avideo),in(tags.location,(Salford,London)))

Response

Returns all Sources which have a format attribute equal to urn:x-nmos:format:video and a tags attribute with a key of location, which has values of Salford or London.

Ancestry Queries (OPTIONAL)

Query APIs MAY support Source and Flow ancestry queries against their API resources where the ancestry trait is specified in the RAML documentation. A 501 (Not Implemented) HTTP status code MUST be returned where an ancestry query is attempted against a Query API which does not implement it.

Sources and Flows list their parents in an array. A Query API implementing ancestry tracking MAY be queried using query.ancestry_... query parameters in order to identify parents or children of a given Source or Flow.

Examples

Example 1: Children Of A Source

Request

GET /x-nmos/query/v1.1/sources?query.ancestry_id=c1398579-15bc-468e-91ec-df5bbefe1cd3&query.ancestry_type=children

Response

Headers

X-Ancestry-Generations: 4

Payload

Returns Sources which list an ID of c1398579-15bc-468e-91ec-df5bbefe1cd3 in their parents attribute array. These children are the first generation descendants.
For each first generation descendant found, the ID of these Sources is used to look up the next generation by the parents arrays of further Sources.
This process continues up to the generation limit returned in the X-Ancestry-Generations header. The server MAY refuse to honour a requested query.ancestry_generations value using a 400 (Bad Request) error code if it is deemed to be too resource intensive.

Notes

Responses do not include the Source with ID specified in the ancestry_id parameter.

Example 2: Parents Of A Flow

Request

GET /x-nmos/query/v1.1/flows?query.ancestry_id=ad14888a-3a98-444c-8aa8-4d87b77cbaa1&query.ancestry_type=parents&query.ancestry_generations=2

Response

Headers

X-Ancestry-Generations: 2

Payload

Returns Flows which are listed in the parents array of the Flow identified by the ID ad14888a-3a98-444c-8aa8-4d87b77cbaa1. These parents are the first generation ancestors.
For each first generation ancestor found, their parents array is used to identify the next generation of ancestors to be returned.
This process continues up to the generation limit returned in the X-Ancestry-Generations header (in this case limited to 2 by a query parameter). The server MAY refuse to honour a requested query.ancestry_generations value using a 400 (Bad Request) error code if it is deemed to be too resource intensive.

Notes

Responses do not include the Flow with ID specified in the ancestry_id parameter.