Working Draft - September 2024
One of the disadvantages of GraphQL’s request/response model is that applications which retrieve large datasets may suffer from latency. However not all requested data may be of equal importance, and in some use cases it may be possible for applications to act on a subset of the requested data.
Today applications which seek to ensure that data of high importance can be retrieved quickly have a few options:
- Splitting queries
- Prefetching
However each of these solutions imposes some undesirable trade-offs on applications.
Given a query where some fields are expensive and non-essential, non-essential fields can be fetched in a separate query issued after an initial query. In the context of lists, this may mean fetching only the first few items in a list and issuing a follow-up pagination request for additional items. By separating the requests based on data priority, we have achieved a faster delivery time to essential data. However, query splitting imposes several trade-offs on applications:
Increased latency for lower priority fields, as now these fields have to wait for the original query to complete and have added network round trips Client resource contention. Issuing multiple requests can increase contention on scarce resources like battery and antenna. Increased cost. Processing additional requests can increase server costs by putting pressure on both the middle tier and the data layer. For example a follow-up query may need to load a subset of the same data loaded by the initial query, putting additional pressure on data stores.
This technique involves optimistically fetching data based on a prediction that a user will execute an action. Prefetching can be one of the most effective ways of reducing latency. However, a significant tradeoff with prefetching that a lot of applications cannot afford is increased server cost due to incorrect predictions. With an unsophisticated prefetch algorithm, applications can easily overfetch by a factor of 10 fold.
This proposal would introduce @stream and @defer directives which clients could use to communicate the relative priority of requested data to GraphQL implementations. Furthermore this proposal would enable GraphQL APIs to split requested data across multiple response payloads in order of priority. The goal of this proposal is to enable applications to reduce latency without increasing server cost or resource contention.
While both incremental delivery and GraphQL subscriptions send multiple payloads over time, incremental delivery is not intended to enable applications to respond to real-time changes. Consequently streams opened for incremental delivery are expected to be short-lived. Implementations are not required to reflect interleaving mutations which occur during incremental delivery. Assuming there are no interleaving mutations, combining together the various payloads in an incrementally delivered response should produce the same output as if that response was not delivered incrementally.
Facebook has been using Incremental Delivery at scale since 2017, including on major surfaces such as news feed. This proposal captures the key concepts that we have found to be useful.
GraphQL servers will not be required to implement @defer and/or @stream. If they are implemented, they will be required to follow the proposed specification. Servers that do not implement @defer and/or @stream should not expose these directives in their schema. Queries containing these directives that are sent to an unsupported server should fail validation.
The @defer directive may be specified on a fragment spread to imply de-prioritization, that causes the fragment to be omitted in the initial response, and delivered as a subsequent response afterward. A query with @defer directive will cause the request to potentially return multiple responses, where non-deferred data is delivered in the initial response and data deferred delivered in a subsequent response. @include and @skip take precedence over @defer.
if: Boolean- When
truefragment may be deferred, if omitted defaults totrue.
- When
label: String- A unique label across all
@deferand@streamdirectives in an operation. - This
labelshould be used by GraphQL clients to identify the data from patch responses and associate it with the correct fragment. - If provided, the GraphQL Server must add it to the payload.
- A unique label across all
The @stream directive may be provided for a field of List type so that the backend can leverage technology such asynchronous iterators to provide a partial list in the initial response, and additional list items in subsequent responses. @include and @skip take precedence over @stream.
if: Boolean- When
truefield may be streamed, if omitted defaults totrue.
- When
label: String- A unique label across all
@deferand@streamdirectives in an operation. - This
labelshould be used by GraphQL clients to identify the data from patch responses and associate it with the correct fragments. - If provided, the GraphQL Server must add it to the payload.
- A unique label across all
initialCount: Int- The number of list items the server should return as part of the initial response.
When an operation contains @defer or @stream directives, the GraphQL execution will return multiple payloads. The first payload is the same shape as a standard GraphQL response. Any fields that were only requested on a fragment that is deferred will not be present in this payload. Any list fields that are streamed will only contain the initial list items.
Each subsequent payload will be an object with the following properties:
pending: A list of objects that informs clients of payloads that will be incrementally delivered. Contains thepathandlabelof the correspondingdeferorstreamdirective, and a uniqueidthat can be used to associate with futureincrementalandcompleteobjects.incremental: A list of objects containing the data that is being delivered incrementally. Contains adatafield for data from a@defer, or anitemsfield for list items from a@stream. Thepathof this data is determined by concatenating thepathfrom the associatedpendingobject with the optionalsubPathfield.completed: A list of objects that informs clients that all of the incremental data for the associated@deferor@streamhas been delivered. Clients should use theidto determine the associated@deferor@streamfrom a previouspendingobject.hasNext: A boolean that is present andtruewhen there are more payloads that will be sent for this operation. The last payload in a multi-payload response must returnhasNext: false.hasNextmust not be present for single-payload responses to preserve backwards compatibility.errors: An array that will contain any field errors that are produced while executing the deferred or streamed selection set. If the error did not bubble above anincrementalobject boundary, it will be present on theincrementalobject. If an error bubbles above the boundary, causing the@deferor@streamto fail, it will be present on thecompletedobject.extensions: For implementers to extend the protocol.
Note: The label field is not a unique identifier for payloads. There may be multiple payloads with the same label for either payloads for @stream, or payloads from a @defer fragment under a list field. The combination of label and path will be unique among all payloads.
The ability to defer/stream parts of a response can have a potentially significant impact on application performance. Developers generally need clear, predictable control over their application's performance. It is highly recommended that the GraphQL server honor the @defer and @stream directives on each execution. However, the specification will allow advanced use-cases where the server can determine that it is more performant to not defer/stream. Therefore, GraphQL clients should be able to process a response that ignores the defer/stream directives.
This also applies to the initialCount argument on the @stream directive. Clients should be able to process a streamed response that contains a different number of initial list items than what was specified in the initialCount argument.
query {
person(id: "cGVvcGxlOjE=") {
...HomeWorldFragment @defer(label: "homeWorldDefer")
name
films @stream(initialCount: 2, label: "filmsStream") {
title
}
}
}
fragment HomeWorldFragment on Person {
homeworld {
name
}
}Response Payloads
Payload 1
{
"data": {
"person": {
"name": "Luke Skywalker",
"films": [
{ "title": "A New Hope" },
{ "title": "The Empire Strikes Back" }
]
}
},
"pending": [
{ "id": 0, "label": "homeWorldDefer", "path": ["person"] },
{ "id": 1, "label": "filmsStream", "path": ["person", "films"] }
],
"hasNext": true
}
Payload 2
{
"incremental":[
{
"id": "0",
"data": {
"homeworld": {
"name": "Tatooine"
}
}
}
],
"completed": [{"id": "0"}],
"hasNext": true
}Payload 3
{
"incremental":[
{
"id": "1",
"items": [{"title": "Return of the Jedi"}]
}
],
"completed": [{"id": "1"}],
"hasNext": false
}See more examples in graphql/defer-stream-wg#69
- Make GraphQL a great choice for applications which demand responsiveness.
- Enable interoperability between different GraphQL clients and servers without restricting implementation.
- Enable a strong tooling ecosystem (including GraphiQL).
- Provide concrete guidance to implementers.
- Provide guidance to developers evaluating whether to adopt incremental delivery.
The goal of incremental delivery is to prioritize the delivery of essential data. Even though incremental delivery delivers data over time, the response describes the data at a particular point in time. Therefore, it is not necessary to reflect real time changes to the data model in incremental delivery. Implementers of @defer and @stream are not obligated to address interleaving mutations during the execution of @defer and @stream.
GraphQL Subscription is an event-oriented approach to capture real time data changes. It intends to describe interesting events that happen over a period of time and delivers updated values that “invalidate” previous values.
For GraphQL communications built on top of HTTP, a natural and compatible technology to leverage is HTTP chunked encoding to implement a stream of responses for incremental delivery.
Supporting @defer can add complexity to type-generating clients. Separate types will need to be generated for the different deferred fragments. These clients will need to use the pending and completed fields to determine which fragments have been fulfilled to ensure the application is using the correct types.
The GraphQL spec does not currently support object identification or consistency. It is currently possible for the same object to be returned in multiple places in a query. If that object changes while the resolvers are running, the query could return inconsistent results. @defer/@stream does not increase the likelihood of this, as the server still attempts to resolve everything as fast as it can. The only difference is some results can be returned to the client sooner. This proposal does not attempt to address this issue.
This is currently a risk in GraphQL servers that do not implement any kind of query limiting as arbitrarily complex queries can be sent. Adding @defer may add some overhead as the server will now send parts of the query earlier than it would have without @defer, but it does not allow for any additional resolving that was not previously possible.
The first experimental implementation of @defer was in Apollo Server and only supported @defer on fields. When applying this to production code, we quickly found that most use-cases were to defer a group of multiple fields, and block the corresponding UI from rendering until all fields in the group have been resolved. This requires the client to track the loading state of each individual field, showing the loading indicator until it receives the asynchronous payloads for each field. Alternatively, a client could render the UI for each individual field as soon as it's ready. However, this could lead to a bad user experience, with the UI repainting and reflowing many times as the data is loaded.
By supporting @defer on fragments, the client is signaling to the server to return the results of the fields in the fragment, when the entire fragment has finished resolving. The client only needs to wait for this payload before rendering the corresponding UI.
For these reasons, this proposal only supports @defer on fragment spreads and inline fragments. If only a single field needs to be deferred, an inline fragment with @defer can easily wrap the field. By not allowing @defer on fields, we hope to discourage bad user experiences caused by many fields loading independently, and provide an easy upgrade path from deferring a single field to deferring many fields.
The GraphQL WG is not ruling out supporting @defer on fields in the future if additional use-cases are discovered, but it is no longer being considered for this proposal.
With incremental delivery, where multiple responses are delivered in one request, client code could re-render its UI multiple times in a short period of time. This could degrade performance of the application, negating the performance gains from using @defer or @stream. There are a few approaches that the spec allows that could be taken to mitigate this.
- Server sends batched responses. The incremental delivery spec directly supports returning batches of data. This gives the server the flexibility to determine when it is beneficial to group incremental responses together. If several responses are ready at the same time, the server can deliver them together. The server may also have knowledge of how long resolvers will take to resolve and could choose to debounce. It is also worth noting that a naive debouncing algorithm on the server could also result in degraded performance by introducing latency.
An example batched response:
query {
person(id: "cGVvcGxlOjE=") {
...HomeWorldFragment @defer(label: "homeWorldDefer")
name
films @stream(initialCount: 1, label: "filmsStream") {
title
}
}
}
fragment HomeWorldFragment on Person {
homeworld {
name
}
}Response Payloads
Payload 1
{
"data": {
"person": {
"name": "Luke Skywalker",
"films": [
{ "title": "A New Hope" },
]
}
},
"pending": [
{ "id": 0, "label": "homeWorldDefer", "path": ["person"] },
{ "id": 1, "label": "filmsStream", "path": ["person", "films"] }
]
"hasNext": true
}
Payload 2
{
"incremental":[
// data for `homeWorldDefer` and `filmsStream` batched in same incremental array
{
"id": "0",
"data": {
"homeworld": {
"name": "Tatooine"
}
},
},
{
"id": "1",
"items": [
// multiple streamed items batched into same incremental object
{"title": "The Empire Strikes Back"},
{"title": "Return of the Jedi"}
]
}
],
"completed": [{"id": "0"}, {"id": "1"}],
"hasNext": false
}- Server can ignore
@defer/@stream. This approach allows the GraphQL server to treat@deferand@streamas hints. The server can ignore these directives and include the deferred data in previous responses. This requires clients to be written with the expectation that deferred data could arrive in either its own incrementally delivered response or part of a previously delivered response. Clients must interpret the lack of apendingobject for a given@deferor@streamas an indication that the data has already been delivered and will not be delivered in a future incremental payload.