The management server should only send updates to the Envoy client when
the resources in the :ref:`DiscoveryResponse <envoy_api_msg_DiscoveryResponse>` have changed. Envoy replies
to any :ref:`DiscoveryResponse <envoy_api_msg_DiscoveryResponse>` with a :ref:`DiscoveryRequest <envoy_api_msg_DiscoveryRequest>` containing the
ACK/NACK immediately after it has been either accepted or rejected. If
the management server provides the same set of resources rather than
waiting for a change to occur, it will cause Envoy and the management
server to spin and have a severe performance impact.
Within a stream, new :ref:`DiscoveryRequests <envoy_api_msg_DiscoveryRequest>` supersede any prior
:ref:`DiscoveryRequests <envoy_api_msg_DiscoveryRequest>` having the same resource type. This means that
the management server only needs to respond to the latest
:ref:`DiscoveryRequest <envoy_api_msg_DiscoveryRequest>` on each stream for any given resource type.
Resource hints
^^^^^^^^^^^^^^
The :ref:`resource_names <envoy_api_field_DiscoveryRequest.resource_names>` specified in the :ref:`DiscoveryRequest <envoy_api_msg_DiscoveryRequest>` are a hint.
the expectations during warming. The management server may be able to
infer all the required EDS/RDS resources from the :ref:`node <envoy_api_msg_Core.Node>`
identification in the :ref:`DiscoveryRequest <envoy_api_msg_DiscoveryRequest>`, in which case this hint may
be discarded. An empty EDS/RDS :ref:`DiscoveryResponse <envoy_api_msg_DiscoveryResponse>` is effectively a
nop from the perspective of the respective resources in the Envoy.
When a `Listener` or `Cluster` is deleted, its corresponding EDS and
RDS resources are also deleted inside the Envoy instance. In order for
EDS resources to be known or tracked by Envoy, there must exist an
applied `Cluster` definition (e.g. sourced via CDS). A similar
relationship exists between RDS and `Listeners` (e.g. sourced via
LDS).
For EDS/RDS, Envoy may either generate a distinct stream for each
resource of a given type (e.g. if each :ref:`ConfigSource <envoy_api_msg_core.ConfigSource>` has its own
distinct upstream cluster for a management server), or may combine
together multiple resource requests for a given resource type when they
are destined for the same management server. While this is left to
implementation specifics, management servers should be capable of
handling one or more :ref:`resource_names <envoy_api_field_DiscoveryRequest.resource_names>` for a given resource type in
each request. Both sequence diagrams below are valid for fetching two
EDS resources `{foo, bar}`:
|Multiple EDS requests on the same stream| |Multiple EDS requests on
distinct streams|
Resource updates
^^^^^^^^^^^^^^^^
As discussed above, Envoy may update the list of :ref:`resource_names <envoy_api_field_DiscoveryRequest.resource_names>` it
presents to the management server in each :ref:`DiscoveryRequest <envoy_api_msg_DiscoveryRequest>` that
ACK/NACKs a specific :ref:`DiscoveryResponse <envoy_api_msg_DiscoveryResponse>`. In addition, Envoy may later
issue additional :ref:`DiscoveryRequests <envoy_api_msg_DiscoveryRequest>` at a given :ref:`version_info <envoy_api_field_DiscoveryRequest.version_info>` to
update the management server with new resource hints. For example, if
Envoy is at EDS version **X** and knows only about cluster ``foo``, but
then receives a CDS update and learns about ``bar`` in addition, it may
issue an additional :ref:`DiscoveryRequest <envoy_api_msg_DiscoveryRequest>` for **X** with `{foo,bar}` as
`resource_names`.
..figure:: diagrams/cds-eds-resources.svg
:alt:CDS response leads to EDS resource hint update
There is a race condition that may arise here; if after a resource hint
update is issued by Envoy at **X**, but before the management server
processes the update it replies with a new version **Y**, the resource
hint update may be interpreted as a rejection of **Y** by presenting an
**X**:ref:`version_info <envoy_api_field_DiscoveryResponse.version_info>`. To avoid this, the management server provides a
``nonce`` that Envoy uses to indicate the specific :ref:`DiscoveryResponse <envoy_api_msg_DiscoveryResponse>`
each :ref:`DiscoveryRequest <envoy_api_msg_DiscoveryRequest>` corresponds to:
..figure:: diagrams/update-race.svg
:alt:EDS update race motivates nonces
The management server should not send a :ref:`DiscoveryResponse <envoy_api_msg_DiscoveryResponse>` for any
:ref:`DiscoveryRequest <envoy_api_msg_DiscoveryRequest>` that has a stale nonce. A nonce becomes stale
following a newer nonce being presented to Envoy in a
:ref:`DiscoveryResponse <envoy_api_msg_DiscoveryResponse>`. A management server does not need to send an
update until it determines a new version is available. Earlier requests
at a version then also become stale. It may process multiple
:ref:`DiscoveryRequests <envoy_api_msg_DiscoveryRequest>` at a version until a new version is ready.
..figure:: diagrams/stale-requests.svg
:alt:Requests become stale
An implication of the above resource update sequencing is that Envoy
does not expect a :ref:`DiscoveryResponse <envoy_api_msg_DiscoveryResponse>` for every :ref:`DiscoveryRequests <envoy_api_msg_DiscoveryRequest>`
- Allows the protocol to communicate on the wire in terms of
resource/resource name deltas ("Delta xDS"). This supports the goal
of scalability of xDS resources. Rather than deliver all 100k
clusters when a single cluster is modified, the management server
only needs to deliver the single cluster that changed.
- Allows the Envoy to on-demand / lazily request additional resources.
For example, requesting a cluster only when a request for that
cluster arrives.
An Incremental xDS session is always in the context of a gRPC
bidirectional stream. This allows the xDS server to keep track of the
state of xDS clients connected to it. There is no REST version of
Incremental xDS yet.
In the delta xDS wire protocol, the nonce field is required and used to
pair a :ref:`DeltaDiscoveryResponse <envoy_api_msg_DeltaDiscoveryResponse>`
to a :ref:`DeltaDiscoveryRequest <envoy_api_msg_DeltaDiscoveryRequest>`
ACK or NACK. Optionally, a response message level :ref:`system_version_info <envoy_api_field_DeltaDiscoveryResponse.system_version_info>`
is present for debugging purposes only.
:ref:`DeltaDiscoveryRequest <envoy_api_msg_DeltaDiscoveryRequest>` can be sent in the following situations:
- Initial message in a xDS bidirectional gRPC stream.
- As an ACK or NACK response to a previous :ref:`DeltaDiscoveryResponse <envoy_api_msg_DeltaDiscoveryResponse>`. In this case the :ref:`response_nonce <envoy_api_field_DiscoveryRequest.response_nonce>` is set to the nonce value in the Response. ACK or NACK is determined by the absence or presence of :ref:`error_detail <envoy_api_field_DiscoveryRequest.error_detail>`.
- Spontaneous :ref:`DeltaDiscoveryRequests <envoy_api_msg_DeltaDiscoveryRequest>` from the client. This can be done to dynamically add or remove elements from the tracked :ref:`resource_names <envoy_api_field_DiscoveryRequest.resource_names>` set. In this case :ref:`response_nonce <envoy_api_field_DiscoveryRequest.response_nonce>` must be omitted.
In this first example the client connects and receives a first update
that it ACKs. The second update fails and the client NACKs the update.
Later the xDS client spontaneously requests the "wc" resource.
..figure:: diagrams/incremental.svg
:alt:Incremental session example
On reconnect the Incremental xDS client may tell the server of its known
resources to avoid resending them over the network. Because no state is
assumed to be preserved from the previous stream, the reconnecting
client must provide the server with all resource names it is interested
in.
..figure:: diagrams/incremental-reconnect.svg
:alt:Incremental reconnect example
Resource names
^^^^^^^^^^^^^^
Resources are identified by a resource name or an alias. Aliases of a
resource, if present, can be identified by the alias field in the
resource of a :ref:`DeltaDiscoveryResponse <envoy_api_msg_DeltaDiscoveryResponse>`. The resource name will be
The client can send either an alias or the name of a resource in the
:ref:`resource_names_subscribe <envoy_api_field_DeltaDiscoveryRequest.resource_names_subscribe>` field of a :ref:`DeltaDiscoveryRequest <envoy_api_msg_DeltaDiscoveryRequest>` in
order to subscribe to a resource. Both the names and aliases of
resources should be checked in order to determine whether the entity in
question has been subscribed to.
A :ref:`resource_names_subscribe <envoy_api_field_DeltaDiscoveryRequest.resource_names_subscribe>` field may contain resource names that the
server believes the client is already subscribed to, and furthermore has
the most recent versions of. However, the server *must* still provide
those resources in the response; due to implementation details hidden
from the server, the client may have "forgotten" those resources despite
When a client loses interest in some resources, it will indicate that
with the :ref:`resource_names_unsubscribe <envoy_api_field_DeltaDiscoveryRequest.resource_names_unsubscribe>` field of a
:ref:`DeltaDiscoveryRequest <envoy_api_msg_DeltaDiscoveryRequest>`. As with :ref:`resource_names_subscribe <envoy_api_field_DeltaDiscoveryRequest.resource_names_subscribe>`, these
may be resource names or aliases.
A :ref:`resource_names_unsubscribe <envoy_api_field_DeltaDiscoveryRequest.resource_names_unsubscribe>` field may contain superfluous resource
names, which the server thought the client was already not subscribed
to. The server must cleanly process such a request; it can simply ignore
these phantom unsubscriptions.
REST-JSON polling subscriptions
-------------------------------
Synchronous (long) polling via REST endpoints is also available for the
xDS singleton APIs. The above sequencing of messages is similar, except
no persistent stream is maintained to the management server. It is
expected that there is only a single outstanding request at any point in
time, and as a result the response nonce is optional in REST-JSON. The
