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docs: add cluster warming information #541

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Mar 14, 2018
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docs: add cluster warming information #541

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docs: add cluster warming information
mattklein123 Mar 13, 2018
813fd09
Merge remote-tracking branch 'origin/master' into warming_docs
mattklein123 Mar 14, 2018
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mattklein123 Mar 14, 2018
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3 changes: 2 additions & 1 deletion docs/root/configuration/cluster_manager/cluster_stats.rst
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Expand Up @@ -19,7 +19,8 @@ statistics. Any ``:`` character in the stats name is replaced with ``_``.
cluster_added, Counter, Total clusters added (either via static config or CDS)
cluster_modified, Counter, Total clusters modified (via CDS)
cluster_removed, Counter, Total clusters removed (via CDS)
total_clusters, Gauge, Number of currently loaded clusters
active_clusters, Gauge, Number of currently active (warmed) clusters
warming_clusters, Gauge, Number of currently warming (not active) clusters

Every cluster has a statistics tree rooted at *cluster.<name>.* with the following statistics:

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23 changes: 23 additions & 0 deletions docs/root/intro/arch_overview/cluster_manager.rst
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Expand Up @@ -24,3 +24,26 @@ distribution.

* Cluster manager :ref:`configuration <config_cluster_manager>`.
* CDS :ref:`configuration <config_cluster_manager_cds>`.

Cluster warming
---------------

When clusters are initialized both at server boot as well as via CDS, they are "warmed." This means
that clusters do not become available until the following operations have taken place.
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It is not immediately clear what availability implies.. Does cluster unavailability imply that:

  • routes that reference the cluster fail validate_clusters check, and
  • listeners that reference the cluster remain in the warming state?


* Initial service discovery load (e.g., DNS resolution, EDS update, etc.).
* Initial active :ref:`health check <arch_overview_health_checking>` pass if active health checking
is configured. Envoy will send a health check request to each discovered host to determine its
initial health status.

The previous items ensure that Envoy has an accurate view of a cluster before it begins using it
for traffic serving.

When discussing cluster warming, the cluster "becoming available" means:

* For newly added clusters, the cluster will not appear to exist to the rest of Envoy until it has
been warmed. I.e., HTTP routes that reference the cluster will result in either a 404 or 503
(depending on configuration).
* For updated clusters, the old cluster will continue to exist and serve traffic. When the new
cluster has been warmed, it will be atomically swapped with the old cluster such that no
traffic interruptions take place.