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website: document multi-DC, caching, clarify prepared queries and 

multi-DC
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Mitchell Hashimoto 2018-06-06 15:10:52 -07:00 committed by Jack Pearkes
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51
website/source/docs/connect/index.html.md
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@ -64,3 +64,54 @@ APIs are all made to the local Consul agent over a loopback interface, and all
local caching, background updating, and support blocking queries. As a result,
most API calls operate on purely local in-memory data and can respond
in microseconds.
## Agent Caching and Performance
To enable microsecond-speed responses on
[agent Connect API endpoints](/api/agent/connect.html), the Consul agent
locally caches most Connect-related data and sets up background
[blocking queries](/api/index.html#blocking-queries) against the server
to update the cache in the background. This allows most API calls such
as retrieving certificates or authorizing connections to use in-memory
data and respond very quickly.
All data cached locally by the agent is populated on demand. Therefore,
if Connect is not used at all, the cache does not store any data. On first
request, the data is loaded from the server and cached. The set of data cached
is: public CA root certificates, leaf certificates, and intentions. For
leaf certificates and intentions, only data related to the service requested
is cached, not the full set of data.
Further, the cache is partitioned by ACL token and datacenters. This is done
to minimize the complexity of the cache and prevent bugs where an ACL token
may see data it shouldn't from the cache. This results in higher memory usage
for cached data since it is duplicated per ACL token, but with the benefit
of simplicity and security.
With Connect enabled, you'll likely see increased memory usage by the
local Consul agent. The total memory is dependent on the number of intentions
related to the services registered with the agent accepting Connect-based
connections. The other data (leaf certificates and public CA certificates)
is a relatively fixed size per service. In most cases, the overhead per
service should be relatively small: single digit kilobytes at most.
The cache does not evict entries due to memory pressure. If memory capacity
is reached, the process will attempt to swap. If swap is disabled, the Consul
agent may begin failing and eventually crash. Cache entries do have TTLs
associated with them and will evict their entries if they're not used. Given
a long period of inactivity (3 days by default), the cache will empty itself.
## Multi-Datacenter
Connect currently only works for service-to-service connections wtihin a
single Consul datacenter. Connect may be enabled on multiple Consul datacenters,
but only services within the same datacenters can establish Connect-based
connections.
CA configurations and intentions are both local to their respective datacenters;
they are not replicated across datacenters.
Multi-datacenter support for Connect is under development and will be
released as a feature of Consul Enterprise in late 2018. This feature will
facilitate intention replication, datacenter constraints on intentions,
CA state replication, multi-datacenter certificate rotations, and more.

6
website/source/docs/connect/proxies.html.md
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@ -133,6 +133,12 @@ service.
}
```
-> **Note:** Connect does not currently support cross-datacenter
service communication. Therefore, prepared queries with Connect should
only be used to discover services within a single datacenter. See
[Multi-Datacenter Connect](/docs/connect/index.html#multi-datacenter) for
more information.
### Dynamic Upstreams
If an application requires dynamic dependencies that are only available