Add some docs on VTGate buffering for MoveTables and Reshard traffic …

…switching

Signed-off-by: Matt Lord <mattalord@gmail.com>

content/en/docs/20.0/concepts/move-tables.md

@@ -2,7 +2,7 @@
 title: MoveTables
 ---
 
-MoveTables is a new workflow based on VReplication. It enables you to relocate tables between keyspaces, and therefore physical MySQL instances, without downtime.
+MoveTables is a workflow based on VReplication. It enables you to relocate tables between keyspaces, and therefore physical MySQL instances, without downtime.
 
 ## Identifying Candidate Tables
 

content/en/docs/20.0/reference/features/vtgate-buffering.md

@@ -82,6 +82,19 @@ endeavor to handle errors appropriately if/when they occur (e.g. unplanned
 outages/fail overs, etc.)
 {{< /warning >}}
 
+## What happens during a MoveTables or Reshard SwitchTraffic or ReverseTraffic with Buffering
+
+Fundamentally Vitess will:
+
+ * Hold up and buffer any queries sent to the `PRIMARY` tablet for each shard involved.
+ * Perform the traffic switching work so that application traffic against the tables (MoveTables) or shards (Reshard) are transparently switched to the new keyspace (MoveTables) or shards (Reshard).
+ * Drain the buffered queries to the new keyspace or shards — or if the switch failed then back to the original keyspace or shards.
+
+{{< warning >}}
+This process is not guaranteed to eliminate errors to the application, but
+rather reduce them or make them less frequent. The application should still
+endeavor to handle errors appropriately if/when they occur.
+{{< /warning >}}
 
 ## How does it work?
 

content/en/docs/20.0/reference/vreplication/movetables.md

@@ -80,13 +80,21 @@ It is too expensive to get real-time row counts of tables, using _count(*)_, say
 
 [`switchtraffic`](../../programs/vtctldclient/vtctldclient_movetables/vtctldclient_movetables_switchtraffic/) switches traffic forward for the `tablet-types` specified. You can switch all traffic with just one command, and this is the default behavior. Note that you can now switch replica, rdonly, and primary traffic in any order.
 
+{{< info >}}
+Note that VTGate can [buffer queries](../../features/vtgate-buffering/) when switching traffic which can virtually eliminate any visible impact on application users.
+{{</ info >}}
+
 </div>
 
 #### ReverseTraffic
 <div class="cmd">
 
 [`reversetraffic`](../../programs/vtctldclient/vtctldclient_movetables/vtctldclient_movetables_reversetraffic/) switches traffic in the reverse direction for the `tablet-types` specified. The traffic should have been previously switched forward using `SwitchTraffic` for the `cells` and `tablet-types` specified.
 
+{{< info >}}
+Note that VTGate can [buffer queries](../../features/vtgate-buffering/) when switching traffic which can virtually eliminate any visible impact on application users.
+{{</ info >}}
+
 </div>
 
 #### Cancel

content/en/docs/20.0/reference/vreplication/reshard.md

@@ -64,13 +64,21 @@ It is too expensive to get real-time row counts of tables, using _count(*)_, say
 
 [`SwitchTraffic`](../../programs/vtctldclient/vtctldclient_movetables/vtctldclient_movetables_switchtraffic) switches traffic forward for the `tablet-types` specified. This replaces the previous `SwitchReads` and `SwitchWrites` commands with a single one. It is now possible to switch all traffic with just one command, and this is the default behavior. Also, you can now switch replica, rdonly and primary traffic in any order: earlier you needed to first `SwitchReads` (for replicas and rdonly tablets) first before `SwitchWrites`.
 
+{{< info >}}
+Note that VTGate can [buffer queries](../../features/vtgate-buffering/) when switching traffic which can virtually eliminate any visible impact on application users.
+{{</ info >}}
+
 </div>
 
 #### ReverseTraffic
 <div class="cmd">
 
 [`ReverseTraffic`](../../programs/vtctldclient/vtctldclient_movetables/vtctldclient_movetables_reversetraffic/) switches traffic in the reverse direction for the `tablet-types` specified. The traffic should have been previously switched forward using `SwitchTraffic` for the `cells` and `tablet_types` specified.
 
+{{< info >}}
+Note that VTGate can [buffer queries](../../features/vtgate-buffering/) when switching traffic which can virtually eliminate any visible impact on application users.
+{{</ info >}}
+
 </div>
 
 #### Cancel