Docs: High Density Integration Pattern

Documentation explaining how to use the new advanced allocation features
to run multiple concurrent game sessions in a single `GameServer`
instance.

Closes googleforgames#1239

build/includes/website.mk

@@ -80,6 +80,7 @@ site-images: $(site_path)/static/diagrams/gameserver-reserved.puml.png
 site-images: $(site_path)/static/diagrams/canary-testing.puml.png
 site-images: $(site_path)/static/diagrams/allocation-player-capacity.puml.png
 site-images: $(site_path)/static/diagrams/reusing-gameservers.puml.png
+site-images: $(site_path)/static/diagrams/high-density.puml.png
 
 # generate pngs from dot files
 %.dot.png: %.dot

site/content/en/docs/Integration Patterns/high-density-gameservers.md

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+---
+title: "High Density GameServers"
+linkTitle: "High Density GameServers"
+date: 2021-08-31
+weight: 70
+description: >
+  How to run multiple concurrent game sessions in a single GameServer process.
+---
+
+{{< alpha title="Allocation Player Filter and Allocation State Filter" gate="PlayerAllocationFilter,StateAllocationFilter" >}}
+
+Depending on the setup and resource requirements of your game server process, sometimes it can be a more economical 
+use of resources to run multiple concurrent game sessions from within a single `GameServer` instance.
+
+The tradeoff here is that this requires more management on behalf of the integrated game server process and external 
+systems, since it works around the common Kubernetes and/or Agones container lifecycle.
+
+Utilising the new allocation `gameServerState` filter as well as the existing ability to edit the 
+`GameServer` Labels at both [allocation time]({{% ref "/docs/Reference/gameserverallocation.md" %}}), and from 
+within the game server process, [via the SDK]({{% ref "/docs/Guides/Client SDKs/_index.md#setlabelkey-value" %}}), 
+means Agones is able to atomically remove a `GameServer` from the list of potentially allocatable 
+`GameServers` at allocation time, and then return it back into the pool of allocatable `GameServers` if and when the 
+game server process deems that is has room to host another game session. 
+
+<a href="../../../diagrams/high-density.puml.png" target="_blank">
+<img src="../../../diagrams/high-density.puml.png" alt="High Density Allocation Diagram" />
+</a>
+
+{{< alert title="Info" color="info">}}
+To watch for Allocation events, there is the initial `GameServer.status.state` change from `Ready` to `Allocated`,
+but it is also useful to know that the value of `GameServer.metadata.annotations["agones.dev/last-allocated"]` will
+change with each allocation, regardless of if there is a state change or not.
+{{< /alert >}}
+
+## Example `GameServerAllocation`
+
+The below `Allocation` will first attempt to find a `GameServer` from the `Fleet` `simple-udp` that is already 
+Allocated and also has the label `agones.dev/sdk-available` with the value of `true`.
+
+The above condition indicates that the matching game server process behind the matched `GameServer` record is able to 
+accept another game session at this time.
+
+If the above Allocated `GameServer` does not exist, then allocated a Ready `GameServer` from the `simple-udp` `Fleet`.
+
+Whichever condition is met, once allocation is made against a `GameServer`, its label of `agones.dev/sdk-available` 
+will be set to the value of `false`, thereby removing it from any future allocations with the below schema.
+
+It will then be up to the game server process to decide on if and when it is appropriate to set the 
+`agones.dev/sdk-available` value back to `true`, thereby indicating that it can accept another concurrent 
+gameplay session. 
+
+```yaml
+apiVersion: "allocation.agones.dev/v1"
+kind: GameServerAllocation
+spec:
+  selectors:
+    - matchLabels:
+        agones.dev/fleet: simple-udp
+        agones.dev/sdk-available: "true" # this is important
+      gameServerState: Allocated # new state filter: allocate from Allocated servers
+    - matchLabels:
+        agones.dev/fleet: simple-udp
+      gameServerState: Ready # Allocate out of the Ready Pool (which would be default, so backward compatible)
+  metadata:
+    labels:
+      agones.dev/sdk-available: "false" # this removes it from the pool
+```
+
+{{< alert title="Info" color="info">}}
+It's important to note that the labels that the `GameServer` process use to add itself back into the pool of 
+allocatable instances, must start with the prefix `agones.dev/sdk-`, since only labels that have this prefix are 
+available to be [updated from the SDK]({{% ref "/docs/Guides/Client SDKs/_index.md#setlabelkey-value" %}}).
+{{< /alert >}}
+
+## Next Steps
+
+* View the details about [using the SDK]({{% ref "/docs/Guides/Client SDKs/_index.md#setlabelkey-value" %}}) to set 
+  labels on the `GameServer`.
+* Check all the options available on [`GameServerAllocation`]({{% ref "/docs/Reference/gameserverallocation.md" %}}).

site/static/diagrams/high-density.puml

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+@startuml
+participant Matchmaker
+participant Agones
+participant "Game Server\nProcess" as Binary
+participant SDK
+participant "GameServer\nResource" as GameServer
+box "Game Server Pod"
+  participant Binary
+  participant SDK
+end box
+
+== GameServer Start ==
+
+Agones -> GameServer: GameServer created through\na <i>Fleet</i> configuration
+activate GameServer
+GameServer -> Binary: Agones creates a Pod with the\nconfigured Game Server Container
+activate Binary
+activate SDK
+Binary -> SDK: SDK.WatchGameServer()
+note right
+Use the SDK Watch function
+to watch and react to allocation
+events
+end note
+
+Binary -> SDK: SDK.Ready()
+note right
+  Call <i>Ready()</i> when the
+  Game Server can take player
+  connections and is able to
+  be allocated.
+end note
+GameServer <-- SDK: Update to <i>Ready</i> State
+
+== No allocated <i>GameServers</i> ==
+
+Matchmaker -> Agones: Create: <i>GameServerAllocation</i>
+note left
+  The <i>GameServerAllocation</i> is implemented to
+  optionally select an already allocated <i>GameServer</i>
+  with <i>metadata["agones.dev/sdk-available"] = "true"</i>
+  if one exists. At this stage, one does not, so
+  Agones will allocate a <i>Ready</i> <i>GameServer</i>.
+end note
+Agones -> GameServer: Finds a <i>Ready</i> <i>GameServer</i>,\nsets it to <i>Allocated</i> State\nand <i>metadata["agones.dev/sdk-available"] = "false"</i>
+note left
+By setting the label "agones.dev/last-allocated" to "false"
+this remove the `GameServer` from possibly being
+re-allocated until it knows it can handle
+another session.
+end note
+Matchmaker <-- Agones : <i>GameServerAllocation</i> is returned\nwith <i>GameServer</i> details\nincluding IP and port to connect to.
+
+SDK --> Binary: Sends SDK.WatchGameServer()\nevent for Allocation.
+Binary -> SDK: SDK.SetLabel("available", "true")
+note right
+Since this game process can handle <i>n</i>
+concurrent sessions, set this label to match
+optional allocation label selectors, so it can be
+re-allocated.
+(See example below for more details)
+end note
+SDK --> GameServer: Sets <i>metadata["agones.dev/sdk-available"] = "true"</i>
+
+== Allocated <i>GameServers</i> with room for more sessions ==
+
+Matchmaker -> Agones: Create: <i>GameServerAllocation</i>
+note left
+  The <i>GameServerAllocation</i> will this time
+  find the Allocated <i>GameServer</i> with the label
+  <i>metadata["agones.dev/sdk-available"] = "true"</i>,
+  indicating that it can accept more
+  concurrent game sessions.
+end note
+Agones -> GameServer: Finds the Allocated <i>GameServer</i>\nwith <i>metadata["agones.dev/sdk-available"] = "true"</i>\n and set <i>metadata["agones.dev/sdk-available"] = "false"</i>.
+note right
+  This is the a <i>GameServer</i> that has room
+  for another concurrent game session.
+end note
+Matchmaker <-- Agones: returns <i>Allocated GameServer</i> record
+
+SDK --> Binary: Sends SDK.WatchGameServer()\nevent for Allocation.
+
+alt <i>GameServer</i> can accept more concurrent sessions
+  Binary -> SDK: SDK.SetLabel("available", "true")
+  SDK --> GameServer: Sets <i>metadata["agones.dev/sdk-available"] = "true"</i>.
+end alt
+note right
+  If the <i>GameServer</i> can accept
+  more concurrent sessions, reset the
+  <i>"agones.dev/sdk-available"</i>
+  back to "true"
+end note
+
+== <i>GameServer</i> has completed <i>n</i> number of complete sessions ==
+
+Binary -> SDK: SDK.Shutdown()
+note left
+The <i>GameServer</i> process tracks the number of sessions
+that it hosts, and after <i>n</i> number, calls <i>Shutdown()</i>
+to delete the <i>GameServer</i> resource and backing Pod.
+end note
+SDK --> GameServer: Update to <i>Shutdown</i> state.
+Agones -> GameServer: Deletes GameServer resource\n and backing Pod.
+destroy Binary
+destroy SDK
+destroy GameServer
+@enduml