Add a billing flow documentation page (#328)

billing_flow.md

@@ -0,0 +1,125 @@
+
+# Billing Flow
+
+Here are a couple of diagrams that illustrate the billing flow. Setting up means getting a new AWS Marketplace subscription or doing the same flow with an existing subscription. 
+
+
+## Setting up a subscription with a single workspace
+
+Here is the base case where a user has a single workspace. The assignment of the subscription to the workspace is done automatically. Workspace selection dialog is not shown.
+
+![diagram](images/subscription_setup_single_workspace.png)
+
+<details>
+
+<summary>source</summary>
+
+```plantuml
+@startuml subscription_setup_single_workspace
+
+actor User as U
+participant "FixBackend API" as FB
+participant "AWS Marketplace" as AMP
+participant AwsMarketplaceHandler as AH
+participant AWS as AWS
+collections SubscriptionRepo as S
+collections WorkspaceRepo as WS
+
+U -> FB: Clicks on the AWS Marketplace button
+FB -> U: Redirect link to AWS Marketplace
+U -> AMP: Clicks "Set up your account"
+AMP -> U: Redirect Fix Backend callback endpoint
+U -> FB: Sends the callback to FixBackend
+FB -> FB: Self-redirect to use the SameSite cookie
+FB -> AH: Create a new subscription
+AH -> AWS: Resolve customer information
+AWS -> AH: Returns the customer information
+AH -> S: Create a new subscription
+AH -> WS: Add subscription to workspace
+AH -> FB: (subscription, workspace_assigned=true)
+FB -> U: Redirect to "/" with a success message
+@enduml
+```
+
+</details>
+
+## Setting up subscription with multiple workspaces
+
+A bit more complex case where a user has multiple workspaces. The subscription is not assigned to any workspace by default. Instead, the user is prompted to select a workspace to assign the subscription to.
+
+
+![diagram](images/subscription_setup_multiple_workspaces.png)
+
+<details>
+
+<summary>source</summary>
+
+```plantuml
+@startuml subscription_setup_multiple_workspaces
+
+actor User as U
+participant "FixBackend API" as FB
+participant "AWS Marketplace" as AMP
+participant AwsMarketplaceHandler as AH
+participant AWS as AWS
+collections SubscriptionRepo as S
+collections WorkspaceRepo as WS
+
+U -> FB: Clicks on the AWS Marketplace button
+FB -> U: Redirect link to AWS Marketplace
+U -> AMP: Clicks "Set up your account"
+AMP -> U: Redirect Fix Backend callback endpoint
+U -> FB: Sends the callback to FixBackend
+FB -> FB: Self-redirect to use the SameSite cookie
+FB -> AH: Create a new subscription
+AH -> AWS: Resolve customer information
+AWS -> AH: Returns the customer information
+AH -> S: Create a new subscription
+note over AH: Subscription is not assigned to any workspace \nbecause the user has multiple workspaces
+AH -> FB: (subscription, workspace_assigned=false)
+FB -> U: Redirect to /subscription/choose-workspace
+loop until receives 200 OK
+    U -> FB: add subscription to workspace
+end
+FB -> FB: validate permissions to update billing
+FB -> S: validate that the subscription was created by the current user
+FB -> WS: Add subscription to workspace
+FB -> U: 200 OK
+@enduml
+```
+
+</details>
+
+## Updating the product tier
+
+
+![diagram](images/product_tier_update.png)
+
+<details>
+
+<summary>source</summary>
+
+```plantuml
+@startuml product_tier_update
+
+actor User as U
+participant "FixBackend API" as FB
+participant "BillingEntryService" as B
+collections WorkspaceRepo as WS
+queue DomainEvents as Q
+
+U -> FB: selects the produuct tier
+FB -> FB: validates user permissions
+FB -> B: update the product tier
+B -> B: checks the subscription if paid product tier
+B -> WS: update the product tier
+B -> Q: publish ProductTierChanged
+B -> FB: updated workspace
+FB -> B: get product tier
+B -> FB: returns the product tier
+FB -> U: 200 OK
+
+@enduml
+```
+
+</details>