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Architecture

Data Structures

%%{ init: { "er" : { "layoutDirection" : "LR" } } }%%
erDiagram
    User ||--o{ Vote : submits
    Vote o{--|| Link : on
    Link o{--|| Score : has
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Link

The hostname String that represents the website, for example "www.google.com" or "blog.myspecialplace.com". See here for a good explanation of the different pieces in the URL:
Structure and components of a URL

Right now all voting just happens on the hostname, but there could be a future where voting happens on full URL paths. Like voting on individual Medium articles for example.

Vote

An Integer that's either a +1 or -1, stored with a Timestamp when the vote was made. Always associated with a User and a Link.

Timestamp

Represented everywhere as an RFC 3339 string with the specific format 2023-02-02T09:36:03Z.

Score

An enum that represents how good a website Link is. It has 4 possible values:

Enum Definition
Good Sum of all votes >= 20
Bad Sum of all votes <= -10
Controversial (-10 < Sum of all votes < 20) && (Number of votes > 50)
NoScore If none of the above

The score is calculated in the API and exposed to the extension through the /scores request.

In the future this will probably need to be tweaked for more nuanced scoring, like weighting recent votes higher.

User

Identified by a UUID. I wanted a passwordless system and this seemed like a flexible choice. Has a number of properties:

  • is_banned: Boolean
  • created_at: Timestamp

Settings

System wide configuration that can change the behaviour of everything.

  • voting_is_disabled: Boolean
  • maximum_votes_per_user_per_day: 10

The idea behind voting_is_disabled is in case there's a spam armaggedon and all voting needs to be stopped.

API

Request Response
GET /scores?for=[link1, link2, ...] [{link: Link, score: Score}]
POST /vote {link, vote, user_id}

Database

I decided to go with a NoSQL database for two reasons:

  1. It'd be cool to learn.
  2. My extremely basic understanding of NoSQL leads me to believe that it's better suited for what this is trying to do. DynamoDB on AWS seems cheap enough and if this extension actually gets used and needs to scale then future Tom won't be boned.

The access patterns are reasonably well defined:

Runtime Access Patterns Description Table - Filter
Get vote summaries for a Link Summaries are sum_of_votes & count_of_votes Table:Discontent - PK=link#<link>, SK=link#<link>
Get all votes for a Link To calculate sum_of_votes & count_of_votes Table:Discontent - PK=link#<link>, SK.startswith(user#)
Get vote for a Link and user To make sure a user can't vote twice Table:Discontent - PK=link#<link>, SK=user#<user_id>
Get vote for a Link and user To auto select the correct vote button Table:Discontent - PK=link#<link>, SK=user#<user_id>
Get vote summaries for a User To limit the number of submissions in a time range Table:Discontent - PK=day#<date>, SK=user#<user_id>
Get banned state for a User Prevent banned users from submitting more votes Table:Discontent - PK=user#<user_id>, SK=user#<user_id>

The following are analysis access patterns, not really part of regular usage.

Analysis Access Patterns Description Table & Filter
Get User details To carry out abuse investigations Table:Discontent - PK=user#<user_id>, SK=user#<user_id>,
Get all votes for a user To carry out abuse investigations GSI:UserVotes - PK=user#<user_id>, SK.within(timerange)
Get top users by daily count of votes To identify possible abuse GSI:DailyUserHistory - PK=<day>, SK.top(N)
Get top links by daily count of votes To identify possible abuse GSI:DailyLinkHistoryByCountOfVotes - PK=<day>, SK.top(N)
Get top links by daily sum of votes To create a best links leaderboard GSI:DailyLinkHistoryBySumOfVotes - PK=<day>, SK.top(N)
Get top links by daily count of votes To create a controversial links leaderboard GSI:DailyLinkHistoryByCountOfVotes - PK=<day>, SK.top(N)

Sequence diagrams

Get scores for links

sequenceDiagram
    actor Extension
    participant API
    participant Database
    Extension->>API: GET /scores?for=[link1, link2, ...]
		activate API
		API->>API: Validate request
    alt Request Error
        API->>Extension: Request Error (Invalid params / authentication...)
    end
		API->>Database: BatchGetItem(Table:Discontent - PK,SK=<link, ...>)
		Note over API,Database: If a link does not yet exist in the table, it's not returned
		activate Database
    alt Database Error
        Database->>API: Database Error (connection / server...)
        API->>Extension: Server Error
    end
		Database->>API: Return [{link, sum_of_votes, count_of_votes}, ...]
		API->>API: Calculate scores
		API->>Extension: Return [{link, score}, ...]
    deactivate Database
    deactivate API
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Submit a vote for a link

sequenceDiagram
    actor Extension
    participant API
    participant Database
    Extension->>API: POST /vote {link, vote, user_id}`
		activate API
		API->>API: Validate parameters
    alt Invalid parameters
        API->>Extension: Error: Invalid parameters
    end
		API->>Database: Check user history & settings. GetBatchItems(___________)
		Note over API,Database: Voting disabled? GetItem(PK=settings, SK=settings)
		Note over API,Database: Too many votes? GetItem(PK=date, SK=user_id)
		Note over API,Database: User exists? User banned? GetItem(PK=user_id, SK=user_id)
		Note over API,Database: Already voted? GetItem(PK=link, SK=user_id)
		activate Database
    alt Database Error
        Database->>API: Database Error (connection / server...)
        API->>Extension: Server Error
    end
		Database->>API: Return UserHistory & Settings
		deactivate Database
		API->>API: Check user history & Settings
    alt Failed
        API->>Extension: 403 Forbidden: Too many votes / banned / voting disabled
    end
		activate Database
		API->>Database: Submit vote. BatchWriteItems(_________________)
    alt If user does not exist
		Note over API,Database: Put(PK=user_id, SK=user_id | not_banned,created_at)
		Note over API,Database: <run [First time user voting on link]>
		else First time user voting on link
		Note over API,Database: Put(PK=link, SK=user_id | vote)
		Note over API,Database: Update(PK=link, SK=link | count_of_votes++, sum_of_votes+=vote)
		Note over API,Database: -- Add history
		Note over API,Database: Update(PK=day, SK=link | count++, sum+=vote)
		Note over API,Database: Update(PK=day, SK=user | count++, sum+=vote)
    else If user already voted on link
		Note over API,Database: Put(PK=link, SK=user_id | vote)
		Note over API,Database: Update(PK=link, SK=link | sum_of_votes+=(-old_vote+new_vote))
		Note over API,Database: -- Undo old history
		Note over API,Database: Update(PK=old_day, SK=link | count--, sum-=old_vote)
		Note over API,Database: Update(PK=old_day, SK=user | count--, sum-=old_vote)
		Note over API,Database: -- Add history
		Note over API,Database: Update(PK=day, SK=link | count++, sum+=vote)
		Note over API,Database: Update(PK=day, SK=user | count++, sum+=vote)
		end
		activate Database
    alt Database Error
        Database->>API: Database Error (connection / server...)
        API->>Extension: Server Error
    end
		Database->>API: Return success
		API->>Extension: Return success
    deactivate Database
    deactivate API
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Testing

Some manual tests are required right now: