Leader Election

• Imagine a situation where we might want our database to connect to a third party service (stripe, plaid, youtube, Google Analytics, etc.)

• That would look like this:

Third Party Service             Database
[                 ]<-------->[   Master   ]

• This isn't ideal: so we'll put our own service which gates out certain functionality.

Third Party Service           Your Service           Database
[                 ]<-------->[   Leader   ]<------->[        ]

• But here, we only have server that is a single point of failure.
• Let's make it more redundant by adding more servers.

Third Party Service           Your Service           Database
[                 ]<-------->[   Leader   ]<------->[        ]
[                 ]<-------->[   Leader   ]<------->[        ]
[                 ]<-------->[   Leader   ]<------->[        ]
[                 ]<-------->[   Leader   ]<------->[        ]
[                 ]<-------->[   Leader   ]<------->[        ]

• But now, we want every operation to only occur once.

• We can't charge a person twice for the same invoice.

• Leader Election has a server elect one of the servers as a leader, which is the only server that connects to the database.

Third Party Service           Your Service           Database
[                 ]<-------->[   Leader   ]<------->[        ]
[                 ]--------->[  Follower  ]<------->[        ]
[                 ]--------->[  Follower  ]<------->[        ]
[                 ]--------->[  Follower  ]<------->[        ]

• Now, all writes to the leader have to propogate to the followers in case of failure, and when the leader dies, then the followers have to relect a new leader.

• Consensus building (Leader Election) is actually quite hard.

• It's hard to share some state.

• We need a Consensus Algorithm to achieve this leader election.