Here is a simple event stream:

const events = [
    { type: 'account opened', credit: 5000 },
    { type: 'cheque deposited', credit: 50000 },
    { type: 'bill paid', debit: 10000, payee: 'AT&T' }
]

This event stream is located in memory but let’s ignore that for now.

We’d like to know what the current account balance is, given above transactions.

const rules = (account, event) => {
    if (event.type === 'account opened') {
        account.balance = event.credit
    }
    else if (event.type === 'cheque deposited') {
        account.balance += event.credit
    }
    else if (event.type === 'bill paid') {
        account.balance -= event.debit
    }
    return account
}

const account = events.reduce(rules, {})

console.log(account.balance)

Prints out 45000 which is exactly what you’d expect: 5000 + 50000 - 10000 = 45000

Run it on JS Bin

Since this is a toy example, it is missing a number of important features:

• Data is not saved anywhere

• Retrieving the current balance requires looping over all events in the stream

• There is no type checking performed

• There is no mechanism for other components to be notified when a new event is saved

These are all issues that Persistr solves.

Here’s what the above code looks like using Persistr. It’s a one-liner to retrieve the current value of a domain object:

const account = await stream.object('Bank Account').latest()
console.log(account.balance)

Persistr is about making event-sourcing simple.

Have any questions or comments about this post? Email me at sasha@persistr.com

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