Chapter 12 checkpoint

Chapter 12 checkpoint: Mint.com

Every question from this chapter’s lessons. Your first answer to each is the one that counts; afterward you can practice again as often as you like. Passing needs 75% right on the first attempt.

0 of 16 answered0 right on the first attempt

Question 1 of 16

Mint ingests 5 billion transactions a month. Using 400 requests per second ≈ 1 billion a month, how many transactions per second is that on average?

Your answertransactions/s

Answers within 5% count.

Question 2 of 16

Each transaction takes about 50 bytes, and 5 billion arrive each month. How much new transaction data is that per month, in GB?

Your answer

Answers within 5% count, in any unit you choose.

Question 3 of 16

Unlike most systems in this course, Mint has a 10:1 write-to-read ratio. Why?

Budgets are rewritten on every page viewReads are all served by a CDN, so they are not countedUsers make transactions every day, but few visit the site every dayEvery read writes an audit log entry

Question 4 of 16

How should transaction amounts be stored?

As free text, such as “USD 12.50”As a binary floating-point numberAs a decimal type or as integer centsAs whole dollars, rounding the cents away

Question 5 of 16

Extraction is at-least-once, so the same transaction can arrive several times. What should the unique constraint be on?

A hash of merchant, amount and dateThe account and the provider’s transaction idThe time the transaction was receivedNo constraint; check in Redis first

Question 6 of 16

A hotel stay first appears as a pending charge of 320.00 USD and later posts as 345.82 USD. The user’s spending now counts both. What went wrong?

The two amounts are in different currenciesThe queue delivered the same message twiceThe posted version was added but the pending one it replaces was never removedFloating-point rounding doubled the amount

Question 7 of 16

The Budget Service must hear about each new transaction. How does the extraction service avoid a dual write?

Write the event to an outbox table in the same database transaction, and relay it afterwardWrite the rows, then publish the eventUse two-phase commit between the database and the queuePublish the event first, then write the rows

Question 8 of 16

A posted transaction arrives from the bank. Put the steps of its ingestion in order.

Begin a database transaction
Commit
Insert the posted row, doing nothing if it already exists
Write an outbox event only if the row was inserted

Question 9 of 16

In envelope encryption, what do the data key and the root key each do?

Both are the same key, stored twiceThe root key encrypts each record; data keys protect the root keyA data key encrypts each record; the root key encrypts the data keysThe root key hashes the records so they can be compared

Question 10 of 16

Why must an aggregator encrypt the credentials it uses to reach banks, rather than hash them?

Compliance rules forbid hashingEncryption is faster than hashingIt has to recover and use them to fetch data; a hash cannot be reversedHashes of credentials would collide

Question 11 of 16

What does NIST SP 800-38D require of the IV used with AES-GCM?

It should be the same for every recordIt must be at least 256 bitsIt must never be reused with the same keyIt must be kept secret like the key

Question 12 of 16

An audit log chains each entry’s hash to the previous one. An insider edits an old entry and recomputes every later hash, so the chain checks out. What would have exposed the edit?

Auditors holding earlier head hashes, which the rewritten log no longer matchesNothing; hash chains always detect editsEncrypting the logUsing SHA-512 instead of SHA-256

Question 13 of 16

A seller-to-category map holds 50,000 sellers at about 240 bytes each. How much memory does it take, in MB?

Your answer

Answers within 5% count, in any unit you choose.

Question 14 of 16

The budget template gives food 20% of income. For a user earning 5,000 USD a month, what food budget does it recommend, in USD?

Your answerUSD

Answers within 1% count.

Question 15 of 16

A transaction’s seller is not in the seed map, but many users have overridden that seller’s category. What does the primer’s Category Service do?

Uses the most recent overrideLeaves it uncategorizedUses the most common override, and caches it in the seller mapRe-categorizes every past transaction from that seller

Question 16 of 16

You want monthly spending to update as transactions arrive rather than in a batch. What does the lesson suggest?

Keep floating-point running totalsRe-run the full SQL aggregation on every page viewRun the MapReduce job every minuteConsume outbox events into integer-cent counters, and reconcile them with the batch job