Assignment-week4-databases-AliaTaher #53
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| const DATABASE_NAME = "databaseWeek4"; | ||
| const COLLECTION_NAME = "population"; |
| if (fromAccount.balance < amount) { | ||
| console.log("Insufficient balance transfer aborted."); | ||
| await session.abortTransaction(); | ||
| return; | ||
| } |
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Early returns and throwings are always encouraged!
Week4/homework/transfer.js
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| const toBalance = toAccount.balance + amount; | ||
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| const fromChange = { | ||
| change_number: fromAccount.account_changes.length + 1, |
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suggestion: using fromAccount.account_changes.length + 1 to know the next change_number is OK.
However, in real life one account may have a lot more change records. We don't really care what those changes are, we just want to know the maximum change number. Fetching all changes with the account can be expensive (since you need to transfer more bytes in the network. When you download something big from Internet, it is always slower than dowloading smaller files), and we try to only fetch the columns we need.
One solution is to have a function to get only the maximum change number (you have learned MAX sql syntax), and when you fetch the account data you don't fetch the changes column.
Week4/homework/transfer.js
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| }; | ||
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| const toChange = { | ||
| change_number: toAccount.account_changes.length + 1, |
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Same change number suggestion applies here.
Week4/homework/transfer.js
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| await collection.updateOne( | ||
| { account_number: from }, | ||
| { | ||
| $set: { balance: fromBalance }, |
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Needs work: if there's only one user updating the database at any certain time, using set won't cause any problem. However, if multiple users are trying to update the same data at the same time without knowing each other's action (concurrent), using set will cause big problem, because set will overwrite a value completely.
Let's assume there are three users: A, B and C. Both A and B are trying to send money to C, and they don't know each other. Currently C has 1000 in his balance. A and B start transaction at the same time.
In A's transaction, the current balance is 1000. A sends 500 to C, so fromBalance in A's transaction is 1500.
In B's transaction, the current balance is also 1000. B sends 300 to C, so fromBalance in B's transaction is 1300.
From C's point of view, his final balance should be 1800, right?
However, if A commits the transaction earlier, C's balance will first be 1500. Does B know this happened? NOOOOOO. When B starts the transaction he will have a snapshot of the start point, and this snapshot won't include changes from other transactions during B's transaction process. If the database system follows "last write win" principle, which is very common in real life, then after B commits his transaction, C's balance would be 1300, if set is used.
Could you try to find another way to convey the same change in the database but without this overwrite risk?
…r and update balance with $inc
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