Does erasure coding increase data durability? Lets assume a node has a 0.81% annual failure rate. According to the calculation done by Backblaze, erasure coding can achieve 11 nines durability vs 3-copy replication can achieve 6 nines durability. What other techniques do you think are important to improve the scalability and durability of an object store such as S3? 88

Foreign exchange in payment Have you wondered what happens under the hood when you pay with USD online and the seller from Europe receives EUR (euro)? This process is called foreign exchange. Suppose Bob (the buyer) needs to pay 100 USD to Alice (the seller), and Alice can only receive EUR. The diagram below illustrates the process. 1. Bob sends 100 USD via a third-party payment provider. In our example, it is Paypal. The money is transferred from Bobs bank account (Bank B) to Paypals account in Bank P1. 2. Paypal needs to convert USD to EUR. It leverages the foreign exchange provider (Bank E). Paypal sends 100 USD to its USD account in Bank E. 89 3. 100 USD is sold to Bank Es funding pool. 4. Bank Es funding pool provides 88 EUR in exchange for 100 USD. The money is put into Paypals EUR account in Bank E. 5. Paypals EUR account in Bank P2 receives 88 EUR. 6. 88 EUR is paid to Alices EUR account in Bank A.

Now lets take a close look at the foreign exchange (forex) market. It has 3 layers: Retail market. Funding pools are parts of the retail market. To improve efficiency, Paypal usually buys a certain amount of foreign currencies in advance. Wholesale market. The wholesale business is composed of investment banks, commercial banks, and foreign exchange providers. It usually handles accumulated orders from the retail market. Top-level participants. They are multinational commercial banks that hold a large number of certificates of deposit from different countries. They exchange these certificates for foreign exchange trading. When Bank Es funding pool needs more EUR, it goes upward to the wholesale market to sell USD and buy EUR. When the wholesale market accumulates enough orders, it goes upward to top-level participants. Steps 3.1-3.3 and 4.1-4.3 explain how it works. If you have any questions, please leave a comment.

What foreign currency did you find difficult to exchange? And what company have you used for foreign currency exchange? 90