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I am quite new to Bitcoins and have two questions.
Lets say person A owns 1 Bitcoin. A wants to send .5 BTC to person B and .5 BTC to person C in a single transaction. Is this possible to do within one single transaction ?
Later on if Person C sends the 0.5 Bitcoin it obtained from A to Person B, will it be possible for person B to verify that the Bitcoin which it now received from Person B and the one which it already received from Person A, were from the same source?
Yes, sending a bitcoin value to multiple outputs/recipients in a single transaction is possible.
See here: https://en.bitcoin.it/wiki/Transaction
And yes, person C will be able to verify that the two bitcoin values originated both from A, since the entire blockchain is publicly readable.
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I am going to share a question that I was asked in a data science interview. I failed the interview as I couldn't answer the question and still now I don't have any idea how to solve it.
Below a data table is given. It's about number of users of a mobile game.
Users Assigned = Newly added users on a day.
Active users = Total users active on a day.
The question is- What % of users are still playing the game daily?
Options are
Almost 5%
Almost 10% to 15%.
And I was ask to show the step by step processes.
It was my first-ever interview in life, so I was nervous and couldn't solve within given time. I hope someone will answer me here and will explain it clearly.
On day 0 you have 4051 users, all of which are active.
On day 1 you have 4095 added users meaning 4051+4095, of which 5522 are active.
Given that registration seems to indicate activity, we have to exclude those that registered today, meaning
Day 1 active users = (5522-4095) / (4051+4095) = 0.1751780015 => 18%
Continue this until day 6 when you have 6820 active with a total playerbase of 4051+4095+3868+3921+4010+4070+4001
Day 6 = (6820-4001) / (4051+4095+3868+3921+4010+4070+4001) = 0.1006210737 =>10%
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Why does bitcoin have miners? Why don't the users mine their own blocks. I understand it may be to ensure the currency is traded, but in any non transaction blockchain case do you need miners or is it fine to let the users mine their own blocks?
I couldn't find any reason as to why bitcoin really needs miners other than for adding coin to the market.
The only distinction between a user and a miner is that a miner is doing computational work to get bitcoin, and a user is using bitcoin to make transactions. A user could easily do the computational work as well, but then they would just be a miner. The reason not everyone is a miner is because not everyone wants to do computational work, many just want to make transactions.
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I was recently asked the following question in an interview.
"How would you design a system to keep track of a million items at xyz.com ?
The xyz.com could update the prices maybe 2-3 times a day or once per month, so no guarentee on frequency.
Your system should show accurate prices for >95% of items at any given point of time and aim for 99%.
Also scale for 1billion items etc..
"
I asnwered along the lines of creating a distributed system app that would categorize items by priority (based on historical price fluctuations and 80/20 % rule etc) and do API calls more frequently for these.
But I was not allowed to use API calls.
I suggested scraping html content. (But the website can block my ip for such high load)
I basically want to know the resources that would help me anwering these type of questions. Prefer full length courses (Distributed systems ?) or books rather than quick-fix blogs.
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We have a ruby-on-rails platform (w/ postgreSQL db) for people to upload various products to trade. Of course, many of these products listed are the same, while they are described differently by the consumer (either through spelling, case etc.) "lots of duplicates"
For the purposes of analytics and a better UX, we're aiming to create an evolving "master product list", or "whitelist", if you will, that will have users select from an existing list of products they are uploading, OR request to add a new one. We also plan to enrich each product entry with additional information from the web, that would be tied to the "master product".
Here are some methods we're proposing to solve this problem:
A) Take all the "items" listed in the website (~90,000), de-dupe as much as possible by running select "distinct" queries (while maintaining a key-map back to original data by generating an array of item keys from each distinct listing in a group-by.)
THEN
A1) Running this data through mechanical turk, and asking each turk user to list data in a uniform format.
OR
A2) Running each product entry through the Amazon products API and asking the user to identify a match.
or
A3) A better method?
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In chubby the process of each log that write to db is one instance of the paxos process, and this instance may have many proposers, so it will have a master selected process.
why it has many proposers?
Having a single leader (aka a "Distinguished Proposer") is an optimization for Paxos. The whole point of Paxos is to decide a value when there are multiple proposers proposing different things.
The whole point of having multiple proposers is in case the "Distinguished Proposer" fails; something has to pick up that new role. Paxos, when it decides a value, implicitly decides between proposers.
As in your other question, I suggest looking at the other Paxos questions, perhaps read Paxos Made Simple.