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Possible Duplicate:
How can I read information from the blockchain?
Can anyone tell me from where exactly sites like
take their information. How bitcoinmonitor knows what transactions have been done right now?
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They monitor the public blockchain from multiple Bitcoin servers. There's a very simple rule for which chain is valid -- whatever chain has the most work is the valid one.
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You can get most of the information provided by those services through the use of the Bitcoin API. You might need to do some bookkeeping (for tracking transaction details and so forth), but it shouldn't be too hard. Other information needs to be extracted using additional tools, for example some packet sniffers for discovering transaction's relaters' IPs and so forth.
So if for instance I want to create a service, similar to above mentioned, to what servers I have to connect. As far as I know all the information is stored in .dat files on the computer, where bitcoin is installed. – Salvador Dali – 2012-09-21T00:48:08.080
1@SalvadorDali: You could just connect to a random set of nodes, but there's an advantage to connecting to the nodes closest to the large mining pools. You can find them by looking at which nodes tend to be first to announce a newly-found block. It doesn't matter that much so long as you are well connected as the mining pools are careful to keep themselves well connected too. – David Schwartz – 2012-09-21T18:18:10.300