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For example, if I update a global variable in one goroutine, then read the variable in another goroutine, can I get the newest value?
Another question is, can "atomic.Load*" and "atomic.Store*" ensure visibility?
Without explicit synchronization between goroutines, there is no guarantee that you will see the latest value of a shared variable. The Go memory model describes this:
https://golang.org/ref/mem
Atomic load/store have memory barriers, and they do guarantee you will see the latest value, though the Go memory model does not explicitly state this.
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I wanted to know who will create the block ids for blocks in hadoop either HDFS client or Name node.Please let me know.
The NameNode allocates the block ID and gives it to the client. The client then uses this block ID while communicating with a DataNode to write data into the block.
Apache JIRA HDFS-4645 documents the current design for allocation of block IDs. It uses a monotonically increasing ID starting from a specific constant. If you're interested in seeing the code for this, refer to the BlockIdManager and SequentialBlockIdGenerator classes in the Apache Hadoop codebase.
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What are the "under the hood" differences? What are the practical differences?
Is there any difference from a users perspective?
I know you could use def to define a method, but could you define a message?
When you send a message to an object, the object will (usually) respond by executing a method with the same name as that message.
You cannot define messages. You just send them.
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I'm trying to write a small dictionary Ruby app where I can look up existing entries, add and remove entries (ALL ENTRIES ARE PUT INTO A HASH). So right now I'm creating a Dictionary class and I'm not quite sure what exactly should be 'initialized' method.
I'm still pretty new to Ruby, so if someone could explain what should be initialized at the beginning of a class I'd be extremely grateful.
How do you know what to initialize?
Time and experience. In general, #new should:
Initialize instance variables.
Configure any object defaults.
Register itself with observers.
Housekeeping tasks needed to reach a minimal "ready" state.
Sometimes an object needs to do a lot more work to be "ready" (whatever that means for your class), and #new can just as easily do too little as too much. Finding the right balance for your application is what matters, so feel free to open new questions here or on Code Review Stack Exchange with more concrete questions and some actual code.
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I tried to find the usage of NtqueryDirectoryFile() function.
http://msdn.microsoft.com/en-us/library/windows/hardware/ff556633%28v=vs.85%29.aspx
I refered the above link for this. But, They mentioned about Zwxxx (). I couldn't understand Ntxx and ZWxxx. Can anyone explain about this.
Basically Zw functions are equivalent of Nt functions except they don't do security checks, so are used inside kernel mode only, to avoid the performance penalty.
You can only call Nt functions from user mode, through ntdll.dll. As Michael pointed out Zw functions are just mnemonics to Nt counterparts in user mode.
#Adriano provided a nice link: http://msdn.microsoft.com/en-us/library/windows/hardware/ff565438(v=vs.85).aspx
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I need to screenscrape a bunch of pages and store them in a database using ActiveRecord. I messed around using EventMachine and Typhoeus but I get flaky results, mostly mysteriously empty records getting saved.
What's the trick? I had the best results with scraping pages and writing them to disk, then inserting them into the DB, but I'd really like to do it in a single pass.
Is anybody doing this successfully?
My guess is your threads are sharing an AR connection. This will cause problems, and anyway concurrency won't get records into your database any faster. I suggest you download concurrently and insert them in a single thread (like you've been doing.)