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• The application Map and Reduce functions are loaded at run-time using the Go plugin package, from files whose names end in .so.
• If you change anything in the mr/ directory, you will probably have to re-build any MapReduce plugins you use, with something like go build -buildmode=plugin ../mrapps/wc.go

这两条是对Map和Reduce函数的解释，通过.so在运行时加载。

• This lab relies on the workers sharing a file system. That's straightforward when all workers run on the same machine, but would require a global filesystem like GFS if the workers run on different machines.

这些worker共享同一个文件系统，如果当worker跑在不同的机器上时，需要全局的文件系统。

• A reasonable naming convention for intermediate files is mr-X-Y, where X is the Map task number, and Y is the reduce task number.

建议将中间文件名命名为mr-X-Y，X是Map任务数量，Y是reduce任务数

The worker's map task code will need a way to store intermediate key/value pairs in files in a way that can be correctly read back during reduce tasks. One possibility is to use Go's encoding/json package. To write key/value pairs to a JSON file:

worker的map任务需要存储时可以使用json

• The map part of your worker can use the ihash(key) function (in worker.go) to pick the reduce task for a given key.

• You can steal some code from mrsequential.go for reading Map input files, for sorting intermedate key/value pairs between the Map and Reduce, and for storing Reduce output in files.

可以从串行化的程序中参考到一些思路。

• The master, as an RPC server, will be concurrent; don't forget to lock shared data.

对于RPC服务，可能会有并发问题。注意加锁。

• Use Go's race detector, with go build -race and go run -race. test-mr.sh has a comment that shows you how to enable the race detector for the tests.

使用Go并发检测器

• Workers will sometimes need to wait, e.g. reduces can't start until the last map has finished. One possibility is for workers to periodically ask the master for work, sleeping with time.Sleep() between each request. Another possibility is for the relevant RPC handler in the master to have a loop that waits, either with time.Sleep() or sync.Cond. Go runs the handler for each RPC in its own thread, so the fact that one handler is waiting won't prevent the master from processing other RPCs

• The master can't reliably distinguish between crashed workers, workers that are alive but have stalled for some reason, and workers that are executing but too slowly to be useful. The best you can do is have the master wait for some amount of time, and then give up and re-issue the task to a different worker. For this lab, have the master wait for ten seconds; after that the master should assume the worker has died (of course, it might not have).

注意对worker就行心跳检测

• To test crash recovery, you can use the mrapps/crash.go application plugin. It randomly exits in the Map and Reduce functions.

To ensure that nobody observes partially written files in the presence of crashes, the MapReduce paper mentions the trick of using a temporary file and atomically renaming it once it is completely written. You can use ioutil.TempFile to create a temporary file and os.Rename to atomically rename it.

• test-mr.sh runs all the processes in the sub-directory mr-tmp, so if something goes wrong and you want to look at intermediate or output files, look there.

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