- 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
這兩條是對Map和Reduce函數的解釋，通過.so在運行時加載。
- 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.
這些worker共享同一個文件系統，如果當worker跑在不同的機器上時，需要全局的文件系統。
- 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.
建議將中間文件名命名爲mr-X-Y，X是Map任務數量，Y是reduce任務數
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任務需要存儲時可以使用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.
可以從串行化的程序中參考到一些思路。
- The master, as an RPC server, will be concurrent; don't forget to lock shared data.
對於RPC服務，可能會有併發問題。注意加鎖。
- 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併發檢測器
- 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).
注意對worker就行心跳檢測
- 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.