Today s plan Any questions about lock server lab Reviewing event driven programming Outline structure of the remaining labs Common libasync libarpc nfsloop programming idioms writing rpc client code writing async functions that call RPCs writing rpc server code Flash Event driven programming Achieve I O concurrency for communication efficiently Threads give cpu and i o concurrency Never quote clear when you ll context switch cpu i o concurrency State machine style execution Lots of threads request handling state machines in parallel Single address space no context switch overhead efficient Have kernel notify us of I O events that we can handle w o blocking The point this preserves the serial natures of the events Programmer sees events functions occuring one at a time Simplifies locking but when do you still need it libasync handles most of the busywork draw amain select on board again e g write ability events are usually boring libarpc translates to events that the programmer might care about rpcs ccfs architecture draw block diagram on the board OS app ccfs blockserver ccfs app OS lockserver ccfs communicate through RPC you ll be writing clients and servers include names of RPCs on the little lines real apps can be structured just like this okws chord dhash Synchronous RPC Example 1 Sketch this on the board and use it to show evolution Making RPCs Already saw basic framework in Lab 1 libarpc provides an rpc compiler protocol x C and h Provides un marshalling of structs into strings External Data Representation XDR rfc1832 Example 2 libraries to help handle the network axprt asynchronous transport write clients aclnt aclnt handles all bookkeeping formatting etc for us e g which cb gets called write servers asrv svccb Asynchronous RPC needs a callback Example 3 Note Cite as Robert Morris course materials for 6 824 Distributed Computer Systems Engineering Spring 2006 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 1 Need to split code into separate functions need to declare prototypes 2 return values passed in by aclnt as arguments e g clnt stat 3 cb must keep track of where results will be stored 4 Actually must split everything that uses an async function How do we translate this into a stub function Need to provide our own callback Example 4 translate RPC results error into something the app can use Server side Setup involves listening on a socket allocating a server with dispatch cb Example 5 dispatch svccb sbp switch to dispatch on sbp proc call sbp reply res You must not block when handling proc you don t need to reply right away but blocking would be bad Managing memory with svccb Use getarg type to get pointer to argument svccb managed Use getres type to get a pointer to a reply struct svccb managed sbp reply causes the sbp to get deleted Writing user level NFS servers classfsd code will allow you to mount a local NFS server w o root nfsserv udp handles tedious work we register a dispatch function Similar to generic RPC server but use nfscall instead of svccb Adds features like nc error You ll need to do multiple operations to handle each RPC draw RPC issue timeline os kernel ccfs lockserver blockserver Not unlike how we might operate get an e mail from friend can you make it to my wedding check class calendar on web check research deadlines send IM to wife research ticket prices reply Or Amazon com login Example 6 An aside on locking No locking etc needed usually e g to increment a variable When do you need locking When an operation involving multiple stages Be careful about callbacks that are supposed to happen later e g delaycb send grant Parallelism and Example 7a Example 7b Example 7c Example 7d loops synchronous code serialized and async parallelism but yet better parallelism Summary Cite as Robert Morris course materials for 6 824 Distributed Computer Systems Engineering Spring 2006 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY Events programming gives programmer a view that is roughly consistent with what happens Can build abstractions to handle app level events Need to break up state and program flow but always know when there s a wait and have good control over parallelism Cite as Robert Morris course materials for 6 824 Distributed Computer Systems Engineering Spring 2006 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY
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