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<!doctype html public "-//W30//DTD W3 HTML 2.0//EN"><HTML><TITLE>PROGRAMMING METHODOLOGY GROUP</TITLE><center><!WA0><A HREF="http://www.pmg.lcs.mit.edu/"><!WA1><IMG SRC=http://www.lcs.mit.edu/web_project/Brochure/pmg/pmgline.gif></a></center><p><center><table border><tr> <td><!WA2><img src=http://www.lcs.mit.edu/web_project/Brochure/pmg/liskov.gif></td> <td width=100 rowspan=2><br></td> <td><!WA3><img src=http://www.lcs.mit.edu/web_project/Brochure/pmg/shrira.gif></td></tr><tr><td> <!WA4><A HREF="http://www.pmg.lcs.mit.edu/~liskov/index.html"><address><b>Barbara Liskov</b></a>,<br>NEC Professor <br>of Software Science and Engineering</address> </td><td><address><b>Liuba Shrira</b>,<br>Research Associate</address></td></tr></table></center><body><p>The Programming Methodology group is focused on distributedcomputing. Specific challenges include making it easier forprogrammers to implement distributed applications, andsupporting secure and convenient data- and code-sharing indistributed environments.<p>Our current research is illustrated by <!WA5><A HREF="http://www.pmg.lcs.mit.edu/Thor.html">Thor</a>, anobject-oriented database system. Thor provides users with auniverse of objects that are stored reliably and are highlylikely to be accessible when needed; to achieve thesegoals, object storage is replicated. Objects in theuniverse can be shared by client programs written indifferent programming languages; controlled sharing isprovided via an access control mechanism.<p>Clients can locate objects either by navigation or throughqueries. Each object has an encapsulated state and a set ofmethods. Users can extend Thor by defining new object typesand new implementations of these types; the definitions aregiven in a new object-oriented programming language called<!WA6><A HREF="http://www.pmg.lcs.mit.edu/Theta.html">Theta</a>. Clients interact with Thor by calling methods ofobjects; calls run inside Thor to ensure that the objectscannot be corrupted by misbehaving client programs, whichmight be implemented in languages with unsafe features(such as explicit pointer arithmetic). Thor is beinginstalled on DEC Alphas connected by a local area network.<p>Other work includes Harp, a highly available file systemthat runs inside a network file server such as NFS; clientscontinue to use the file server as usual, but files arereplicated at several server machines. If one machinecrashes, the system is reconfigured on-the-fly and usersstill have access to their files. The replication techniqueused in Harp is also being used in Thor.<p>Future plans include the use of Thor as a basis fordistributed implementation of applications. Suchapplications are now based on the client-server model, inwhich components explicitly exchange messages. We will soonconduct experiments to confirm our belief that practicaldistributed applications will become easier by using Thoras a "blackboard" on which clients write information, suchas a request for a kind of service.</BODY><p><!WA7><a href="http://www.lcs.mit.edu/web_project/Brochure/contents.html"><!WA8><img align=left src=http://www.lcs.mit.edu/web_project/Brochure/icons/contents_motif.gif></a><!WA9><a href="http://www.lcs.mit.edu/web_project/Brochure/pdos/pdos.html"><!WA10><img align=left src=http://www.lcs.mit.edu/web_project/Brochure/icons/previous_group_motif.gif></a><!WA11><a href="http://www.lcs.mit.edu/web_project/Brochure/sig/sig.html"><!WA12><img align=left src=http://www.lcs.mit.edu/web_project/Brochure/icons/next_group_motif.gif></a></HTML>⌨️ 快捷键说明
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