ll,uli. rt:al-t.iliif,{lat,al]aseapplications miseinsdety-criticd installations andmilitary systems where enforcing security t> irtt~Ml to th(, successor the enterprise. A secure real-time (liiti]l)~<(, svstrm has to simultaneously satisfy two require-II1(J1l Is guarauree data security and minimize the number ,,1 Ititw,d transaction deadlines. We investigate here the p,ukmuance implications, in terms of missed deadlines, of gu;uallt.veiug secorit.y in a real-time database system. In lwtitulal. w{' focus on the concurrency control aspects of 1111> 15>11(, (hlr lll;l]tl ({)jlt.ril)~it,iollsu et hefollowing: First, we iden-Lt]"vwhich a.umug the previously proposed real-time concur-IIIK.K((lrlt.rol protocols" are capable of providing protection ,i~;iil~si })[}th (Im:(t aud imlirect (covert channels) means of lllla!ltt]~mzrd Mress to data. Second. using a detailed simolimou model of a firm-deadline real-time database system, wv profile the real-time performance of a representative set ,)1 illtw, s~~(ur[ (om.orrmi(:y control protocols. Our exper-IILIIIIIIs show that. a prioritized optimistic concurrency control protocol. OPT-WAIT, provides the best overall perforii,ium Third. we propose and evaluate a novel durd ap-lmMrlI to secure t,rimsaction concurrency control that al-1,,~~> tll(' rml-time (latabase system to simultaneously use tlIffIw,III c(ulrvlrreucy control mechanisms for guaranteeing (,{II IIIV ,UI{I fijr imprcn,iug real-time performance. By appropriately ckoosing these different mechanisms, we have IML(OII al)h, to (Iesign hyfwui concurrency control algorithms 111,11 lJII)\.1~1{1 (,v(,II Iwttw performance than OPT-WAIT.

A significant body of literature is available on distributed transaction commit protocols. Surprisingly, however, the relative merits of these protocols have not been studied with respect to their quantitative impact on transaction processing performance. In this paper, using a detailed simulation model of a distributed database system, we profile the transaction throughput performance of a representative set of commit protocols. A new commit protocol, OPT, that allows transactions to “optimistically” borrow uncommitted data in a controlled manner is also proposed and evaluated. The new protocol is easy to implement and incorporate in current systems, and can coexist with most other optimizations proposed earlier. For example, OPT can be combined with current industry standard protocols such as Presumed Commit and Presumed Abort. The experimental results show that distributed commit processing can have considerably more influence than distributed data processing on the throughput performance and that the choice of commit protocol clearly affects the magnitude of this influence. Among the protocols evaluated, the new optimistic commit protocol provides the best transaction throughput performance for a variety of workloads and system configurations. In fact, OPT's peak throughput is often close to the upper bound on achievable performance. Even more interestingly, a three-phase (i.e., non-blocking) version of OPT provides better peak throughput performance than all of the standard two-phase (i.e., blocking protocols evaluated in our study.