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If a recursive program contains a rule with a negative literal, the following problem can arise Recall that when we make an inference by using a ground instantiation of a rule, for each negative literal notq in the rule body we check that q is not present in the set of facts I This test assumes that q cannot be inferred later However, in the xed-point iteration, the set of facts I grows in each iteration, and even if q is not present in I at one iteration, it may appear in I later Thus, we may have made an inference in one iteration that can no longer be made at an earlier iteration, and the inference was incorrect We require that a recursive program should not contain negative literals, in order to avoid such problems Instead of creating a view for the employees supervised by a speci c manager Jones, we can create a more general view relation empl that contains every tuple (X, Y ) such that X is directly or indirectly managed by Y , using the following program (also shown in Figure 57): empl(X, Y ) : manager(X, Y ) empl(X, Y ) : manager(X, Z), empl(Z, Y ) To nd the direct and indirect subordinates of Jones, we simply use the query empl(X, Jones ) which gives the same set of values for X as the view empl-jones Most Datalog implementations have sophisticated query optimizers and evaluation engines that can run the preceding query at about the same speed they could evaluate the view empl-jones The view empl de ned previously is called the transitive closure of the relation manager If the relation manager were replaced by any other binary relation R, the preceding program would de ne the transitive closure of R

Web caching is used to reduce latency and the amount of traffic when downloading web content Assuming a web cache server is deployed, when a user accesses a web site, the content that is downloaded is cached on the cache server Subsequent access to the same content is then delivered from the local cache server versus downloading the content from the original server The Web Cache Communications Protocol (WCCP) allows the security appliances to interact with external web cache and/or filtering servers

The McGraw Hill Companies, 2001

To understand the benefits that WCCP provides, I ll go through the process that the appliance goes through when using WCCP: 1 The user opens a web page, where the connection (or connections) makes its way to the appliance 2 The appliance intercepts the web connection request, encapsulates it in a Generic Routing Encapsulation (GRE) packet to prevent modification by intermediate devices, and forwards it to the web cache server 3 If the content is cached in the server, it responds to the user directly with the content 4 If the content is not cached in the server, a response is sent to the appliance, and the appliance allows the user s connection to proceed to the original web server For step 3 during the redirection process, the appliance doesn t add the connection to the state table and therefore doesn t perform any TCP state tracking, doesn t randomize the TCP sequence number in the TCP header, doesn t perform Cut-through Proxy

decomposed into R1(A, B) and R2(A, C, D, E) Now, neither of the dependencies in F contains only attributes from (A, C, D, E) so we might be misled into thinking R2 satis es BCNF In fact, there is a dependency AC D in F + (which can be inferred using the pseudotransitivity rule from the two dependencies in F ), which shows that R2 is not in BCNF Thus, we may need a dependency that is in F + , but is not in F , to show that a decomposed relation is not in BCNF An alternative BCNF test is sometimes easier than computing every dependency in F + To check if a relation Ri in a decomposition of R is in BCNF, we apply this test: For every subset of attributes in Ri , check that + (the attribute closure of under F ) either includes no attribute of Ri , or includes all attributes of Ri If the condition is violated by some set of attributes in Ri , consider the following functional dependency, which can be shown to be present in F + : ( + ) Ri The above dependency shows that Ri violates BCNF, and is a witness for the violation The BCNF decomposition algorithm, which we shall see in Section 762, makes use of the witness