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Figure 12-4 shows a diagrammatic representation of three hardware circuits: a half adder, a full adder, and a 2-bit carry ripple adder. The first of these has two input wires, x and y, and sets the sum wire high if exactly one of these is high. If both x and y are high, then the sum is low, and the carry wire is high instead. Thus, the circuit computes the 2-bit sum of the inputs. Likewise, a full adder computes the sum of three Boolean inputs, which, since it is at most three, can still be represented by 2 bits. A 2-bit carry ripple adder is formed by composing a half adder and a full adder together, wiring the carry from the first adder to one of the inputs of the second adder. The overall circuit has four inputs and three outputs.

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Neither option is very appealing. Both imply that you lose data. If you had been executing in ARCHIVELOG mode, on the other hand, you simply would have found another disk and restored the affected files from Saturday s backup onto it. Then, you would have applied the archived redo logs and, ultimately, the online redo logs to them (in effect replaying the week s worth of transactions in fastforward mode). You lose nothing. The data is restored to the point of the failure. People frequently tell me they don t need ARCHIVELOG mode for their production systems. I have yet to meet anyone who was correct in that statement. I believe that a system is not a production system unless it is in ARCHIVELOG mode. A database that is not in ARCHIVELOG mode will, some day, lose data. It is inevitable; you will lose data (not might, but will) if your database is not in ARCHIVELOG mode. We are using RAID-5, so we are totally protected is a common excuse. I ve seen cases where, due to a manufacturing error, all disks in a RAID set froze, all at about the same time. I ve seen cases where the hardware controller introduced corruption into the data files, so people safely protected corrupt data with their RAID devices. RAID also does not do anything to protect you from operator error, one of the

most common causes of data loss. RAID does not mean the data is safe, it might be more available, it might be safer, but data solely on a RAID device will be lost someday; it is a matter of time. If we had the backups from before the hardware or operator error and the archives were not affected, we could have recovered. The bottom line is that there is no excuse for not being in ARCHIVELOG mode on a system where the data is of any value. Performance is no excuse; properly configured archiving adds little to no overhead. This, and the fact that a fast system that loses data is useless, means that even if archiving added 100 percent overhead, you still need to do it. A feature is overhead if you can remove it and lose nothing important; overhead is like icing on the cake. Preserving your data, and making sure you don t lose your data isn t overhead it s the DBA s primary job! Only a test or maybe a development system should execute in NOARCHIVELOG mode. Most development systems should be run in ARCHIVELOG mode for two reasons: This is how you will process the data in production; you want development to act and react as your production system would. In many cases, the developers pull their code out of the data dictionary, modify it, and compile it back into the database. The development database holds the current version of the code. If the development database suffers a disk failure in the afternoon, what happens to all of the code you compiled and recompiled all morning It s lost.

Figure 12-4. Three simple hardware circuits The following code models these circuit components. This uses relational modeling, where each circuit is modeled not as a function but as a propositional logic predicate that relates its input wires to its output wires: let sumBit x y = (x ^^^ y) let carryBit x y = (x &&& y) let halfAdder x y sum carry = (sum === sumBit x y) &&& (carry === carryBit x y) let fullAdder x y z sum carry = let xy = (sumBit x y) (sum === sumBit xy z) &&& (carry === (carryBit x y ||| carryBit xy z))