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CHAPTER 6 LOCKING AND LATCHING
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A one-way hash function takes a variable-length input string, the data, and converts it to a fixed-length (generally smaller) output string called a hash value. The hash value serves as a unique identifier (like a fingerprint) of the input data. You can use the hash value to verify whether data has been changed or not. Note that a one-way hash function is a hash function that works in one direction. It is easy to compute a hash value from the input data, but it is hard to generate data that hashes to a particular value.
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http://download.oracle.com/docs/cd/E11882_01/appdev.112/e10577/d_crypto.htm#ARPLS65700 We can use these hashes or checksums in the same way that we used our version column. We simply compare the hash or checksum value we obtain when we read data out of the database with that we obtain before modifying the data. If someone modified the row s values after we read it out, but before we updated it, then the hash or checksum will almost certainly be different. There are many ways to compute a hash or checksum. I ll list four of these and demonstrate one in this section. All of these methods are based on supplied database functionality. OWA_OPT_LOCK.CHECKSUM: This method is available on Oracle8i version 8.1.5 and up. There is a function that, given a string, returns a 16-bit checksum, and another function that, given a ROWID, will compute the 16-bit checksum of that row and lock it at the same time. Possibilities of collision are 1 in 65,536 strings (the highest chance of a false positive). DBMS_OBFUSCATION_TOOLKIT.MD5: This method is available in Oracle8i version 8.1.7 and up. It computes a 128-bit message digest. The odds of a collision are about 1 in 3.4028E+38 (very small). DBMS_CRYPTO.HASH: This method is available in Oracle 10g Release 1 and up. It is capable of computing a Secure Hash Algorithm 1 (SHA-1) or MD4/MD5 message digests. It is recommended that you use the SHA-1 algorithm. ORA_HASH: This method is available in Oracle 10g Release 1 and up. This is a built-in SQL function that takes a varchar2 value as input and (optionally) another pair of inputs that control the return value. The returned value is a number by default a number between 0 and 4294967295.
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Note An array of hash and checksum functions are available in many programming languages, so there may be
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others at your disposal outside the database. That said, if you use built-in database capabilities, you will have increased your portability (to new languages, new approaches) in the future.
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The following example shows how you might use the ORA_HASH built-in function in Oracle 10g and above to compute these hashes/checksums. The technique would also be applicable for the other three listed approaches; the logic would not be very much different, but the APIs you call would be. First, we ll start by getting rid of the column we used in the previous example ops$tkyte%ORA11GR2> alter table dept drop column last_mod; Table altered.
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CHAPTER 6 LOCKING AND LATCHING
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and then have our application query and display the information for department 10 . Note that while we query the information, we compute the hash using the ORA_HASH built-in. This is the version information that we retain in our application. Following is our code to query and display: ops$tkyte%ORA11GR2> ops$tkyte%ORA11GR2> ops$tkyte%ORA11GR2> ops$tkyte%ORA11GR2> variable variable variable variable deptno number dname varchar2(14) loc varchar2(13) hash number
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ops$tkyte%ORA11GR2> begin 2 select deptno, dname, loc, 3 ora_hash( dname || '/' || loc ) hash 4 into :deptno, :dname, :loc, :hash 5 from dept 6 where deptno = 10; 7 end; 8 / PL/SQL procedure successfully completed. ops$tkyte%ORA11GR2> select :deptno, :dname, :loc, :hash 2 from dual; :DEPTNO :DNAME :LOC :HASH ---------- ---------- ---------- ---------10 Accounting NEW YORK 2721972020 As you can see, the hash is just some number. It is the value we would want to use before updating. To update that row, we would lock the row in the database as it exists right now, and then compare the hash value of that row with the hash value we computed when we read the data out of the database. The logic for doing so could look like the following: ops$tkyte%ORA11GR2> exec :dname := lower(:dname); PL/SQL procedure successfully completed. ops$tkyte%ORA11GR2> update dept 2 set dname = :dname 3 where deptno = :deptno 4 and ora_hash( dname || '/' || loc ) = :hash 5 / 1 row updated. ops$tkyte%ORA11GR2> select dept.*, 2 ora_hash( dname || '/' || loc ) hash 3 from dept 4 where deptno = :deptno; DEPTNO DNAME LOC HASH ---------- ---------- ---------- ---------10 accounting NEW YORK 2818855829 Upon re-querying the data and computing the hash again after the update, we can see that the hash value is different. If someone had modified the row before we did, our hash values would not have compared. We can see this by attempting our update again, using the old hash value we read out the first time:
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