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The transaction is said to have been committed at the point where the updated dbpointer is written to disk We now consider how the technique handles transaction and system failures First, consider transaction failure If the transaction fails at any time before db-pointer is updated, the old contents of the database are not affected We can abort the transaction by just deleting the new copy of the database Once the transaction has been committed, all the updates that it performed are in the database pointed to by dbpointer Thus, either all updates of the transaction are re ected, or none of the effects are re ected, regardless of transaction failure Now consider the issue of system failure Suppose that the system fails at any time before the updated db-pointer is written to disk Then, when the system restarts, it will read db-pointer and will thus see the original contents of the database, and none of the effects of the transaction will be visible on the database Next, suppose that the system fails after db-pointer has been updated on disk Before the pointer is updated, all updated pages of the new copy of the database were written to disk Again, we assume that, once a le is written to disk, its contents will not be damaged even if there is a system failure Therefore, when the system restarts, it will read db-pointer and will thus see the contents of the database after all the updates performed by the transaction The implementation actually depends on the write to db-pointer being atomic; that is, either all its bytes are written or none of its bytes are written If some of the bytes of the pointer were updated by the write, but others were not, the pointer is meaningless, and neither old nor new versions of the database may be found when the system restarts Luckily, disk systems provide atomic updates to entire blocks, or at least to a disk sector In other words, the disk system guarantees that it will update db-pointer atomically, as long as we make sure that db-pointer lies entirely in a single sector, which we can ensure by storing db-pointer at the beginning of a block Thus, the atomicity and durability properties of transactions are ensured by the shadow-copy implementation of the recovery-management component As a simple example of a transaction outside the database domain, consider a textediting session An entire editing session can be modeled as a transaction The actions executed by the transaction are reading and updating the le Saving the le at the end of editing corresponds to a commit of the editing transaction; quitting the editing session without saving the le corresponds to an abort of the editing transaction Many text editors use essentially the implementation just described, to ensure that an editing session is transactional A new le is used to store the updated le At the end of the editing session, if the updated le is to be saved, the text editor uses a le rename command to rename the new le to have the actual le name The rename, assumed to be implemented as an atomic operation by the underlying le system, deletes the old le as well Unfortunately, this implementation is extremely inef cient in the context of large databases, since executing a single transaction requires copying the entire database Furthermore, the implementation does not allow transactions to execute concurrently with one another There are practical ways of implementing atomicity and durability that are much less expensive and more powerful We study these recovery techniques in 17.

The basic threat detection feature of the appliance is new in version 8 With this feature enabled, the appliance monitors the dropped packet rate and security events and, if it sees a threat, the appliance generates a log message with a log identifier number of 730100 The kinds of security events or dropped packet rates that the appliance monitors include Matches on deny statements in ACLs Malformed packets (for example, invalid IP header values or an incorrect header length) Packets that fail application layer inspection policies defined by the Modular Policy Framework (MPF) or that inherit in the application inspection process itself (For example, if a specified URL in a policy was seen, causing an HTTP connection to be reset, or if a wiz command was executed on an SMTP/ESMTP connection respectively) Defined connection limits that have been exceeded, which includes global system values as well as limits you ve defined with MPF or the static/nat commands Seeing unusual ICMP packets or connections

To model an enterprise more accurately, the database designer may choose to place certain constraints on a particular generalization One type of constraint involves determining which entities can be members of a given lower-level entity set Such membership may be one of the following: Condition-de ned In condition-de ned lower-level entity sets, membership is evaluated on the basis of whether or not an entity satis es an explicit condition or predicate For example, assume that the higher-level entity set ac-

24:

The McGraw Hill Companies, 2001

Observations The reason this technique is called a drop-in is that it provides additional sales rewards for a defined range in this case, starting at $2,000,000 and terminating at $5,000,000

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Examining the combined rate of all security-related packet drops in this bulleted list An interface became overloaded, causing packet drops A scanning attack was detected (For example, the TCP three-way handshake failed, or the first packet in a TCP connection was not a SYN this is discussed in the Scanning Threat Detection section later in the chapter) An incomplete connection was detected (For example, the TCP three-way handshake failed, or UDP traffic is only seen in one direction of a connection)