How Exchange Server database transactions work

The process that database transactions use is the same process used by all mailbox servers, regardless of whether or not continuous replication has been enabled. Database transactions are operations that prompt a database update. These updates can include operations like sending or receiving an email message, updating a calendar item or creating a new task.

Any time a database transaction occurs, the database page that the transaction is being performed against is read from the database file and copied to an area of the server's memory known as the ESE cache. Any updates to the database resulting from the current transaction are performed within the ESE cache but are not written directly to the database.

Cached database pages that have been updated but not written to disk are referred to as dirty pages. If additional transactions perform operations against dirty pages, then the operation is performed within the ESE cache. In such a case, Exchange uses a mechanism called the version store to make sure the database remains in a consistent state.

Next, the transaction is recorded in a transaction log file. Depending on the size of the transaction, this process may involve closing the current log file and starting another one. Because the log file is only 1 MB, Exchange may consider the excessively large transactions as multiple log files. Each time a log file reaches the 1 MB limit, it is closed and a new log file starts.

When a backup is running against an Exchange server, the contents of the log files are written to the database. At this point, the checkpoint file is advanced so that it references the oldest log file that has not yet been committed to the database.

Exchange 2007 Replication Introduction

The original RTM release of Exchange Server 2007 offered two different types of continuous replication: local continuous replication and cluster continuous replication. Local continuous replication provides a degree of fault tolerance on a single mailbox server. The basic premise is that LCR creates a secondary copy of an Exchange Server database on a separate volume than the server. In contrast, CCR uses a similar method to create a separate copy of the database and stores it on a separate Exchange server.

When Microsoft released Service Pack 1 for Exchange Server 2007, it introduced a third type of continuous replication known as standby continuous replication (SCR). SCR is similar to CCR; however, CCR can only create one replica of a protected database. SCR can create multiple replicas of a database.

Even though Exchange Server 2007 provides three different types of continuous replication, all three work in basically the same way. Continuous replication is based on a technique called log shipping, which involves copying each log file to the location where the backup database is stored while the log file is built. The log file is then replayed against the replica database, bringing the database into a current state.

Microsoft classifies continuous replication as a storage group-level operation, but I think it's more accurate to classify it as a database-level operation. Log files reside at the storage group level, but continuous replication, in any form, limits the protected storage group to a single database. Therefore, continuous replication could be considered a database-level operation because it involves a single database.

Firewall

A firewall is a part of a computer system or network that is designed to block unauthorized access while permitting authorized communications. It is a device or set of devices configured to permit, deny, encrypt, decrypt, or proxy all (in and out) computer traffic between different security domains based upon a set of rules and other criteria.

Firewalls can be implemented in either hardware or software, or a combination of both. Firewalls are frequently used to prevent unauthorized Internet users from accessing private networks connected to the Internet, especially intranets. All messages entering or leaving the intranet pass through the firewall, which examines each message and blocks those that do not meet the specified security criteria.

There are several types of firewall techniques:

1. Packet filter: Looks at each packet entering or leaving the network and accepts or rejects it based on user-defined rules. Packet filtering is fairly effective and transparent to users, but it is difficult to configure. In addition, it is susceptible to IP spoofing.
2. Application gateway: Applies security mechanisms to specific applications, such as FTP and Telnet servers. This is very effective, but can impose a performance degradation.
3. Circuit-level gateway: Applies security mechanisms when a TCP or UDP connection is established. Once the connection has been made, packets can flow between the hosts without further checking.
4. Proxy server: Intercepts all messages entering and leaving the network. The proxy server effectively hides the true network addresses.

Switches

A switch is a device that forwards and filters OSI layer 2 datagrams (chunk of data communication) between ports (connected cables) based on the MAC addresses in the packets.^[6] This is distinct from a hub in that it only forwards the packets to the ports involved in the communications rather than all ports connected. Strictly speaking, a switch is not capable of routing traffic based on IP address (OSI Layer 3) which is necessary for communicating between network segments or within a large or complex LAN. Some switches are capable of routing based on IP addresses but are still called switches as a marketing term. A switch normally has numerous ports, with the intention being that most or all of the network is connected directly to the switch, or another switch that is in turn connected to a switch.

Switch is a marketing term that encompasses routers and bridges, as well as devices that may distribute traffic on load or by application content (e.g., a Web URL identifier). Switches may operate at one or more OSI model layers, including physical, data link, network, or transport (i.e., end-to-end). A device that operates simultaneously at more than one of these layers is called a multilayer switch.

Overemphasizing the ill-defined term "switch" often leads to confusion when first trying to understand networking. Many experienced network designers and operators recommend starting with the logic of devices dealing with only one protocol level, not all of which are covered by OSI. Multilayer device selection is an advanced topic that may lead to selecting particular implementations, but multilayer switching is simply not a real-world design concept.

Setting Up a DMZ in a Virtual World

While everyone agrees that virtualization solves a number of problems, most folks will agree that it often forces us to reconsider how we do things. Security is a prime example of this and we have been spending serious cycles on understanding the implications of this and the impact for you, our customers. The newest example of this activity is a joint white paper developed by Cisco and VMware on Deploying a DMZ in a virtualized environment with the Cisco Nexus 1000V and VMware vSphere 4.

Remember, if you want to play with this for yourself, download the 60-day eval at http://www.cisco.com/go/1000veval.