Your education on AD begins with the components that deliver its services. This month: the store.

Active Directory by Design

Your education on AD begins with the components that deliver its services. This month: the store.

Now that Windows 2000 is on the shelves and many machines are coming pre-loaded, we’ll see a strong, continuous push by Microsoft to induce businesses to move over to the latest and greatest. Most prudent companies will take small steps and continue to use the operating Windows NT 4.x services. Essentially, the only thing that will change is that the workstations and servers will be running Win2K code. However, many of us will continue to use the old supported NetBIOS-based services, at least until our support staffs can get their arms around the new services and architecture to determine if and how they want to migrate to a complete Win2K environment.

While that issue works its way through your organization’s pipeline, your best strategy to be ready for the big move is to learn as much as you can before you’re called on to take the slings and arrows.

Squarely at the center of the multitude of services available in Win2K is Active Directory acting as the repository for information. Recently, I’ve discussed directory services in general; now I want to talk about AD specifically and in detail. Obviously, AD is a complex and broad subject, so this discussion will take place over the next several months.

I want to start with an overview of the components and services that are combined to deliver AD services in the context of the terms that have been standardized in X.500. AD isn’t an X.500 directory, but it does follow the general X.500 modular architecture. It’s useful to use X.500 terms when discussing the various directory service components as a reference point when you later compare specific competing products to each other. This month I give an overview of AD Directory System Agent (DSA) components or—as it’s referred to in AD—the store.

AD--Active Directory
DC--domain controller
DIB--Data Information Base
DIT-- Directory Information Tree
DSA--Directory System Agent
ESE--Extensible Store Engine
FSMO--flexible single master operations
GC--global catalog
MMC--Microsoft Management Console
RPC--remote procedure call

The AD Database

The underlying foundation of any directory, or any application for that matter, is the database, which is referred to in X.500 as the Data Information Base (DIB). AD refers to the DIB as the store; it’s based upon the Exchange JET storage engine. Microsoft is quick to point out that it’s not the same JET engine that has been used in past releases of Exchange (or Access, for that matter), but a new version called the Extensible Store Engine (ESE), which shares design with the JET engine, but not code. The main characteristics of the database are that it’s transaction-based, and it employs log and checkpoint files to verify the transactions. Microsoft has claimed success in tests with up to 1.5 million objects in the store. The largest store I’ve heard about in a real organization is just under 500,000 objects.

As you can imagine, backing up these files is critical to the successful restoration of an AD. This process should certainly be in the design documents. The backup process becomes more complex as you add new domains. Remember, each domain is only a partition of the directory store, and at least one DC from each domain must be backed up. The process is similar to the method for making sure that a distributed Exchange messaging system is properly backed up. I’ll cover this topic in more detail in a future column.

Unlike the SAM database, which was part of the registry, the AD store is located in the NTDS.DIT file on a NTFS partition on the domain controllers. AD’s distributed store can be replicated on many different servers within the same domain and also partitioned into many different sections through different domains. Essentially, a domain is a partition of the AD store. AD doesn’t support the creation of DIB partitions smaller than the domain boundary.

Between the AD DSA and the DIB is the database layer, which provides an access point into the actual ESE database. In common Microsoft fashion, the database layer is an abstraction layer between the data store, and applications that are interested in the information in the store. This layer can be accessed by the DSA or through other APIs such as MAPI. It forms the Directory Information Tree (DIT) structure for the DSA from the flat information held in the directory store.

The DIB is the actual repository for the data that the directory is comprised of. However, data isn’t useful until it can be organized and accessed by interested agents, such as applications and users. The real core of this responsibility and of the directory itself is the Directory System Agent. The DSA in AD terminology is a Domain Controller (DC); it’s contained in the NTDSA.DLL that runs on DCs. This DLL is installed and uninstalled with the dcpromo, the Active Directory installation wizard, which finally allows us to create and demote DCs without having to reinstall the entire operating system (one of the great new features of Win2K). The DSA Domain Controller is responsible for providing updates to other DCs. These updates are organized by type. The DSA is further broken down into several functions, which are referred to as roles within the Domain Controller.

Operation: FSMO

Although all DSAs working together are designed to provide multi-master replication of attributes and objects within the directory, some of the information in the DIB can’t afford to have replication conflicts. Therefore, some of the DSA functions are still single master-based. These are called flexible single master operations (FSMO), and Microsoft has defined five specific operations that rely upon them:

  • Schema Master—The schema is the actual structure of the DIB or store. This determines where the attributes will be located in the database. The schema for a given directory service must be consistent throughout the entire forest. There can be only one FSMO schema master within the entire forest of the directory.
  • Relative ID Master—Each domain uses this master to assign unique security IDs to user, group, or computer objects. This FSMO is also used when an object is moved from one domain to another, to ensure that the RID remains unique across the forest.
  • PDC Emulator Master—This FSMO provides compatibility with the NT 4.x authentication requests in a mixed-mode environment. This operation is what allows AD to look just like a PDC to down-level clients. It’s also used in a pure native mode environment to resolve password inconsistencies. As with its NT 4.x virtual PDC counterpart, there can be only one PDC Emulator Master in each Domain.
  • Domain Naming Master—This FSMO is responsible for managing the addition and deletion of the domain names and controlling the uniqueness of those names. If the administrator’s client machine can’t connect to the DC assigned to this FSMO role, you won’t be able to add or remove a domain. There’s only one Domain Naming Master FSMO within a forest.
  • Infrastructure Master—One FSMO in each domain is responsible for enforcing group-to-user references between domains as they’re updated in the natural administration process. Microsoft advises that if you have multiple DCs in a domain, don’t place a Global Catalog on this machine, or the group-to-user resolution process won’t work properly.

I know you’ve been hearing a lot about the multi-master replication model for AD. However, as you can see, several components still use the master/slave database model. As you can imagine, this has a fundamental impact on where you place these machines and on the type of machines you use to support these services. You can’t count on multiple instances of the FSMOs to create redundancies in the system. These machines are best placed in locations that are physically close (this side of routers) to the administrators who are interested in them, and they also need to be closely monitored for availability.

All FSMOs are determined automatically during the installation process. However, as the network grows and the location of these servers becomes less than optimal, you can transfer these FSMO roles to other machines. Just make sure you’re moving them to machines you’re confident will remain operational. Treat them as you’d treat a PDC in NT 4.x.

These singular roles are normally transferred through AD MMC snap-ins, but the roles can be seized from downed machines if necessary. Again, this is similar to an NT 4.x BDC being promoted when a PDC fails. You seize these roles with ntdsutil, a utility that comes with the Windows 2000 Resource Kit. That utility is also used for other database management tasks such as moving, compacting, repairing, and checking the integrity of the AD data store.

As with the BDC-to-PDC analogy, use caution when taking this step. If the original FSMO comes back online, unpredictable results can occur, and they won’t be pretty. If you have an FSMO machine that goes down and you want to seize the role for another machine, make sure you demote the original DC component before bringing it back online. If you still want it to be the FSMO for that role, then you need to run dcpromo again to make it a DC and transfer the role using the appropriate snap-in. This should remind you of the reintroduction of a repaired PDC into a system that has had a BDC promoted while the original PDC was under repair.

DC Roles

There are also generic roles that DCs play in AD that are critical to the communication and synchronization of the data store. Every DC automatically participates in intra-site automatically through remote procedure calls (RPCs). One DC within each site, defined as a group of domain controllers within one or more subnets connected with fast links, is configured as a bridgehead server. These servers act as a replication focal point to other islands of self-contained fast connectivity. Replication is a topic I’ll cover in length in a future column.

The other major role DCs may play is in holding a copy of the Global Catalog (GC). This is an assigned role, and any number of DCs within a domain may hold a copy of the GC. The placement of GCs is determined largely by the characteristics of the physical network. The GC is an index file that contains all of the objects in the entire directory forest. This index information is stored in the NTDS.DIT file along with the other directory information. However, it doesn’t contain all of the attributes in the forest, only the ones Microsoft has placed in there by default. As the Administrator, you can add other attributes if you want them included in the index.

The default base of attributes in the GC can’t be removed; they’re used by services to find other services they need to function properly. This index file also allows users and applications to locate directory objects without any knowledge of the domains and across a discontiguous namespace.

The DSA in a directory implementation is where most of the work takes place to keep the system properly converged and available to provide accurate and useful information to requestors. Microsoft has taken the X.500 DSA model and created a more modular implementation, using DCs as DSAs. The most important thing to consider with this DC model is that it isn’t a complete multi-master database model. There are singular and relatively singular roles that some DCs are chosen to play within the system. To ensure operational availability, take care in where you locate these physically and in the hardware you choose to run them on.


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