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A distributed directory service makes logically organized information available through cooperating systems. In the X.500 model, a client can send requests to one or more servers, which may each hold part of the directory information. LDAP is a protocol for accessing directory services built on X.500 data and service models; it is not another name for the directory service itself.
What makes a directory service distributed?
A directory service provides access to organized information through a directory model. It is distributed when cooperating systems provide that access rather than one system acting as the sole holder and provider of all the information. The standards describe this broadly: they do not require one particular way to divide, copy, or synchronize data.
In the X.500 model, a person or other entity seeking information is a directory user. A client acts on the user’s behalf and communicates with directory servers. The server may answer from information it holds or participate in providing access to information held elsewhere.
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- Directory user: A person or another entity requesting information or a directory operation.
- Directory User Agent (DUA): The client that sends requests on the user’s behalf.
- Directory System Agent (DSA): A server that responds to requests and may hold a fragment of the directory information.
- Directory Information Base (DIB): The information held by the directory. It includes user information as well as administrative and operational information.
- Directory Information Tree (DIT): The hierarchical organization of directory entries used by LDAP.
What is the difference between a directory service and LDAP?
A directory service is the information service and model; LDAP is one protocol used to access it. RFC 4511 puts it this way: “LDAP provides access to distributed directory services that act in accordance with X.500 data and service models.” That distinction matters because LDAP describes how clients interact with a directory, not every architectural choice a directory service makes.
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LDAP represents entries in a DIT and defines protocol operations for accessing directory services. The service may be distributed across servers, but the protocol name alone does not tell you how information is partitioned, replicated, or kept consistent.
How do distribution, replication, and consistency differ?
Distribution concerns how cooperating systems provide access to the directory. Information may be divided among servers, so different servers hold different fragments. Replication means maintaining copies, or shadows, of some information on multiple servers. Synchronization mechanisms can update copied DIT fragments, but their behavior depends on the protocol and implementation.
Consistency describes how updates become visible across copies. A distributed directory does not automatically provide immediate consistency, and a replica is not necessarily current at every moment. RFC 3384 discusses multiple consistency models. Its eventual-consistency model describes updates propagating to reachable replicas along paths of eventual connectivity; it is a model, not a guarantee that applies to every directory product.
What should you check when evaluating a directory implementation?
The standards establish useful questions for comparing architectures, but they do not rank products or establish a universal performance result. For a particular implementation, check its documentation for the answers:
- Is information partitioned among servers, replicated across them, or both?
- How do updates propagate, and which servers accept writes?
- What consistency or convergence guarantees does the implementation document?
- How do clients locate servers and respond to referrals or unavailable servers?
- How are schema, access control, administration, and operational information handled?
These details are implementation-specific. Standards describing directory models or protocol behavior should not be treated as proof of a product’s current conformance, security properties, or operational guarantees.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Standards behind the terminology
The core references for these concepts include RFC 4511 on LDAP, RFC 4512 on LDAP directory information models, RFC 3384 on directory replication requirements, and RFC 4533 on LDAP content synchronization. The cited RFCs date from 2002 and 2006; the ITU-T X.500 contents page cited for the model is from February 2001. They explain the concepts, but current conformance and product-specific behavior should be checked against current standards editions and the implementation’s own documentation.
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