SQL Server Architecture – Are you ready ?

Explaining SQL Server Architecture in one diagram or Image is very difficult.

Even if we get the same from any blog or website, it is not so easy to understand.
Many of us till date did not even know, what are different engines and their use.

“I have mentioned this considering my situation when I served few years with SQL Server, so please don’t laugh”

I have been planning to prepare such an article for long time and now have come to the conclusion that it can be worked out in Five different phases, just to avoid any gap/confusion. Overall aim is to help understanding SQL Server end to end.

Remember one thing here that each phase is INDIRECTLY related with each other.

A. Logical & Physical Architecture.
B. Recovery Models Architecture
C. SQL Server Engine / DB Engine
There are four major components of DB engine, these are
 – Client Protocols
 – Relational Engine (also known as Query processor)
 – Storage Engine
D. Security Architecture
E. Backup Architecture

Phase 1.  Here you will learn about logical and physical architecture of SQL server.


Phase 2.  I don’t believe that without understanding Recovery Models one can learn SQL server Basics.


Besides that, if you need deep knowledge, I’m a big fan of the Inside SQL Server books: Inside Microsoft SQL Server 2005: The Storage Engine : Inside Microsoft SQL Server 2005: T-SQL Programming : Inside Microsoft SQL Server 2005: T-SQL Querying o Inside Microsoft SQL Server 2005: Query Tuning and Optimization

Here I have tried to describe the overview of System default databases.

Master : The master database holds information for all databases located on the SQL Server instance and is theglue that holds the engine together. Because SQL Server cannot start without a functioning masterdatabase, you must administer this database with care.
MSDB: The msdb database stores information regarding database backups, SQL Agent information, DTS packages, SQL Server jobs, and some replication information such as for log shipping.
tempdb: The tempdb holds temporary objects such as global and local temporary tables and stored procedures.
model: The model is essentially a template database used in the creation of any new user database created in the instance.
resource: The resoure Database is a read-only database that contains all the system objects that are included with SQL Server. SQL Server system objects, such as sys.objects, are physically persisted in the Resource database, but they logically appear in the sys schema of every database. The Resource database does not contain user data or user metadata.


What is a transaction and what are ACID properties?
A transaction is a logical unit of work in which, all the steps must be performed or none. ACID stands for Atomicity, Consistency, Isolation, Durability. These are the properties of a transaction. For more information and explanation of these properties, see SQL Server books online or any RDBMS fundamentals text book.
Explain different isolation levels
An isolation level determines the degree of isolation of data between concurrent transactions. The default SQL Server isolation level is Read Committed. Here are the other isolation levels (in the ascending order of isolation): Read Uncommitted, Read Committed, Repeatable Read, Serializable. See SQL Server books online for an explanation of the isolation levels. Be sure to read about SET TRANSACTION ISOLATION LEVEL, which lets you customize the isolation level at the connection level.
CREATE INDEX myIndex ON myTable(myColumn)

What type of Index will get created after executing the above statement?
Non-clustered index. Important thing to note: By default a clustered index gets created on the primary key, unless specified otherwise.
What’s the maximum size of a row?
8060 bytes. Don’t be surprised with questions like ‘what is the maximum number of columns per table’. Check out SQL Server books online for the page titled: “Maximum Capacity Specifications”.
Explain Active/Active and Active/Passive cluster configurations
Hopefully you have experience setting up cluster servers. But if you don’t, at least be familiar with the way clustering works and the two clusterning configurations Active/Active and Active/Passive. SQL Server books online has enough information on this topic and there is a good white paper available on Microsoft site.
Explain the architecture of SQL Server
This is a very important question and you better be able to answer it if consider yourself a DBA. SQL Server books online is the best place to read about SQL Server architecture. Read up the chapter dedicated to SQL Server Architecture.
What is lock escalation?
Lock escalation is the process of converting a lot of low level locks (like row locks, page locks) into higher level locks (like table locks). Every lock is a memory structure too many locks would mean, more memory being occupied by locks. To prevent this from happening, SQL Server escalates the many fine-grain locks to fewer coarse-grain locks. Lock escalation threshold was definable in SQL Server 6.5, but from SQL Server 7.0 onwards it’s dynamically managed by SQL Server.
What’s the difference between DELETE TABLE and TRUNCATE TABLE commands?
DELETE TABLE is a logged operation, so the deletion of each row gets logged in the transaction log, which makes it slow. TRUNCATE TABLE also deletes all the rows in a table, but it won’t log the deletion of each row, instead it logs the deallocation of the data pages of the table, which makes it faster. Of course, TRUNCATE TABLE can be rolled back.
Explain the storage models of OLAP
Check out MOLAP, ROLAP and HOLAP in SQL Server books online for more infomation.
What are the new features introduced in SQL Server 2000 (or the latest release of SQL Server at the time of your interview)?

What changed between the previous version of SQL Server and the current version?
This question is generally asked to see how current is your knowledge. Generally there is a section in the beginning of the books online titled “What’s New”, which has all such information. Of course, reading just that is not enough, you should have tried those things to better answer the questions. Also check out the section titled “Backward Compatibility” in books online which talks about the changes that have taken place in the new version.
What are constraints? Explain different types of constraints.
Constraints enable the RDBMS enforce the integrity of the database automatically, without needing you to create triggers, rule or defaults.


For an explanation of these constraints see books online for the pages titled: “Constraints” and “CREATE TABLE”, “ALTER TABLE”
What is an index? What are the types of indexes? How many clustered indexes can be created on a table? I create a separate index on each column of a table. what are the advantages and disadvantages of this approach?
Indexes in SQL Server are similar to the indexes in books. They help SQL Server retrieve the data quicker.

Indexes are of two types. Clustered indexes and non-clustered indexes. When you craete a clustered index on a table, all the rows in the table are stored in the order of the clustered index key. So, there can be only one clustered index per table. Non-clustered indexes have their own storage separate from the table data storage. Non-clustered indexes are stored as B-tree structures (so do clustered indexes), with the leaf level nodes having the index key and it’s row locater. The row located could be the RID or the Clustered index key, depending up on the absence or presence of clustered index on the table.

If you create an index on each column of a table, it improves the query performance, as the query optimizer can choose from all the existing indexes to come up with an efficient execution plan. At the same t ime, data modification operations (such as INSERT, UPDATE, DELETE) will become slow, as every time data changes in the table, all the indexes need to be updated. Another disadvantage is that, indexes need disk space, the more indexes you have, more disk space is used.


Although, I would recommend you to spent some time on the Microsoft site, they have a lot of information available on SQL Server architecture.

SQL Server 2000: http://msdn.microsoft.com/en-us/library/aa933154(SQL.80).aspx

SQL Server 2005: http://www.microsoft.com/Sqlserver/2005/en/us/default.aspx

SQL Server 2008: http://www.microsoft.com/sqlserver/2008/en/us/overview.aspx


Amit 🙂


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