SQLOSSQLOS is the base component in the SQL Server architecture. It implements functions normally associated with the Operating System - thread scheduling, memory management, I/O management, buffer pool management, resource management, synchronization primitives and locking, and deadlock detection. Because the requirement of SQL Server is highly specialized, SQL Server implements its own memory and thread management system, rather than using the generic one already implemented in the Operating System. It divides all the operations it performs into a series of Tasks - both background maintenance jobs as well as processing requests from clients. Internally, a pool of worker threads is maintained, onto which the tasks are scheduled. A task is associated with the thread till it is completed, only after its completion is the thread freed and returned to the pool. If there are no free threads to assign the task to, the task is temporarily blocked. Each worker thread is mapped onto either an Operating System thread or a fiber, which are user mode threads, which is used to implement co-operative multitasking. Using the latter, even though all the book-keeping jobs of thread management has to be implemented in SQLOS (in addition to the native OS implementation), it can optimize for the particular use. SQLOS also includes synchronization primitives for locking as well as monitoring for the worker threads to detect if they have entangled themselves into a deadlock and takes necessary measures to recover from the situation.
SQLOS handles the memory requirements of SQL Server as well. Reducing disc I/O is one of the primary goals of specialized memory management in SQL Server. As such, it maintains a buffer pool, handled by SQLOS, which is used to cache data pages from the disc as well as to satisfy the memory requirements for other components, including query processor, and other internal data structures.[1] SQLOS also has to take care that the memory allocated is used efficiently, as such it monitors all the memory allocated from the buffer pool, ensuring that the components return unused memory to the pool, as well as shuffling data in and out of the cache to make room for newer data. For changes that are made to the data in buffer, SQLOS writes the data back to the disc lazily, that is when the disc subsystem is either free, or there have significant number changes made to the cache, while still serving requests from the cache. For this, it implements a Lazy Writer, which handles the task of writing the data back to persistent storage.
SQL Server normally supports up to 2 GB memory on x86 hardware, though it can be configured to use up to 64 GB if Address Windowing Extension is used on supporting OS. For x64 hardware, it supports 8 TB of memory, and 7 TB for IA-64 systems. However, when running x86 versions of SQL Server on x64 hardware, it can access 4 GB of memory without any special configuration.
Relational engineThe Relational engine is the component, which implements the relational data store, using the capabilities provided by SQLOS, which is exposed to this layer via the private SQLOS API. It implements the type system, to define the types of the data that can be stored in the tables, as well as the different types of data items (such as tables, indexes, logs etc) that can be stored. It includes the Storage Engine, which handles the way data is stored on persistent storage devices, as well as implement methods for fast access to the data. The storage engine implements log-based transaction so as to ensure that any changes to the data are ACID compliant. It also includes the query processor, which is the component that allows data to be retrieved. The specification of what needs to be retrieved is provided in the form of a SQL query, which it optimizes and translates into the sequence of operations needed to retrieve the data. The operations are then scheduled on to the worker threads, which are scheduled for execution by SQLOS.
Protocol layerProtocol layer implements the external interface to SQL Server. All operations that can be invoked on SQL Server are communicated to it via a Microsoft-defined format, called Tabular Data Stream (TDS). TDS packets can be encased in other physical transport dependent protocols, including TCP/IP, Named pipes, and Shared memory. Consequently, access to SQL Server is available over these protocols. In addition, the SQL Server API is also exposed over web services.
Data storageThe main unit of data storage is a database, which is a collection of tables with typed columns. SQL Server supports different data types, including primary types such as Integer, Float, Decimal, Char (including character strings), Varchar (variable length character strings), binary (for unstructured binary blob of data), Text (for textual data) among others. It also allows user-defined composite types (UDTs) to be defined and used. SQL Server also makes server statistics available as virtual tables and views (called Dynamic Management Views or DMVs). A database can also contain other objects including views, stored procedures, indexes and constraints, in addition to tables, along with a transaction log. An SQL Server database can contain a maximum of 231 objects, and can span multiple OS-level files with a maximum file size of 220 TB.[1] The data in the database are stored in primary data files with an extension .mdf. Secondary data files, identified with an .ndf extension, are used to store optional metadata. Log files are identified with the .ldf extension.
Storage space allocated to a database is divided into sequentially numbered pages, each 8 KB in size. A page is the basic unit of I/O for SQL Server operations. A page is marked with a 96-byte header which stores metadata about the page including the page number, page type, free space on the page and the ID of the object that owns it. Page type defines the data contained in the page - data stored in the database, index, allocation map which holds information about how pages are allocated to tables and indexes, change map which holds information about the changes made to other pages since last backup or logging, or contain large data types such as image or text. While page is the basic unit of an I/O operation, space is actually managed in terms of an extent which consists of 8 pages. A database object can either span all 8 pages in an extent ("uniform extent") or share an extent with up to 7 more objects ("mixed extent"). A row in a database table cannot span more than one page, so is limited to 8 KB in size. However, if the data exceeds 8 KB and the row contains Varchar or Varbinary data, the data in those columns are moved to a new page (or possible a sequence of pages, called Allocation unit) and replaced with a pointer to the data.
For physical storage of a table, its rows are divided into a series of partitions (numbered 1 to n). The partition size is user defined; by default all rows are in a single partition. A table is split into multiple partitions in order to spread a database over a cluster. Rows in each partition are stored in either B-tree or heap structure. If the table has an associated index to allow fast retrieval of rows, the rows are stored in-order according to their index values, with a B-tree providing the index. The data is in the leaf node of the leaves, and other nodes storing the index values for the leaf data reachable from the respective nodes. If the index is non-clustered, the rows are not sorted according to the index keys. An indexed view has the same storage structure as an indexed table. A table without an index is stored in an unordered heap structure. Both heaps and B-trees can span multiple allocation units.
Buffer managementSQL Server buffers pages in RAM to minimize disc I/O. Any 8 KB page can be buffered in-memory, and the set of all pages currently buffered is called the buffer cache. The amount of memory available to SQL Server decides how many pages will be cached in memory. The buffer cache is managed by the Buffer Manager. Either reading from or writing to any page copies it to the buffer cache. Subsequent reads or writes are redirected to the in-memory copy, rather than the on-disc version. The page is updated on the disc by the Buffer Manager only if the in-memory cache has not been referenced for some time. While writing pages back to disc, asynchronous I/O is used whereby the I/O operation is done in a background thread so that other operations do not have to wait for the I/O operation to complete. Each page is written along with its checksum when it is written. When reading the page back, its checksum is computed again and matched with the stored version to ensure the page has not been damaged or tampered with in the mean time.
Logging and TransactionSQL Server ensures that any change to the data is ACID-compliant, i.e., it uses transactions to ensure that any operation either totally completes or is undone if fails, but never leave the database in an intermediate state. Using transactions, a sequence of actions can be grouped together, with the guarantee that either all actions will succeed or none will. SQL Server implements transactions using a write-ahead log. Any changes made to any page will update the in-memory cache of the page, simultaneously all the operations performed will be written to a log, along with the transaction ID which the operation was a part of. Each log entry is identified by an increasing Log Sequence Number (LSN) which ensure that no event overwrites another. SQL Server ensures that the log will be written onto the disc before the actual page is written back. This enables SQL Server to ensure integrity of the data, even if the system fails. If both the log and the page were written before the failure, the entire data is on persistent storage and integrity is ensured. If only the log was written (the page was either not written or not written completely), then the actions can be read from the log and repeated to restore integrity. If the log wasn't written, then also the integrity is maintained, even though the database is in a state when the transaction as if never occurred. If it was only partially written, then the actions associated with the unfinished transaction is discarded. Since the log was only partially written, the page is guaranteed to have not been written. So, the data is still in a consistent state. Removing the unfinished log entries means as if the transaction is undone. SQL Server ensures consistency between the log and the data every time an instance is restarted.
Concurrency and lockingSQL Server allows multiple clients to use the same database concurrently. As such, it needs to control concurrent access to shared data, to ensure data integrity - when multiple clients update the same data, or clients attempt to read data that is in the process of being changed by another client. SQL Server provides two modes of concurrency control: pessimistic concurrency and optimistic concurrency. When pessimistic concurrency control is being used, SQL Server controls concurrent access by using locks. Locks can be either shared or exclusive. Exclusive lock grants the user exclusive access to the data - no other user can access the data as long as the lock is held. Shared locks are used when some data is being read - multiple users can read from data locked with a shared lock, but not acquire an exclusive lock. The latter would have to wait for all shared locks to be released. Locks can be applied on different levels of granularity - on entire tables, pages, or even on a per-row basis on tables. For indexes, it can either be on the entire index or on index leaves. The level of granularity to be used is defined on a per-database basis by the database administrator. While a fine grained locking system allows more users to use the table or index simultaneously, it requires more resources. So it does not automatically turn into higher performing solution. SQL Server also includes two more lightweight mutual exclusion solutions - latches and spinlocks - which are less robust than locks but are less resource intensive. SQL Server uses them for DMVs and other resources that are usually not busy. SQL Server also monitors all worker threads that acquire locks to ensure that they do not end up in deadlocks - in case they do, SQL Server takes remedial measures, which in many cases is to kill one of the threads entangled in a deadlock and rollback the transaction it started. To implement locking, SQL Server contains the Lock Manager. The Lock Manager maintains an in-memory table that manages the database objects and locks, if any, on them along with other metadata about the lock. Access to any shared object is mediated by the lock manager, which either grants access to the resource or blocks it.
SQL Server also provides the optimistic concurrency control mechanism. Also called multi version concurrency control, it allows a new version of a row to be created whenever the row is updated, as opposed to overwriting the row, i.e., a row is additionally identified by the ID of the transaction that created the version of the row. Both the old as well as the new versions of the row are stored and maintained, though the old versions are moved out of the database into a system database identified as Tempdb. When a row is in the process of being updated, any other requests are not blocked (unlike locking) but are executed on the older version of the row. If the other request is an update statement, it will result in two different versions of the rows - both of them will be stored by the database, identified by their respective transaction IDs.
Data retrievalThe main mode of retrieving data from an SQL Server database is querying for it. The query is expressed using a variant of SQL called T-SQL, a dialect Microsoft SQL Server shares with Sybase SQL Server due to its legacy. The query declaratively specifies what is to be retrieved. It is processed by the query processor, which figures out the sequence of steps that will be necessary to retrieve the requested data. The sequence of actions necessary to execute a query is called a query plan. There might be multiple ways to process the same query. For example, for a query that contains a join statement and a select statement, executing join on both the tables and then executing select on the results would give the same result as selecting from each table and then executing the join, but result in different execution plans. In such case, SQL Server chooses the plan that is supposed to yield the results in the shortest possible time. This is called query optimization and is performed by the query processor itself.
SQL Server includes a cost-based query optimizer which tries to optimize on the cost, in terms of the resources it will take to execute the query. Given a query, the query optimizer looks at the database schema, the database statistics and the system load at that time. It then decides which sequence to access the tables referred in the query, which sequence to execute the operations and what access method to be used to access the tables. For example, if the table has an associated index, whether the index should be used or not - if the index is on a column which is not unique for most of the columns (low "selectivity"), it might not be worthwhile to use the index to access the data. Finally, it decides whether to execute the query concurrently or not. While a concurrent execution is more costly in terms of total processor time, because the execution is actually split to different processors might mean it will execute faster. Once a query plan is generated for a query, it is temporarily cached. For further invocations of the same query, the cached plan is used. Unused plans are discarded after some time.
SQL Server also allows stored procedures to be defined. Stored procedures are parameterized T-SQL queries, that are stored in the server itself (and not issued by the client application as is the case with general queries). Stored procedures can accept values sent by the client as input parameters, and send back results as output parameters. They can also call other stored procedures, and can be selectively provided access to. Unlike other queries, stored procedures have an associated name, which is used at runtime to resolve into the actual queries. Also because the code need not be sent from the client every time (as it can be accessed by name), it reduces network traffic and somewhat improves performance. Execution plans for stored procedures are also cached as necessary.
SQL CLRMicrosoft SQL Server 2005 includes a component named SQL CLR via which it integrates with .NET Framework. Unlike most other applications that leverage .NET Framework functionality, SQL Server itself hosts the .NET Framework runtime, i.e., memory, threading and resource management requirements of .NET Framework are satisfied by SQLOS itself, rather than the underlying Windows operating system. SQLOS provides deadlock detection and resolution services for .NET code as well. With SQL CLR, stored procedures and triggers can be written in any managed .NET language, including C# and VB.NET. Managed code can also be used to define UDTs which can be persisted in the database. Managed code is compiled to .NET assemblies and after being verified for type safety, registered at the database. After that, they can be invoked like any other procedure. However, only a subset of the Base Class Library is available, when running code under SQL CLR. Most APIs relating to user interface functionality are not available.
When writing code for SQL CLR, data stored in SQL Server databases can be accessed using the ADO.NET APIs like any other managed application that accesses SQL Server data. However, doing that creates a new database session, different from the one in which the code is executing. To avoid this, SQL Server provides some enhancements to the ADO.NET provider that allows the connection to be redirected to the same session which already hosts the running code. Such connections are called context connections and are set by setting context connection parameter to true in the connection string. SQL Server also provides several other enhancements to the ADO.NET API, including classes to work with tabular data or a single row of data as well as classes to work with internal metadata about the data stored in the database. It also provides access to the XML features in SQL Server, including XQuery support. These enhancements are available only when running code under SQL CLR, and not available generally.
ServicesSQL Server also includes an assortment of add-on services. While these are not essential for the operation for the database system, these provide value added services on top of the core database management system. These services either run as a part of some SQL Server component or out-of-process as Windows Service and presents their own API to control and interact with them.
Service BrokerThe Service Broker, which runs as a part of the database engine, provides a reliable messaging and message queuing platform for SQL Server applications. Used inside an instance, it is used to provide an asynchronous programming environment. For cross instance applications, Service Broker communicates over TCP/IP and allows the different components to be synchronized together, via exchange of messages.
Replication ServicesSQL Server Replication Services are used by SQL Server to replicate and synchronize database objects, either in entirety or a subset of the objects present, across replication agents, which might be other database servers across the network, or database caches on the client side. Replication follows a publisher/subscriber model, i.e., the changes are sent out by one database server ("publisher") and are received by others ("subscribers"). SQL Server supports three different types of replication:
Transaction replicationEach transaction made to the publisher database (master database) is synced out to subscribers, who update their databases with the transaction. Transactional replication synchronizes databases in near real time.
Merge replicationChanges made at both the publisher and subscriber databases are tracked, and periodically the changes are synchronized bi-directionally between the publisher and the subscribers. If the same data has been modified differently in both the publisher and the subscriber databases, synchronization will result in a conflict which has to be resolved - either manually or by using pre-defined policies.
Snapshot replicationSnapshot replication published a copy of the entire database (the then-snapshot of the data) and replicates out to the subscribers. Further changes to the snapshot are not tracked.
Analysis ServicesSQL Server Analysis Services adds OLAP and data mining capabilities for SQL Server databases. The OLAP engine supports MOLAP, ROLAP and HOLAP storage modes for data. Analysis Services supports the XML for Analysis standard as the underlying communication protocol. The cube data can be accessed using MDX queries. Data mining specific functionality is exposed via the DMX query language. Analysis Services includes various algorithms - Decision trees, clustering algorithm, Naive Bayes algorithm, time series analysis, sequence clustering algorithm, linear and logistic regression analysis, and neural networks - for use in data mining.
Reporting ServicesSQL Server Reporting Services is a report generation environment for data gathered from SQL Server databases. It is administered via a web interface. Reporting services features a web services interface to support the development of custom reporting applications. Reports are created as RDL files.
Reports can be designed using recent versions of Microsoft Visual Studio(including Visual Studio.NET 2003 onwards) with Business Intelligence Development Studio, installed or with the included Report Builder. Once created, RDL files can be rendered in a variety of formats including Excel, PDF, CSV, XML, TIFF (and other image formats), and HTML Web Archive.
Notification ServicesSQL Server Notification Services is a platform for generating notifications, which are sent to Notification Services subscribers. A subscriber registers for a specific event or transaction (which is registered on the database server as a trigger); when the event occurs, Notification Services uses Service Broker to send a message to the subscriber informing about the occurrence of the event.
Integration ServicesSQL Server Integration Services is used to integrate data from different data sources. It is used for the ETL capabilities for SQL Server for data warehousing needs. Integration Services includes GUI tools to build data extraction workflows integration various functionality such as extracting data from various sources, querying data, transforming data including aggregating, duplication and merging data, and then loading the transformed data onto other sources, or sending e-mails detailing the status of the operation.
Full Text Search ServiceSQL Server Full Text Search service is a specialized indexing and querying service for unstructured text stored in SQL Server databases. The full text search index can be created on any column with character based text data. It allows for words to be searched for in the text columns. While it can be performed with the SQL LIKE operator, using SQL Server Full Text Search service can be more efficient. Full Text Search (FTS) allows for inexact matching of the source string, indicated by a Rank value which can range from 0 to 1000 - a higher rank means a more accurate match. It also allows linguistic matching ("inflectional search"), i.e., linguistic variants of a word (such as a verb in a different tense) will also be a match for a given word (but with a lower rank than an exact match). Proximity searches are also supported, i.e., if the words searched for do not occur in the sequence they are specified in the query but are near each other, they are also considered a match. T-SQL exposes special operators that can be used to access the FTS capabilities.
The Full Text Search engine is divided into two processes - the Filter Daemon process (msftefd.exe) and the Search process (msftesql.exe). These processes interact with the SQL Server. The Search process includes the indexer (that creates the full text indexes) and the full text query processor. The indexer scans through text columns in the database. It can also index through binary columns, and use iFilters to extract meaningful text from the binary blob (for example, when a Microsoft Word document is stored as an unstructured binary file in a database). The iFilters are hosted by the Filter Daemon process. Once the text is extracted, the Filter Daemon process breaks it up into a sequence of words and hands it over to the indexer. The indexer filters out noise words, i.e., words like A, And etc, which occur frequently and are not useful for search. With the remaining words, an inverted index is created, associating each word with the columns they were found in. SQL Server itself includes a Gatherer component that monitors changes to tables and invokes the indexer in case of updates.
When a full text query is received by the SQL Server query processor, it is handed over to the FTS query processor in the Search process. The FTS query processor breaks up the query into the constituent words, filters out the noise words, and uses an inbuilt thesaurus to find out the linguistic variants for each word. The words are then queried against the inverted index and a rank of their accurateness is computed. The results are returned to the client via the SQL Server process.
The SQL Server Full Text Search service architecture
EditionsMicrosoft makes SQL Server available in multiple versions, with different feature sets and targeting different users. These versions are:
SQL Server Express EditionSQL Server Express Edition is a scaled down, free edition of SQL Server, which includes the core database engine. While there are no limitations on the number of databases or users supported, it is limited to using one processor, 1 GB memory and 4 GB database files. The entire database is stored in a single .mdf file, and thus making it suitable for XCOPY deployment. It is intended as a replacement for MSDE. The Full-Text Search Service is made available as a free add-on to SQL Server Express edition as well. In addition, a stripped down version of SQL Server Management Studio is also available for this edition.
SQL Server Workgroup EditionSQL Server Workgroup Edition includes the core database functionality but does not include the additional services.
SQL Server Standard EditionSQL Server Standard edition includes the core database engine, along with the stand-alone services. It differs from Enterprise edition in that it supports less number of active instances (number of nodes in a cluster) and does not include some high-availability functions such as hot-add memory (allows memory to be added while the server is still running), and parallel indexes.
SQL Server Enterprise EditionSQL Server Enterprise Edition is the full-featured version of SQL Server, including both the core database engine and add-on services, while including a range of tools for creating and managing an SQL Server cluster.
SQL Server Developer EditionSQL Server Developer Edition includes the same features as SQL Server Enterprise Edition, but is limited by the license to be only used as a development and test system, and not as production server. This version is available to download by students free of charge as a part of Microsoft's DreamSpark program.
Tools:
SQLCMDSQLCMD is a command line application that comes with Microsoft SQL Server, and exposes the management features of SQL Server. It allows SQL queries to be written and executed from the command prompt. It can also act as a scripting language to create and run a set of SQL statements as a script. Such scripts are stored as a .sql file, and are used either for management of databases or to create the database schema during the deployment of a database.
Visual StudioMicrosoft Visual Studio includes native support for data programming with Microsoft SQL Server. It can be used to write and debug code to be executed by SQL CLR. It also includes a data designer that can be used to graphically create, view or edit database schemas. Queries can be created either visually or using code. Visual Studio 2008 onwards, Visual Studio provides intellisense for SQL queries as well.
SQL Server Management StudioSQL Server Management Studio is a GUI tool included with SQL Server 2005 for configuring, managing, and administering all components within Microsoft SQL Server. The tool includes both script editors and graphical tools which work with objects and features of the server. A version of SQL Server Management Studio is also available for SQL Server Express Edition, for which it is known as SQL Server Management Studio Express (SSMSE).
A central feature of SQL Server Management Studio is the Object Explorer, which allows the user to browse, select, and act upon any of the objects within the server. It can be used to visually observe and analyze query plans and optimize the database performance, among others. SQL Server Management Studio can also be used to create a new database, alter any existing database schema by adding or modifying tables and indexes or analyze performance. It includes the query windows which provides a GUI based interface to write and execute queries.
Microsoft SQL Server Management Studio showing a query, the results of the query, and the Object Explorer pane while connected to a SQL Server database engine instance.Business Intelligence
Development StudioBusiness Intelligence Development Studio (BIDS) is the IDE from Microsoft used for developing data analysis and Business Intelligence solutions utilizing the Microsoft SQL Server Analysis Services, Reporting Services and Integration Services. It is based on the Microsoft Visual Studio development environment but customizes with the SQL Server services-specific extensions and project types, including tools, controls and projects for reports (using Reporting Services) and data mining structures such as Cubes, Roles and Multidimensional views (using Analysis Services).
See Also
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