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Before a connection can be opened, its ConnectionString property has to be initialized, typically by passing it to the connection constructor Although you can assemble this connection string by hand, it is error prone because it is subject to various insertion traps, and the exact keys that various providers accept are many and hard to remember For these reasons, it is common to either externalize entire connection strings in configuration files (discussed next) or use a ConnectionStringBuilder object from the appropriate provider namespace This object contains all the known connection keys as properties that can be safely set, avoiding passing values of incorrect type or misspelling the key names Consider the following example (this time, using SQL Server running on localhost and referencing the company database; initially, before that database is created, you should remove the InitialCatalog reference from your connection string): open SystemData open SystemData.

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If your program is using ODBC or JDBC when you submit these statements, they (silently) inject a commit after each UPDATE. Consider the impact of this if the system fails after the first UPDATE and before the second. You ve just lost $1,000! I can sort of understand why ODBC does this. The developers of SQL Server designed ODBC, and this database demands that you use very short transactions due to its concurrency model (writes block reads, reads block writes, and locks are a scarce resource). What I can t understand is how this got carried over into JDBC, an API that is supposed to support the enterprise. It is my belief that the very next line of code after opening a connection in JDBC should always be Connection conn = DriverManager.getConnection ("jdbc:oracle:oci:@database","scott","tiger"); conn.setAutoCommit (false); This returns control over the transaction back to you, the developer, which is where it belongs. You can then safely code your account transfer transaction and commit it after both statements have succeeded. Lack of knowledge of your API can be deadly in this case. I ve seen more than one developer unaware of this autocommit feature get into big trouble with his application when an error occurred.

One of the really nice features of Oracle is its ability to transparently handle distributed transactions I can update data in many different databases in the scope of a single transaction When I commit, either I commit the updates in all of the instances or I commit none of them (they will all be rolled back) I need no extra code to achieve this; I simply commit A key to distributed transactions in Oracle is the database link A database link is a database object that describes how to log into another instance from your instance However, the purpose of this section is not to cover the syntax of the database link command (it is fully documented), but rather to expose you to its very existence.

SqlClient let connStr = new SqlConnectionStringBuilder(DataSource="localhost", IntegratedSecurity=true, InitialCatalog="company") On the other hand, not all keys accepted by the various providers are contained in these builder objects, and it is sometimes necessary to add custom key/value pairs You can do this by using the Add method For instance, for an OleDb provider, user credentials can be given as follows: connStrAdd("User Id", "your_user_id") connStrAdd("Password", "your_password") This naturally requires extra care to ensure that the proper keys are assigned You can now access the resulting connection string by reading the ConnectionString property and use it to create a connection object: let conn = new SqlConnection(connStrConnectionString) You can also store your connection strings in configuration files (webconfig for web applications or YourProgramexeconfig for regular applications) The main advantage here is that connection details can be configured without affecting the application Consider the following configuration file: < xml version='1.

Once you have a database link set up, accessing remote objects is as easy as this: select * from T@another_database; This would select from the table T in the database instance defined by the database link ANOTHER_DATABASE Typically, you would hide the fact that T is a remote table by creating a view of it, or a synonym For example, I can issue the following and then access T as if it were a local table: create synonym T for T@another_database; Now that I have this database link set up and can read some tables, I am also able to modify them (assuming I have the appropriate privileges, of course) Performing a distributed transaction is now no different from a local transaction All I would do is this: update local_table set x = 5; update remote_table@another_database set y = 10; commit; That s it.

Oracle will commit either in both databases or in neither It uses a two-phase commit protocol (2PC) to do this 2PC is a distributed protocol that allows for a modification that affects many disparate databases to be committed atomically It attempts to close the window for distributed failure as much as possible before committing In a 2PC between many databases, one of the databases.