![]() If the access is a read - which is what we did with a select after staring both transactions - then a shared lock is created. Since an access can be a read or written we also distinguish the access type. With deferred transactions no locks are acquired until the database is accessed. The default is deferred and it is the type of transaction in the example above. There are 3 types of transactions in SQLite: deferred, immediate, and exclusive. After we’ve ended both shell sessions we can see the 3 animals: cat, dog and bird. Only when we end the transaction from the second shell can we commit the transaction from the first shell. We then try to commit from the first shell and it also says the database is locked. We get an error saying the database is locked. Where things get interesting is when the second shell tries to insert a fish. We then insert an animal named bird and we observe that only the first shell can see the bird, whereas, a select from the second shell cannot see a bird. We issue a select in both transactions and we can see the two animals in the database. Then on two different shells we open the same database file and enter into two different transactions. Before we open any transactions we insert a cat and a dog. In the example above we create an example animals database called animals.db where we have a single column called name where we insert names of animals. $ Explanation of the two transaction example S1 > SELECT name FROM animals all locks are relinquished S2 > COMMIT TRANSACTION s2 ends transaction and releases shared lock s1 > COMMIT TRANSACTION s1 can to from pending to exclusive locks S1 > COMMIT TRANSACTION s1 cannot write since s1 has a shared lock Error: database is locked S2 > INSERT INTO animals(name) VALUES( "fish") s2 cannot write since s1 has reserved lock Error: database is locked S1 > INSERT INTO animals(name) values( "bird") s1 has a reserved lock s2 > SELECT name FROM animals S1 > SELECT name FROM animals s1 and s2 have shared locks S2 > SELECT name FROM animals s2 has a shared lock cat Sqlite > INSERT INTO animals(name) values( "dog") Sqlite > INSERT INTO animals(name) values( "cat") $ sqlite3 animals.db "CREATE TABLE animals(name) " $ sqlite3 animals.db Each of these shells will be in their own transaction. Here we will create a database called animals.db and we will then open two shells which will try to read and write to the database at the same time. SQLite Database Example with Transactions The COMMIT TRANSACTION or END will end the transaction or if error will ROLLBACK which we won’t discuss here.Once started, nothing else can modify the database. The BEGIN TRANSACTION command will start a transaction.In SQLite you can start and end a transaction with the SQL commands below. Beginning and Ending a Transaction in SQLite We can consider this transaction to be atomic in the sense that all the commands will execute successfully or it will fail. In the context of a database a transaction is a grouping of SQL commands. ![]() Let’s first define a database transaction which is essentially to carry out or conduct to a conclusion. In this post we will go over transactions in SQLite which prohibits changes made to the database unless they are inside a transaction. With that said it’s pretty typical that multiple processes or threads have read and write access to a database. In the world of software these days it is expected that applications do multiple things at once. SQLite Transactions Part 1 | Lloyd Rochester's Geek Blog Lloyd Rochester's Geek Blog Home About Navigate
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