X can subsequently do a SELECT against the records that Y modified but X will see the older unmodified entries because Y's changes are all invisible to X while X is holding a read transaction. Then Y comes along and runs an UPDATE statement to modify the database. X starts a read transaction using BEGIN followed by one or more SELECT statements. Any write transactions that commit while the read transaction is active are still invisible to the read transaction, because the reader is seeing a snapshot of database file from a prior moment in time.Īn example: Suppose there are two database connections X and Y. When a read transaction starts, that reader continues to see an unchanging "snapshot" of the database file as it existed at the moment in time when the read transaction started. In WAL mode, SQLite exhibits "snapshot isolation". That means that readers can continue to read the old, original, unaltered content from the original database file at the same time that the writer is appending to the write-ahead log. It can do this because changes do not overwrite the original database file, but rather go into the separate write-ahead log file. WAL mode permits simultaneous readers and writers. WAL mode is enabled by running "PRAGMA journal_mode=WAL". Later, after the transaction commits, those changes will be moved from the WAL file back into the original database in an operation called "checkpoint". Instead, changes go into a separate "write-ahead log" or "WAL" file. In WAL mode, changes are not written to the original database file. Since version 3.7.0, SQLite also supports "WAL mode".
You can view the logs in the Windows event viewer. I downloaded the latest version of Valentina Server yesterday.
I did the test on Windows 10, Xojo 2015R4. Writing is denied immediately without waiting for the timeout. Meanwhile, on the second session,click "go page2" then "Insert 1 line". The import starts and will take several minutes.
Open another session on a web browser (http: //127.0.0: 9000) With valentina server, the writing is rejected immediately, without waiting for the timeout. Usually SQLITE refuses writing at the end of this timeout. Of course, the second can not be imported because the engine is already locked on the first writing.īUT it seems that the server does not respecte the timeout. With a session, I run an import of one million lines.ĭuring this import, another session tries to import a single line. I do a test with Valentina Server for SQLITE (licence 5 connexions) and Xojo.
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