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This commit is contained in:
Joey Hess 2018-10-30 11:40:11 -04:00
parent 9e41a86706
commit 718915e9fc
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1 changed files with 9 additions and 11 deletions
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20
Database/Handle.hs
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@ -44,11 +44,7 @@ type TableName = String
{- Sqlite only allows a single write to a database at a time; a concurrent {- Sqlite only allows a single write to a database at a time; a concurrent
- write will crash. - write will crash.
- -
- While a DbHandle serializes concurrent writes from - MultiWrter works around this limitation.
- multiple threads. But, when a database can be written to by
- multiple processes concurrently, use MultiWriter to make writes
- to the database be done robustly.
-
- The downside of using MultiWriter is that after writing a change to the - The downside of using MultiWriter is that after writing a change to the
- database, the a query using the same DbHandle will not immediately see - database, the a query using the same DbHandle will not immediately see
- the change! This is because the change is actually written using a - the change! This is because the change is actually written using a
@ -57,9 +53,10 @@ type TableName = String
- you can't rely on seeing values you've just written anyway, as another - you can't rely on seeing values you've just written anyway, as another
- process may change them. - process may change them.
- -
- When a database can only be written to by a single process, use - When a database can only be written to by a single process (enforced by
- SingleWriter. Changes written to the database will always be immediately - a lock file), use SingleWriter. Changes written to the database will
- visible then. - always be immediately visible then. Multiple threads can write; their
- writes will be serialized.
-} -}
data DbConcurrency = SingleWriter | MultiWriter data DbConcurrency = SingleWriter | MultiWriter
@ -105,9 +102,10 @@ queryDb (DbHandle _ _ jobs) a = do
{- Writes a change to the database. {- Writes a change to the database.
- -
- In MultiWriter mode, catches failure to write to the database, - In MultiWriter mode, writes can fail if another write is happening
- and retries repeatedly for up to 10 seconds, which should avoid - concurrently. So write failures are caught and retried repeatedly
- all but the most exceptional problems. - for up to 10 seconds, which should avoid all but the most exceptional
- problems.
-} -}
commitDb :: DbHandle -> SqlPersistM () -> IO () commitDb :: DbHandle -> SqlPersistM () -> IO ()
commitDb h wa = robustly Nothing 100 (commitDb' h wa) commitDb h wa = robustly Nothing 100 (commitDb' h wa)