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# Concurrency, locking, and versioning

## Concurrency

The `FileIndex` object is “stateless” and should be share-able between threads.

A `Reader` object (which underlies the `Searcher` object) wraps open files and often individual methods rely on consistent file cursor positions (e.g. they do two `file.read()`s in a row, so if another thread moves the cursor between the two read calls Bad Things would happen). You should use one Reader/Searcher per thread in your code.

Readers/Searchers tend to cache information (such as field caches for sorting), so if you can share one across multiple search requests, it’s a big performance win.

## Locking

Only one thread/process can write to an index at a time. When you open a writer, it locks the index. If you try to open a writer on the same index in another thread/process, it will raise `whoosh.store.LockError`.

In a multi-threaded or multi-process environment your code needs to be aware that opening a writer may raise this exception if a writer is already open. Whoosh includes a couple of example implementations (`whoosh.writing.AsyncWriter` and `whoosh.writing.BufferedWriter`) of ways to work around the write lock.

While the writer is open and during the commit, the index is still available for reading. Existing readers are unaffected and new readers can open the current index normally.

### Lock files

Locking the index is accomplished by acquiring an exclusive file lock on the `<indexname>_WRITELOCK` file in the index directory. The file is not deleted after the file lock is released, so the fact that the file exists does not mean the index is locked.

## Versioning

When you open a reader/searcher, the reader represents a view of the current version of the index. If someone writes changes to the index, any readers that are already open will not pick up the changes automatically. A reader always sees the index as it existed when the reader was opened.

If you are re-using a Searcher across multiple search requests, you can check whether the Searcher is a view of the latest version of the index using `whoosh.searching.Searcher.up_to_date()`. If the searcher is not up to date, you can get an up-to-date copy of the searcher using `whoosh.searching.Searcher.refresh()`:

    # If 'searcher' is not up-to-date, replace it
    searcher = searcher.refresh()

(If the searcher has the latest version of the index, `refresh()` simply returns it.)

Calling `Searcher.refresh()` is more efficient that closing the searcher and opening a new one, since it will re-use any underlying readers and caches that haven’t changed.

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