MVStore

Open a store in exclusive mode. For a file-based store, the parent directory must already exist.

Create and open the store.

This method is called before writing to a map.

Put the page in the cache.

Close the file and the store. Unsaved changes are written to disk first.

Close the file and the store, without writing anything. This will stop the background thread. This method ignores all errors.

Commit the changes.

For in-memory stores, this method increments the version.

For persistent stores, it also writes changes to disk. It does nothing if there are no unsaved changes, and returns the old version. It is not necessary to call this method when auto-commit is enabled (the default setting), as in this case it is automatically called from time to time or when enough changes have accumulated. However, it may still be called to flush all changes to disk.

Try to increase the fill rate by re-writing partially full chunks. Chunks with a low number of live items are re-written.

If the current fill rate is higher than the target fill rate, nothing is done.

Please note this method will not necessarily reduce the file size, as empty chunks are not overwritten.

Only data of open maps can be moved. For maps that are not open, the old chunk is still referenced. Therefore, it is recommended to open all maps before calling this method.

Compact by moving all chunks next to each other.

Compact the store by moving all chunks next to each other, if there is free space between chunks. This might temporarily increase the file size. Chunks are overwritten irrespective of the current retention time. Before overwriting chunks and before resizing the file, syncFile() is called.

Compact the store by moving all live pages to new chunks.

Get the auto-commit delay.

Get the maximum memory (in bytes) used for unsaved pages. If this number is exceeded, unsaved changes are stored to disk.

Get the cache.

Get the maximum cache size, in MB. Note that this does not include the page chunk references cache, which is 25% of the size of the page cache.

Get the amount of memory used for caching, in MB. Note that this does not include the page chunk references cache, which is 25% of the size of the page cache.

Get the current fill rate (percentage of used space in the file). Unlike the fill rate of the store, here we only account for chunk data; the fill rate here is how much of the chunk data is live (still referenced). Young chunks are considered live.

Get the current version of the data. When a new store is created, the version is 0.

Get the file store.

Get the name of the given map.

Get the set of all map names.

Get the metadata map. This data is for informational purposes only. The data is subject to change in future versions.

The data in this map should not be modified (changing system data may corrupt the store). If modifications are needed, they need be synchronized on the store.

The metadata map contains the following entries:

 chunk.{chunkId} = {chunk metadata}
 name.{name} = {mapId}
 map.{mapId} = {map metadata}
 root.{mapId} = {root position}
 setting.storeVersion = {version}

Get the oldest version to retain in memory, which is the manually set retain version, or the current store version (whatever is older).

Get the store header. This data is for informational purposes only. The data is subject to change in future versions. The data should not be modified (doing so may corrupt the store).

Get the store version. The store version is usually used to upgrade the structure of the store after upgrading the application. Initially the store version is 0, until it is changed.

Get the estimated memory (in bytes) of unsaved data. If the value exceeds the auto-commit memory, the changes are committed.

The returned value is an estimation only.

Get the oldest version to retain in memory (for in-memory stores).

Check whether a given map exists.

Check whether there are any unsaved changes.

Whether the store is read-only.

Open a map with the default settings. The map is automatically create if it does not yet exist. If a map with this name is already open, this map is returned.

Open a map with the given builder. The map is automatically create if it does not yet exist. If a map with this name is already open, this map is returned.

Open an old, stored version of a map.

Read a page.

Increment the number of unsaved pages.

Remove a map. Please note rolling back this operation does not restore the data; if you need this ability, use Map.clear().

Remove a page.

Rename a map.

Revert to the beginning of the current version, reverting all uncommitted changes.

Revert to the beginning of the given version. All later changes (stored or not) are forgotten. All maps that were created later are closed. A rollback to a version before the last stored version is immediately persisted. Rollback to version 0 means all data is removed.

Set the maximum delay in milliseconds to auto-commit changes.

To disable auto-commit, set the value to 0. In this case, changes are only committed when explicitly calling commit.

The default is 1000, meaning all changes are committed after at most one second.

Set the read cache size in MB.

How long to retain old, persisted chunks, in milliseconds. Chunks that are older may be overwritten once they contain no live data.

The default value is 45000 (45 seconds) when using the default file store. It is assumed that a file system and hard disk will flush all write buffers within this time. Using a lower value might be dangerous, unless the file system and hard disk flush the buffers earlier. To manually flush the buffers, use MVStore.getFile().force(true), however please note that according to various tests this does not always work as expected depending on the operating system and hardware.

The retention time needs to be long enough to allow reading old chunks while traversing over the entries of a map.

This setting is not persisted.

Whether empty space in the file should be re-used. If enabled, old data is overwritten (default). If disabled, writes are appended at the end of the file.

This setting is specially useful for online backup. To create an online backup, disable this setting, then copy the file (starting at the beginning of the file). In this case, concurrent backup and write operations are possible (obviously the backup process needs to be faster than the write operations).

Update the store version.

How many versions to retain for in-memory stores. If not set, 5 old versions are retained.

Force all stored changes to be written to the storage. The default implementation calls FileChannel.force(true).

Commit and save all changes, if there are any, and compact the store if needed.