Hi all,
sorry for the cross posting.
We would like to add MBTiles support to GWC.
Follows a description of the work with the main issues\limitations.
I would like to have community feedback on this, by the way is there a better way to propose this work ?
* MBtiles and SQLitle *
MBtiles is a specification that describe how to store tiles in an SQLite database, this will allow us to store many tiles in a single SQLite
file avoiding us file systems headaches:
https://github.com/mapbox/mbtiles-spec/blob/master/1.1/spec.md.
We can rely on GeoTools gt-mbtiles module for reading and writing MBTiles, this way most of the work of implementing this blobstore will be
managing SQLite connections and SQLite files.
SQLite files cannot be managed as simple files. When connections to an SQLite database are open we should not delete, move or switch the
associated file. Databases files can be filled with "empty space" after deleting an huge amount of data or can become fragmented after
frequent inserts, updates or delete operations.
SQLite documentation warns us against putting databases files on a shared file system if multiple process need access to it (which is our
case). Unless we can rely on a distributed lock mechanism SQLite databases files should not be used with shared stores.
* VACUUM and DiskQuota *
To remove the fragmented space (or the empty space), the VACUUM command needs to be executed. Although, performing a VACUUM command as a few
drawbacks:
- During a VACUUM twice the size of the original database file is required in disk.
- During the VACUUM operation no access to the database is allowed.
- The VACUUM operation copies the whole database which can take minutes.
For these reasons the VACUUM command cannot be performed after each operation. When possible we will avoid creating fragmented space. For
example, during a truncate operation we may prefer remove a whole SQLilte file instead of deleting part of is content. Another consequence
of the fragmented space is that DiskQuota will not be compatible with this blobstore.
* MBTiles Granularity *
Reading and writing tiles on an SQLite database will be slower than writing on a file system but will allow us to avoid file system
headaches. In order to limit the amount of contention on each single MBTiles file we will allow users to decide the granularity of the files
so that instead of having a single file for each single layer we will allow users to have more granularity.
MBTiles force us to have at least a file per layer and format. If we want to support more CRSs we will also need a file for each CRSs. By
configuration it will be possible to configure the granularity of the database files. By default we will have a granularity per layer, crs,
format and zoom level. As an instance something like this could be offered:
<blobstore>
<file>/path/to/{grid}/{dim}/{tileset}/{z}/{x}-{y}.sqlite</file>
<xcount>1000</xcount>
<ycount>1000</ycount>
</blobstore>
In this case we should include the {x}, {y} and {z} replacements in the template determining the file to use. In the previous example, tile
(z,x,y)=(15,3024,1534) would be stored in a file named /path/to/g/mytileset/15/3000-1000.sqlite3 and tile (5,2,8) would be stored in a file
named /path/to/g/mytileset/5/0-0.sqlite3.
With more databases files we have more performance but we will have also more files to manage on the file system. In addition we can couple
this with the in-memory cache in order to improve tile serving performance.
* Connection Pooling and Performance *
SQLite allow multiple readers but only allow one writer at the time which will block the entire database. At most only one connection should
be open to each SQLite database, the total number of open connections is limited by the number of open files allowed by the OS (in linux
this is controlled by the ulimit). A connection pool that will control the number of open connections and that will be responsible to manage
the connections will be implemented.
* Replace Operation *
As said before, if the cache is running we cannot simply switch SQLite files, we need to make sure that all connections are closed. A
replace operation will be created for this propose. The replace operation will first copy the new file side by side the old one, then block
the requests to the old file, tear down the store, delete the old one, rename the new file to current one, reopen the new db file and start
serving requests again. Should be almost instant. A REST entry point for this operation will be created (with the possibility to send the
new file with the request).
Regards,
--
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