This document is intended to provide general hints, tips and advice to administrators of Xapian systems. It assumes that you have installed Xapian on your system, and are familiar with the basics of creating and searching Xapian databases.
The intended audience is system administrators who need to be able to perform general management of a Xapian database, including tasks such as taking backups and optimising performance. It may also be useful introductory reading for Xapian application developers.
The document is targetted at Xapian version 1.0.0.
Xapian databases hold all the information needed to perform searches in a set of tables:
- A posting list table, which holds a list of all the documents indexed by each term in the database.
- A position list table, which holds a list of all the word positions in each document which each term occurs at.
- A record table, which holds the document data associated with each document in the database.
- A termlist table, which holds a list of all the terms which index each document.
- A value table, which holds the "values" (used for sorting, collapsing, and other match-time calculations) associated with each document in the database.
Each of these tables is held in a separate file, allowing an administrator to see how much data is being used for each of the above purposes. It is not always necessary to fully populate these tables: for example, if phrase searches are never going to be performed on the database, it is not necessary to store any positionlist information. However, the tables must be present, even if they contain no information.
If you look at a Xapian database, you will see that each of these tables actually uses 2 or 3 files. For example, for a "flint" format database the position list table is stored in the files "position.baseA", "position.baseB" and "position.DB".
Of these files, only the ".DB" file actually stores the data. The ".baseA" and ".baseB" files are used to keep track of where to start looking for that data in the ".DB" file. Often, only one of the ".baseA" and ".baseB" files will be present; each of these files refers to a revision of the database, and there may be more than one valid revision of the database stored in the ".DB" file at once.
The ".DB" file is structured as a set of blocks, which have a default size of 8Kb (though this can be set, either through the Xapian API, or with some of the tools detailed later in this document). The first block is used for header information, so a ".DB" file with only a single entry will be 16Kb in size.
Changing the blocksize may have performance implications, but it is hard to tell whether these will be positive or negative for a particular combination of hardware and software without doing some profiling.
Xapian ensures that all modifications to its database are performed atomically. This means that:
- From the point of view of a separate process (or a separate database object in the same process) reading the database, all modifications made to a database are invisible until the modifications is committed.
- The database on disk is always in a consistent state.
- If the system is interrupted during a modification, the database should always be left in a valid state. This applies even if the power is cut unexpectedly, as long as the disk does not become corrupted due to hardware failure.
Committing a modification requires several calls to the operating system to make it flush any cached modifications to the database to disk. This is to ensure that if the system fails at any point, the database is left in a consistent state. Of course, this is a fairly slow process (since the system has to wait for the disk to physically write the data), so grouping many changes together will speed up the throughput considerably.
Many modifications can be explicitly grouped into a single transaction, so that lots of changes are applied at once. Even if an application doesn't explicitly protect modifications to the database using transactions, Xapian will group modifications into transactions, applying the modifications in batches.
Note that it is not currently possible to extend Xapian's transactions to cover multiple databases, or to link them with transactions in external systems, such as an RDBMS.
Finally, note that it is possible to compile Xapian such that it doesn't make modifications in an atomic manner, in order to build very large databases more quickly (search the Xapian mailing list archives for "DANGEROUS" mode for more details). This isn't yet integrated into standard builds of Xapian, but may be in future, if appropriate protections can be incorporated.
Xapian implements a "single writer, multiple reader" model. This means that, at any given instant, there is only permitted to be a single object modifying a database, but there may (simultaneously) be many objects reading the database at once.
Xapian enforces this restriction using lock-files. For a flint database, each Xapian database directory contains a lock file named "flintlock". The lock-file will always exist, but will be locked using fcntl() when the database is open for writing. If a writer exits without being given a chance to clean up (for example, if the application holding the writer is killed), the fcntl() lock will be automatically released by the operating system. Under Microsoft Windows, we use a different locking technique, but with the same features.
Xapian databases contain a revision number. This is essentially a count of the number of modifications since the database was created, and is needed to implement the atomic modification functionality. It is stored as a 32 bit integer, so there is a chance that a very frequently updated database could cause this to overflow. The consequence of such an overflow would be to throw database errors.
This isn't likely to be a practical problem, since it would take nearly a year for a database to reach this limit if 100 modifications were committed every second, and no normal Xapian system will commit more than once every few seconds. However, if you are concerned, you can use the "xapian-compact" tool to make a fresh copy of the database with the revision number set to 1.
For a "flint" database, the revision number of each table can be displayed by the "xapian-check" tool.
Xapian should work correctly over a network file system. However, there are a large number of potential issues with such file systems, so we recommend extensive testing of your particular network file system before deployment.
Be warned that Xapian is heavily IO dependent, and therefore performance over a network file system is likely to be slow unless you've got a very well tuned setup.
Xapian needs to be able to create a lock file in a database directory when modifications are being performed. On some network files systems (eg, NFS) this requires a lock daemon to be running.
As of release 1.0.0, you should use the flint format (which is now the default). The quartz format is now deprecated and support is scheduled for removal in 1.1.0.
If you wish to store meta-data or other information relating to the Xapian database, it is reasonable to wish to put this in files inside the Xapian database directory, for neatness. For example, you might wish to store a list of the prefixes you've applied to terms for specific fields in the database.
Xapian's "flint" backend doesn't perform any operations which will break this technique, so as long as you don't choose a filename that Xapian uses itself, there should be no problems. However, be aware that new versions of Xapian may use new files in the database directory, and it is also possible that new backend formats may not be compatible with the technique (eg, it is possible that a future backend could store its entire database in a single file, not in a directory).
- The simplest way to perform a backup is to temporarily halt modifications, take a copy of all files in the database directory, and then allow modifications to resume. Read access can continue while a backup is being taken.
- If you have a filesystem which allows atomic snapshots to be taken of directories (such as an LVM filesystem), an alternative strategy is to take a snapshot and simply copy all the files in the database directory to the backup medium. Such a copy will always be a valid database.
- Progressive backups are not easily possible; modifications are typically spread throughout the database files.
Even though Xapian databases are often automatically generated from source data which is stored in a reliable manner, it is usually desirable to keep backups of Xapian databases being run in production environments. This is particularly important in systems with high-availability requirements, since re-building a Xapian database from scratch can take many hours. It is also important in the case where the data stored in the database cannot easily be recovered from external sources.
Xapian databases are managed such that at any instant in time, there is at least one valid revision of the database written to disk (and if there are multiple valid revisions, Xapian will always open the most recent). Therefore, if it is possible to take an instantaneous snapshot of all the database files (for example, on an LVM filesystem), this snapshot is suitable for copying to a backup medium. Note that it is not sufficient to take a snapshot of each database file in turn - the snapshot must be across all database files simultaneously. Otherwise, there is a risk that the snapshot could contain database files from different revisions.
If it is not possible to take an instantaneous snapshot, the best backup strategy is simply to ensure that no modifications are committed during the backup procedure. While the simplest way to implement this may be to stop whatever processes are used to modify the database, and ensure that they close the database, it is not actually necessary to ensure that no writers are open on the database; it is enough to ensure that no writer makes any modification to the database.
Because a Xapian database can contain more than one valid revision of the database, it is actually possible to allow a limited number of modifications to be performed while a backup copy is being made, but this is tricky and we do not recommend relying on it. Future versions of Xapian are likely to support this better, by allowing the current revision of a database to be preserved while modifications continue.
Progressive backups are not recommended for Xapian databases: Xapian database files are block-structured, and modifications are spread throughout the database file. Therefore, a progressive backup tool will not be able to take a backup by storing only the new parts of the database. Modifications will normally be so extensive that most parts of the database have been modified, however, if only a small number of modifications have been made, a binary diff algorithm might make a usable progressive backup tool.
When designing an indexing strategy, it is often useful to be able to check the contents of the database. Xapian includes a simple command-line program, "delve", to allow this.
For example, to display the list of terms in document "1" of the database "foo", use:
delve foo -r 1
It is also possible to perform simple searches of a database. Xapian includes another simple command-line program, "quest", to support this. "quest" is only able to search for un-prefixed terms, the query string must be quoted to protect it from the shell. To search the database "foo" for the phrase "hello world", use:
quest -d foo '"hello world"'
If you have installed the "Omega" CGI application built on Xapian, this can also be used with the built-in "godmode" template to provide a web-based interface for browsing a database. See Omega's documentation for more details on this.
Xapian databases normally have some spare space in each block to allow new information to be efficiently slotted into the database. However, the smaller a database is, the faster it can be searched, so if there aren't expected to be many further modifications, it can be desirable to compact the database.
Xapian includes a tool, "xapian-compact" for compacting "flint" format databases. This tool makes a copy of a database, and takes advantage of the sorted nature of the source Xapian database to write the database out without leaving so much space for future modifications. This can result in a large space saving.
The downside of these tools is that future modifications may take a little longer, due to needing to reorganise the database to make space for them. However, modifications are still possible, and if many modifications are made, the database will eventually adjust itself.
The tools have an option ("-F") to perform a "fuller" compaction. This option compacts the database as much as possible, but it violates the design of the Btree format slightly to achieve this, so it is not recommended if further modifications are at all likely in future. If you do need to modify a "fuller" compacted database, we recommend you run xapian-compact on it without "-F" first.
While taking a copy of the database, it is also possible to change the blocksize. If you wish to profile search speed with different blocksizes, this is the recommended way to generate the different databases (but remember to compact the original database as well, for a fair comparison).
When building an index for a very large amount of data, it can be desirable to index the data in smaller chunks (perhaps on separate machines), and then merge the chunks together into a single database. This can also be performed using the "xapian-compact" tool, simply by supplying several source database paths.
Normally, merging works by reading the source databases in parallel, and writing the contents in sorted order to the destination database. This will work most efficiently if excessive disk seeking can be avoided; if you have several disks, it may be worth placing the source databases and the destination database on separate disks to obtain maximum speed.
The "xapian-compact" tool supports an additional option, "--multipass", which is useful when merging more than 3 databases. This will cause the databases to be grouped and merged into temporary databases, which is usually faster, but requires more disk space for the temporary files.
Xapian includes a command-line tool to check that a flint database is self-consistent. This tool, "xapian-check", runs through the entire database, checking that all the internal nodes are correctly connected. It can also be used on a single table in a flint database, by specifying the prefix of the table: for example, for a database "foo", the command:
xapian-check foo/position.
will check the position list table.
It is possible to convert a quartz database to a flint database using the "copydatabase" example program included with Xapian. This is a lot slower to run than "quartzcompact" or "xapian-compact", since it has to perform the sorting of the term occurrence data from scratch, but should be faster than a reindex from source data since it doesn't need to perform the tokenisation step. It is also useful if you no longer have the source data available.
The following command will copy a database from "SOURCE" to "DESTINATION", creating the new database at "DESTINATION" as a flint database.
copydatabase SOURCE DESTINATION
Due to a bug in the flint position list encoding in 0.9.x which made flint databases non-portable between platforms, we had to make an incompatible change in the flint format. It's not easy to write an upgrader, but you can convert a database using the following procedure (although it might be better to rebuild from scratch if you want to use the new UTF-8 support in Xapian::QueryParser, Xapian::Stem, and Xapian::TermGenerator).
Run the following command in your Xapian 0.9.x installation to copy your 0.9.x flint database "SOURCE" to a new quartz database "INTERMEDIATE"
copydatabase SOURCE INTERMEDIATE
Then run the following command in your Xapian 1.0.0 installation to copy your quartz database to a 1.0.0 flint database "DESTINATION":
copydatabase INTERMEDIATE DESTINATION