+ </sect1>
+
+ <sect1 id="shadow-registers">
+ <title>Safe Updating - Using Shadow Registers</title>
+
+ <sect2 id="shadow-registers-description">
+ <title>Description</title>
+
+ <para>
+ The &zebra; server supports <emphasis>updating</emphasis> of the index
+ structures. That is, you can add, modify, or remove records from
+ databases managed by &zebra; without rebuilding the entire index.
+ Since this process involves modifying structured files with various
+ references between blocks of data in the files, the update process
+ is inherently sensitive to system crashes, or to process interruptions:
+ Anything but a successfully completed update process will leave the
+ register files in an unknown state, and you will essentially have no
+ recourse but to re-index everything, or to restore the register files
+ from a backup medium.
+ Further, while the update process is active, users cannot be
+ allowed to access the system, as the contents of the register files
+ may change unpredictably.
+ </para>
+
+ <para>
+ You can solve these problems by enabling the shadow register system in
+ &zebra;.
+ During the updating procedure, <literal>zebraidx</literal> will temporarily
+ write changes to the involved files in a set of "shadow
+ files", without modifying the files that are accessed by the
+ active server processes. If the update procedure is interrupted by a
+ system crash or a signal, you simply repeat the procedure - the
+ register files have not been changed or damaged, and the partially
+ written shadow files are automatically deleted before the new updating
+ procedure commences.
+ </para>
+
+ <para>
+ At the end of the updating procedure (or in a separate operation, if
+ you so desire), the system enters a "commit mode". First,
+ any active server processes are forced to access those blocks that
+ have been changed from the shadow files rather than from the main
+ register files; the unmodified blocks are still accessed at their
+ normal location (the shadow files are not a complete copy of the
+ register files - they only contain those parts that have actually been
+ modified). If the commit process is interrupted at any point during the
+ commit process, the server processes will continue to access the
+ shadow files until you can repeat the commit procedure and complete
+ the writing of data to the main register files. You can perform
+ multiple update operations to the registers before you commit the
+ changes to the system files, or you can execute the commit operation
+ at the end of each update operation. When the commit phase has
+ completed successfully, any running server processes are instructed to
+ switch their operations to the new, operational register, and the
+ temporary shadow files are deleted.
+ </para>
+
+ </sect2>
+
+ <sect2 id="shadow-registers-how-to-use">
+ <title>How to Use Shadow Register Files</title>
+
+ <para>
+ The first step is to allocate space on your system for the shadow
+ files.
+ You do this by adding a <literal>shadow</literal> entry to the
+ <literal>zebra.cfg</literal> file.
+ The syntax of the <literal>shadow</literal> entry is exactly the
+ same as for the <literal>register</literal> entry
+ (see <xref linkend="register-location"/>).
+ The location of the shadow area should be
+ <emphasis>different</emphasis> from the location of the main register
+ area (if you have specified one - remember that if you provide no
+ <literal>register</literal> setting, the default register area is the
+ working directory of the server and indexing processes).
+ </para>
+
+ <para>
+ The following excerpt from a <literal>zebra.cfg</literal> file shows
+ one example of a setup that configures both the main register
+ location and the shadow file area.
+ Note that two directories or partitions have been set aside
+ for the shadow file area. You can specify any number of directories
+ for each of the file areas, but remember that there should be no
+ overlaps between the directories used for the main registers and the
+ shadow files, respectively.
+ </para>
+ <para>
+
+ <screen>
+ register: /d1:500M
+ shadow: /scratch1:100M /scratch2:200M
+ </screen>
+
+ </para>
+
+ <para>
+ When shadow files are enabled, an extra command is available at the
+ <literal>zebraidx</literal> command line.
+ In order to make changes to the system take effect for the
+ users, you'll have to submit a "commit" command after a
+ (sequence of) update operation(s).
+ </para>
+
+ <para>
+
+ <screen>
+ $ zebraidx update /d1/records
+ $ zebraidx commit
+ </screen>
+
+ </para>
+
+ <para>
+ Or you can execute multiple updates before committing the changes:
+ </para>
+
+ <para>
+
+ <screen>
+ $ zebraidx -g books update /d1/records /d2/more-records
+ $ zebraidx -g fun update /d3/fun-records
+ $ zebraidx commit
+ </screen>
+
+ </para>
+
+ <para>
+ If one of the update operations above had been interrupted, the commit
+ operation on the last line would fail: <literal>zebraidx</literal>
+ will not let you commit changes that would destroy the running register.
+ You'll have to rerun all of the update operations since your last
+ commit operation, before you can commit the new changes.
+ </para>
+
+ <para>
+ Similarly, if the commit operation fails, <literal>zebraidx</literal>
+ will not let you start a new update operation before you have
+ successfully repeated the commit operation.
+ The server processes will keep accessing the shadow files rather
+ than the (possibly damaged) blocks of the main register files
+ until the commit operation has successfully completed.
+ </para>
+
+ <para>
+ You should be aware that update operations may take slightly longer
+ when the shadow register system is enabled, since more file access
+ operations are involved. Further, while the disk space required for
+ the shadow register data is modest for a small update operation, you
+ may prefer to disable the system if you are adding a very large number
+ of records to an already very large database (we use the terms
+ <emphasis>large</emphasis> and <emphasis>modest</emphasis>
+ very loosely here, since every application will have a
+ different perception of size).
+ To update the system without the use of the the shadow files,
+ simply run <literal>zebraidx</literal> with the <literal>-n</literal>
+ option (note that you do not have to execute the
+ <emphasis>commit</emphasis> command of <literal>zebraidx</literal>
+ when you temporarily disable the use of the shadow registers in
+ this fashion.
+ Note also that, just as when the shadow registers are not enabled,
+ server processes will be barred from accessing the main register
+ while the update procedure takes place.
+ </para>
+
+ </sect2>
+
+ </sect1>
+
+
+ <sect1 id="administration-ranking">
+ <title>Relevance Ranking and Sorting of Result Sets</title>
+
+ <sect2 id="administration-overview">
+ <title>Overview</title>
+ <para>
+ The default ordering of a result set is left up to the server,
+ which inside &zebra; means sorting in ascending document ID order.
+ This is not always the order humans want to browse the sometimes
+ quite large hit sets. Ranking and sorting comes to the rescue.
+ </para>
+
+ <para>
+ In cases where a good presentation ordering can be computed at
+ indexing time, we can use a fixed <literal>static ranking</literal>
+ scheme, which is provided for the <literal>alvis</literal>
+ indexing filter. This defines a fixed ordering of hit lists,
+ independently of the query issued.
+ </para>
+
+ <para>
+ There are cases, however, where relevance of hit set documents is
+ highly dependent on the query processed.
+ Simply put, <literal>dynamic relevance ranking</literal>
+ sorts a set of retrieved records such that those most likely to be
+ relevant to your request are retrieved first.
+ Internally, &zebra; retrieves all documents that satisfy your
+ query, and re-orders the hit list to arrange them based on
+ a measurement of similarity between your query and the content of
+ each record.
+ </para>
+
+ <para>
+ Finally, there are situations where hit sets of documents should be
+ <literal>sorted</literal> during query time according to the
+ lexicographical ordering of certain sort indexes created at
+ indexing time.
+ </para>
+ </sect2>
+
+
+ <sect2 id="administration-ranking-static">
+ <title>Static Ranking</title>
+
+ <para>
+ &zebra; uses internally inverted indexes to look up term frequencies
+ in documents. Multiple queries from different indexes can be
+ combined by the binary boolean operations <literal>AND</literal>,
+ <literal>OR</literal> and/or <literal>NOT</literal> (which
+ is in fact a binary <literal>AND NOT</literal> operation).
+ To ensure fast query execution
+ speed, all indexes have to be sorted in the same order.
+ </para>
+ <para>
+ The indexes are normally sorted according to document
+ <literal>ID</literal> in
+ ascending order, and any query which does not invoke a special
+ re-ranking function will therefore retrieve the result set in
+ document
+ <literal>ID</literal>
+ order.
+ </para>
+ <para>
+ If one defines the
+ <screen>
+ staticrank: 1
+ </screen>
+ directive in the main core &zebra; configuration file, the internal document
+ keys used for ordering are augmented by a preceding integer, which
+ contains the static rank of a given document, and the index lists
+ are ordered
+ first by ascending static rank,
+ then by ascending document <literal>ID</literal>.
+ Zero
+ is the ``best'' rank, as it occurs at the
+ beginning of the list; higher numbers represent worse scores.
+ </para>
+ <para>
+ The experimental <literal>alvis</literal> filter provides a
+ directive to fetch static rank information out of the indexed &acro.xml;
+ records, thus making <emphasis>all</emphasis> hit sets ordered
+ after <emphasis>ascending</emphasis> static
+ rank, and for those doc's which have the same static rank, ordered
+ after <emphasis>ascending</emphasis> doc <literal>ID</literal>.
+ See <xref linkend="record-model-alvisxslt"/> for the gory details.
+ </para>
+ </sect2>
+
+
+ <sect2 id="administration-ranking-dynamic">
+ <title>Dynamic Ranking</title>
+ <para>
+ In order to fiddle with the static rank order, it is necessary to
+ invoke additional re-ranking/re-ordering using dynamic
+ ranking or score functions. These functions return positive
+ integer scores, where <emphasis>highest</emphasis> score is
+ ``best'';
+ hit sets are sorted according to <emphasis>descending</emphasis>
+ scores (in contrary
+ to the index lists which are sorted according to
+ ascending rank number and document ID).
+ </para>
+ <para>
+ Dynamic ranking is enabled by a directive like one of the
+ following in the zebra configuration file (use only one of these a time!):
+ <screen>
+ rank: rank-1 # default TDF-IDF like
+ rank: rank-static # dummy do-nothing
+ </screen>
+ </para>
projects / idzebra-moved-to-github.git / blobdiff