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  <header>
    <copyright>
      <year>1998</year><year>2013</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
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    </legalnotice>

    <title>wrap_log_reader</title>
    <prepared>Esko Vierum&auml;ki</prepared>
    <responsible>Esko Vierum&auml;ki</responsible>
    <docno></docno>
    <approved>nobody</approved>
    <checked>no</checked>
    <date>98-09-21</date>
    <rev>A</rev>
    <file>wrap_log_reader.sgml</file>
  </header>
  <module>wrap_log_reader</module>
  <modulesummary>A function to read internally formatted wrap disk logs</modulesummary>
  <description>
    <p><c>wrap_log_reader</c> is a function to read internally formatted
      wrap disk logs, refer to disk_log(3). <c>wrap_log_reader</c> does not
      interfere with disk_log activities; there is however a known bug in this
      version of the <c>wrap_log_reader</c>, see chapter <c>bugs</c> below.
      </p>
    <p>A wrap disk log file consists of several files, called index files. 
      A log file can be opened and closed. It is also possible to open just one index file
      separately. If an non-existent or a non-internally formatted file is opened,
      an error message is returned. If the file is corrupt, no attempt to repair it
      will be done but an error message is returned. 
      </p>
    <p>If a log is configured to be distributed, there is a possibility that all items 
      are not loggen on all nodes. <c>wrap_log_reader</c> does only read the log on
      the called node, it is entirely up to the user to be sure that all items are read.
      </p>
  </description>
  <datatypes>
    <datatype>
      <name name="continuation"/>
      <desc><p>Continuation returned by
         <c>open/1,2</c> or <c>chunk/1,2</c>.</p>
      </desc>
    </datatype>
  </datatypes>
  <funcs>
    <func>
      <name name="chunk" arity="1"/>
      <name name="chunk" arity="2"/>
      <fsummary>Read a chunk of objects written to a wrap log.</fsummary>
      <type name="chunk_ret"/>
      <desc>
        <p>This function makes it possible to efficiently read the
          terms which have been appended to a log. It minimises disk
          I/O by reading large 8K chunks from the file.
          </p>
        <p>The first time <c>chunk</c> is called an initial
          continuation returned from the <c>open/1</c>, <c>open/2</c> must be provided.
          </p>
        <p>When <c>chunk/3</c> is called, <c><anno>N</anno></c> controls the
          maximum number of terms that are read from the log in each
          chunk.  Default is <c>infinity</c>, which means that all the
          terms contained in the 8K chunk are read. If less than
          <c><anno>N</anno></c> terms are returned, this does not necessarily mean
          that end of file is reached.
          </p>
        <p>The <c>chunk</c> function returns a tuple
          <c>{<anno>Continuation2</anno>, <anno>Terms</anno>}</c>, where <c><anno>Terms</anno></c> is a list
          of terms found in the log. <c><anno>Continuation2</anno></c> is yet
          another continuation which must be passed on into any
          subsequent calls to <c>chunk</c>. With a series of calls to
          <c>chunk</c> it is then possible to extract all terms from a
          log.
          </p>
        <p>The <c>chunk</c> function returns a tuple
          <c>{<anno>Continuation2</anno>, <anno>Terms</anno>, <anno>Badbytes</anno>}</c> if the log is opened
          in read only mode and the read chunk is corrupt. <c><anno>Badbytes</anno></c>
          indicates the number of non-Erlang terms found in the chunk.
          Note also that the log is not repaired.
          </p>
        <p><c>chunk</c> returns <c>{<anno>Continuation2</anno>, eof}</c> when the end of the log is
          reached, and <c>{error, <anno>Reason</anno>}</c> if an error occurs.
          </p>
        <p>The returned continuation may or may not be valid in the next call to
          <c>chunk</c>.  This is because the log may wrap and delete
          the file into which the continuation points. To make sure
          this does not happen, the log can be blocked during the
          search.
          </p>
      </desc>
    </func>
    <func>
      <name name="close" arity="1"/>
      <fsummary>Close a log</fsummary>
      <desc>
        <p>This function closes a log file properly.
          </p>
      </desc>
    </func>
    <func>
      <name name="open" arity="1"/>
      <name name="open" arity="2"/>
      <fsummary>Open a log file</fsummary>
      <type name="open_ret"/>
      <desc>
        <p><c><anno>Filename</anno></c> specifies the name of the file which is to be read. </p>
        <p><c><anno>N</anno></c> specifies the index of the file which is to be read.
          If <c><anno>N</anno></c> is omitted the whole wrap log file will be read; if it
          is specified only the specified index file will be read.
          </p>
        <p>The <c>open</c> function returns <c>{ok, <anno>Continuation</anno>}</c> if the
          log/index file was successfully opened. The <c><anno>Continuation</anno></c>
          is to be used when chunking or closing the file. 
          </p>
        <p>The function returns <c>{error, <anno>Reason</anno>}</c> for all errors.
          </p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Bugs</title>
    <p>This version of the <c>wrap_log_reader</c> does not detect if the <c>disk_log</c>
      wraps to a new index file between a <c>wrap_log_reader:open</c> and the first 
      <c>wrap_log_reader:chunk</c>.
      In this case the chuck will actually read the last logged items in the log file,
      because the opened index file was truncated by the <c>disk_log</c>.
      </p>
  </section>

  <section>
    <title>See Also</title>
    <p><seealso marker="disk_log">disk_log(3)</seealso></p>
  </section>
</erlref>

