path: root/lib/kernel/doc/src/wrap_log_reader.xml

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<erlref>
  <header>
    <copyright>
      <year>1998</year><year>2016</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
      limitations under the License.
    
    </legalnotice>

    <title>wrap_log_reader</title>
    <prepared>Esko Vierum&auml;ki</prepared>
    <responsible>Esko Vierum&auml;ki</responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date>1998-09-21</date>
    <rev>A</rev>
    <file>wrap_log_reader.sgml</file>
  </header>
  <module>wrap_log_reader</module>
  <modulesummary>A service to read internally formatted wrap disk logs.
  </modulesummary>
  <description>
    <p>This module makes it possible to read internally formatted
      wrap disk logs, see
      <seealso marker="disk_log"><c>disk_log(3)</c></seealso>.
      <c>wrap_log_reader</c> does not
      interfere with <c>disk_log</c> activities; there is however a bug in this
      version of the <c>wrap_log_reader</c>, see section
      <seealso marker="#bugs">Known Limitations</seealso>.</p>
    <p>A wrap disk log file consists of many files, called index files. A log
      file can be opened and closed. Also, a single index file can be opened
      separately. If a non-existent or non-internally formatted file is opened,
      an error message is returned. If the file is corrupt, no attempt is made
      to repair it, but an error message is returned.</p>
    <p>If a log is configured to be distributed, it is possible that all items
      are not logged on all nodes. <c>wrap_log_reader</c> only reads the log on
      the called node; it is 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>Enables to efficiently read the
          terms that are appended to a log. Minimises disk
          I/O by reading 64 kilobyte chunks from the file.</p>
        <p>The first time <c>chunk()</c> is called, an initial
          continuation returned from <c>open/1</c> or <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. Defaults to <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>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 that  must be passed on to 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>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.
          Notice that the log is not repaired.</p>
        <p>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 either is or is not valid in the next call
          to this function. This is because the log can wrap and delete
          the file into which the continuation points. To ensure
          this does not occur, the log can be blocked during the search.</p>
      </desc>
    </func>

    <func>
      <name name="close" arity="1"/>
      <fsummary>Close a log.</fsummary>
      <desc>
        <p>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 to be
          read.</p>
        <p><c><anno>N</anno></c> specifies the index of the file to be read.
          If <c><anno>N</anno></c> is omitted, the whole wrap log file is read;
          if it is specified, only the specified index file is read.</p>
        <p>Returns <c>{ok, <anno>Continuation</anno>}</c> if the
          log/index file is opened successfully.
          <c><anno>Continuation</anno></c>
          is to be used when chunking or closing the file.</p>
        <p>Returns <c>{error, <anno>Reason</anno>}</c> for all errors.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Known Limitations</title>
    <marker id="bugs"/>
    <p>This version of <c>wrap_log_reader</c> does not detect if
      <c>disk_log</c> wraps to a new index file between a call to
      <c>wrap_log_reader:open()</c> and the first call to
      <c>wrap_log_reader:chunk()</c>.
      If this occurs, the call to <c>chunk()</c> reads the last logged
      items in the log file, as the opened index file was truncated by
      <c>disk_log</c>.</p>
  </section>

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