Write-ahead log

const wal = require('internal').wal

This module provides functionality for administering the write-ahead logs.Most of these functions only return sensible values when invoked with the MMFiles engine being active.

Configuration

retrieves the configuration of the write-ahead loginternal.wal.properties()

Retrieves the configuration of the write-ahead log. The result is a JSONarray with the following attributes:

  • allowOversizeEntries: whether or not operations that are bigger than asingle logfile can be executed and stored
  • logfileSize: the size of each write-ahead logfile
  • historicLogfiles: the maximum number of historic logfiles to keep
  • reserveLogfiles: the maximum number of reserve logfiles that ArangoDBallocates in the background
  • syncInterval: the interval for automatic synchronization of not-yetsynchronized write-ahead log data (in milliseconds)
  • throttleWait: the maximum wait time that operations will wait beforethey get aborted if case of write-throttling (in milliseconds)
  • throttleWhenPending: the number of unprocessed garbage-collectionoperations that, when reached, will activate write-throttling. A value of0 means that write-throttling will not be triggered.

Examples

  1. arangosh> require("internal").wal.properties();

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configures the write-ahead loginternal.wal.properties(properties)

Configures the behavior of the write-ahead log. properties must be a JSONJSON object with the following attributes:

  • allowOversizeEntries: whether or not operations that are bigger than asingle logfile can be executed and stored
  • logfileSize: the size of each write-ahead logfile
  • historicLogfiles: the maximum number of historic logfiles to keep
  • reserveLogfiles: the maximum number of reserve logfiles that ArangoDBallocates in the background
  • throttleWait: the maximum wait time that operations will wait beforethey get aborted if case of write-throttling (in milliseconds)
  • throttleWhenPending: the number of unprocessed garbage-collectionoperations that, when reached, will activate write-throttling. A value of0 means that write-throttling will not be triggered.

Specifying any of the above attributes is optional. Not specified attributeswill be ignored and the configuration for them will not be modified.

Examples

  1. arangosh> require("internal").wal.properties({ 
  2. ........>    allowOverSizeEntries: true,
  3. ........> logfileSize: 32 * 1024 * 1024 });

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Flushing

flushes the currently open WAL logfileinternal.wal.flush(waitForSync, waitForCollector)

Flushes the write-ahead log. By flushing the currently active write-aheadlogfile, the data in it can be transferred to collection journals anddatafiles. This is useful to ensure that all data for a collection ispresent in the collection journals and datafiles, for example, when dumpingthe data of a collection.

The waitForSync option determines whether or not the operation shouldblock until the not-yet synchronized data in the write-ahead log wassynchronized to disk.

The waitForCollector operation can be used to specify that the operationshould block until the data in the flushed log has been collected by thewrite-ahead log garbage collector. Note that setting this option to _true_might block for a long time if there are long-running transactions andthe write-ahead log garbage collector cannot finish garbage collection.

Examples

  1. arangosh> require("internal").wal.flush();

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