3.0.1 • Public • Published


Maintain an index into a log file as it is appended to and rotated


npm install --save live-index


var LiveIndex = require("live-index")

Constructor LiveIndex([options])

Instantiates a LiveIndex instance. The options object takes the following properties:

  • pathToWatch: The path to a log file you want to watch and index.
  • indexer: A function you define to update the index. See the indexer delegagte documentation below for what gets passed to this function.
  • tailChunkSize: Set the size in bytes that should be read at a time from the data files. You can use this to control the maximum size of the chunks dispatched to your indexer function.
  • rotationPollInterval: Poll for data file rotation every specified number of milliseconds, instead of relying on rename events. Try this if indexing stops after a rotation.

If both pathToWatch and indexer are defined, then .watch() is automatically called for you.

Method setIndexer(indexer)

Sets indexer as the indexer delegate. See the indexer delegate documentation below for what gets passed to this function.

Method setIndexStorageObject(obj)

Sets the object to be used for in memory storage of index identifiers and file positions. This overrides the default Map instance.

The object specified by the obj parameter must have .get(id, [callback]) and .set(id, val, [callback]) methods. They may optionally be asynchronous; in which case they must either call callback in the standard error-first manner, or return a promise.

Method setPathToWatch(path)

Set the path of the file to watch once .watch() is called.

Method watch()

Begin watching the files that have been added to the index.

Method addStaticDataFile(path, [callback])

Appends a file given by path to the list of data files and indexes it. Calls the callback once indexing is complete. If callback is not specified a promise is returned instead.

Method readStreamBetweenIndexes(startIdentifier, endIdentifier, [callback])

Asynchronously returns a Readable stream starting from the data file and position specified by startIdentifier and ends at the data file and position specified by endIdentifier. This stream will automatically span across multiple data files. If any or both of the identifiers are not in the index, then undefined is returned instead of a stream object. If the index for endIdentifier comes before startIdentifier, an error is thrown.

The callback parameter is standard error-first. If it is omitted, a promise that resolves to the stream returned instead.

Delegate indexer(chunk, addIndex, processedTo)

Your indexer function must handle these parameters:

  • chunk: The Buffer object containing the data your indexer will look at to create indexes.
  • addIndex: A callback to create an index entry or link.
  • processedTo: A callback you can optionally call to indicate how much of the chunk you were able to process.

Callback: addIndex(identifier, location)

Call the addIndex() function from your indexer to add entries to the index. There are two ways to call this function. In the first form, location is an integer specifying the byte offset in the chunk that the index entry represents.

In the second form, location is a string, specifying another identifier that this index should link to. When you look up identifier later, it will automatically be resolved to the index entry specified by location.

Callback: processedTo(positionInChunk)

Calling this in your indexer function to tell the LiveIndex how much of the chunk you were able to process. The next time your indexer function is called, the passed chunk will contain what you were unable to process.


Patches are welcome! This module is written in ES2015. Use npm run compile to compile the source. All changes should include a test case or modification to an existing test case. Use npm test to run the tests.

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