level-min v2.0.2

level-min

💡 A light-weight full-text search library for Node.js, aimed to minimize the workload of developers. Using LevelDB as storage backend. Chinese version Readme

• With a built-in text processing procedure: Tokenizer, Porter Stemmer and Stopwords filter.
• Full-text search using TF-IDF algorithm and Cosine Similarity.
• Highly configurable index-schema to construct a more flexible inverted index.
• Multi-language support, including Chinese, English, Japanese, French, Italian, Russian, Spanish, Portuguese, etc.

This package is provided for developers who would like to deploy their node.js based application on an entry-level server/VPS.

Ease of deployment and light-weight are all the ambitions of this package.

If you require more features for a more complex application, maybe ElasticSearch will be a better potential solution.

Installation

$ npm install level-min

In general, a valid node-gyp installation is needed.

Quick Start

const Min = require("level-min");

// Initialize level-min instance by a database address and a object that contain the leveldb configuration
// An absolute path like database address is strongly recommended (if it is relative, the dbAddress=path.join(process.cwd(), InputDbAddress))
const min = new Min("data",options);

// An example of how to store and index a document.
// Using such a schema, only the title and content in the value object will be indexed.
// So that those useless indexes of the value (i.e. a field of imgUrl) will not be stored.
min.put("Document1",{
    title:"Introduction of level-min",
    content:"This is a long text. Balabala. ",
    imgUrl:"http://just-for-example.url.com/img.jpg"
},{
    keyWeight:0,
    valueWeightCalc:true,
    defaultValueWeight:0,
    valueWeights: { title: 5, content: 1 }
})

min.put("example","A meaningless string.")

// In practice, the result may be affected since both put() and search are async .
// Maybe Promise.all() is a good solution.
let result = await min.search("Introduction");

API

• Min() / min.setDB()
• min.put()
• min.del()
• min.get()
• min.search()
• Setting of tokenizer

Min(dbAddress,[options,]) / min.setDB(dbAddress,[options,])

Both of these functions will switch the leveldb instance inside Min to the given address.

Using as const min = Min(location) is highly recommended.

options(Same as the content at leveldown#db.open)

The optional options argument may contain:

• createIfMissing (boolean, default: true): If true, will initialize an empty database at the specified location if one doesn't already exist. If false and a database doesn't exist you will receive an error in your open() callback and your database won't open.

• errorIfExists (boolean, default: false): If true, you will receive an error in your open() callback if the database exists at the specified location.

• compression (boolean, default: true): If true, all compressible data will be run through the Snappy compression algorithm before being stored. Snappy is very fast and shouldn't gain much speed by disabling so leave this on unless you have good reason to turn it off.

• cacheSize (number, default: 8 * 1024 * 1024 = 8MB): The size (in bytes) of the in-memory LRU cache with frequently used uncompressed block contents.

Advanced options

The following options are for advanced performance tuning. Modify them only if you can prove actual benefit for your particular application.

• writeBufferSize (number, default: 4 * 1024 * 1024 = 4MB): The maximum size (in bytes) of the log (in memory and stored in the .log file on disk). Beyond this size, LevelDB will convert the log data to the first level of sorted table files. From the LevelDB documentation:

Larger values increase performance, especially during bulk loads. Up to two write buffers may be held in memory at the same time, so you may wish to adjust this parameter to control memory usage. Also, a larger write buffer will result in a longer recovery time the next time the database is opened.

• blockSize (number, default 4096 = 4K): The approximate size of the blocks that make up the table files. The size related to uncompressed data (hence "approximate"). Blocks are indexed in the table file and entry-lookups involve reading an entire block and parsing to discover the required entry.

• maxOpenFiles (number, default: 1000): The maximum number of files that LevelDB is allowed to have open at a time. If your data store is likely to have a large working set, you may increase this value to prevent file descriptor churn. To calculate the number of files required for your working set, divide your total data by 'maxFileSize'.

• blockRestartInterval (number, default: 16): The number of entries before restarting the "delta encoding" of keys within blocks. Each "restart" point stores the full key for the entry, between restarts, the common prefix of the keys for those entries is omitted. Restarts are similar to the concept of keyframes in video encoding and are used to minimise the amount of space required to store keys. This is particularly helpful when using deep namespacing / prefixing in your keys.

• maxFileSize (number, default: 2* 1024 * 1024 = 2MB): The maximum amount of bytes to write to a file before switching to a new one. From the LevelDB documentation:

... if your filesystem is more efficient with larger files, you could consider increasing the value. The downside will be longer compactions and hence longer latency/performance hiccups. Another reason to increase this parameter might be when you are initially populating a large database.

Notes:

Both put() and del() have used the db.batch() of level to keep the Atomicity inside the db's transactions. Hence, skip those functions offered by level-min and operate the leveldb directly can be dangerous.

min.put(key,value,[options,])

put() is an instance method on an existing database object, used to store new entries, or overwrite existing entries in the LevelDB store. At the same time, the indexes based on the input are also generated. A promise is returned.

The key should be string. The value may better be String, Array or Object. Other object types result in an unexpected error. Keys may not be null or undefined and objects converted with toString() should not result in an empty-string. Values may not be null or undefined.

The optional options argument may contain:

• keyWeight (Number, default: 1): If the value of given keyWeight is less than or equal to zero(<=0), the tokens inside the input key will not be indexed.
• valueWeightCalc (boolean, default: false): If false, the tokens inside the value will not be counted. Switch it to true if it is needed.
• defaultValueWeight (Number, default: 1): If the value of given valueWeightCalc is true, its value will be calculated as the default weights of tokens inside value.
• valueWeights (object, default: {}): The values for those spec key/index when the value is an Array/Object. For example, the value is an object -> {a:text, b:text,...} and the valueWeights is {a:3}, the tokens inside field a will be calculated as a token-frequency of 3, others' token-frequency will follow the defaultValueWeight.

min.del(key)

Delete the record of key inside the db. If there are any associated indexes that are generated together before, they will be removed as well. A Promise is returned.

min.get(key)

get() is the primary method for fetching data from the store. The key should be a String. If it doesn't exist in the store then the promise will be rejected. A not-found err object will be of type 'NotFoundError' so you can err.type == 'NotFoundError' or you can perform a truthy test on the property err.notFound.

min.get("example").then(info=>{
    ...
}).catch(e=>{
    if(e.type === "NotFoundError"){
        //there is no such key in the db
    }
})

min.search(content, [options,])

search() function will tokenize the input and then query them inside the db. If multiple results are returned, their scores will be calculated using tf-idf algorithm and Cosine Similarity. The top-k of the results sorted by their scores in a descending order will finally be returned via promise.

An example for the search:

let options={
    limit:10, // when we only want the top 10 results
    cosineSimilarity:true // if you want to combine the tf-idf and cosine similarity , false :tf-idf alone , true: combine
}
min.search("Shakespeare").then(results=>{
    // results in order
}).catch(e=>{
    //exception
})

What if you want to use some customized text processing components rather than the integrated one?

A serious of APIs are offered to let the developers customize there own Tokenizer, Stopwords and Stemmer.

min.tokenizer.setCustomStopwords(["avi","1080"]);

This function, min.tokenizer.setCustomStopwords(), will accept an array of stopwords which will play a role as token-filter in the following steps.

min.tokenizer.setCustomTokenizer(tokenizer);

min.tokenizer.setCustomTokenizer() will accept an object that with a function named tokenize() that can split the input sentence into an array of tokens. Or an error will be logged in console.

min.tokenizer.setCustomStemmer(stemmer);

min.tokenizer.setCustomStemmer() will accept an object that with a function named stem() that accept a token and return a processed string. Or an error will be logged in console.

If the developer want to switch the tokenizer or the stemmer back to the integrated one, min.tokenizer.configTokenizer() can be used.

An example:

min.tokenizer.configTokenizer({
    tokenizer: true,
    stemmer: true,
    stopword: true,
    customTokenizer: false,
    customStopword: false,
    customStemmer: false
});

When the stemmer and stopword options switch to false, the Text Processing Procedure will not contain these procedures anymore.

Warning: the modification of the attributes in the above configuration can result in an unexpected and serious consequences. So I strongly advice that the option tokenizer should always be true.

✨ Develop Plan

• TF-IDF Similarity based full text search
• Integrated Text Processing Procedure: Tokenizer, Porter Stemmer and Stopwords filter
• Direct operation on the raw leveldb
• Support for custom text processing component
• Support for Cosine Similarity

About the Text Processing Procedure

The built-in text processing procedures in level-min including Tokenizer, Porter Stemmer and Stopwords filter that are offered by various npm libraries.

As a result of the multi-language supporting, there are many versions of those integrated tokenizers, stemmers and stopwords filter since they are from different packages.

In practice, which lib will be used in a specific input is more likely to depend on the language-detect packages. Hence, the indexes generated by the text processing procedure may be slightly different than what is being indexed.

To solve this issue, a configurable language preference is provided (as above).