[Quick Start] Lucene

Lucene is a full-text search library in Java.It does so by adding content to a full-text index. It then allows you to perform queries on this index, returning results ranked by either the relevance to the query or sorted by an arbitrary field such as a document’s last modified date.The content you add to Lucene can be from various sources, like a SQL/NoSQL database, a filesystem, or even from websites.

Searching & Indexing:

Lucene is fast because it uses indexes for searching instead of searching for the text directly. It uses a Inverted index for searching as it inverts the page centric searching(page->word) to keyword centric searching(word->page). It is similar to searching for keyword related pages at the end of the book instead of searching all pages for the keyword.

Document is the unit of search and index. It consists of one or more fields(key-value pair). Indexing involves adding Documents to an IndexWriter, and searching involves retrieving Documents from an index via an IndexSearcher.

IndexProcess

Query:

Querying is done by its own mini language. The Lucene query language allows the user to specify which field(s) to search on, which fields to give more weight to (boosting), the ability to perform boolean queries (AND, OR, NOT) and other functionality.

SearchProcess

More on how to use Lucene in programs can be found in tutorialspoint

 

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Kernel Panic

Kernel Panic is a term used to denote when a OS cant boot properly or crashed because of a fatal error. This could be because of many reasons like unsupported hardware or missing/corrupted drive or because of unstable updates.It is equivalent of Windows Blue Screen error for UNIX and UNIX based systems.

Kernel Panic sometimes might be caused because of Kernel Oops(In case of error, Kernel will kill the offending process and continue to run). This can be identified with the call trace stack displayed at the time of startup.In these cases, users will be displayed with the grub menu.

Error1: Simplest reason for this panic might be because of less space in /boot folder.Easiest way to fix this issue will be to boot the old kernel from the available list and users can purge old kernels through

sudo apt-get autoremove

and also remove unstable updates.

Error2: “Kernel Panic – not syncing: VFS: Unable to mount root fs on unknown-block(0,0)”
In this case, grub update is needed. Boot old kernel(if available) or use Live CD to boot and run the following commands from any one of the below approach.

Fix 1: Run

dpkg –configure -a

This will update the initramfs hook and fix the broken kernel.
Fix 2: Run

update-initramfs -u -k version

update-grub

Fix 3: This fix can be done when grub is looking in a different path. Find the correct the filesystem where grub is installed using following commands in GRUB prompt(Shift +C during startup),

ls # Should show your drives and partitions hd0,1=sda1
ls (hd0,1)/ # Should include ‘vmlinuz’ and ‘initrd’
ls (hd0,1)/boot/grub # Should see a lot of *.mod files

and then install grub with following commands after booting old kernel/using LiveCD,

sudo mount /dev/sda1 /mnt
sudo grub-install –root-directory=/mnt /dev/sda

Note: 2nd line should have the partition number.

[Quick Scan]Promises – JavaScript

Promises provide a simpler alternative for executing, composing, and managing asynchronous operations when compared to traditional callback-based approaches. They also allow you to handle asynchronous errors using approaches that are similar to synchronous try/catch
Promise represents the result of a asynchronous transactions.  A Promise represents an operation that hasn’t completed yet, but is expected in the future. It allows you to associate handlers to an asynchronous action’s eventual success value or failure reason. This lets asynchronous methods return values like synchronous methods: instead of the final value, the asynchronous method returns a promise of having a value at some point in the future.

A promise is in one of three different states:

  • pending – The initial state of a promise.
  • fulfilled – The state of a promise representing a successful operation.
  • rejected – The state of a promise representing a failed operation.
Once a promise is fulfilled or rejected, it is immutable (i.e. it can never change again).
In short- Helps you write/manage asynchronous calls easily

Brotli – Small Bread for Chrome is coming !

Google Chrome may be one of the fastest browsers around, but that doesn’t mean there isn’t room for improvement. Starting in the very near future, Chrome’s getting Brotli (“small bread” in Swiss German), a new page compression algorithm that decreases load times.

Brotli, which was revealed in September as a replacement for Chrome’s outgoing Zopfli algorithm, uses a more efficient data format to improve the compression of scripts by up to 26 percent. That should mean faster website rendering and better space utilization in most case, but the potential applications extend beyond mere page content. Compression engineer Zoltan Szabadka sees Brotli being used in image optimization and website pre-fetching on unreliable connections, and perhaps even font compression in scenarios where high-resolution typography isn’t a necessity (e.g., smartphones and other small-screen devices). Subsequently, Google expects all users (but especially those on mobile) to see “lower data transfer fees and reduced battery use.”

Apache Thrift: Networking Stack

Thrift’s networking stack can be represented as follows,

Capture

 Transport:

It provides a simple abstraction for reading/writing from/to the network. This enables Thrift to decouple the underlying transport from the rest of the system (like serialization/deserialization).

Protocol:

This specifies how datatypes use the underlying Transport to encode/decode themselves. Thus the protocol implementation governs the encoding scheme and is responsible for (de)serialization. Some examples of protocols in this sense include JSON, XML, plain text, compact binary etc.

Processor:

Service-specific processor implementations are generated by the compiler. The input and output streams are represented by Protocol objects. The Processor essentially reads data from the wire (using the input protocol), delegates processing to the handler (implemented by the user) and writes the response over the wire (using the output protocol).

Server:

A Server pulls together all of the various features described above.

* Create a transport
* Create input/output protocols for the transport
* Create a processor based on the input/output protocols
* Wait for incoming connections and hand them off to the processor

Apache Thrift: Introduction

Thrift is an interface definition language and binary communication protocol that is used to define and create services for numerous languages. It is used as a remote procedure call(RPC) framework and was developed at Facebook for “scalable cross-language services development”.  The Apache Thrift software framework combines a software stack with a code generation engine to build services that work efficiently and seamlessly between almost any language.

 

There’s a lot of repeated work you have to do when you’re writing a server – primarily designing a protocol and writing code to serialize and deserialize messages on the protocol, but also dealing with sockets and managing concurrency, and writing clients in many languages. Thrift automatically does all of this, given a description of the functions you want to expose from your server to clients. It’s also useful for serializing data on disk or into shared memory (where many of the same problems come up).