The distributed Bitcoin mining network performs quadrillions of calculations every second that maintain the integrity of its blockchain. Other blockchains aren’t remotely as secure, but they innovate much faster. Sidechains, an innovation proposed and developed by the startup Blockstream, allow for the best of both worlds; the creation of new blockchains “pegged” to Bitcoin, so that value can be transferred between them, which can conceivably be automatically secured by Bitcoin miners via “merged mining.”
It’s the IBM “blockchain”. Basically Apache Kafka queue service, where they have modified the partitions. Each partition is an ordered, immutable sequence of messages which are continuously appended. They added some “nodes” to clean the inputs and voila; blockchain! We should add that there are no blocks, but batches of transactions are renamed to fit the hype better. Since everything gets written in one queue at the end of the day, IBM offers the bluemix cloud server (priced at 120.000$ per year) to host the service. Smaller test packages with a couple of input cleaning nodes go reportedly for 30.000$.
It is different with a private blockchain (or closed) since the members of the network are selected before being able to download the protocol and therefore use the proposed service by the network. The mining capabilities and the system of consensus as a whole are centralized within the hands of the same entity. A network based on a private blockchain is therefore not decentralized in itself.
Blockchain was invented by Satoshi Nakamoto in 2008 to serve as the public transaction ledger of the cryptocurrency bitcoin.[1] The invention of the blockchain for bitcoin made it the first digital currency to solve the double-spending problem without the need of a trusted authority or central server. The bitcoin design has inspired other applications,[1][3] and blockchains which are readable by the public are widely used by cryptocurrencies. Private blockchains have been proposed for business use. Some marketing of blockchains has been called "snake oil".[9]
In a cooperative consensus algorithm, there is a fixed number of voters. Voters cannot leave and join randomly. All voters know each other and every voter has only one vote. If the majority agree on the value of the data, then the system is working as designed. This can handle over 30,000 transactions per second. Scaling the number of voters can be an issue, because every vote proposed by a voter must be delivered to every other voter in the consortium.

Tú, o el usuario en cuestión de las sidechains, envía los bitcoins a una dirección Bitcoin específica, sabiendo que, una vez mandados, estarán fuera de tu control y fuera del control de cualquier otra persona. Estarán completamente inmovilizados y sólo se podrán desbloquear si alguien puede demostrar que no se están utilizando en ningún otro lugar.

However, even this would have its own separate value and wouldn't necessarily solve any issue especially if a market is deemed to be, well, worthless. The two-way peg isn't perfect however. Especially since SPV can theoretically be tricked into crediting more coins than were originally deposited. If the attack will then transfer those coins back onto the parent it would take coins from another user on the Sidechain to fund the imbalance. And in the process create a permanent dissilience between the two chains. In order to strengthen the security of a Sidechain beyond just SPV, it would require the parent to soft fork and upgrade its core wallet software so that both chains can then validate transfers between them.
The top 10 Ethereum decentralized apps (DApps) have daily active user counts in the thousands. Compare this with a centralized platform like Facebook, which has over a billion daily users, and you can see just how small scale blockchain use still remains. For a detailed comparison, read “State of the DApps: 5 Observations From Usage Data (April 2018)”.
Sidechains, just like any other Blockchain, need their own miners to help protect them from nefarious actors and attacks which people would like to leverage against the network. However, since wealth isn't actually created on the Sidechain there is far less incentive for miners to actually work on it and help protect it. Because of this, transaction fees are the basic reward that is offered to miners. However, these often equate to mere pennies.

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If you’ve been keeping track of developments in the bitcoin industry, you’d know that the blockchain refers to the public ledger of transactions associated with the cryptocurrency. As the bitcoin ecosystem has grown in size and scale throughout the years, the blockchain has also increased considerably in length and storage size, prompting debates on whether or not to increase its block size limit.
Sidechain is a chain of blocks based on the main parental blockchain. Sidechains realize the new financial ecosystems via integration into Bitcoin. Relatively new to Bitcoin, the sidechain is an extension that enables the ability both to build a link between BTC and an altcoin and to create new independent services that work via the main Bitcoin blockchain. Using sidechains allows for the creation of various types of smart contracts, stocks, derivatives, etc. It is possible to develop a limitless number of Bitcoin or Ethereum-based sidechains with different tasks and features, assets of which will depend on the main blockchain’s volatility. It allows traditional blockchains to support several kinds of assets, payments, smart contracts and also to increase the level of security and anonymity of transactions.

Elements Alpha functions as a sidechain to Bitcoin’s testnet, though the peg mechanism currently works through a centralized protocol adapter, as described in the Sidechains whitepaper. It relies on an auditable federation of signers to manage the testnet coins transferred into the sidechain via the “Deterministic Pegs” Element, and to produce blocks via the “Signed Blocks” Element. This makes it possible to immediately explore the new chain’s possibilities, using different security trade-offs. They plan to, in a later release, upgrade the protocol adapter to support fully decentralized merge-mining of the sidechain, and ultimately to phase in the full 2-way peg mechanism.

Using Rootstock as an example, in order to transfer assets from one chain to the other a user on the parent first has to send their coins to a special output address where they will consequently become locked and un-spendable. Once the transaction is completed, SPV then confirms it across the chains and after waiting out a contest period, which is just a secondary method to help prevent double spending, the equivalent amount will be credited and spendable on the Sidechain and vice versa.
Smart contracts are immutable pieces of code and their outcomes are irreversible. Hence, formal verification of their code is very important before deploying them. It’s very hard to verify smart contracts in the Ethereum Virtual Machine (EVM). A business can’t afford to deploy faulty but immutable smart contracts and suffer the consequences of their irreversible outcome. This article details the challanges: “Fundamental challenges with public blockchains”.

“The only reason the banks have gotten to the point of thinking about permissioned ledger is because they finally reached the stage of bargaining, third stage in five stages of grief, for industry they’re about to lose. They start with denial, and the basis of denial is, well, this thing isn’t gonna work, it’s gonna die any day soon, and it doesn’t. And then they say, it’s just silly money and it doesn’t have any value, until it does; and no one else is gonna play with it, except they are; serious investors won’t put money into this, except they did; and it still refuses to die. We go from denial to bargaining. Somewhere in between might be anger, some depression, and eventually they’re going to reach acceptance, but it’s gonna take a long time. 
External Account, which stores ETH balance – This contains the address of the User that was created using the Web3.js API, e,g, personal.newAccount(…). These accounts are used for executing smart contract transactions. ETH is your incentive received for using your account to mine transactions. The address of the account is the public key, and the password of the account is the private key.

The main point of a side-chain is to allow cryptocurrency networks to scale and interact with one-another. For example alt-coins and Bitcoin run on separate chains, however side chains allow for these separate currencies to be transferred through these two-way 'portal's or interfaces via a fixed conversion amount. Added benefits of side-chains are different asset classes like,stocks, bonds etc being integrated through a converted price onto the main chain, along with additional functionality like smart contracts,unique D-Apps, micro-payments and security updates that can be later incorporated into the primary network from these side-chains.
@tetsu – not sure what you mean. My reading of the sidechains paper is that the worst case scenario is that an attacker manages to “reanimate” Bitcoins on the main blockchain that had been sent to the sidechain… but that would be the attacker stealing the coins from the rightful owner on the sidechain. From Bitcoin’s perspective, the coins were always going to be reanimated…. so the risk is entirely borne by the holder(s) on the sidechain. Am I missing something?
2) Yea, blockchain could be a suboptimal MQ Series, a slower append only persistent wire that has a lot of ready-made tools for audit and security analysis (ecosystem argument). As blockchain ecosystem grows all kinds of data transformation tools will appear (e.g. we are working on such). Inside blockchain could be tuned to be less PoW intensive and to cut blocks faster. Besides, the variations of PoS or a hybrid PoW + PoS scheme are emerging which could use the fact that inside, as you say, all network participants can have clear identities, unlike on the public bitcoin’s blockchain.
"Proof of Work" used by Bitcoin is a competitive consensus algorithm. Each node races to solve a difficult puzzle first. Doing so earns the right to produce a block and you are rewarded in Bitcoin. The block is where the transaction (value of data) is written and confirmed. However, this race is a waste of time and money for those that don’t win. You get nothing unless you are the first to solve the puzzle. Since no one wants to lose, nodes started working together to solve the puzzle and share the reward based on your computational power (the hash rate).
The idea emerged that the Bitcoin blockchain could be in fact used for any kind of value transaction or any kind of agreement such as P2P insurance, P2P energy trading, P2P ride sharing, etc. Colored Coins and Mastercoin tried to solve that problem based on the Bitcoin Blockchain Protocol. The Ethereum project decided to create their own blockchain, with very different properties than Bitcoin, decoupling the smart contract layer from the core blockchain protocol, offering a radical new way to create online markets and programmable transactions known as Smart Contracts.
Start mining on node 1 by using the function miner.start(1), where 1 refers to the number of threads. Note that the miner.start(n) function will always return "null." Unless you have many CPU cores, keep the thread number low to avoid high CPU usage. Note that mining without any pending transaction can still earn your default account incentive (ETH). It creates empty blocks, thus strengthening the integrity of the blockchain tree.
Pegged sidechains employ a two-way peg to transfer assets between chains, and they consist of providing proof of possession in the transferring transactions. The idea is to enable the capability of locking an asset on an original parent chain, which can then be transferred to a sidechain before eventually being redeemed on the original chain. Notably, the original asset on the parent chain is locked in a specific output address and is not destroyed like early implementations of sidechains.
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My chief concern is not with the concept of side chains per se (yet). I have still much to learn about how they are being considered. I am only concerned with the way the concept is being presented here. However, I am sure that much of this was due to space restrictions as much as anything. The concept of side chains is an intriguing one. It is also clearly attempting to address a major problem with the whole Bitcoin scheme- namely the verification latency it introduces for transactions. This is only one of the hurdles facing Bitcoins acceptance into the world of commerce, but it is a considerable one.

Blockchain technology can be used to create a permanent, public, transparent ledger system for compiling data on sales, tracking digital use and payments to content creators, such as wireless users [65] or musicians.[66] In 2017, IBM partnered with ASCAP and PRS for Music to adopt blockchain technology in music distribution.[67] Imogen Heap's Mycelia service has also been proposed as blockchain-based alternative "that gives artists more control over how their songs and associated data circulate among fans and other musicians."[68][69] Everledger is one of the inaugural clients of IBM's blockchain-based tracking service.[70]
Since extension blocks can be implemented via soft forks, the features of the extension blocks are essentially opt-in for users. Even in the case of extension blocks with a larger block size limit, users are not forced to upgrade and validate or propagate blocks that are much larger in size. Those who wish to enjoy the level of decentralization offered by 1MB blocks can continue to do so, while those who would like to experiment with much larger block size limits can do so on an opt-in basis.
Blockstream believes that to be secure, blockchain systems must be built with open source technology. Towards that goal, we've created the Elements Project, a community of people extending and improving the Bitcoin codebase. As open source, protocol-level technology, developers can use Elements to extend the functionality of Bitcoin and explore new applications of the blockchain. Join the expanding group of individual and corporate developers using Elements to build robust, advanced, and innovative blockchains.
A consortium blockchain is part public, part private. This split works at the level of the consensus process: on a consortium chain, a pre-selected group of nodes control the consensus process, but other nodes may be allowed to participate in creating new transactions and/or reviewing it. The specific configuration of each consortium chain (i.e., which nodes have the power to authorize transactions via the consensus process, which can review the history of the chain, which can create new transactions, and more) is the decision of each individual consortium.