Glossary

Coordinator

Moderate

In blockchain technology, a coordinator is a specialized client that allows nodes to verify the validity of their copy of the ledger against specific transactions.

What Is a Coordinator?

Blockchains are essentially a complicated series of transactions kept in a distributed ledger. A decentralized network of validator nodes then verifies this ledger. While this is the most simplistic way to look at a blockchain, there are numerous other elements of its functionality. Additionally, every blockchain is different. For example, the core functionalities of the Bitcoin and Ethereum blockchain might be the same. However, they have very different applications.
In blockchain technology, a coordinator performs the function of a tracking and checking system for validator nodes. The coordinator issues milestones throughout a given amount of time. These milestones are essentially transactions recorded on the blockchain that validator nodes use in order to verify that their copy of the ledger is correct. It is important to note that not all blockchains feature a coordinator client.
A good example of a blockchain that used a coordinator client for ledger verification is IOTA. The IOTA blockchain depended on this coordinator client in order to add a secondary level of security and ensure the validity of all copies of the distributed ledger. However, the IOTA foundation has now implemented the next step in its roadmap, and the coordinator client is completely eradicated from the system.
It is not surprising that IOTA took that next step. Many blockchain and cryptocurrency enthusiasts believe that coordinators hinder the decentralized power of the technology. As they essentially have a concentrated power to influence all copies of the ledger in the network simultaneously, they are regarded as a centralization feature. In this sense, blockchains that have a coordinator client cannot be fully decentralized. 
Decentralization is at the heart of blockchain technology in terms of ideology, so a centralizing factor is not a desirable feature. This is why IOTA, for example, only relied on its coordinator in the very beginning. The goal of implementing such a client was to allow the developers enough time to work on the system while ensuring it was secure from the very beginning. 
Nowadays, as blockchain technology grows and develops, new consensus mechanisms come to life. These make it easier for blockchains to be fully decentralized. If we continue with the IOTA example, the network now relies solely on its Directed Acyclic Graph (DAG) for transaction verification and node validation. In this sense, coordinator clients might not be a viable security solution for much longer. Even so, for projects that are just starting out, or ones that actually prefer a level of centralization in the blockchain, a coordinator client can be a helpful way to ensure the security of the whole network. 
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