Hyperledger Fabric Peer is the node type responsible for endorsing transactions and recording them on the Blockchain for the Hyperledger Fabric permissioned blockchain framework. The Hyperledger Fabric Peer can either use a goleveldb or a CouchDB database for holding the ledger data.
This Peer can receive transaction requests, which it checks and signs, endorsing them. These endorsements can then be sent to the Ordering Service (one or more Orderer nodes), which will package them and return blocks that the Peer can then commit to their own Ledger.
A blockchain network is comprised primarily of a set of peer nodes (or, simply, peers). Peers are a fundamental element of the network because they host ledgers and smart contracts. Recall that a ledger immutably records all the transactions generated by smart contracts (or chaincode). Smart contracts and ledgers are used to encapsulate the shared processes and shared information in a network, respectively. These aspects of a peer make them a good starting point to understand a Hyperledger Fabric network.
Other elements of the blockchain network are of course important: ledgers and smart contracts, orderers, policies, channels, applications, organizations, identities, and membership, and you can read more about them in their own dedicated sections. This section focusses on peers and their relationship to those other elements in a Hyperledger Fabric network.
A blockchain network is comprised of peer nodes, each of which can hold copies of ledgers and copies of smart contracts. In this example, the network N consists of peers P1, P2 and P3, each of which maintains their own instance of the distributed ledger L1. P1, P2, and P3 use the same chaincode, S1, to access their copy of that distributed ledger.
Peers can be created, started, stopped, reconfigured, and even deleted. They expose a set of APIs that enable administrators and applications to interact with the services that they provide. We’ll learn more about these services in this section.
Three K8S secrets containing:
Let’s look at a peer in a little more detail. We can see that it’s the peer that hosts both the ledger and chaincode. More accurately, the peer actually hosts instances of the ledger, and instances of chaincode. Note that this provides a deliberate redundancy in a Fabric network — it avoids single points of failure. We’ll learn more about the distributed and decentralized nature of a blockchain network later in this section.
A peer hosts instances of ledgers and instances of chaincodes. In this example, P1 hosts an instance of ledger L1 and an instance of chaincode S1. There can be many ledgers and chaincodes hosted on an individual peer.
Because a peer is a host for ledgers and chaincodes, applications and administrators must interact with a peer if they want to access these resources. That’s why peers are considered the most fundamental building blocks of a Hyperledger Fabric network. When a peer is first created, it has neither ledgers nor chaincodes. We’ll see later how ledgers get created, and how chaincodes get installed, on peers.
Although this section is about peers rather than channels, it’s worth spending a little time understanding how peers interact with each other, and with applications, via channels — a mechanism by which a set of components within a blockchain network can communicate and transact privately.
These components are typically peer nodes, ordered nodes and applications and, by joining a channel, they agree to collaborate to collectively share and manage identical copies of the ledger associated with that channel. Conceptually, you can think of channels as being similar to groups of friends (though the members of a channel certainly don’t need to be friends!). A person might have several groups of friends, with each group having activities they do together. These groups might be totally separate (a group of work friends as compared to a group of hobby friends), or there can be some crossover between them. Nevertheless, each group is its own entity, with “rules” of a kind.
Channels allow a specific set of peers and applications to communicate with each other within a blockchain network. In this example, application A can communicate directly with peers P1 and P2 using channel C. You can think of the channel as a pathway for communications between particular applications and peers. (For simplicity, orderers are not shown in this diagram, but must be present in a functioning network.)
We see that channels don’t exist in the same way that peers do — it’s more appropriate to think of a channel as a logical structure that is formed by a collection of physical peers. It is vital to understand this point — peers provide the control point for access to, and management of, channels.
Now that you understand peers and their relationship to ledgers, chaincodes, and channels, you’ll be able to see how multiple organizations come together to form a blockchain network.
Blockchain networks are administered by a collection of organizations rather than a single organization. Peers are central to how this kind of distributed network is built because they are owned by — and are the connection points to the network for — these organizations.
Peers in a blockchain network with multiple organizations. The blockchain network is built up from the peers owned and contributed by the different organizations. In this example, we see four organizations contributing eight peers to form a network. The channel C connects five of these peers in the network N — P1, P3, P5, P7, and P8. The other peers owned by these organizations have not been joined to this channel, but are typically joined to at least one other channel. Applications that have been developed by a particular organization will connect to their own organization’s peers as well as those of different organizations.
It’s really important that you can see what’s happening in the formation of a blockchain network. The network is both formed and managed by the multiple organizations who contribute resources to it. Peers are the resources that we’re discussing in this topic, but the resources an organization provides are more than just peers. There’s a principle at work here — the network literally does not exist without organizations contributing their individual resources to the collective network. Moreover, the network grows and shrinks with the resources that are provided by these collaborating organizations.
You can see that (other than the ordering service) there are no centralized resources — in the example above, the network, N, would not exist if the organizations did not contribute their peers. This reflects the fact that the network does not exist in any meaningful sense unless and until organizations contribute the resources that form it. Moreover, the network does not depend on any individual organization — it will continue to exist as long as one organization remains, no matter which other organizations may come and go. This is at the heart of what it means for a network to be decentralized.
Applications in different organizations, as in the example above, may or may not be the same. That’s because it’s entirely up to an organization as to how its applications process their peers’ copies of the ledger. This means that both application and presentation logic may vary from organization to organization even though their respective peers host exactly the same ledger data.
Applications connect either to peers in their organization, or peers in another organization, depending on the nature of the ledger interaction that’s required. For ledger-query interactions, applications typically connect to their own organization’s peers. For ledger-update interactions, we’ll see later why applications need to connect to peers representing every organization that is required to endorse the ledger update.
Now that you’ve seen how peers from different organizations come together to form a blockchain network, it’s worth spending a few moments understanding how peers get assigned to organizations by their administrators.
Peers have an identity assigned to them via a digital certificate from a particular certificate authority. You can read lots more about how X.509 digital certificates work elsewhere in this guide but, for now, think of a digital certificate as being like an ID card that provides lots of verifiable information about a peer. Each and every peer in the network is assigned a digital certificate by an administrator from its owning organization.
When a peer connects to a channel, its digital certificate identifies its owning organization via a channel MSP. In this example, P1 and P2 have identities issued by CA1. Channel C determines from a policy in its channel configuration that identities from CA1 should be associated with Org1 using ORG1.MSP. Similarly, P3 and P4 are identified by ORG2.MSP as being part of Org2.
Whenever a peer connects using a channel to a blockchain network, a policy in the channel configuration uses the peer’s identity to determine its rights. The mapping of identity to the organization is provided by a component called a Membership Service Provider (MSP) — it determines how a peer gets assigned to a specific role in a particular organization and accordingly gains appropriate access to blockchain resources. Moreover, a peer can be owned only by a single organization and is therefore associated with a single MSP. We’ll learn more about peer access control later in this section, and there’s an entire section on MSPs and access control policies elsewhere in this guide. But for now, think of an MSP as providing linkage between an individual identity and a particular organizational role in a blockchain network.
To digress for a moment, peers, as well as everything that interacts with a blockchain network, acquire their organizational identity from their digital certificate and an MSP. Peers, applications, end users, administrators, and orderers must have an identity and an associated MSP if they want to interact with a blockchain network. We give a name to every entity that interacts with a blockchain network using an identity — a principal. You can learn lots more about principals and organizations elsewhere in this guide, but for now, you know more than enough to continue your understanding of peers!
Finally, note that it’s not really important where the peer is physically located — it could reside in the cloud, or in a data center owned by one of the organizations, or on a local machine — it’s the identity associated with it that identifies it as being owned by a particular organization. In our example above, P3 could be hosted in Org1’s data center, but as long as the digital certificate associated with it is issued by CA2, then it’s owned by Org2.
We’ve seen that peers form the basis for a blockchain network, hosting ledgers and chaincode which can be queried and updated by peer-connected applications. However, the mechanism by which applications and peers interact with each other to ensure that every peer’s ledger is kept consistent is mediated by special nodes called orderers, and it’s to these nodes we now turn our attention.
An update transaction is quite different from a query transaction because a single peer cannot, on its own, update the ledger — updating requires the consent of other peers in the network. A peer requires other peers in the network to approve a ledger update before it can be applied to a peer’s local ledger. This process is called consensus, which takes much longer to complete than a simple query. But when all the peers required to approve the transaction do so, and the transaction is committed to the ledger, peers will notify their connected applications that the ledger has been updated. You’re about to be shown a lot more detail about how peers and orderers manage the consensus process in this section.
Specifically, applications that want to update the ledger are involved in a 3-phase process, which ensures that all the peers in a blockchain network keep their ledgers consistent with each other. In the first phase, applications work with a subset of endorsing peers, each of which provides an endorsement of the proposed ledger update to the application, but do not apply the proposed update to their copy of the ledger. In the second phase, these separate endorsements are collected together as transactions and packaged into blocks. In the final phase, these blocks are distributed back to every peer where each transaction is validated before being applied to that peer’s copy of the ledger.
As you will see, orderer nodes are central to this process, so let’s investigate in a little more detail how applications and peers use orderers to generate ledger updates that can be consistently applied to a distributed, replicated ledger.
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