THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
Clustering and Partitioning
Picture 1.
A cluster in Ignite is a set of interconnected nodes where server nodes (Node 0, Node 1, etc. in Picture 1) are organized in a logical ring and client nodes (applications) are connected to them. Both server and client nodes can execute all range of APIs supported in Ignite. The main difference between the two is that the clients do not store data.
Data that is stored on the server nodes is represented in a form of key-value pairs. The pairs in their turn are located in specific partitions which belong to individual Ignite caches as it's shown in Picture 2:
Picture 2.
To ensure data consistency and comply with the high-availability principle, server nodes are capable of storing a primary as well as backup copies of data. Basically, there is always a primary copy of a partition with all its key-value pairs in the cluster and might be 0 or more backup copies of the same partition depending on the configuration parameters.
Each cluster node (servers and clients) are aware of all primary and backup copies of every partition. This information is collected and broadcasted to all the nodes from a coordinator (the oldest server node) via internal partition map exchange messages.
However, all the data related requests/operations (get, put, SQL, etc.) go to primary partitions except for some read operations when CacheConfiguration.readFromBackup is enabled. If it's an update operation (put, INSERT, UPDATE) then Ignite ensures that both the primary and backup copies are updated and stay in a consistent state.
Transactions
This section dives into the details of Ignite transactional protocol. High-level principles and features are described in Ignite technical documentation.
Two-Phase Commit Protocol
A single transaction in distributed systems usually spans across several server nodes which imposes additional requirements for the sake of data consistency. For instance, it is obligatorily to detect and handle situations when a transaction was not fully committed due to a partial outage or cluster nodes loss. Ignite relies on two-phase commit for handling this and many other situations in order to ensure data consistency cluster-wide.
As the protocol name suggests, a transaction is executed in two phases. The "prepare" phase goes first:
Picture 3.
- Transaction coordinator (aka. near node or application that runs a transaction) send "prepare" message (step 1) all primary nodes participating in the transaction.
- The primary nodes forward the message to nodes that hold a backup copy of data if any (step 2) and acquire all the required data locks.
- The primary nodes acknowledge (step 4) that all the locks are required and they are ready to commit the transaction.
Right after that, the transaction coordinator executes the second phase by sending "commit" message:
Picture 4.
Once the backup and primary copies are updated, the transaction coordinator gets acknowledged and assumes that the transaction is finished.
This is how the 2-phase commit works in a nutshell. Below we will see how the protocol tolerates failures, distinguishes pessimistic and optimistic transaction and does many other things.
Near Node, Remote Node and DHT
The transaction coordinator is also known as a near node among Ignite community and committers. The transaction coordinator (near node) initiates a transaction, tracks its state, sends over "prepare" and "commit" message, orchestrates the overall transaction process. Usually, a near node is a client node that connects our applications to the cluster. The application issues
Picture 5.
In contrast to the Near involved in the transaction partitions are called Remote. They keep "their" part of the cache. Physically, every node allocates at part DHT (Distributed hash table) and through afiniti function, any transaction participant may determine its partition and nodes. It should be noted that DHT keeps the buckets to the level of partitioning, and then use B + tree.
Overview
Content Tools
Apps
