Motivation

When building a cluster of Ignite nodes, users need to be able to establish some restrictions on the member nodes based on cluster invariants in order to avoid breaking the consistency of the cluster. Such restrictions may include: having the same product version across the cluster, having consistent table and memory configurations, enforcing a particular cluster state.

Description

Problem statement

This document describes the process of a new node joining a cluster, which includes a validation step where a set of rules are applied to determine whether the incoming node is able to enter the current topology. These rules may include node-local information (e.g. node version) as well as cluster-wide information (e.g. data encryption algorithm), which means that the validation component may require access to the Meta Storage (it is assumed that Meta Storage contains the consistent cluster-wide information, unless some other mechanism is proposed). The problem is that, according to the Node Lifecycle description, a cluster can exist in a "zombie" state, during which the Meta Storage is unavailable. This means the the validation process can be split into 2 steps:

1. "Pre-init" validation: a joining node tries to enter the topology on the network level and gets validated against its local properties.
2. "Post-init" validation: the cluster has received the "init" command, which activates the Meta Storage, and the joining node can be validated against the cluster-wide properties.

Apart from the 2-step validation, there are also the following questions that need to be addressed:

1. Where will the whole process happen: on the joining node itself or on an arbitrary remote node.
2. How to negate possible security issues if a not yet fully validated node gets access to the Meta storage.
3. How to deal with different configurations of the Meta Storage: the "most recent" configuration should be consistently delivered to all nodes in a cluster.

Terminology

Init command

The "init" command is supposed to move the cluster from the "zombie" state into the "active" state. It is supposed to have the following characteristics (note that the "init" command has not been specified at the moment of writing and is out of scope of this document, so all statements are approximate and can change in the future):

1. It should deliver the following information: addresses of the nodes that host the Meta Storage Raft group, Meta Storage version and a cluster tag (described below).
2. It should deliver this information atomically, i.e. either all nodes enter the "active" state or none. As a possible solution, it can be implemented similarly to a two-phase commit: first, a "prepare" message is broadcasted to all nodes in the current topology. The initiator node remembers the topology members at the start of the prepare phase and restarts the operation (or sends additional messages) until the topology is stable. After the prepare phase is finished, the commit message is broadcasted. Until the commit phase finishes, no new nodes are allowed to enter the cluster.

Initialized and empty nodes

This document uses a notation of "initialized" and "empty" nodes. An initialized node is a node that has received the "init" message sometime in its lifetime and therefore possesses the cluster tag and the meta storage version. An empty node has never received the "init" command and does not possess the aforementioned properties.

Meta Storage version

Meta Storage version is a property that should be used to compute the most "recent" state of a given Meta Storage configuration. A possible implementation can be a monotonically increasing counter, which is increased every time the Meta Storage configuration (e.g. addresses of nodes that host the Meta Storage Raft group) is updated.

Cluster tag

A cluster tag is a string that uniquely identifies a cluster (e.g. a UUID). It is generated once per cluster and is distributed across the nodes during the "init" phase. The purpose of a cluster tag is to understand whether a joining node used to be a member of another cluster, in which case its Meta Storage version is not comparable and the joining node should be rejected. Together with the Meta Storage version, it creates a partial ordering that allows to compare different configuration versions.

Validation approaches

Local validation

Local validation approach requires the joining node to retrieve some information from a random node/Meta Storage and deciding to join the cluster based on that information.

This approach has the following pros and cons:

1. When using in the rolling upgrade scenario, it might be easier to maintain backward compatibility: a newer joining node already knows about the requirements of the older nodes in the cluster.
2. Possible security issues: if a node is able to allow itself to join, it might be easier to compromise the cluster.

Remote validation

Remote validation approach requires the joining node to send some information about itself  to a remote node, which decides whether to allow the new node to join or not.

This approach has the following pros and cons:

1. This approach is used in Ignite 2, which may be more familiar to users and developers.
2. It may be more secure, since a node can't join without notifying at least one valid node.
3. Harder to support backward compatibility.

Discussion needed: At the time of writing this document, it is assumed that validation protocol is going to be remote.

Implementation details

Based on the initialization status of the nodes in a cluster, there can be several possible scenarios of new nodes entering the topology:

1. An empty node enters a cluster where all nodes are empty.
2. An empty node enters a cluster where all nodes are initialized.
3. An initialized node enters a cluster where all nodes are empty.
4. An initialized node with a Meta Storage of version X enters a cluster where all nodes have the Meta Storage of version Y.

Common logic

A joining node tries to enter the topology. It is possible to piggyback on the transport of the membership protocol in order to exchange validation messages before allowing to send membership messages (similar to the handshake protocol). During this step it sends some information (cluster tag, Meta Storage version, node version) to a random node and gets validated (more details below). After this step is complete, the joining node becomes visible through the Topology Service, therefore establishing an invariant that visible topology will always consist of nodes that have passed the first validation step. Possible issues: there can be a race condition when multiple conflicting nodes join at the same time, in which case only the first node to join will be valid. This can be considered expected behavior, because such situations can only occur during the initial set up of a cluster, which is a manual process and requires manual intervention anyway.

Empty node joins a cluster

If an empty node tries to join a cluster the following process is proposed:

1. It connects to a random node, sends the available local validation information and enters the topology, if it gets accepted.
2. The following scenarios can then happen:
   1. The random node is initialized. The joining node should then retrieve the information, that was broadcasted by the "init" command, become initialized and finish the join process.
   2. The random node is empty because the cluster has not yet been initialized. In this case the node finishes the join process and remains in the "zombie" state until the "init" command arrives.
   3. The random node is empty because it hasn't finished the join process itself. In this case the random node should send a corresponding message, and the joining node should choose another random node and repeat the process.

Initialized node joins a cluster

If an initialized node tries to join a cluster the following process is proposed:

1. It connects to a random node and sends the available local validation information (including the cluster tag and the Meta Storage version).
2. The following scenarios can then happen:
   1. The random node is initialized and the cluster tags do not match. The joining node must be rejected.
   2. The random node is initialized, the cluster tags match, local Meta Storage version is "smaller" than the remote. The node joins the topology and updates its Meta Storage configuration, thus ending the joining process.
   3. The random node is initialized, the cluster tags match, local Meta Storage version is "larger" than the remote. Discussion needed: what is the expected behavior in this case?
   4. The random node is empty because the cluster has not yet been initialized. The joining node should initiate a process similar to sending the "init" command. Discussion needed: what to do if another "init" command is running in parallel?
   5. The random node is empty because it hasn't finished the join process itself. In this case the random node should send a corresponding message, and the joining node should choose another random node and repeat the process.

Changes in API (WIP)

NetworkTopologyService

Current TopologyService  will be renamed to NetworkTopologyService . It is proposed to extend this service to add validation handlers that will validate the joining nodes on the network level.

/**
 * Class for working with the cluster topology on the network level.
 */
public interface NetworkTopologyService {
    /**
     * This topology member.
     */
    ClusterNode localMember();

    /**
     * All topology members.
     */
    Collection<ClusterNode> allMembers();

    /**
     * Handlers for topology events (join, leave).
     */
    void addEventHandler(TopologyEventHandler handler);

    /**
     * Returns a member by a network address
     */
    @Nullable ClusterNode getByAddress(NetworkAddress addr);

    /**
     * Handlers for validating a joining node.
     */
    void addValidationHandler(TopologyValidationHandler handler);
}

TopologyService

The new service will have the same API, but will work on top of the Meta Storage, and will provide methods to work with the list of validated nodes. In addition to that, it will perform the validation of incoming nodes against the Meta Storage, based on the registered validation handlers.

/**
 * Class for working with the cluster topology on the Meta Storage level. Only fully validated nodes are allowed to be present in such topology.
 */
public interface TopologyService {
    /**
     * This topology member.
     */
    ClusterNode localMember();

    /**
     * All topology members.
     */
    Collection<ClusterNode> allMembers();

    /**
     * Handlers for topology events (join, leave).
     */
    void addEventHandler(TopologyEventHandler handler);

    /**
     * Returns a member by a network address
     */
    @Nullable ClusterNode getByAddress(NetworkAddress addr);

    /**
     * Handlers for validating a joining node.
     */
    void addValidationHandler(TopologyValidationHandler handler);
}

TopologyService will depend on the MessagingService  (to respond and listen to validation requests) and on the MetaStorageManager (for interacting with the Meta Storage).

Risks and Assumptions

1. "Init" command is not fully specified and can influence the design.
2. Proposed implementation does not discuss message encryption and security credentials.
3. Two-layered topology view may be confusing to use.

Discussion Links

// Links to discussions on the devlist, if applicable.

Reference Links

1. IEP-73: Node startup
2. https://github.com/apache/ignite-3/blob/main/modules/runner/README.md
3. IEP-67: Networking module

Tickets

Unable to render Jira issues macro, execution error.