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Riak Core Broadcast
The following describe the broadcast subsystems implemented by
riak_core_broadcast. It is intended for those wishing to use the
subsystem as well as support and modify it.
riak_core_broadcast provides a generalized mechanism for ensured
eventual delivery of messages to all nodes in a riak_core
cluster. The development of this subsystem was driven primarily by the
implementation of Cluster Metadata, however it was built for use in
other applications as well. Because of this generalization, it is
required the client application/subsystem provide certain guarantees
in order to ensure 100% reliability when delivering messages.
Broadcasted messages are opaque to riak_core_broadcast. The only
requirement is that each message must be uniquely identifiable. The
storage of messages is the responsibility of the application/subsystem
using riak_core_broadcast. For more details about what is required
when using riak_core_broadcast see
Broadcast Handlers.
To deliver messages riak_core_broadcast sends them along the paths
of a directed, possibly cyclic, graph of all nodes in the cluster. In
most cases, however, the graph will be a spanning tree. Only in the
cases where membership changes or failures occur will a less optimal
path be used. Keep reading for more details.
riak_core_broadcast provides the following guarantee regarding
message delivery -- assuming the application/subsystem using it meets
the necessary requirements (see
Broadcast Handlers):
A broadcasted message will be delivered to all nodes *at least
once*. There is one exception: when a node the a broadcast originates on is
removed before it can deliver the payload to at least one other
node, this guarantee is not met.
Exploration of ways to mitigate the exception mentioned are being tracked in this issue.
riak_core_broadcast is a hybrid of deterministic broadcast and
stochastic epidemic protocols: a self-healing spanning tree. The
primary path for the delivery of messages is to
broadcast using the spanning tree but when links fail, they are healed
by using pull-based gossip [1]. Sometimes, the spanning tree may have
additional links -- making it a directed , and possibly cyclic, graph
while it is being healed or when membership changes. In addition, the
entire mechanism is backed by a standard anti-entropy epidemic [1]
The implementation is based upon "Plumtree" [2] , which specifies an algorithm for a self-healing spanning tree. The paper [2] has a clear and concise description of the algorithm so none will be recreated here.
Plumtree was chosen, primarily, because of its efficiency in the stable case while maintaining reliability. When the graph of nodes is a proper spanning tree, each node will receive the payload exactly once. In failure cases, nodes may receive the payload more than once, but the act of doing so heals the tree and removes redundant links.
riak_core_broadcast extends what is specified in the Plumtree
Protocol. The requirements of the implementation in [2] and
riak_core_broadcast differ in several ways. The following describes
those differences and their effect on the implementation.
In large-scale, partially connected eventual delivery protocols it is
common to separate responsibilities into two parts: the peer service
and the message overlay. The latter is what Plumtree implements and it
is responsible for providing a reliable and hopefully efficient
delivery mechanism for most messages. The former is responsible for
ensuring delivery of critical data like membership, among other things
not relevant to this document and may do so less efficiently at the
cost of safety. The message overlay relies on the peer service and
makes certain assumptions about it. These assumptions are the source
of many of the differences between the implementation specified in [2]
and riak_core_broadcast. The Peer Service in [2] is HyParView
[3]. In riak_core it is the existing riak_core_gossip.
The assumptions made in [2] are discussed in section 3.3 of the paper. The "Connectivity" requirement of the service is not one that can be provided by Riak. Namely, a node must be able to be a singleton during a long lived partition and still ensure the delivery to all nodes. The implementation in [2] assumes in this case that the peer service will fail this node out of the overlay and that failure to deliver messages to it or sent from it is acceptable. We do not.
The paper also is focused on clusters on the order of tens of
thousands of nodes, whereas Riak cluster, typically range from 5 to
100. The Peer Service for riak_core_broadcast is fully-connected --
each node is aware of every other node in the cluster and can
communicate with them directly outside of riak_core_broadcast. This
affects how membership is handled.
The reliability of Plumtree is measured in [2] by determining how many
healthy nodes receive the message, where healthy nodes are defined to
be nodes that have not failed while there are protocol messages
related to the broadcast "in the network". The reliability of
riak_core_broadcast, however, must define healthy nodes as those
that have not permanently failed or have been removed from the
cluster.
When considering dynamic membership, the definition of healthy nodes
in [2] is extended to include joined nodes that did so while there
were still protocol messages related to the broadcast "in the
network". riak_core_broadcast, extends this further to include any
node which may join in the future, despite protocol messages no longer
being "in the network".
This requirement can be thought of as "Historical Delivery".
Plumtree is a reliable protocol in the face of massive node failures,
which was the focus of the research, however, it is not tolerant to
network partitions. This is hinted at in the "Connectivity"
requirement [2] of Plumtree's Peer Service and can be proven using a
formal model. To
address this, riak_core_broadcast extends how the set of unsent lazy
(or "i have") messages is maintained and adds an additional message:
"ignore i have".
The original algorithm removes the record of an "i have" message, used to
indicate it needs to sent, as soon as one attempt is made to send
it. This is a problem when messages are dropped by the
network. riak_core_broadcast maintains this record after the attempt
and periodically scans the set of them and re-sends.
The record is removed from the set when an "ignore i have" message is received. This message is sent by the peer who received the "i have" and indicates that no graft is necessary.
In cases where a large percentage of nodes fail, the union of all sets of lazy links between nodes, which heal the spanning tree, will no longer contain links between all nodes. When this happens, messages may not be delivered to all healthy nodes even under the less strict definition used in [2]. Although, [2] discusses a similar failure scenario, they are not quite the same because of the Peer Service, however the differences are not worth exploring further.
Although high failure percentages are not so common in riak_core
applications, it is still an unacceptable failure
scenario. riak_core_broadcast uses a classic anti-entropy epidemic
[1] to ensure that each node eventually checks-in with every other
node in the cluster and delivers any missing messages. These are
referred to as "Exchanges".
This, also, ensures that riak_core_broadcast maintains
Historical Delivery.
Since the Peer Service between the two implementations differ their
generation of the set of initial eager and lazy peers for each node do
as well. riak_core_broadcast uses the
riak_core_util:build_tree/3. See riak_core_broadcast:init_peers/1
for more details.
Since riak_core_gossip is fully-connected, unlike HyParView [3], how
the system deals with membership changes has also been
modified. riak_core_broadcast simply regenerates the initial peers,
as if it were restarted w/ the new membership information, and allows
the protocol to modify things as needed.
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