go

Alternator - Client-side load balancing - Go

Introduction

As explained in the toplevel README, DynamoDB applications are usually aware of a single endpoint, a single URL to which they connect - e.g., http://dynamodb.us-east-1.amazonaws.com. But Alternator is distributed over a cluster of nodes and we would like the application to send requests to all these nodes - not just to one. This is important for two reasons: high availability (the failure of a single Alternator node should not prevent the client from proceeding) and load balancing over all Alternator nodes.

One of the ways to do this is to provide a modified library, which will allow a mostly-unmodified application which is only aware of one "enpoint URL" to send its requests to many different Alternator nodes.

Our intention is not to fork the existing AWS client library for Go. Rather, our intention is to provide a small library which tacks on to the existing "aws-sdk-go" library which the application is already using, and makes it do the right thing for Alternator.

The alternator_lb.go library

The AlternatorNodes class defined in alternator_lb.go can be used to easily change any application using aws-sdk-go from using Amazon DynamoDB to use Alternator: While DynamoDB only has one "endpoint", this class helps us balance the requests between all the nodes in the Alternator cluster.

Using the library

To use this class, simply replace the code which creates a AWS SDK session.Session Instead of the usual way of creating a Seassion, like:

sess := session.Must(session.NewSessionWithOptions(session.Options{
            SharedConfigState: session.SharedConfigEnable,

Use an AlternatorNodes object, which keeps track of the live Alternator nodes, to create a session with the following commands:

alternator_nodes := NewAlternatorNodes("http", 8000, []string {"127.0.0.1"})
sess := alternator_nodes.session("dog.scylladb.com", "alternator", "secret_pass")

Then, the rest of the applicaton can use this session normally - call db := dynamodb.New(sess) and then send DynamoDB requests to db; As usual, this db object is thread-safe and can be used from multiple threads.

The parameters to NewAlternatorNodes() indicate a list of known Alternator nodes, and their common scheme (http or https) and port. This list can contain one or more nodes - we then periodically contact these nodes to fetch the full list of nodes using Alternator's /localnodes request. In the session() method, one needs to pick a "fake domain" which doesn't really mean anything (except it will be used as the Host header, and be returned by the DescribeEndpoints request), and the key and secret key for authentication to Alternator.

Every request performed on this new session will pick a different live Alternator node to send it to. Despite us sending different requests to different nodes, Go will keep these connections cached and reuse them when we send another request to the same node.

(TODO: figure out the limitations of this caching. Where is it documented?).

Example

This directory also contains two trivial examples of using alternator_lb.go, try.go. This example opens a session using NewAlternatorNodes(), as described above, and then uses it 20 times in a loop - and we'll see that every request will be sent to a different node.