In Chapter 6, we saw the value of replicating stateless services for reliability, redundancy, and scaling. This chapter considers sharded services. With the replicated services that we introduced in Chapter 6, each replica was entirely homogeneous and capable of serving every request. In contrast to replicated services, with
sharded services, each replica, or shard, is only capable of serving a
subset of all requests. A load-balancing node, or root, is responsible
for examining each request and distributing each request to the appropriate
shard or shards for processing. The contrast between replicated and
sharded services is represented in Figure 7-1.
Figure 7-1. Replicated service versus sharded service
Replicated services are generally used for building stateless services,
whereas sharded services are generally used for building stateful services.
The primary reason for sharding the data is because the size of the state
is too large to be served by a single machine. Sharding enables you to
scale a service in response to the size of the state that needs to be served.
Sharding is not exclusively for stateful services, though.
Sometimes sharding can also be useful for stateless services for the purpose
of isolation. Failures in distributed systems can sometimes come from the
input requests to the system (“poison requests”) in a replicated system—if ...
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