Cryptocurrencies, implemented with blockchain protocols, promise to become a global payment system if they can overcome performance limitations. Rapidly advancing architectures improve on latency and throughput, but most require all participating servers to process all transactions. Several recent works propose to shard the system, such that each machine would only process a subset of the transactions. However, we identify a denial-of-service attack that is exposed by these solutions - an attacker can generate transactions that would overload a single shard, thus delaying processing in the entire system. Moreover, we show that in common scenarios, these protocols require most node operators to process almost all blockchain transactions. We present Ostraka, a blockchain node architecture that shards (parallelizes) the nodes themselves. We prove that replacing a unified node with an Ostraka node does not affect the security of the underlying consensus mechanism. We evaluate analytically and experimentally block propagation and processing in various settings. Ostraka allows nodes in the network to scale, without costly coordination. In our experiments, Ostraka nodes' transaction processing rate grows linearly with the addition of resources.
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