Standardize a Stellar RPC contract

[tomerweller]

Currently, the main stellar-core consumer, horizon, is heavily dependent on the internals of the stellar-core implementation. Specifically, it directly accesses the stellar-core postgresql database.

Problems with current implementation:

• This is an implicit contract. Any change to the stellar-core schema can potentially break horizon. Not to mention bigger changes like swapping out the persistence layer. (Horizon does not work with stellar-core if that uses sqlite instead of postgresql)
• Performance: horizon can (and does) affect stellar-core's performance by creating a load on it's database.

A standardized Stellar RPC contract can solve the above and open a path for diversity of implementations.

Some potential stellar-rpc requests:

• Serving ledger bundles (header + txs + tx results + metas) for ingestion.
• Transaction submission.

Some bad stellar-rpc requests:

• Serving live ledger entries. As this can be costly (performance wise) and these can be inferred from the ledger bundles.

Format: given that stellar uses XDR extensively, we can use ONC which is a minimal standard for creating XDR request/response messages.

Transport: HTTP and/or IPC (ethereum's json-rpc, for example, supports both).

Prior art:

• stellar-core already exposes an http interface that includes transaction submission and some administrative commands. It does not, however, provide access to any ledger entries or historical data.
• In the ethereum ecosystem there's a standard json-rpc contract, supported in both main implementations: geth and parity. This allows applications to switch the underlying node implementation without changing application code.