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doc: Add RPC interface guidelines

Wladimir J. van der Laan 4 years ago
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RPC interface guidelines

A few guidelines for introducing and reviewing new RPC interfaces:

- Method naming: use consecutive lower-case names such as `getrawtransaction` and `submitblock`

- *Rationale*: Consistency with existing interface.

- Argument naming: use snake case `fee_delta` (and not, e.g. camel case `feeDelta`)

- *Rationale*: Consistency with existing interface.

- Use the JSON parser for parsing, don't manually parse integers or strings from
arguments unless absolutely necessary.

- *Rationale*: Introduces hand-rolled string manipulation code at both the caller and callee sites,
which is error prone, and it is easy to get things such as escaping wrong.
JSON already supports nested data structures, no need to re-invent the wheel.

- *Exception*: AmountToValue can parse amounts as string. This was introduced because many JSON
parsers and formatters hard-code handling decimal numbers as floating point
values, resulting in potential loss of precision. This is unacceptable for
monetary values. **Always** use `AmountToValue` and `ValueToAmount` when
inputting or outputting monetary values. The only exceptions to this are
`prioritisetransaction` and `getblocktemplate` because their interface
is specified as-is in BIP22.

- Missing arguments and 'null' should be treated the same: as default values. If there is no
default value, both cases should fail in the same way.

- *Rationale*: Avoids surprises when switching to name-based arguments. Missing name-based arguments
are passed as 'null'.

- *Exception*: Many legacy exceptions to this exist, one of the worst ones is
`getbalance` which follows a completely different code path based on the
number of arguments. We are still in the process of cleaning these up. Do not introduce
new ones.

- Try not to overload methods on argument type. E.g. don't make `getblock(true)` and `getblock("hash")`
do different things.

- *Rationale*: This is impossible to use with `bitcoin-cli`, and can be surprising to users.

- *Exception*: Some RPC calls can take both an `int` and `bool`, most notably when a bool was switched
to a multi-value, or due to other historical reasons. **Always** have false map to 0 and
true to 1 in this case.

- Don't forget to fill in the argument names correctly in the RPC command table.

- *Rationale*: If not, the call can not be used with name-based arguments.

- Set okSafeMode in the RPC command table to a sensible value: safe mode is when the
blockchain is regarded to be in a confused state, and the client deems it unsafe to
do anything irreversible such as send. Anything that just queries should be permitted.

- *Rationale*: Troubleshooting a node in safe mode is difficult if half the
RPCs don't work.

- Add every non-string RPC argument `(method, idx, name)` to the table `vRPCConvertParams` in `rpc/client.cpp`.

- *Rationale*: `bitcoin-cli` and the GUI debug console use this table to determine how to
convert a plaintext command line to JSON. If the types don't match, the method can be unusable
from there.

- A RPC method must either be a wallet method or a non-wallet method. Do not
introduce new methods such as `getinfo` and `signrawtransaction` that differ
in behavior based on presence of a wallet.

- *Rationale*: as well as complicating the implementation and interfering
with the introduction of multi-wallet, wallet and non-wallet code should be
separated to avoid introducing circular dependencies between code units.