3. Example

blockchain::fetch_last_height() takes no arguments and returns the height of the last block.

# Blank destination
destination = worker_uuid()
command = "blockchain.fetch_last_height"
# Random uint32_t
id = rand()
# Blank parameter list
data = buffer()

The load balancer will query a random worker, route our request to the worker, the worker will perform the request, send back a reply with the same ID as the request, and finally the load balancer will forward the reply back onto the client.

Every Obelisk reply always begins with a 4 byte error_code. If 0 then no error occured, otherwise the value will be set to a libbitcoin error code value.

uint32_t val = deserial.read_4_bytes();
std::error_code ec;
if (val)
    ec = static_cast<bc::error::error_code_t>(val);

See libbitcoin’s <bitcoin/error.hpp> for a list of possible error_code values and their meanings.

Then the result return values come afterwards:

size_t last_height = deserial.read_4_bytes();

The full C++ code:

template <typename Deserializer>
bool read_error_code(Deserializer& deserial,
    size_t data_size, std::error_code& ec)
{
    if (data_size < 4)
    {
        log_error() << "No error_code in response.";
        return false;
    }
    uint32_t val = deserial.read_4_bytes();
    if (val)
        ec = static_cast<bc::error::error_code_t>(val);
    return true;
}

// ...

void wrap_fetch_last_height(const data_chunk& data,
    blockchain::fetch_handler_last_height handle_fetch)
{
    if (data.size() != 8)
    {
        log_error() << "Malformed response for blockchain.fetch_history";
        return;
    }
    std::error_code ec;
    auto deserial = make_deserializer(data.begin(), data.end());
    bool read_success = read_error_code(deserial, data.size(), ec);
    BITCOIN_ASSERT(read_success);
    size_t last_height = deserial.read_4_bytes();
    handle_fetch(ec, last_height);
}

The serialization of values is the same as in Bitcoin. See libbitcoin’s <bitcoin/utility/serializer.hpp> for more info on how types are serialized.