Estimating Gas Costs with Speculative Execution

Version 1.5 of the Casper Node includes a new JSON RPC endpoint called speculative_exec. This endpoint allows developers to send a Deploy to a single node, which will execute the Deploy without committing the results to global state and, therefore, not incurring the associated costs. Observing the execution results of the Deploy gives a rough estimate of the potential cost for sending the Deploy without speculative execution.

In addition to the Deploy in question, speculative_exec also accepts a [block_identifier] for a specific block height or hash to speculate on. If you do not provide a block identifier, the Deploy will be executed on the most recent block.

Sending a Speculative Execution Deploy using the Rust CLI Casper Client

The Rust CLI Casper client includes a speculative-exec option that will flag a normal put-deploy for execution but not commitment to global state. The following command shows an example:

casper client put-deploy /
--node-address <HOST:PORT> /
--chain-name <CHAIN_NAME> /
--secret-key <PATH> /
--session-path <PATH>  /
--payment-amount <PAYMENT_AMOUNT_IN_MOTES>
--speculative-exec <BLOCK HEIGHT OR HASH>

You should receive execution_results that show a cost.

{
  "jsonrpc": "2.0",
  "id": -4571113357017152230,
  "result": {
    "api_version": "1.0.0",
    "block_hash": "6ca035b08de092e7f5e8fff771b880c5b4d7463a8f7a9b108888aaad958e5b0f",
    "execution_result": {
      "Success": {
        "effect": {
          <Deploy effects removed for conciseness.>
        },
        "transfers": [],
        "cost": "87300473670"
      }
    }
  }
}

note

Cost estimates acquired through speculative_exec may vary from the cost of sending the same Deploy to a Casper network. Speculative execution is a tool to help narrow down the potential cost of sending a Deploy, but many factors can cause the actual cost to vary.