Transaction vs Call

There are two ways we can invoke functions.

1. Call

2. Send a transaction (data modification)

A transaction is necessary, if a value in a Smart Contract needs to be updated (state change).

A call is done, if a value is read.

Transactions cost Ether (gas), need to be mined and therefore take a while until the value is reflected, which you will see later. Calls are virtually free and instant.

transaction followed by call

Sending a transaction to a function

Anytime we send a transaction to the network it takes time to be process (it must be confirmed by other nodes and mined).

When we send a transaction to a function, we are returned a transaction hash that identifies the transaction.

• We do not get a return value back, even if the function has a return component in it.

• Costs money.

Transactions

Transactions always start with an externally owned account signing a transaction and submitting it to the network. The returned value is a transaction receipt.

The outcome of the transaction is unknown because nothing has happened yet except that a request has been submitted to the network for verification in the future.

A typical approach to coping with this fact of life is to wait for the transaction to be mined, then inspect the transaction log for useful output, inspect the success/fail status and draw your own conclusions, or inspect a read-only function to discover the new state after the transaction was mined.