Design Pattern Solidity: Free up unused storage
Why you should clean up after yourself
You may or may not be used to a garbage collectors in your previous programming language. There is no such thing in Solidity and even if there was a similar concept, you would still be better off managing state data yourself. Only you as a programmer can know exactly which data will not be used in the future anymore.
You may be wondering 'Why should I care?'. Let us take a look at the reasons for releasing unused data.
1. Gas costs
The obvious is answer is that you can receive gas refunds for releasing unused storage. In the yellow paper on page 25 'Appendix G. Fee Schedule', you can read the gas costs for each instruction. As you might know, SSTORE will generally create the most costs in your contracts with a significant cost of 20,000 gas per instruction. On the contrary, if you look at R_sclear:
Refund given when the storage value is set to zero from non-zero.
15,000 gas refund means you can actually get 75% of your storing costs back! That is a large amount, do not forget about this. And the solution is simple, just set a value back to 0 once you are sure it will not be used anymore.
2. Network bloating
If the gas costs did not convince you, I hear you. It seems like a design flaw in Ethereum that you can just store data and leave it there forever without ever paying for it again. But that does not mean should store everything forever. Storing useless data only forces every Ethereum node to store your data for all eternity. What a waste of resources!
In fact, new rental models have been discussed, even sophisticated methods including revival methods. I do not see those coming any time soon (within the next 1-2 years), but if your contract should be as future-proof as possible, you better keep your storage usage low!
How to free up unused data
Now to the practical part. Freeing up data is really easy and there is even a special keyword delete in Solidity to help you.
Deleting simple types
For simple types like integers, it does not really matter if your write delete myInteger or myInteger = 0. It will have the same effect. Depending on the context, you may choose one way over another. Generally, if it is about some calculations = 0 is easier to read and if it is only about freeing up storage delete will be easier to read.
Deleting arrays
But for deleting arrays, you always want to use delete. It will automatically create an array of length 0 for dynamic arrays or set each item of the array to 0 for static arrays. If you want to delete just a single item like delete myArray[index], you should be aware that this will create a gap in the array. To avoid the gap and if you do not care about the order of the items, what you can do instead is myArray[index] = myArray[myArray.length - 1]; followed by myArray.pop(). Pop removes the last element and also implicitly calls delete on the removed element.
Deleting structs
Instead of having to write myStructInstance = new MyStruct(0,0,address(0)), you can just write delete myStructInstance. It will automatically clear out each entry of your struct with one exception:
Deleting mappings
If you have a mapping in a struct or in your state in general, Solidity cannot delete it, because it does not know the keys for the mapping. Since the keys are arbitrary and not stored along, the only way to delete structs is to know the key for each stored value. A value can then be deleted by delete myMapping[myKey].
Solidity Developer
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Avoiding out of gas for Truffle tests
How you do not have to worry about gas in tests anymore
You have probably seen this error message a lot of times: Error: VM Exception while processing transaction: out of gas Disclaimer : Unfortunately, this does not always actually mean what it is saying when using Truffle , especially for older versions. It can occur for various reasons and might be...
Design Pattern Solidity: Stages
How you can design stages in your contract
Closely related to the concept of finite-state machines, this pattern will help you restrict functions in your contract. You will find a lot of situations where it might be useful. Any time a contract should allow function calls only in certain stages. Let's look at an example: contract Pool {...
Web3 1.2.5: Revert reason strings
How to use the new feature
A new Web3 version was just released and it comes with a new feature that should make your life easier. With the latest version 1.2.5, you can now see the the revert reason if you use the new handleRevert option. You can activate it easily by using web3.eth.handleRevert = true . Now when you use...
Gaining back control of the internet
How Ocelot is decentralizing cloud computing
I recently came across an ambitious company that will completely redefine the way we are using the internet. Or rather, the way we are using its underlying infrastructure which ultimately is the internet. While looking at their offering, I also learned how to get anonymous cloud machines, you...
Devcon 5 - Review
Impressions from the conference
I had a lot to catch up on after Devcon. Also things didn't go quite as planned, so please excuse my delayed review! This year's Devcon was certainly stormy with a big typhoon warning already on day 1. Luckily (for us, not the people in Tokyo), it went right past Osaka. Nevertheless, a lot of...
Devcon 5 - Information, Events, Links, Telegram
What you need to know
Devcon 5 is coming up soon and there are already lots of events available, information about Osaka and more. Here is a short overview: Events Events Calendar Events Google Docs Events Kickback Most events are in all three, but if you really want to see all, you will have to look at all three...
Design Pattern Solidity: Off-chain beats on-chain
Why you should do as much as possible off-chain
As you might have realized, Ethereum transactions are anything but cheap. In particular, if you are computing complex things or storing a lot of data. That means sometimes we cannot put all logic inside Solidity. Instead, we can utilize off-chain computations to help us. A very simple example...
Design Pattern Solidity: Initialize Contract after Deployment
How to use the Initializable pattern
There are a few reasons why you might want to initialize a contract after deployment and not directly by passing constructor arguments. But first let's look at an example: contract MyCrowdsale { uint256 rate; function initialize(uint256 _rate) public { rate = _rate; } } What's the advantage over...
Consensys Blockchain Jobs Report
What the current blockchain job market looks like
Consensys published their blockchain jobs report which you can checkout in their Blockchain Developer Job Kit. The most interesting aspects are Blockchain developer jobs have been growing at a rate of 33x of the previous year according to LinkedIns jobs report Typical salary is about...
Provable — Randomness Oracle
How the Oraclize random number generator works
One particularly interesting approach by Provable is the usage of a hardware security device, namely the Ledger Nano S. It uses a trusted execution environment to generate random numbers and provides a Provable Connector Contract as interface. How to use the Provable Randomness Oracle? Use the...
Solidity Design Patterns: Multiply before Dividing
Why the correct order matters!
There has been a lot of progress since the beginning of Ethereum about best practices in Solidity. Unfortunately, I have the feeling that most of the knowledge is within the circle of experienced people and there aren’t that many online resources about it. That is why I would like to start this...
Devcon 5 Applications closing in one week
Devcon 5 Applications closing
Watch out for the Devcon 5 applications. You only have one week left to apply either as Buidler Student Scholarship Press Devcon is by far the biggest and most impressive Ethereum conference in the world. And it's full of developers! I am especially excited about the cool location this year in...
Randomness and the Blockchain
How to achieve secure randomness for Solidity smart contracts?
Update 2023 : Ethereum transitioned to Proof of Stake! If you are interested in the randomness there, you can now use the updated info over at https://soliditydeveloper.com/prevrandao. When we talk about randomness and blockchain, these are really two problems: How to generate randomness in smart...