The Checks-Effects-Interactions Pattern: A Key to Secure Smart Contracts

Discussing the practice and its secuirty implications.

When it comes to smart contract secuirty, there are different ways in which we try to enhance our safety, be it by smart contract auditing, using newly developed AI tools, or any other method you like. There have been recent advancements in almost every method. We use newly developed methodologies and techniques to ensure no vulnerability remains unnoticed.

Many advanced practices try to reduce the risk of protocol getting compromised. Still, the most important part of your secuirty journey is secuirty oriented development. You should create smart contracts keeping secuirty as a preference for coding. This can reduce a lot of work in the later stages of development.

There are various things involved in security-oriented development, keeping yourself updated with the latest trends in Web3 hacks, keeping yourself skilled as per new development practices, etc. One practice that has helped a lot when creating smart contracts is the Checks-Effects-Interactions Pattern. What is it?

What is Checks-Effects-Interactions?

The Checks-Effects-Interaction is a pattern used in the context of smart contract development, ensuring the secuirty and integrity of smart contracts. It helps in building robust and secure smart contracts. This pattern revolves around how the structure of a smart contract should be distributed in 3 distinct sections. Let’s see what all these sections comprise.

Checks:- The first step in this pattern is to perform all the necessary checks before executing any state-changing actions. The state here means the data we are concerned with. These Checks ensure that the conditions required for the execution of the contract are met. For example, while performing any ERC20 transaction, there should be a check about the authorisation of the transaction initiator. These checks reduce the risks of potential vulnerabilities minimising the chances of unintended consequences.

Effects:- This section is where all the state-changing operations are performed. This is the next step from checks. In checks, we refrained from making any state-changing actions. Once we are through the checks sections, it implies that all the necessary conditions required for transactions are met, and now we can modify the state of the blockchain by updating the concerned variables. This section is where the understanding of the underlying principle of the protocol comes into use. Since we are dealing with making changes to the state based on the functionality, the logic of the code would be correct, allowing users to perform transactions seamlessly and without any discrepancy.

Interactions:- After we finish the checks and effects, the next section is Interactions. This involves calling external contracts or interacting with other components of the protocol. This typically includes invoking other smart contract functions and external actions like making API calls or interacting with off-chain systems. THis part should be given extra since the interactions should be performed safely, and all the potential risks and dependencies should be properly managed. This also involves properly handling the errors, gas composition and other optimization-related issues.

Advantages of Using the Check-Effect-Interaction Pattern

In the previous part of the blog, we learned what the Check-Effect-Interaction(CEI) system is, its components, and how they are connected to each other. In this section, let’s learn about the security advantages it provides.

  1. Separation of Concerns:- The easiest way to do something is to break it into parts and do each separately and effectively. This is what the CRI pattern allows us to do. It promotes a clear separation of concerns by dividing the smart contract logic into three distinct sections making it easier to analyse code and identify potential vulnerabilities.
  2. Preventing Reentarancy attacks:- Reentarancy has been quite popular for some time and can be beaten using the CEI pattern while creating smart contracts. By following the CEI pattern, we perform checks before any state-changing operation even takes place, This ensures that all necessary conditions are verified and prevents external contracts from manipulating the contract’s state during critical operations, thus mitigating the risk of reentrancy attacks.
  3. Improved contract behaviour predictability:- The CEI patterns help structure our smart contracts. Following the CEI patterns, we organise the code into different sections; thus, it becomes easier for developers to understand the code and determine the behaviour of the contract, making the testing, auditing, and verification part easier.
  4. Reduced Attack Surface:- By enforcing checks before effects, the CEI pattern minimises the attack surface of a smart contract. This means potential vulnerabilities and risks are identified and addressed before executing state-changing operations.


The CEI patterns help developers create reliable smart contracts resistant to secuirty vulnerabilities, attacks and unintended consequences. This pattern helps to provide a clear separation of the smart contracts making it easier to diagnose and promoting a very structured way of developing smart contracts.

The CEI is, without a doubt, very effective when it comes to developing the smart contract efficiently, but this does not guarantee absolute secuirty because secuirty is a vast field, and vulnerability can be found anywhere in the protocol; thus, following a CEI pattern is just a step to reduce the risk of getting hacked. The smart contract developers must also consider other secuirty practices, conduct thorough testing and audits of smart caontracts, and stay updated with the latest secuirty trends.

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