Actor Model
CosmWasm, at its core, is built around the actor model. This prevents some common pitfalls which, for example, Ethereum smart contracts had to fix over the years. The issues mostly revolved around reentrancy issues (opens in a new tab) where we could, for example, call out to another contract before finishing the current execution.
If you are already familiar with the actor model through, for example, programming in languages such as Erlang or Elixir, you can skip this page.
While the term "actor model" sounds fancy, it is actually quite simple. Each contract is an "actor" and plays a role in the system (pun intended).
Each actor has its own state and can only be interacted with via messages. This means a contract can only interact with the outside world via messages and only manipulate its own state.
Picture two people living in two houses, each house has a mailbox. If person A wants to talk to person B, they write a letter, address it to person B, and send it off. The mailman (or in this case, the chain) will put the letter in person B's mailbox.
Person B will then read the letter and can choose to write a response, address it to person A with a note indicating that this is a reply, and send it off once again.
No direct interaction is happening here, nobody visits each other's house which would allow them to rummage around in their belongings (state). They simply exchange messages and put everything the other side needs to know into a message.
As you can see in the simple graphic above, they simply exchange messages addressed to each other, and the messages reference previous messages if need be.
In CosmWasm you can only pass a single message type to a contract endpoint. If you are wondering how to handle multiple message types, check out the enum dispatch page.
But how does that fix reentrancy? In CosmWasm, you can only send out messages at the end of a contract execution as part of the response. This ensures you have already written everything to the state, meaning the state can't suddenly change mid-execution and make your contract exhibit buggy behaviour.
CosmWasm effectively forces you to follow the CEI pattern (Checks, Effects, Interactions) (opens in a new tab) while other similar systems only have this as a "best practice".