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Smart contract as an actor

In previous chapters, we were talking about the actor model and how is it implemented in the blockchain. Now it is time to look closer into the typical contract structure to understand how different features of the actor model are mapped to it.

This will not be a step-by-step guide on contract creation, as it is a topic for the series itself. It would be going through contract elements roughly to visualize how to handle architecture in the actor model.

An explanation how are CosmWasm execution performed can be found in SEMANTICS.md, here I will try to explain it step by step.

The state

As before we would start with the state. Previously we were working with the tg4-group contract, so let's start by looking at its code. Go to cw-plus/contracts/cw4-group/src. The folder structure should look like this:

  src
├──  contract.rs
├──  error.rs
├──  helpers.rs
├──  lib.rs
├──  msg.rs
└──  state.rs

As you may already figure out, we want to check the state.rs first.

The most important thing here is couple of constants: ADMIN, HOOKS, TOTAL, and MEMBERS. Every one of such constants represents a single portion of the contract state - as tables in databases. The types of those constants represent what kind of table is this. The most basic ones are Item<T>, which keeps zero or one element of a given type, and Map<K, T> which is a key-value map.

You can see Item is used to keep an admin, and some other data: HOOKS, and TOTAL. HOOKS is used by the cw4-group to allow subscription to any changes to a group - a contract can be added as a hook, so when the group changes, a message is sent to it. The TOTAL is just a sum of all members' weights.

The MEMBERS in the group contract is the SnapshotMap - as you can imagine it is a Map, with some steroids - this particular one, gives us access to the state of the map at some point of history, accessing it by the blockchain height. height is the count of blocks created since the begging of blockchain, and it is the most atomic time representation in smart contracts. There is a way to access the clock time in them, but everything happening in a single block is considered happening in the same moment.

Other types of storage objects not used in group contracts are:

  • IndexedMap - another map type, which allows accessing values by a variety of keys
  • IndexedSnapshotMap - IndexedMap and SnapshotMap married

What is very important - every state type in the contract is accessed using some name. All of those types are not containers, just accessors to the state. Do you remember that I told you before, that blockchain is our database? And that is correct! All those types are just ORM to this database - when we will use them to get actual data from it, we would pass a special State object to them, so they can retrieve items from it.

You may ask - why all those data for a contract are not auto-fetched by whatever is running it? That is a good question. The reason is, that we want contracts to be lazy with fetching. Copying data is a very expensive operation, and for everything happening on it, someone has to pay - it is realized by gas cost. I told you before, that as a contract developer you don't need to worry about gas at all, but it was only partially true. You don't need to know exactly how gas is calculated, but by lowering your gas cost, you would may execution of your contracts cheaper which is typically a good thing. One good practice to achieve that is to avoid fetching data you will not use in a particular call.

Messages

In a blockchain, contracts communicate with each other by some JSON messages. They are defined in most contracts in the msg.rs file. Take a look at it.

There are three types on it, let's go through them one by one. The first one is an InstantiateMsg. This is the one, which is sent on contract instantiation. It typically contains some data which are needed to properly initialize it. In most cases, it is just a simple structure.

Then there are two enums: ExecuteMsg, and QueryMsg. They are enums because every single variant of them represents a different message which can be sent. For example the ExecuteMsg::UpdateAdmin corresponds to the update_admin message we were sending previously.

Note, that all the messages are attributed with #[derive(Serialize, Deserialize)], and #[serde(rename_all="snake_case")]. Those attributes come from the serde create, and they help us with deserialization of them (and serialization in case of sending them to other contracts). The second one is not required, but it allows us to keep a camel-case style in our Rust code, and yet still have JSONs encoded with a snake-case style more typical to this format.

I encourage you to take a closer look at the serde documentation, like everything it is there, can be used with the messages.

One important thing to notice - empty variants of those enums, tends to use the empty brackets, like Admin {} instead of more Rusty Admin. It is on purpose, to make JSONs cleaner, and it is related to how serde serializes enum.

Also worth noting is that those message types are not set in stone, they can be anything. This is just a convention, but sometimes you would see things like ExecuteCw4Msg, or similar. Just keep in mind, to keep your message name obvious in terms of their purpose - sticking to ExecuteMsg/QueryMsg is generally a good idea.

Entry points

So now, when we have our contract message, we need a way to handle them. They are send to our contract via entry points. There are three entry points in the cw4-group contract:

#[cfg_attr(not(feature = "library"), entry_point)]
pub fn instantiate(
deps: DepsMut,
env: Env,
_info: MessageInfo,
msg: InstantiateMsg,
) -> Result<Response, ContractError> {
// ...
}
#[cfg_attr(not(feature = "library"), entry_point)]
pub fn execute(
deps: DepsMut,
env: Env,
info: MessageInfo,
msg: ExecuteMsg,
) -> Result<Response, ContractError> {
// ..
}
#[cfg_attr(not(feature = "library"), entry_point)]
pub fn query(deps: Deps, _env: Env, msg: QueryMsg) -> StdResult<Binary> {
// ..
}

Those functions are called by the CosmWasm virtual machine when a message is to be handled by contract. You can think about them as the main function of normal programs, except they have a signature that better describes the blockchain itself.

What is very important, is that the names of those entry points (similarly to the main function) are fixed - it is relevant, so the virtual machine knows exactly what to call.

So let's start with the first line. Every entry point is attributed with #[cfg_attr(not(feature = "library"), entry_point)]. It may look a bit scary, but it is just a conditional equivalent of #[entry_point] - the the attribute would be there if and only if the "library" feature is not set. We do this, to be able to use our contracts as dependencies for other contract - the final binary can contain only one copy of each entry point, so we make sure, that only the top-level one is compiled without this feature.

The entry_point attribute is a macro which generates some boilerplate. As the binary is run by WASM virtual machine, it doesn't know much about Rust types - the actual entry point signatures are very inconvenient to use. To overcome this issue, there is a macro created, which generates entry points for us, and those entry points are just calling our functions.

Now take a look at functions arguments. Every single entry point takes as the last argument a message which triggered the execution of it (except for reply - I would explain it later). In addition to that, there are additional arguments provided by blockchain:

  • Deps or DepsMut object is the gateway to the world outside the smart contract context. It allows accessing the contract state, as well as querying other contracts, and also delivers an Api object with a couple of useful utility functions. The difference is, that DepsMut allows updating state, while Deps allows only to look at it.
  • Env object delivers information about the blockchain state in the moment of execution - its height, the timestamp of execution and information about the executing contract itself.
  • MessageInfo object is information about the contract call - it contains the address which sends the message, and the funds send with the message.

Keep in mind, that the signatures of those functions are fixed (except the messages type), so you cannot interchange Deps with DepsMut to update contract state in the query call.

The last portion of entry points is the return type. Every entry point returns a Result type, with any error which can be turned into a string - in case of contract failure, the returned error is just logged. In most cases, the error type is defined for a contract itself, typically using a thiserror crate. Thiserror is not required here, but strongly recommended - using it makes the error definition very straightforward, and also improves the testability of the contract.

The important thing is the Ok part of Result. Let's start with the query because this one is the simplest. The query always returns the Binary object on the Ok case, which would contain just serialized response. The common way to create it is just calling a to_binary method on an object implementing serde::Serialize, and they are typically defined in msg.rs next to messages types.

Slightly more complex is the return type returned by any other entry point - the cosmwasm_std::Response type. This one keeps everything needed to complete contract execution. There are three chunks of information in that.

The first one is an events field. It contains all events, which would be emitted to the blockchain as a result of the execution. Events have a really simple structure: they have a type, which is just a string, and a list of attributes which are just string-string key-value pairs.

You can notice that there is another attributes field on the Response This is just for convenience - most executions would return only a single event, and to make it a bit easier to operate one, there is a set of attributes directly on response. All of them would be converted to a single wasm event which would be emitted. Because of that, I consider events and attributes to be the same chunk of data.

Then we have the messages field, of SubMsg type. This one is the clue of cross-contact communication. Those messages would be sent to the contracts after processing. What is important - the whole execution is not finished, unless the processing of all sub-messages scheduled by the contract finishes. So if the group contract sends some messages as a result of update_members execution, the execution would be considered done only if all the messages sent by it would also be handled (even if they failed).

So when all the sub-messages sent by contract are processed, then all the attributes generated by all sub calls and top-level calls are collected and reported to the blockchain. But there is one additional information - the data. So this is another Binary field, just like the result of a query call, and just like it, it typically contains serialized JSON. Every contract call can return some additional information in any format, You may ask - in this case, why do we even bother returning attributes? It is because of a completely different way of emitting events and data. Any attributes emitted by the contract would be visible on blockchain eventually (unless the whole message handling fails). So if your contract emitted some event as a result of being sub call of some bigger use case, the event would always be there visible to everyone. This is not true for data. Every contract call would return only single data chunk, and it has to decide if it would just forward the data field of one of the sub-calls, or maybe it would construct something by itself. I would explain it in a bit more detail in a while.

Sending submessages

I don't want to go into details of the Response API, as it can be read directly from documentation, but I want to take a bit closer look to the part about sending messages.

The first function to use here is add_message, which takes as an argument the CosmosMsg (or rather anything convertible to it). Message added to response this way, would be sent and processed, and its execution would not affect the result of the contract at all.

The other function to use is add_submessage, taking a SubMsg argument. It doesn't differ much from add_message - SubMsg just wraps the CosmosMsg, adding some info to it: the id field, and reply_on. There is also a gas_limit thing, but it is not so important - it just causes sub-message processing to fail early if the gas threshold is reached.

The simple thing is reply_on - it describes if the reply message should be sent on processing success, on failure, or both.

The id field is an equivalent of order id in our KFC example from the very beginning. If you send multiple different sub-messages, it would be impossible to distinguish them without that field. It would not be even possible to figure out, what type of original message is reply handling! This is why the id field is there - sending a sub-message you can set it to any value, and then on the reply, you can figure out what is happening based on this field.

An important note here - you don't need to worry about some sophisticated way of generating ids. Remember, that the whole processing is atomic, and only one execution can be in progress at once. In most cases, your contract sends a fixed number of sub-messages on very concrete executions. Because of that, you can hardcode most of those ids while sending (preferably using some constant).

To easily create sub-messages, instead of setting all the fields separately, you would typically use helper constructors: SubMsg::reply_on_success, SubMsg::reply_on_error and SubMsg::reply_always.

CosmosMsg

If you took a look at the CosmosMsg type, you could be very surprised - there are so many variants of them, and it is not obvious how do they relate to communication with other contracts.

The message you are looking for this is the WasmMsg (CosmosMsg::Wasm variant). This one is very much similar to what we already know - it has a couple of variants of operation to be performed by contracts: Execute, but also Instantiate (so we can create new contracts in contract executions), and also Migrate, UpdateAdmin, and ClearAdmin - those are used to manage migrations (will tell a bit about them at the end of this chapter).

Another interesting message is the BankMsg (CosmosMsg::Bank). This one allows a contract to transfer native tokens to other contracts (or burn them - equivalent to transferring them to some black whole contract). I like to think about it as sending a message to very special contract responsible for handling native tokens - this is not a true contract, as it is handled by the blockchain itself, but at least to me it simplifies things.

Other variants of CosmosMsg are not very interesting for now. The Custom one is there to allow other CosmWasm-based blockchains to add some blockchain-handled variant of the message. This is a reason why most message related types in CosmWasm are generic over some T - this is just a blockchain-specific type of message. We will never use it in the wasmd. All other messages are related to advanced CosmWasm features, and I will not describe them here.

Reply handling

So now that we know how to send sub-message, it is time to talk about handling the reply. When sub-message processing is finished, and it is requested to reply, the contract is called with an entry point:

#[cfg_attr(not(feature = "library"), entry_point)]
pub fn reply(deps: DepsMut, env: Env, msg: Reply) -> Result<Response, ContractError> {
// ...
}

The DepsMut, and Env arguments are already familiar, but there is the new one, substituting the typical message argument: the cosmwasm_std::Reply.

This is a type representing the execution status of the sub-message. It is slightly processed cosmwasm_std::Response. First important thing it contains, is an id - the same, which you set sending sub-message, so now you can identify your response. The other one is the ContractResult, which is very similar to the Rust Result<T, String> type, except it is there for serialization purposes. You can easily convert it into a Result with an into_result function.

In the error case of ContracResult, there is a string - as I mentioned before, errors are converted to string right after execution. The Ok case contains SubMsgExecutionResponse with two fields: events emitted by sub-call, and the data field embedded on response.

As said before, you never need to worry about forwarding events - CosmWasm would do it anyway. The data however is another story. As mentioned before, every call would return only a single data object. In the case of sending sub-messages and not capturing a reply, it would always be whatever is returned by the top-level message. But it is not a case when reply is called. If a reply is called, then it is a function deciding about the final data. It can decide to either forward the data from the sub-message (by returning None) or to overwrite it. It cannot choose, to return data from the original execution processing - if the contract sends sub-messages waiting for replies, it is supposed to not return any data, unless replies are called.

But what happens, if multiple sub-messages are sent? What would the final data contain? The rule is - the last non-None. All sub-messages are always called in the order of adding them to the Response. As the order is deterministic and well defined, it is always easy to predict which reply would be used.

Migrations

I mentioned migrations earlier when describing the WasmMsg. So migration is another action possible to be performed by contracts, which is kind of similar to instantiate. In software engineering it is a common thing, to release an updated version of applications. It is also a case in the blockchain - SmartContract can be updated with some new features. In such cases, new code is uploaded, and the contract is migrated - so it knows, that from this point, its messages are handled by another, updated contract code.

However, it may be, that the contract state used by the older version of the contract differs from the new one. It is not a problem if some info was added (for example some additional map - it would be just empty right after migration). But the problem is, when the state changes, for example, the field is renamed. In such a case, every contract execution would fail because of (de)serialization problems. Or even more subtle cases, like adding a map, but one which should be synchronized with the whole contract state, not empty.

This is the purpose of the migration entry point. It looks like this:

#[cfg_attr(not(feature = "library"), entry_point)]
pub fn migrate(deps: DepsMut, env: Env, msg: MigrateMsg) -> Result<Response<T>, ContracError> {
// ..
}

MigrateMsg is the type defined by the contract in msg.rs. The migrate entry point would be called at the moment of performing the migration, and it is responsible for making sure, the state is correct after the migration. It is very similar to schema migrations in traditional database applications. And it is also kind of difficult, because of version management involved - you can never assume, that you are migrating a contract from the previous version - it can be migrated from any version, released anytime - even later than that version we are migrating to!

It is worth binging back one issue from the past - the contract admin. Do you remember the --no-admin flag we set previously on every contract instantiation? It made our contract unmigrateable. Migrations can be performed only by contract admin. To be able to use it, you should pass --admin address flag instead, with address being the address which would be able to perform migrations.

Sudo

Sudo is the last possible entry point in CosmWasm, and it is the one we would never use in wasmd. It is equivalent to CosmosMsg::Custom, but instead of being a special blockchain-specific message to be sent and handled by a blockchain itself, it is now a special blockchain-specific message sent by blockchain to contract in some conditions. There are many uses for those, but I will not cover of them, because it is not related to CosmWasm itself. The signature of sudo looks like:

#[cfg_attr(not(feature = "library"), entry_point)]
pub fn sudo(deps: DepsMut, env: Env, msg: SudoMsg) -> Result<Response, ContractError> {
// ..
}

The important difference is, that because sudo messages are blockchain specific, the SudoMsg type is typically defined by some blockchain helper crate, not the contract itself.