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Solana programming

Blockchain core concepts

See L1/Solana.md

Course

  • Solana Development Course - open sourced course to teach students both client & program side Solana development and concepts

SolDev.app

  • SolDev.app - excellent collection of resources, getting better by the day

Tutorials

Learning Rust

Essential

Bonus

Reference

Security

See also: Security.md

Devtools

Walkthrough

Please feel free to directly edit: fix inaccuracies, expand content, condense sections, etc.

Goal of this section: condense the above resources and compile useful tidbits (tips, gotchas) to reference in the future.

Let's also take a look at the Serum code itself. The escrow tutorial is excellent, but it would be interesting to look at a directly relevant real-world program.

Overview

Framing

Let's start by framing what Solana programming is. Doing this will help us form the mental model of what exactly we're programming.

At the highest-level, you have two kinds of Solana programs:

  • an on-chain program. Analogous to "smart contract". Literally runs on the blockchain and executes "Solana computer instructions".
  • an off-chain app. Doesn't run on the blockchain but interacts with the programs that do.

I find this analogous to any client-server interaction model, where the server (backend) is the on-chain program and the client (frontend) is an app that interacts with it. In fact, writing Solana programs is a lot like implementing RPC servers, REST endpoints, [insert distributed / IPC thing here]. On-chain programs can interact with other on-chain programs, just like how any server can also be a client.

Tooling

Solana supports any programming language that can compile to BPF bytecode. However, the vast majority of the ecosystem and tooling is in Rust. All code in this tutorial is written in Rust, unless otherwise specified.

Relevant Rust crates:

  • solana-program for writing on-chain programs
    • in all likelihood, you will also depend on existing programs in the Solana program library which are published as their own crates, e.g. spl-token
  • solana-sdk, solana-client for off-chain programs

On-chain programs

The solana-program crate exposes a macro aptly named entrypoint!

  • It follows the framework over library model, i.e. they call you

  • So it expects you to pass in a function with a very specific signature. This function is where you will define your logic that ultimately runs on the chain once deployed (more on that later).

  • The inputs to this deployed function are passed in via a transaction

  • Solana program flow:

    • Process the input, including deserialization to determine the user's instructions
    • Do your logic / algorithm
    • Return the serialized outputs in a ProgramResult, a thin-wrapper around std::result::Result.

  • See the helloworld example

Anchor

Echo

  • If you've worked through the "Echo program" from the Solana bootcamp, you might be curious how the same program might be written in Anchor.
    • See here for a quick-and-dirty implementation of the above.

Testing

In general, it's a good practice to take a layered approach to testing. For example:

  • for fast and lightweight tests, use regular Rust unit tests. Optionally, use solana-program-test and BanksClient.
  • for something heavier but closer to the "real deal", consider standing SolanaTestValidator inside your integration.rs.
    • with Anchor, consider also writing "end-to-end" tests using (anchor test), which will also test the IDL.
  • finally, deploy to devnet and testnet before mainnet-beta.

Rust's convention is unit tests (#[cfg(test)]) and integration tests (separate tests subdirectory). In addition to this, Anchor supports end-to-end integration tests via anchor test.

Highlights

Here are some differences to highlight about Anchor:

  • Instead of passing a list of AccountInfo and processing them with next_account_info, all the account-passing is handled by Anchor via struct definitions.
    • Notice how they #[derive(Accounts)]: Anchor does the wiring of account passing and parsing for us. We instead just pass in Context<T>).
  • A lot of the checks / constraints are now pulled out of the actual "processing" and pushed into Anchor macros.
    • This allows us to separate business logic from administrative logic.

Reification

  • The process of turning all the account parsing / processing logic into structs is known as reification. We've transformed code into data, thereby reifying it.
  • The inverse is Church encoding. Turning data into code.
  • Read more here if this excites you.

Anchor has sample Escrow program. Their version implements an escrow program that is Paulx++: it supports cancellation, and makes use of more advanced and idiomatic Rust. See the code.

Walkthrough: Serum (WIP)

Let's take a look at something more complicated than Escrow. How about the Serum dex itself!

To begin, let's examine the project structure. It looks like a monorepo: both the on-chain program and the app live in the same codebase.

Ok, now let's look for the entrypoint to the on-chain program. Remember that entrypoint! macro mentioned earlier? It's being called in dex/src/lib.rs:

#[cfg(all(feature = "program", not(feature = "no-entrypoint")))]
use solana_program::entrypoint;
#[cfg(feature = "program")]
use solana_program::{account_info::AccountInfo, entrypoint::ProgramResult, pubkey::Pubkey};

#[cfg(feature = "program")]
#[cfg(not(feature = "no-entrypoint"))]
entrypoint!(process_instruction);
#[cfg(feature = "program")]
fn process_instruction(
    program_id: &Pubkey,
    accounts: &[AccountInfo],
    instruction_data: &[u8],
) -> ProgramResult {
    Ok(state::State::process(
        program_id,
        accounts,
        instruction_data,
    )?)
}

Simple enough. Continuing to follow the white rabbit into the process method of state.rs:

#[cfg_attr(not(feature = "program"), allow(unused))]
impl State {
    #[cfg(feature = "program")]
    pub fn process(program_id: &Pubkey, accounts: &[AccountInfo], input: &[u8]) -> DexResult {
        let instruction = MarketInstruction::unpack(input).ok_or(ProgramError::InvalidArgument)?;
        match instruction {
            MarketInstruction::InitializeMarket(ref inner) => Self::process_initialize_market(
                account_parser::InitializeMarketArgs::new(program_id, inner, accounts)?,
            )?,
            MarketInstruction::NewOrder(_inner) => {
                unimplemented!()
            }
            MarketInstruction::NewOrderV2(_inner) => {
                unimplemented!()
            }
            MarketInstruction::NewOrderV3(ref inner) => {
                account_parser::NewOrderV3Args::with_parsed_args(
                    program_id,
                    inner,
                    accounts,
                    Self::process_new_order_v3,
                )?
            }
        ...

Interesting! So it looks like they deserialize the instructions then pattern match to figure out what the client wants to do.

As you might expect, these are standard instructions you'd run on an order book.

APIs and the bigger picture

One thing that stands out here is the NewOrder, NewOrderV2, NewOrderV3 definitions, with the first two unimplemented. It looks a little funky. It appears to be handling for API versioning / compatibility.

Here is a key point: a Solana program is ultimately an API. As will all APIs, it's important to think carefully about the interface you're providing.

  • What are you going to do when your program logic changes?
  • How are you going to add features (API additions)?
  • How are you going to deprecate functionality (breaking changes)?

Ok, so why is this important?

When there is an entire ecosystem of other programs or client apps that depend on your on-chain program, it's especially critical to consider these points.

Imagine a dApp developer building a UI on top of the Serum interface. How would they feel if Serum was constantly changing the way they implemented order submission, breaking their app? They'd probably stop using Serum.

In the spirit of decentralization, you should consider it your responsibility to think carefully about your consumers everytime you make a change your program. Did the public API change? Is it really encapsulated from the client? A culture of constant breaking changes will damage morale and eventually drive developers away from the ecosystem.

All this said, as with anything there is a trade-off. Solana programming is still very new. Development is rapid and ongoing. That comes hand-in-hand with breaking changes.

In my opinion, the best thing to do is think for yourself: carefully consider the trade-offs and make an informed decision. Don't blindly break APIs, thinking that's ok just because Solana development is new. Don't chain yourself to previous iterations of your code either -- if your app has evolved, your codebase should evolve with it. Solana explicitly spells out some backwards compatibility guidelines.

API Evolution in Serum (WIP)

Take a look at this pull request releasing Dex V3, a breaking change: project-serum/serum-dex#97. Here's the description:

The primary change is to immediately match incoming orders against the book, instead of first buffering them in the request queue. The request queue still exists to reduce breakage, but is always empty. Because of this, we're forced to remove support for the old order placement and cancellation instructions, since they don't provide the bids and asks accounts which would be necessary in order to process them in the new model.

Some more context on the purpose of the above change:

  • orders used to go into a request queue, not the order book directly.
  • a "user" (i.e. either another on-chain program or client app) would explicitly send a "crank turn" instruction to Serum
  • this "crank turn" pulled requests off the queue and matched them in the order book

PR Observations:

  • All affected instructions implemented a corresponding new version "V2" or "V3", e.g. MarketInstruction::CancelOrderV2
  • The old versions of the instruction were then removed by replacing the body with unimplemented!

Solana programming pros and cons

LISP programmers know the value of everything and the cost of nothing

Solana programs are stateless. What does that mean? Programs cannot hold state, so it's passed in via process_instruction.

The consequence is the same input always results in the same output. If you're familiar with the "pure functional programming" paradigm, you'll feel right at home.

Not every chain enforces statelessness. What are the pros / cons to Solana doing this?

Pros:

  • Parallel processing. Sealevel (Solana runtime) knows exactly what data a program depends on and can safely parallelize
  • Stateless-ness and determinism makes it easier to reason about programs, because of referential transparency

Cons:

  • Debugging can be difficult because a lot of data lives outside your program that you have to fetch with RPC
  • APIs for passing around accounts are not that friendly: they're passed as an array, so you have to remember the position-order

Anchor aims to solve some of the cons described. See the above Anchor escrow tutorial as well as angkor wat.

Accounts (WIP)

  • Accounts are just bytearrays &[u8].
  • Accounts have a data field for you to store arbitrary information
  • Executable accounts are programs
  • Types of account "types" in Anchor: Account, Program, Sysvar, different macros
  • Go into how Serum parses the account array into the literal information it needs to execute the order instruction

Highlights

Solana programs are stateless

If the program needs to store state between transactions, it does so using accounts

Programs are constrained to run deterministically, so random numbers are not available

the basic operational unit on solana is an instruction. an instruction is one call into a program. one or more instructions can be bundled into a message. a message plus an array of signatures constitutes a transaction

It's worth reviewing the on-chain programming docs or at least the FAQ. It'll save you a debugging headache.

Gotchas

  • Don't use std::collections::HashMap. You'll get an obscure error because of the "no-randomness" constraint
    • Reason: HashMap<K, V, S = RandomState>. Notice the generic type S defaults to RandomState
    • It may be possible use by substituting a different, non-random S - see the with_hasher constructor (I have not tried this myself)
  • Relatedly, don't use rand crate. If a crate you depend on transitively depends on rand, follow this guide