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Arc 11 covered Days 71–77 of Epoch 3, and it was all about Cross-Program Invocations. In Arc 9, we wrote our first Solana program. In Arc 10, we gave that program a more useful state model with Program Derived Addresses. But our programs were still mostly working alone. They could read and update their own accounts, enforce their own constraints, and respond to instructions sent by a client. They could not directly change state owned by another program or bypass the rules that program enforced. That is an important part of Solana’s security model. The System Program owns the rules for creating accounts and transferring lamports. Token-2022 owns the rules for mints, token accounts, supply, and mint authorities. If our program needs one of those capabilities, it calls the program that owns the operation. That call is a Cross-Program Invocation, or CPI. The Web2 comparison is a service-to-service API call. One service sends a request through another service’s public interface, and the receiving service applies its own rules. A CPI works in a similar way, with one important difference: the outer and inner instructions execute as part of the same Solana transaction. If the inner call fails, the state changes made by the outer instruction are rolled back too. That combination of clear program boundaries and atomic execution is what makes Solana programs composable. Our first CPI called the System Program The arc began with the smallest useful CPI we could build. Our Anchor program accepted a sender, a recipient, and an amount of SOL to transfer. But the program did not edit the sender’s balance directly. Instead, it called the System Program’s transfer instruction. That distinction matters. Accounts on Solana are owned by programs, and the owner program controls how their data may be changed. Our program could not simply reproduce the effect of a System Program transfer by adjusting balances itself. It had to ask the System Program to perform the operation. Every CPI need

Published to Dev.to — Bitcoin Development Series, Part 1 of 4_ Bitcoin is heading toward an August 2026 deadline for BIP-110, a proposed temporary softfork that would restrict Ordinals-style arbitrary data from being embedded in transactions for one year. As of today, miner signaling sits at effectively zero. The proposal is almost certainly going to fail but the mechanics of why and how are worth understanding if you work anywhere near the Bitcoin protocol. This post walks through how soft fork activation works, what BIP-110 specifically proposes, and how to inspect miner signaling yourself with code. What Is a Soft Fork? A soft fork is a backward-compatible change to Bitcoin's consensus rules. Nodes running old software still accept blocks from nodes running the new rules — but not vice versa. This is what makes soft forks safer than hard forks in a permissionless network: you do not force everyone to upgrade on day one. Hard forks, by contrast, change rules in a way that causes old nodes to reject new blocks entirely. They require near-universal coordination, which is why Bitcoin has avoided them. How Soft Fork Activation Works: BIP 9 The dominant activation mechanism used since 2016 is defined in BIP 9 . The process works like this: A proposal is assigned a version bit (bit 0–28) in the block header's nVersion field. Miners signal readiness by setting that bit in blocks they produce. Activation requires 95% of blocks in a 2,016-block retarget window to signal support. There is a starttime and a timeout . If the threshold is not met before timeout , the proposal fails and is discarded. # Simplified BIP 9 state machine logic THRESHOLD = 0.95 # 95% of blocks in a retarget window WINDOW = 2016 # one retarget period def check_activation ( signaling_blocks : int , total_blocks : int ) -> str : ratio = signaling_blocks / total_blocks if ratio >= THRESHOLD : return " LOCKED_IN " # activates after one more window return " STARTED " # still counting print ( check_activati