Augment Code vs Copilot

augment-vs-copilot.md

We are facing some issues and want to understand what is happening analyzing log messages.

Context:

• https://github.com/untillpro/voedger/tree/732311621f1f682168b8b5ecee29ec855260de46

Prompt:

explain Key=1: compareAndSwap

Augment Code, Sonnet 4.5

I'll help you understand the Key=1: compareAndSwap concept. Let me search the codebase for information about this.

Now let me look at the specific context where Key=1: compareAndSwap appears in the elections implementation:

Based on my search through the codebase, I can now explain what Key=1: compareAndSwap means:

Explanation of Key=1: compareAndSwap

This is a log message from the leader election system in the Voedger codebase. Let me break it down:

Context

The message appears in the elections package (pkg/ielections/impl.go), which implements a distributed leader election mechanism using TTL-based storage.

What it means

• Key=1: This is the election key identifier. In the VVM (Voedger Virtual Machine) orchestration context, the key 1 represents the pKeyPrefix_VVMLeader constant used for VVM leader elections
• compareAndSwap: This refers to the atomic Compare-And-Swap operation being performed on the storage

How it works

ok, err := e.storage.CompareAndSwap(key, val, val, int(ttlSeconds))
if err != nil {
    logger.Error(fmt.Sprintf("Key=%v: compareAndSwap failed, will release: %v", key, err))
}

if !ok {
    logger.Error(fmt.Sprintf("Key=%v: compareAndSwap !ok => release", key))
}

Purpose

The CompareAndSwap operation is used to maintain leadership by:

1. Checking if the current value in storage matches the expected value (oldVal)
2. Updating the value to newVal (in this case, same value) if the check succeeds
3. Refreshing the TTL to extend the leadership claim

This happens periodically (every ttlSeconds/2) in a background goroutine to keep the leadership alive.

The atomic operation

// CompareAndSwap performs an atomic compare-and-swap operation similar to Cassandra's UPDATE IF.
// It updates a record's value only if its current value matches oldValue.
//
// Parameters:
//   - pKey: Primary key bytes
//   - cCols: Clustering columns bytes (if any)
//   - oldValue: Expected current value bytes to compare against
//   - newValue: New value bytes to swap in if comparison succeeds
//   - ttlSeconds: Time-to-live in seconds to set/update on the record

Why it matters

• Returns true: Leadership is successfully renewed (value matched and TTL refreshed)
• Returns false: Someone else has taken leadership or the key expired - the current leader must release
• Returns error: Storage operation failed - leadership must be released

This is a critical part of the distributed consensus mechanism that ensures only one VVM instance acts as the leader at any given time.

Copilot, Claude Sonnet 4.5

Perfect! Now I have all the context. Let me provide a clear explanation:

Explanation

Key=1 in the context of compareAndSwap refers to the partition key prefix used in the VVM elections/leadership storage system.

Technical Details

In consts.go, there's an iota enumeration that defines different storage key prefixes:

const (
    pKeyPrefix_null           pKeyPrefix = iota // 0
    pKeyPrefix_VVMLeader                        // 1 - Elections
    pKeyPrefix_SeqStorage_Part                  // 2 - Sequencer partition
    pKeyPrefix_SeqStorage_WS                    // 3 - Sequencer workspace
    pKeyPrefix_AppTTL                           // 4 - Application TTL storage
)

pKeyPrefix_VVMLeader = 1 is the partition key prefix for the VVM elections system, which uses CompareAndSwap for leader election operations.

How it's used

In impl_elections.go, when building storage keys for elections:

func (s *implIElectionsTTLStorage) buildKeys(key TTLStorageImplKey) (pKey, cCols []byte) {
    pKey = make([]byte, utils.Uint32Size)
    binary.BigEndian.PutUint32(pKey, pKeyPrefix_VVMLeader)  // Sets to 1
    cCols = make([]byte, utils.Uint32Size)
    binary.BigEndian.PutUint32(cCols, key)
    return
}

This partition key (value=1) identifies all election-related records in the underlying storage system, allowing the CompareAndSwap operation to atomically update leadership values for distributed coordination.

Prompt for Augment: which value is used to check?

Let me search for the specific value being used in the compareAndSwap operation for the VVM leader election.

Perfect! Now I have the complete picture. Let me trace through the exact value being used:

Now I have all the information needed to answer the question. Let me provide a clear explanation:

Answer: Which value is used to check?

The value being checked in the CompareAndSwap operation is the VVM's IP address as a string.

The flow:

1. When leadership is acquired (line 137 in pkg/vvm/impl_orch.go):

   vvm.leadershipCtx = elections.AcquireLeadership(vvmIdx, vvm.ip.String(), leadershipDurationSeconds)

• Key: vvmIdx (starts at 1, represents the VVM slot number)
• Value: vvm.ip.String() (the IP address of the current VVM instance)

2. When leadership is maintained (line 71 in pkg/ielections/impl.go):

   ok, err := e.storage.CompareAndSwap(key, val, val, int(ttlSeconds))

• oldValue: val (the IP address string that was originally stored)
• newValue: val (the same IP address string - no change)
• Purpose: Verify that the stored value still matches this VVM's IP address

What this means:

The CompareAndSwap checks if the value stored in the leadership key still equals the current VVM's IP address.

• If it matches: This VVM is still the leader, and the TTL is refreshed
• If it doesn't match: Another VVM has taken over leadership (or the key expired), so this VVM must release leadership

Example:

If VVM with IP 192.168.1.100 acquires leadership for key 1:

• Storage stores: Key=1 -> "192.168.1.100"
• Every renewal checks: "Is the value at Key=1 still 192.168.1.100?"
• If yes: Refresh TTL and continue as leader
• If no: Someone else is leader, release and stop

This mechanism ensures that only the VVM that originally acquired leadership can renew it, preventing conflicts in distributed deployments.

Prompt for Copilot: which value is used to compare with?

Found it! Looking at impl.go:

ok, err := e.storage.CompareAndSwap(key, val, val, int(ttlSeconds))

Answer

The value being compared is val (the leadership value), and it's compared with itself (val → val).

Why compare with the same value?

This pattern is used for TTL renewal during leadership maintenance:

1. Old value: val - The current leader's identifier that was stored when leadership was first acquired
2. New value: val - The same identifier (no change)
3. Purpose:
   • Verify we still hold the leadership (the stored value matches our val)
   • Refresh the TTL to extend the leadership duration
   • Detect if someone else has taken over (if the value changed, CompareAndSwap returns false)

The flow

1. Acquire leadership: InsertIfNotExist(key, val, ttlSeconds) - Store initial value
2. Maintain leadership: Every ttlSeconds/2, call CompareAndSwap(key, val, val, ttlSeconds) to:
   • Confirm we're still the leader (value hasn't changed)
   • Reset the TTL clock
3. If CAS fails: Someone changed the value or it expired → release leadership

So the "comparison" is actually a leadership verification + TTL refresh operation in one atomic step.