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

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.