開始之前，建議先閱讀 節點log簡介 ，了解log的格式，比較容易看懂本文。
如果新裝的節點，或是按了Remove all blockchain data，要怎麼知道區塊同步的進度？
查看 %appdata%\Pi Network\docker_volumes\supervisor_logs\stellar-core-stdout---supervisor-xxx_.log ，找"History"訊息。
(不知道 %appdata% 是什麼的人，請看這篇 https://yuanrui919.github.io/mvappdata/ )
1. 首先會檢查本地跟網路上的區塊編號，發現有所落差，會看到類似這樣的log，狀態進入到catchup：
[Ledger INFO] Lost sync, local LCL is 1, network closed ledger 7204156
[Ledger INFO] Changing state LM_BOOTING_STATE -> LM_CATCHING_UP_STATE
2. 接著要向History Archives下載資料，但歷史檔每64個區塊(大約5分鐘)才會更新一次，所以會等最新的歷史檔更新，才會開始下載(7204161-1=7204160，可以被64整除)，於是會看到這樣的log：
[History INFO] Waiting for trigger ledger: 7204156/7204161, ETA: 25s
[History INFO] Waiting for trigger ledger: 7204157/7204161, ETA: 20s
...
[History INFO] Waiting for trigger ledger: 7204160/7204161, ETA: 5s
3. 確定區塊7204161檢查點已經關閉了，就可以開始追趕了，但只往回追256個區塊(7203903~7204159)，並不會下載全部的區塊。這是由CATCHUP_COMPLETE、CATCHUP_RECENT兩個參數所控制的。
[History INFO] Starting catchup after ensuring checkpoint ledger 7204161 was closed on network
[History INFO] Starting catchup with configuration:
lastClosedLedger: 1
toLedger: 7204159
count: 256
4. 下載狀態文件，是json格式。
[History INFO] Catching up to ledger 7204159: Downloading state file history/00/6d/ed/history-006ded3f.json for ledger 7204159
[History INFO] Downloading history archive state: history/00/6d/ed/history-006ded3f.json
[History INFO] Catching up to ledger 7204159: Downloading state file history/00/6d/ec/history-006dec3f.json for ledger 7203903
[History INFO] Downloading history archive state: history/00/6d/ec/history-006dec3f.json
5. 下載區塊XDR檔。
[History INFO] Catching up to ledger 7204159: downloading ledger files 1/5 (20%)
[History INFO] Catching up to ledger 7204159: downloading ledger files 5/5 (100%)
[History INFO] Catching up to ledger 7204159: Succeeded: batch-download-ledger-006dec3f-006ded7f : 5/5 children completed
6. 驗證剛下載內容(注意這裡的中括號和小括號，中括號有包含，小括號沒有包含)。
[History INFO] Verifying ledgers [7203903,7204160)
[History INFO] Verifying ledger [seq=7204159, hash=b60608] against SCP hash
[History INFO] Catching up to ledger 7204159: verifying checkpoint 1/5 (20%)
[History INFO] History chain [7203903,7204159] verified
[History INFO] Catching up to ledger 7204159: Succeeded: verify-ledger-chain
7. 如果本地區塊是從1開始，又會下載一個很大的檔案，有2GB。我不確定是怎麼回事，應該跟歷史檔歸檔的方式有關，XDR檔總是會不斷合併，可能是下載最近一段完整的檔案。
[History INFO] Catching up to ledger 7204159: downloading and verifying buckets: 0/21 (0%)
[History INFO] Catching up to ledger 7204159: downloading and verifying buckets: 1/21 (4%)
...
[History INFO] Catching up to ledger 7204159: Succeeded: download-verify-buckets : 21/21 children completed
8. 寫入本地的bucket。
[History INFO] Applying buckets
[History INFO] Catching up to ledger 7204159: Applying buckets 0%. Currently on level 10
[Bucket INFO] Bucket-apply: 12288 entries in 1.17MB/2.05GB in 0/21 files (0%)
...
[Bucket INFO] Bucket-apply: 22027837 entries in 2.05GB/2.05GB in 21/21 files (100%)
[History INFO] Catching up to ledger 7204159: Applying buckets 100%. Currently on level 0
9. Apply Buckets完成。
[History INFO] ApplyBuckets : done, restarting merges
[History INFO] Catching up to ledger 7204159: Succeeded: download-verify-apply-buckets
10. 這邊4個checkpoint是256個區塊(64*4=256)(7203904~7203967, 7203968~7204031, 7204032~7204095, 7204096~7204159)。
[History INFO] Downloading, unzipping and applying transactions for checkpoint 7203967
[History INFO] Downloading, unzipping and applying transactions for checkpoint 7204031
[History INFO] Downloading, unzipping and applying transactions for checkpoint 7204095
[History INFO] Downloading, unzipping and applying transactions for checkpoint 7204159
11. 開始把區塊寫入DB及XDR檔(7203904~7204159，共256個區塊)。
[History INFO] Catching up to ledger 7204159: Download & apply checkpoints: num checkpoints left to apply:4 (0% done)
[Tx INFO] applying ledger 7203904 (txs:28, ops:66)
[Tx INFO] applying ledger 7203905 (txs:31, ops:31)
...
[Tx INFO] applying ledger 7204159 (txs:23, ops:41)
[History INFO] Catching up to ledger 7204159: Download & apply checkpoints: num checkpoints left to apply:0 (100% done)
12. 但是在下載與驗證歷史檔的過程中，網路上仍有新的區塊產生，這些都在buffer裡面，也要寫下來。
[History INFO] Catching up to ledger 7204159: Applying buffered ledgers: Running: apply-buffered-ledgers
[History INFO] Scheduling buffered ledger-close: [seq=7204160, prev=b60608, txs=24, ops=49, sv: [ SIGNED@testnet2 txH: fa44fb, ct: 1651505354, upgrades: [ ] ]]
[History INFO] Catching up to ledger 7204159: Applying buffered ledgers: Waiting before starting apply ledger 7204160
[Tx INFO] applying ledger 7204160 (txs:24, ops:49)
...
[History INFO] Scheduling buffered ledger-close: [seq=7204454, prev=eb1aee, txs=18, ops=41, sv: [ SIGNED@testnet3 txH: a22be3, ct: 1651507075, upgrades: [ ] ]]
[History INFO] Catching up to ledger 7204159: Applying buffered ledgers: Waiting before starting apply ledger 7204454
[Tx INFO] applying ledger 7204454 (txs:18, ops:41)
13. 在處理buffer裡的區塊的過程中，網路上仍有新的區塊產生。如果某些因素造成buffer跟網路的最新區塊仍有延遲，Catchup會失敗，要再重複一次下載歷史檔的程序。
[History INFO] (discontiguous; will fail and restart): Catching up to ledger 7204159: Applying buffered ledgers: apply ledger 7204454
[History WARNING] Expected buffered ledger=7204455, actual=7204456
[History INFO] Catching up to ledger 7204159: Applying buffered ledgers: Failed: apply-buffered-ledgers
[History WARNING] Catchup failed
14. 如果步驟12沒問題，就同步完全了。若結果像步驟13這樣，會不斷重複步驟2,3,4,5,6,10,11,12，直到同步完成。
[History INFO] Catching up to ledger 7204159: Applying buffered ledgers: apply ledger 7204454
[History INFO] Catchup finished
[Ledger INFO] Changing state LM_CATCHING_UP_STATE -> LM_SYNCED_STATE
15. 當其他狀況造成區塊延遲，如網路斷線、停電、重開機等等，Catchup的作業也是跟上面相同。